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CHAPTER NINE
TREE ANATOMY AND PHYSIOLOGY
Trees require sunlight, water, and nutrients in order to thrive, and their vertical and
horizontal growth hinges on their ability to use electromagnetic energy to convert carbon
dioxide into other organic compounds such as sugars. We witness primary and secondary tree
growth anecdotally by observing how heights or diameters of trees increase each year. The
crown of a tree captures energy and transpires, and the root system of a tree collects water
and nutrients and grounds the tree system to prevent catastrophic failure. The bole of a tree
provides vertical support and facilitates upward and downward movement of liquids and
minerals. Tree shape and form is not only determined by genetics but also by the environment
in which it is situated. Individual trees compete for resources with other trees and plants, and
they have adapted as species to the various opportunities nature provides. Their growth
behavior in groups and in reaction to natural and anthropogenic events needs to be
understood by natural resource managers so that sound decisions regarding their
management can be made.
OBJECTIVES
The development of a forest is intimately tied to the growth or demise of individual trees.
The mechanisms which affect the future status of individual trees help us understand how forest
structure might change over time (Ward and Stephens 1993). Models of forest dynamics may
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also help us to understand these transitions and to visualize the potential future conditions in
light of anthropogenic (human-caused) or natural disturbances. The main focus of this chapter is
on tree anatomy and physiology, and biological processes operating at the scale of a cell and up
to the size of a tree. One goal of this book is to help readers learn, comprehend, and articulate the
essential principles of forestry and natural resource management. Therefore, as outcomes of
reading this chapter, the reader should be able to:
 Understand the structure of trees and describe their various functions.
 Understand the needs of trees to survive and the processes by which materials (water,
minerals, etc.) move through trees.
 Understand the photosynthetic process and how trees use carbon dioxide, water, and
minerals to create carbon building blocks and oxygen.
 Understand how and why trees respire, how they respond to signals, why they have
different tolerances to shade, and what nutrients are necessary for unrestricted growth.
 Understand the function of tree root systems.
 Understand the various regeneration processes trees employ.
Ultimately, through the topics presented in this chapter, readers should gain a basic
understanding of the composition of a typical tree, how it grows, and how it maintains itself in
order to contribute to the wide suite of ecosystem services forests provide.
I. TREE ANATOMY
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If one could ascribe a goal for a forest tree or plant, it would be to grow large enough,
and to live long enough, to produce seed and to perpetuate its species. Perhaps unknowingly,
trees also provide a number of provisional and regulatory ecosystem services that are beneficial
to humans and other plant and animal life. In order to facilitate these things, a tree or plant must
compete with other vegetation for resources. Thus, it attempts to secure resources by growing
upward and outward, filling the space with foliage, and growing downward and outward, filling
the space with a root system (Lanner 2002). In order to survive, a tree or plant needs to absorb
energy (sunlight), carbon dioxide (CO2), water (H2O), and nutrients, all the while competing for
light, water, and nutrient resources with neighboring trees and plants, and respiring oxygen back
into the atmosphere. In addition, a tree or plant needs defense mechanisms to be able to survive
droughts, to resist insects and diseases, to survive severe wind events and heavy accumulations
of water and ice, and to recover from animal damage (Figure 1). In other words, trees must be
capable of adapting and taking advantage of resources when they become available to be capable
of self-healing and to be able to develop mechanical structures that resist and distribute stresses
(Bejan et al. 2008). All of this is further complicated by the fact that once established, and
without assistance from humans, a tree or plant cannot move naturally, on their own, to a more
suitable location.
The primary growth of trees is vertical, through division and enlargement of cells out of
the vertical shoots or leaders of a tree. More precisely, vertical growth occurs in the apical or
shoot meristems of tree leaders, which are located in the apex of dormant buds (Figure 2). Apical
meristems are very small sets of undifferentiated tissue (young tissue that we are unable to
determine their structural role) found in the growing tips of buds or roots in plants. At some point
this tissue becomes differentiated into xylem and phloem (secondary meristem or procambium),
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pith (ground meristem), or epidermis (protoderm). Leaf primordia also form in the apical
meristem, and these are lateral outgrowths from the apical meristem that eventually develop into
leaves or needles. In temperate forests, dormant buds are protected by bud scales, and in tropical
forests the apical meristem in trees is generally protected by a looser rosette of embryonic leaves.
In the spring, when the bud axis expands, the primordia enlarge and growth just below the bud
axis forces the apex upward in a surge of growth. Once growth has ceased, the apical meristem
forms a new bud for the next cycle of growth, and the supporting tissue is lignified, making it
woody in nature (Lanner 2002).
Secondary growth in trees is horizontal in nature, and, simply put, as a tree grows in
height, it must also grow in girth in order to maintain itself standing amongst its peers (who are
all competing for light by getting their canopies as high as possible above ground) and against
the forces of wind. Trees grow in diameter through the enlargement of cells in the vascular
cambium (a lateral meristem). The cambium is a multi-cell layer of undifferentiated cells that are
located between the inner bark and the outermost portion of an annual ring. The cambium
comprises two basic types of cells: (a) elongated fusiform initials, which produce xylem and
phloem, and (b) ray initials which give rise to rays. In this cambial zone, cells produced to the
inside of the tree become differentiated into mother xylem (wood) cells (Figure 3). Mitosis
(separation of chromosomes into sets of nuclei) and cytokinesis (division of the nuclei) are the
processes involved in the mitotic phase of cell division from mother cells to daughter cells.
These cells may divide several times before differentiating into distinct, mature xylem cells
(Lachaud et al. 1999). Mature xylem cells, in addition to providing strength, conduct water and
minerals (saps) that may also contain organic compounds, amino acids, and proteins.
Interestingly, the pathway of the ascent of sap within a tree has been understood for over 200
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years (Pickard 1981). The long-term sustainability of some forestry and agroforestry systems
may be threatened in the future by regional climatic changes where rainfall is predicted to
decline up to 30% from current rates during certain seasons of the year. Therefore, it is important
to understand the role of fluid transport in trees and how it may vary according to the season
(David et al. 2004).
Xylem cells produced early in a growing season are large in diameter and have thin cell
walls and are thus relatively soft and weak. These are typically called earlywood, in comparison
to cells produced later in the season (smaller diameter, thicker cell walls that are tougher and
stronger) that are called latewood. If the earlywood is comprised of cells whose diameters are
distinctly larger than the cell diameters of the latewood, the resulting type of wood is considered
ring porous. The contrast between the earlywood and the latewood in many temperate tree
species results in an annual ring from which one might be able to date the age of a tree (Figure
4). If there is little or no differentiation in the size of the earlywood or the latewood cell
diameters, the wood is considered diffuse porous. Here, the contrast between earlywood and
latewood in some temperate hardwoods may be less distinct (Figure 5). Since the growth of trees
is nearly continuous or episodic in tropical forests, annual rings may be absent (Lanner 2002). If
present, the thickness of each ring is representative of the resources available during the growth
period; therefore, one can interpret periods of drought and periods of intense competition for
light and nutrients from the annual rings. The central portion of a tree bole is inactive and has the
sole function of supporting the canopy. Not all trees have it, yet the heartwood of a tree is a
central core of dead xylem in which the cell walls are filled with defensive or waste materials,
tannins, dyes, resins, oils, and other organic compounds.
The pith of a tree is a residual core of cellular material that is generated by a growing
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shoot, and xylem is placed over this tissue to form the bole of a tree. As we mentioned, the initial
xylem cells produced by the cambium are considered mother cells, as are phloem cells, which
are produced just to the outside of the cambium and form the inner bark. As with mother xylem
cells, these mother phloem cells may divide several times before differentiating into distinct,
mature phloem cells (Lachaud et al. 1999). Mature phloem is a group of sieve or tube cells that
serve to distribute photosynthate and other substances produced by the leaves or needles to the
rest of the tree. In contrast to xylem cells, which must be dead and emptied of their contents in
order to function, phloem cells must be alive to perform their function. However, they live only
one year and are crushed between the expansion of xylem and the existing wall of bark in the
second year (Lanner 2002). As a result, bark is essentially an assorted layer of crushed phloem
cells and other cortical remnants or protective cells generated by the cambium (Coder 1999). It is
pushed outward by the formation of new bark and new wood (xylem), and the outer layers
stretch, crack, and eventually slough off (Forest Products Laboratory 1987).
Although we have briefly described cell structure, before we delve too far into tree
growth and growth dynamics, we need to step back and describe the basic building blocks of
wood. Below the bark, a tree is composed mainly of hollow, elongate, spindle-shaped cells that
are parallel to one another within the bole, limbs, and branches. Customarily, wood cells are
called libriform fibers, tracheids, or vessels, and they vary in length both within a tree and
between tree species, with the age of a tree, with the season in which they were formed, and
perhaps due to other environmental factors. Tracheids are the primary water-conducting cells in
conifers, cycads, and ginkgo (Ginkgo biloba) trees and other gymnosperms (naked seed plants).
Tracheids and have thick cell walls, and provide structural support to trees. Vessels are the
water-conducting elements of angiosperms (flowering plants). Both of these are located in the
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xylem area of a tree, and their lignified walls are perforated to allow the flow of water and
minerals from one to the next. The presence of these two types of cells differentiate vascular
plants (those with xylem and phloem) from non-vascular plants. Libriform fibers, thin cells with
simple pits, are the major component of wood in deciduous (hardwood) trees. The general range
of fiber lengths for softwood (coniferous) trees is 3 to 8 millimeters (mm), while the range of
fiber lengths for hardwood (deciduous) trees is generally 1 to 2 mm, but again there is
considerable variation. For example, in one analysis, the fiber length of poplar hybrids (a
hardwood) grown in Washington State ranged from 0.4 to 1.0 mm and seemed to increase with
the age of the trees (DeBell et al. 2002).
The main chemical components of fibers and tracheids are cellulose, hemicellulose, and
lignin, which we described in Chapter 4. Cellulose and hemicellulose are polymers of glucose.
Cellulose is a linear chain of glucose units, and hemicellulose is a polysaccharide. During the
growth of a tree, cellulose fibers are arranged into ordered strands of helically bound (twisted
together) fibrils, which are then organized into larger structural elements that form the walls of
wood fibers. Cellulose molecules are surrounded by lignin and hemicellulose. One of the main
roles of hemicellulose is to facilitate fiber-to-fiber bonding. Lignin is a natural phenolic resin (or
aromatic amorphous molecule, or three-dimensional phenyl-propane polymer) found within cell
walls, yet concentrated toward the outside and between cells. The main role of lignin is to hold
together the fibers and tracheids. In terms of structure, a tree cell consists of a thin primary wall
(Figure 6) and a thicker secondary cell wall consisting of three layers with cellulose fibrils
helically wound at different angles in the various layers. The walls of adjacent cells are bonded
together by the middle lamella (Figure 7), which consists of lignin, calcium, and pectin
substances (Schwarze 2007).
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Xylem rays radiate outward from the pith of a tree and are used for resource storage,
defense, and radial transport of materials between the xylem and phloem. Rays typically account
for 10 to 25% of tree cells (Lachaud et al. 1999). They can be heterocellular, as in oaks, pines,
and firs, or homocellular, as in maples, alders, and sycamores. A tree with a heterocellular ray
system is one where the wood is characterized as having more than one type of ray cell (e.g.,
parenchyma, tracheids, or sheath cells). A tree with a homocellular ray system has only one type
of ray cell (Zuckerman and Davidson 2003). Parenchyma is a type of cell in a tree whose
purpose (e.g., storage, assimilation, or wound healing) relates to the function of a tree rather than
the structure of a tree. Parenchyma is the food storage area of a tree and is more prominent in
angiosperms than in gymnosperms. In conifers, these are represented by resin ducts. In some
trees, a thin band of these is produced at the end or beginning of a growth season and is filled
with calcium oxalate crystals (interestingly, a constituent of human kidney stones and a scale that
forms on beer kegs) (Gourlay et al. 1996). Plasmodesmata (plasma-membrane lined channels in
connected cell walls) facilitate direct intercellular transport of nutrients, signals necessary for
growth and development, proteins, and viruses (Cilia and Jackson 2004).
Apical meristems are two groups of cells where new tree structures are developed. The
apical shoot meristem is responsible for the production of above-ground structures, and branches,
leaves, pollen cones, and seed cones (and other organs) originate from nearby axillary meristems
as small primordia. The shoot meristem consists of three layers (Figure 8), and layering is
maintained by cell divisions that are perpendicular to the surface in the outer two layers (Simon
2001). The apical root meristem is responsible for the production of below-ground structures,
and, thus, this is where the primary axis of the root system is produced, with lateral roots
originating a short distance behind. The root meristem is characterized as having a root cap (root
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tip), an elongation zone, and a differentiated zone. The presence of mycorrhizal fungi, which has
a symbiotic relationship with the root system of a plant, can influence water and nutrient uptake
by trees (Peterson et al. 1984). For most temperate and some tropical trees, mycorrhizal fungi are
found in the rhizosphere of the root system, a narrow region of soil surrounding the mycorrhizal
roots that contains bacteria and sloughed plant cells and that is influenced by root development.
A tree gains from the symbiotic relationship with mycorrhizal fungi a more efficient method of
obtaining soil water and nutrients. Plant growth promotes the development of diverse bacteria
communities, which can either facilitate or inhibit the development of mycorrhiza. Plants
allocate a good portion (10 to 20%) of their photosynthetic assimilate (consumed nutrients) to
mycorrhizal partners, which represents a major route of carbon transfer between the plant and the
soil (Bending et al. 2002).
In the bole of a tree, lateral meristems, the vascular cambium and the cork cambium, are
the two places where cell division occurs. The cork cambium is a one-cell thick layer situated
among the phloem that produces the cork (bark) cells that are used to protect the tree bole,
branches, and limbs and to reduce the losses of waters and sugars. The vascular cambium is a
thin layer of cells that develop both xylem and phloem for the transport of water and sugars in a
tree. With vascular cambium, xylem is produced to the inside of the tree bole, and phloem is
produced to the outside of the tree bole. As we alluded to earlier, the woody part of a tree is
composed of xylem, and the bark or cork part of a tree is composed of phloem and other tissue
material. The tree stem and branching system of a tree have the principle function of supporting
and positioning the canopy in such as way as to maximize the production of photosynthate (a
product of photosynthesis, such as a sugar). However, these portions of a tree also act as a
conducting pathway, allowing sugars to be transported downward to the root system and water
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upward to the canopy (Pickard 1981).
Leaves or needles of a tree contain chlorophyll and are the organ of the tree where
photosynthesis takes place. Pickard (1981) asserts about leaves that:
"Their basic job is to hold a disperse photosynthetic pigment system somewhat perpendicular
to an incident photon beam so that the pigment can transduce the energy of the incoming
photons into a form in which it can be used to synthesize sugar from water and atmospheric
carbon dioxide."
Stomata are cell structures that are contained in the epidermis of tree leaves and needles.
They are involved in the exchange of carbon dioxide (CO2) and water (H2O) between plants and
the atmosphere, thus stomatal conductance (the ability to convey or transmit) is an important
issue in achieving a balance with environmental conditions. Stomata are protected by specialized
sensory organs (guard cells) that respond to environmental conditions and adjust their turgor
(tension created by fluid content) to help control conductance (Mansfield 1998). Stomatal guard
cells respond to light, CO2, vapor pressure deficits, ozone levels, and hormones to adjust the
aperture (opening size) of the stomata (Chavez et al. 2011). Stomatal conductance varies among
and within tree species based on the amount of energy being received, the vapor pressure deficit
between the leaf (or needle) and the atmosphere, soil moisture, and leaf (or needle) temperature,
age, nitrogen content, and chlorophyll concentration (Matsumoto et al. 2005).
II. TRANSPIRATION AND SAP FLOW IN A TREE
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Most people understand that trees need water, nutrients, and sunlight in order to survive.
These resources are critical for the metabolic activity of wood cells, primordia, and parenchyma
during primary or secondary growth of a tree. Water is necessary for most of the metabolic
processes within a tree. Water must migrate through the soil to the root hairs of a tree, through
the cortex of the roots, and across an endodermis (the Casparian strip) or the periderm of the
roots, and into the stele which contains vascular tissue. As we noted, the primary components of
vascular tissue in trees are xylem and phloem, and the tracheids and vessels of the xylem assist in
the transport of water and minerals from the ground. Tree transpiration is a key factor in both
water and carbon exchange (Gartner et al. 2009). Transpiring plants lose water through the cell
walls of leaves or needles, and this loss is replaced with water transported through the xylem.
The cohesion-tension theory was developed to describe water movement up a tree stem and it
suggests that water movement is determined by water loss through transpiration, which creates
tension in the xylem tissue within a tree. This theory has been challenged by some because the
tension involved does not seem to be adequate for moving water upward, against gravity
(Pickard 1981, Meinzer et al. 2001). However, the theory has been debated and discussed for
over a century, even though some now suggest that other processes may also be involved. Not all
trees are alike in their rate of transpiration, transpiration rates seem to adjust to climatic
conditions, and substantial differences in stomatal regulation of water use at the leaf (needle)
level can be found among different tree species (Čermák et al. 1995). In addition, in a study of
maritime pine (Pinus pinaster) in southwestern France, transpiration rates declined with the age
of trees, due to reduced levels of stomatal conductance in taller trees and a reduction in leaf area
in older trees (Delzon and Loustau 2005).
Two types of processes, osmotic and physical, seem to regulate transpiration, which is
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often measured by the flow of sap in trees. The osmotic process is based on anatomical barriers
to sucrose movement and the ability of a tree to adjust the pressure needed to prevent or facilitate
the flow of water. The resulting turgor pressure exerted outward on cell walls makes plants rigid,
and allows plants to stand. The physical process is based on changes in air temperature and the
resulting effects on tree liquids. For example, during periods of time when maples (Acer spp.) are
dormant, and when air temperatures fluctuate above and below the freezing point, the alternating
pressure placed on the trunk and branches causes either the freezing or thawing of sap. When
warming occurs, sap pressure increases and is forced out of small wounds.
Humans have capitalized on these natural processes and devised ways to collect and use
the sap flowing through trees. The collection of maple and birch sap and latexes of tropical
plants are but some examples. Maple sap, for example, is characterized as a dilute solution of
water (95 to 99%), sugar (1 to 5%), and trace amounts of other substances. Sucrose is the
primary sugar found in maple sap, potassium and calcium comprise the majority of the inorganic
elements, and glycine, alanine, asparagines and others are representative of the free amino acids.
A few organic acids are found in maple sap, as are a wide range of phenolic compounds, most of
which appear to be derived from lignin (Perkins and van den Berg 2009). In contrast, the main
sugar components in birch sap are glucose and fructose (Kallio and Ahtonen 1987). Latexes of
tropical plants such as the rubber tree (Hevea brasiliensis), Indian mango (Mangifera indica) and
papaya (Carica papaya) are saps that contain sugars, tannins, resins, proteins and other minerals
and compounds. In many tropical trees, sap flows freely when fruit are cut and harvested (Saby
John et al. 2003).
III. THE PHOTOSYNTHETIC PROCESS
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Photosynthesis is a process that occurs in the leaves or needles of trees, and one whereby
a tree captures electromagnetic energy and uses it to convert carbon dioxide (CO2) into organic
compounds, including sugars. In the presence of enzyme systems associated with chlorophyll in
plants, electromagnetic energy (usually sunlight), CO2, and water combine to form glucose and
oxygen (Figure 9). During this event, light energy (photons) is absorbed by proteins in the
chloroplasts that contain chlorophyll, and this causes a reaction that produces enough energy to
split water into hydrogen and oxygen (Barnes et al. 1998). As a result, oxygen is released by the
plant, and carbon dioxide is converted to sugars through a process called carbon fixation. Most
species of plants require only simple inorganic nutrients to accomplish this process and therefore
are considered autotrophic.
The process of photosynthesis provides the carbon substrate (building blocks) for the
development of vascular tissue, roots, and other parts of a tree. Of the primary colors of light
absorbed by proteins in the chloroplasts that contain chlorophyll, trees absorb more blue (0.4-0.5
m) and red light (0.6-0.7 m) than they do green light (0.5-0.6 m). As a result, more green
light energy is reflected by the crowns of trees, which is why tree crowns appear greenish even
though the percent reflectance of green light energy may be relatively low (about 10%).
Obviously, if electromagnetic energy were to be limited, the rate of photosynthesis would
decline dramatically. The rate of photosynthesis in a plant can also be reduced due to stresses
such as injured leaves (or needles), water deficits, temperature extremes (hot or cold), and
nutrient deficiencies (Méthy et al. 1994). Long-term exposure to elevated rates of ozone (O3) can
also reduce the rate of photosynthesis and thus reduce the growth rate of forests (Matyssek et al.
2007).
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Leaves of temperate deciduous trees can change color in the latter part of the growing
season. In autumn, the level of chlorophyll produced in leaves slows and eventually ceases.
Chlorophyll is then destroyed, and the carotenoids present are allowed to express their color.
Cartenoids are organic pigments within chloroplasts, and they facilitate the energy transfer
process during photosynthesis. Cartenoids absorb blue light and therefore produce shades of
yellow, orange, and brown. While chlorophyll is being destroyed, anthocyanins are synthesized
to further alter the color of leaves and plant tissue. Anthocyanins are pigments that act as a
sunscreen and an antioxidant, and help produce shades of red, blue, and purple in plant tissue.
The difference in the timing of leaf color change and the amount of color expressed in leaves of
tree species is both genetically controlled and related to temperature and moisture conditions of
the environment (U.S. Forest Service 2011).
Leaves and needles of trees have a variety of shapes, arrangements, and surface textures.
They are enclosed in a thin layer of cuticle cells, below which is a set of epidermis cells, and
inside contains the palisade and spongy mesophyll cells (Figure 10 and 11). In addition to
providing protection to the enclosed mesophyll cells, the cuticle also acts to absorb ultraviolet
electromagnetic energy and acts as a defense mechanism (Kinnunen et al. 2001). Chloroplasts
are located in the mesophyll cells of leaves and needles that are located between the protective
upper and lower epidermis. The majority of photosynthetic processes occur in the palisade
mesophyll cells. The spongy mesophyll cells absorb air and perform light-independent
photosynthesis. Vascular tissue laced throughout leaves contains both xylem and phloem cells to
facilitate the transport of water and nutrients. We noted the function of stomata earlier and
suggested that stomata are scattered about the surface of a leaf or needle, linking the outside
world with the parenchyma, and regulated by guard cells. Stomata can be very densely packed
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on a leaf or needle surface. In a study of silver fir (Abies alba) seedlings, stomatal density was in
the range of 6,000 to 7,500 per square centimeter (cm2) (about 38,700 to 48,400 per square inch
(in2)) (Robakowski et al. 2004), in a study of silver birch (Betula pendula) the density was
around 13,000 per cm2 (83,900 per in2) (Pääkkönen et al. 1997), and in a study of Scots pine
(Pinus sylvestris), the density was around 9,700 per cm2 (about 62,600 per in2) (Turunen and
Huttunen 1996).
In autumn, a layer of new cells forms at the base of temperate, deciduous tree species leaf
stems, initiating the process of abscission (the shedding of leaves). As this layer of cells grows, it
clogs and closes the leaf veins that help transport fluids into and out of leaves. Some time after
leaf connecting tissues are sealed, leaves will separate and fall (abscise) from a tree. Trees such
as water oak (Quercus nigra) can retain their dead leaves for several months after they have
effectively died. Although they are often called evergreens, most coniferous trees have needles
that will not indefinitely remain attached to a tree, in fact older needles will change color and
eventually abscise and fall from a tree. Because most conifers retain green needles of staggered
ages that are one, two, and three (or greater) years old, the loss of older needles sometimes goes
unnoticed. The lifespan of a coniferous leaf (needle) can last two to five years, depending on the
species and climatic conditions (e.g., droughts may precipitate the loss of conifer needles). For
example, eastern white pine (Pinus strobus), Austrian pine (Pinus nigra), and Scots pine all
retain needles for about three years, while red pine (Pinus resinosa) retains needles for about
four years (Heimann and Pellitteri 2006).
IV. TREE RESPIRATION
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Trees both produce and consume oxygen. In conjunction with photosynthesis, carbon
fixation occurs in trees only when sunlight is available and with respect to deciduous trees, and
only when leaves or needles are available. However, respiratory activity occurs continuously in
the living tissues of trees, and over the course of a year respiratory activity from the woody part
of a tree can exceed the respiratory activity of foliage (leaves, needles) (Edwards and Hanson
1996). For example, the glucose created through photosynthesis can be broken down into water
and carbon dioxide during cellular respiration, creating energy that is necessary for certain
cellular growth activity. In order to measure total respiration for a whole tree or stand of trees,
both leaf and woody organ (root, trunk, and branch) respiration rates need to be considered.
There are two basic respiration components: growth-related respiration (for construction of new
tissue) and maintenance-related respiration (used to maintain ion gradients and repair
membranes) (Levy and Jarvis 1998). These are a function of tree growth rate (growth-related
respiration) or tree bole biomass (maintenance-related respiration), however estimates are
complicated by the activities of mature and growing woody tissue (Ryan et al. 1994).
Although variation exists vegetatively and geographically, respiration rate per unit
volume in a tree seems to decrease with increasing tree diameter, and maintenance-related
respiration seems to increase with increases in leaf area index, yet seems to decrease with tree
age and geographic latitude (Anekonda et al. 2000, Kim et al. 2007). Respiration also is affected
by the availability of photosynthetic substrates, and the season of year in boreal forests, and is
perhaps weakly correlated with air temperature (Griffiths et al. 2004). Maintenance-related
respiration rates have also been shown to be positively correlated with tree stem temperature and
tree volume (Lavigne et al. 1996). Growth-related respiration should increase with increasing
availability of nitrogen, mainly because growth rates increase as the supply of nitrogen increases
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(Maier 2001). Interestingly, increases in ozone (O3) concentrations in the atmosphere may either
lead to increases in respiration rates (due to increases in metabolic activity in order to repair
injuries) or decreases in respiration rates (due to decreases in growth due to injury) (Kellomäki
and Wang 1998).
V. TREE GROWTH
Trees essentially grow upward and outward through primary and secondary growth
processes. Every period of growth can be described as a coordinated longitudinal and radial
expansion of woody tissue (Coder 1999). Growth periods can be initiated by transitions out of
dormancy, by changes in temperature, and by precipitation events. With each period of growth, a
new sheath of woody material is produced as a layer covering the previous collection of woody
material. Growth occurs in the buds (shoot tips), root tips, and the cambium portion of a tree
(Coder 1999). New wood cells (xylem) are formed on the inside of the cambium, and new bark
cells (phloem) are formed on the outside, and no growth in diameter or length of cells continues
after they are initially formed. In other words, an increase in the size of a tree is due to the
addition of new wood cells, not the enlargement or elongation of old cells (Forest Products
Laboratory 1987).
The sapwood of a tree is the portion of the xylem between the cambium and the
heartwood of a tree, which contains the majority of live-cell volume in a tree stem. The sapwood
part of a tree contributes to the water and sap transport processes, and food is also stored in this
region. In general, the more vigorously-growing tree species will have wider regions of sapwood
(Forest Products Laboratory 1987). Cells in the heartwood area of a tree, which ranges from the
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edge of the sapwood to the pith of a tree, are inactive and do not function in the transport of
water or the storage of food. Extractives are stored in the heartwood, and, in some tree species
this results in darkened wood. The heartwood of some tree species (e.g., western redcedar (Thuja
plicata)) is also resistant to decay, but this is not a universal condition of tree heartwood.
However, the sapwood of all tree species is susceptible to decay. Some species, such as white
oak (Quercus alba), have heartwood that is plugged with ingrowths called tyloses. The resulting
tightly plugged cell structure prevents the passage of liquids through the cell pores. From a forest
products point of view, this condition makes the wood favorable for the production of tight
cooperage (e.g., barrels, kegs, vats) (Forest Products Laboratory 1987).
The cell division, expansion, and differentiation processes of a tree influence the stature
(overall size) and form (architecture) of the tree. The meristematic control of seasonal growth of
bud set and bud flush, along with cambial growth initiation and cessation, also influence tree
growth processes. Other traits of trees that influence their growth capacity include the water use
efficiency of a tree, the carbon storage and allocation processes employed (above- and belowground), and pathogen and insect stress resistance capabilities. Of course, growth rates are
important, as the rate of growth of the shoot meristem influences height growth and the rate of
growth of the vascular cambium influences diameter growth (girth). The condition of the root
system is also an important influence, as the rate of movement of nutrients and water from the
neighboring soil resource into the tree will ultimately affect tree growth (Grattapaglia et al.
2009).
VI. TREE ROOTING
19
Tree root systems are the primary link between the soil and water resources of the land
and the water and nutrient requirements of the tree bole, limbs, and foliage. Root systems
generally form symbiotic relationships with mycorrhiza-forming fungi or nitrogen-fixing
bacteria or actinomycetes. These relationships increase root system efficiency (Lanner 2002).
The outer layer of a root is considered an epidermis (Figure 12), which may contain root hairs
(tubular outgrowths) that extend the surface area of the root and increase the amount of contact
with soil and water resources. The epidermal surface near the tip of a root is where water enters
the tree (Pickard 1981). Below the epidermal layer lies a few layers of cells collectively known
as the cortex. The cortex is where starches (foods) are stored, and it is bound on the inside of the
root by the endodermis. The endodermis is a thin layer of tightly packed cells that act as an
impermeable barrier or water repellant layer (the Casparian strip) through which water can only
move one way, towards the center of the root system. Therefore, a tree root system is a conduit
for water to escape the soil and enter the bole of the plant (Bejan et al. 2008).
The portion of the soil profile where plant roots are found is called the rooting zone
(Ford-Robertson 1971), which on average is just 1 to 2 meters (m) (3.3 to 6.6 feet (ft)) below the
soil surface. However, for some tree species, in some conditions, the rooting depth may vary
widely and extend 20 to 30 m (65.6 to 98.4 ft) (or more) below the surface. For example, the
maximum depth of ponderosa pine (Pinus ponderosa) root systems has been estimated to be
between 1 and 24 m (Stone and Kalisz 1991). The root system can thus be considered a sink or
storage location for carbon acquired through photosynthesis (Tasser and Tappeiner 2005). The
amount of root biomass of a tree is positively correlated with the availability of water and
nitrogen in the soil; however, a negative correlation between root biomass and soil water was
recently observed in South African savannas, which are tropical summer rainfall ecosystems
20
(February and Higgins 2010). Further, a limited rooting zone can reduce sap flow within a tree
when soil moisture levels decrease due to, for example, droughts (Gartner et al. 2009).
The formation and establishment of tree root systems depends largely on the soil in which
the trees have germinated or in which they have been planted (Puhe 2003). The root cap is
responsible for determining the direction of growth, and it is responsive to both stimuli and
irritation. Although a number of factors affect the growth direction of tree roots, they grow in a
general direction consistent with the direction that they emerge from a parent root. However, tree
root growth is affected by the environment in which they are situated. Roots can become
deflected by barriers, stunted by obstacles, affected by anoxia, or impeded by various soil layers.
Factors that impede deep rooting of trees may increase the vulnerability of trees to windthrow
(Puhe 2003). Not all trees have one, but the taproot is the main descending root of a tree. A
taproot can assist in the mechanical stability of a tree and in the development of a straight tree
bole, although trees without taproots are also able to develop straight tree boles (Krause and
Plourde 2008). A taproot can facilitate the exploitation of resources deeper underground (Doi et
al. 2008). Lateral roots that perhaps extend outward from a taproot actually stabilize trees and act
as anchor roots. Lateral roots, on average, extend about 10 to 15 m (32.8 to 49.2 ft) from a tree
bole (yet perhaps as much as 30 to 60 m (98.4 to 196.9 ft)) and are important actors in water and
nutrient acquisition processes (Stone and Kalisz 1991).
The root density of a tree is a function of the underlying soil characteristics,
environmental variables, and tree competition, and is affected by compaction created by heavy
machinery. For example, radiata pine (Pinus radiata) root systems have been shown to be
affected by the competition imposed from nearby Australian oak (Eucalyptus obliqua) root
systems in an Australian study (Bi et al. 1992). A root system is an extensive array of woody
21
material underneath the soil. As an example, the density of roots in jack pine (Pinus banksiana)
stands in Alberta was suggested to be about 47,000 root pieces per squared meter (m2) (about
4,400 per squared feet (ft2)) of soil, straight down, nearly 52,000 per m2 (about 4,800 per ft2) for
an aspen (Populus tremuloides) forest, and about 28,000 per m2 (about 2,600 per ft2) for a white
spruce (Picea glauca) forest (Strong and La Roi 1985). The linear density of fine roots has been
shown to range from about 8 centimeters (cm) of root per cubic centimeter (cm3) of soil (about
4.3 feet (ft) per cubic inch (in3)) in pin oak (Quercus palustris) plantations in the midwestern
United States (Watson and Kelsey 2006), yet about 1 cm of root per cm3 of soil (about 0.5 feet
per in3) in radiata pine forests in Australia (Bi et al. 1992).
Interestingly, although we typically imagine root systems as being located within soils,
and under the ground surface, roots systems have also been found in the canopies of tropical and
temperate trees (Nadkarni 1981, Sanford 1987). These types of root systems can occur through
the formation of adventitious roots arising from damage or injury to above-ground tree organs, or
as an adaptation to limited soil resources. The contribution of these to the total fine root system
of a tree, and to the functioning of a tree, is probably negligible, but much more needs to be
understood about their role in forest dynamics. A coincidental rediscovery of canopy roots in
windthrown, old-growth European beech (Fagus sylvatica) located in central Germany was
recently reported (Hertel 2011). The root systems were noticed in humus pockets that had
accumulated in forks of major branches of tree bole in the central part of a tree crown. The
chemical and morphological traits of these above-ground root systems are different than what
one would find in below-ground root systems, perhaps due to differences in water availability
(greater above ground than below ground).
22
VII. TREE REGENERATION
Tree regeneration is a process whereby plants maintain and perhaps expand their
population. Regeneration can occur through the creation of seed or through the liberation of
vegetative shoots. The life of a seed is best expressed as a complex series of events that begins
with the initiation of a flower, proceeds through stages of seed development, ripening, and
dispersal, and ends with seed germination. Flower initiation, rate, and timing vary with each tree
species. For example, some trees flower in the early spring (cherry) while others flower in late
summer (pine). Most trees that are native to temperate forests flower only once per year while
some native tropical forest trees flower several times per year (Krugman et al. 1974).
Seed-producing plants (spermatophyta) are divided into two botanical groups, the
angiosperms and the gymnosperms. There are a number of characteristics that are used to
describe each group, but the main distinction is that angiosperms (maple, oak, willow, etc.) have
seeds enclosed in a carpel, the female reproductive organ of the flower (Figure 13), while
gymnosperms (pine, spruce, fir) have seeds that are not enclosed in a carpel (Figure 14), and are
generally spirally arranged along a central axis to form a cone (Krugman et al. 1974).
Gymnosperms are therefore said to have naked seeds. Of course, there are some exceptions to
these general rules; for example, one non-coniferous gymnosperm (yew, Taxus brevifolia)
produces seeds that are not located within cones but that are enclosed within a berry-like, cupshaped disk called an aril (Bolsinger and Jaramillo 1990).
Flowers are characteristic reproductive units of trees, yet at times biologists cannot agree
on exactly what they contain (Melzer et al. 2010). Flowers form when a vegetative meristem of a
tree changes its pattern of cell development and begins to form reproductive organs (Krugman et
23
al. 1974). Tree flowers vary in size, shape, color, arrangement, odor, and components. A typical
angiosperm flower may have a stalk (peduncle), a calyx (composed of sepals), a corolla
(composed of petals), stamens (with anthers and filaments), and one or more pistils (containing a
stigma, style, and ovary). A bisexual, or perfect, flower has both a stamen and a pistil, while a
unisexual, or imperfect, flower has only one of these. The calyx and the corolla are designed to
protect the delicate stamen and pistil structure, and their color or odor may attract the necessary
pollinators to ensure transfer of pollen to the ovary. Embryosac formation is facilitated by
meiosis in the megasporangia. In angiosperms, pollen is formed through meiosis in the
microsporangia, or anthers, of male flowers. A typical gymnosperm flower consists of a small
cone (strobili) that lacks the calyx, corolla, stamen, and pistil structures of angiosperm flowers.
In gymnosperms, pollen is generally formed through meiosis in pollen sacs found beneath each
staminate cone scale (Krugman et al. 1974). Some species of trees, such as quaking aspen
(Populus tremuloides), may only produce unisexual male or female reproductive organs on
different individual trees, thus the trees may be considered either male or female. This is called a
dioecious condition; however, for a given tree species, a certain small percentage of trees may
actually produce flowers that contain both male and female organs. If the male and female
reproductive organs can be found on the same plant, they are called monoecious plants.
Interestingly, most gymnosperms are monoecious.
A gametophyte is a phase in the life cycle of a tree where sperm and eggs are produced.
In general, male and female gametophytes in both angiosperms and gymnosperms are just a few
cells in size, and each cell contains a single set of chromosomes. The gametophytes produce
through cell division (mitosis) male and female gametes. The combination of male and female
gametes produces a multicellular zygote, containing two sets of chromosomes. The gamete-
24
bearing phase of reproduction in gymnosperms is relatively short. Pollen grains produce sperm
cells and then pollen is transported to unfertilized seeds of the same species. Once inside the
ovule, fertilization (ovule development) occurs, and the resulting zygote develops into an embryo
and eventually a mature seed. Flowers of angiosperms produce both microspores, which become
pollen grains, and megaspores, which become the egg cells contained in the ovules. Pollen grains
are the male cells situated on the stamen of the flower, and they are produced by the anther. Tree
pollen is dispersed mainly by wind and insects, since the source and the desired destination of
pollen are in very close proximity. Ideally, pollen finds its way to the carpel, or pistil, of the
flower, which contains the egg cells. Gymnosperm reproductive cones are either male or female
organs, and as we noted, the ovules are not enclosed in a carpel (Melzer et al. 2010).
Gymnosperms typically develop a temporary herbaceous male cone that produces and releases
pollen and a more permanent woody female cone that contains the ovules. For species of pines
(Pinus spp.), the latter of these are called pine cones (Figure 15). Tree pollen is again dispersed
mainly by the wind and insects, yet, since the source and the desired destination of pollen are not
as close in proximity as angiosperm flowers, birds and other mammals may facilitate transport of
the pollen. In addition to pines, various species of spruce, fir, redwood (Sequoia sempervirens),
and other coniferous trees with hardened cones are representative of this group.
Germination of seed includes the events leading to the emergence of the embryo and its
subsequent development to the point where it becomes independent of the food resources
provided. If a seed does not germinate shortly after dispersal from the tree, it may remain in a
quiescent state (a persistent state of viability). Food reserves in seeds are mainly lipids (fats) and
carbohydrates, and the concentrations of each vary by tree species. The events leading to the
emergence of an embryo include the absorption of water and subsequent swelling of the seed
25
coat, enzymatic activity that signals the use of stored food reserves, respiration and assimilation
of nutrients, and cell enlargement. The latter of these events represents the plant growth that we
typically observe, and includes the growth of the root system and the growth of the plumule
(lower hypocotyl stem, upper epicotyl stem, and leaves). After the roots begin to establish
themselves (Figure 16), either a hypocotyl emerges upward with the cotyledons attached (typical
of epigeal germination) or cotyledons remain below the ground surface (typical of hypogeal
germination) (Krugman et al. 1974).
Seed production strategies can be considered sexual or asexual. Sexual processes, in
general, involve male sperm and female egg cells that unite to form a zygote (Barnes et al. 1998).
Typically, when viable seeds have been developed, they disperse from the tree, germinate in the
soil or litter layer of a forest floor, and become established as young tree seedlings. While
asexual reproduction processes are important in the development of fast-growing forest
plantations, sexual reproduction of tree species seems to be essential for the natural survival as
low gene flow between plant populations can cause inbreeding or poor adaptation to a changing
environment (Rasmussen and Kollmann 2004). In a natural environment, core populations of tree
species generally exhibit higher levels of genetic diversity than peripheral populations, yet there
are many exceptions to this rule (Pautasso 2009). In sexual reproduction of trees, the distance of
seed dispersal from a parent tree can be very extensive or may be very limited, and thus locally
or widely dispersed based on seed weight and buoyancy in air (Barnes et al. 1998). The timing of
reproduction by trees is controlled by environmental factors such as air temperature and wind
speed, as both pollination and dispersal of seed are affected by weather conditions. Some seeds,
for example, are designed to float with wind currents across the landscape. Other seeds, such as
those of eastern redcedar (Juniperus virginiana), may be digested by birds and then transported
26
considerable distances from the parent tree before being deposited on the ground. Some seeds are
relatively heavy (Figure 17), or perhaps encased inside heavy fruit, and, therefore, once detached
from the parent tree, they may be dispersed very short distances. The mechanism of seed
dispersal is one of the traits of a tree species that enables it to compete successfully in its niche
(Barnes et al. 1998).
Seed can become available through an active seed bank (viable for about a year) or a
dormant or persistent seed bank (viable for more than a year). Seed from some tree species can
remain viable in the soil for many years after dispersal; however, the role of a persistent seed
bank varies, and germination of seed may only occur in the soil (from persistent seed) or in the
litter layer (from new seed) of a forest floor (Deiller et al. 2003), or on hosts such as logs or
stumps (Figure 18). Tree species that regenerate through seed in areas where disturbances such
as fire or clearcuts have occurred are typically called pioneer species. Tree species that become
established through seed under an existing forest canopy and work their way into the canopy
when opportunities become available are typically called gap-phase species. The seedlings of
other tree species may have become established through seed and may persist in the understory
of a forest canopy for many years before opportunities to grow into the canopy become available
(Barnes et al. 1998).
Trees must emerge from a juvenile life phase before they are capable of producing seed.
Trees must also obtain a certain size and structure that is specific to their species before they are
able to reproduce, and the amount of time required to reach this condition varies by species.
Some trees, such as jack pine and lodgepole pine (Pinus contorta), may be able to reproduce
before they are ten years old, while other pines may require twenty to thirty years to mature. The
middle ages of the typical lifespan of a tree species are generally the most productive from a seed
27
generation perspective. On an annual basis, seed production will vary, and environmental
conditions may delay flowering (Krugman et al. 1974). Sexual reproduction processes can be
affected by both biotic (insect pollination) and abiotic (climate and soil conditions) factors
(Rasmussen and Kollmann 2004). For example, the volume of annual crops of oak (Quercus
spp.) seeds (acorns) is usually not constant from one year to the next. Further, Ayari et al. (2011)
suggest that the seed production of tree species such as Aleppo pine (Pinus halepensis), which
grows naturally in the Mediterranean Basin, can be affected by geographical factors such as
latitude, longitude, and elevation. Low air temperatures may be necessary to condition a tree to
respond normally to flower initiation processes, and adequate levels of photosynthesis may be
necessary for flower initiation and development. The quality of the underlying soil is often
positively correlated with seed production as well (Krugman et al. 1974). Therefore, in some
cases flowering may be restricted in certain forest conditions and in areas with low soil quality.
Vegetative reproduction processes are typically considered to be asexual reproduction
processes. These include cloning processes and the development of vegetative shoots from
existing plants. A cloning process is usually conducted in a laboratory or tree nursery. Here,
genetically identical stems are derived from a plant, and each becomes a ramet (an independent
member) of the original plant (Barnes et al. 1998). Ramets are allowed to grow a root system and
are then planted in the ground during a forest regeneration process. For some commerciallyimportant coniferous species, cloning and rooted cutting regeneration processes have been
devised to reproduce coniferous trees (Bettinger et al. 2009). In the forest, vegetative shoots can
also form in the roots and boles of many deciduous trees after damage or after harvesting
activities. Regeneration could occur as a result of root shoots, or meristems, which may facilitate
rapid development of clonal colonies after the removal of an existing forest through disturbance
28
(e.g., wildfire) or harvest activity (Stone and Kalisz 1991). One form of reproduction through
stump sprouts (coppice) is often used to regenerate deciduous forests. Dispersal of genetic
material is severely limited in this case; however, regenerated seedlings may be less vulnerable
to stresses than those created from seed due to the soil, water, and nutrient resources that become
available through the established root and stem systems of a larger plant (Deiller et al. 2003). All
angiosperms have the capability to reproduce vegetatively once they become established in a
forest, yet most coniferous trees (gymnosperms) are unable to naturally reproduce in this
manner. Redwood is one coniferous tree species exception, as it has the ability to sprout from
stumps during any season of the year through the use of dormant or adventitious buds (Olson et
al. 1990).
VIII. TREE TOLERANCE TO SHADE
The future status of a live tree is influenced by many natural and anthropogenic factors.
In natural settings, two of the main factors of influence are the current canopy position of the tree
and the ability of the tree to tolerate levels of light that are sub-optimal for tree growth (Ward
and Stephens 1993). Trees require electromagnetic energy (light) in order to grow and endure,
yet many species have adapted to conditions where the availability of this resource is low, thus
they all are to some extent tolerant of low levels of light. The term shade tolerance, therefore,
refers to the ability of a tree to continue to become established, survive, and thrive in shaded
conditions where the availability of direct electromagnetic energy is low. Shade tolerance can
also refer to the ability of a species of tree to continuously and successfully compete with other
trees for resources and to regenerate under a contiguous canopy. Alternatively, shade tolerance
29
represents the ability of a tree species to persist in a low-resource environment (Ward and
Stephens 1993). Tree species that can compete well in fully shaded conditions are shade tolerant,
while that that require full sunlight and limited competition are shade intolerant. Between these
two extremes are trees that are intermediate in their ability to develop and compete for resources
in shaded conditions (Martin and Gower 1996).
While we tend to describe the ability of trees to tolerate shade on a simple continuum
(Figure 19), in reality a number of factors are important in determining whether a tree will
survive and grow under partial sunlight conditions. In nature, low-light conditions suggest that
shade is being created by other tree or plant species, yet the concept of shade tolerance in trees
could be extended to conditions where shade is created by man-made structures. Shade tolerant
tree species do not require full sunlight, and a lack of competition for it, to survive. However,
some of these tree species will grow better in partially shaded conditions than when completely
exposed to the sun. Other factors, such as the age of a tree, the quality of the site where a tree
grows, and even the region in which a tree is growing may influence the tolerance of a tree to
shaded conditions. For example, an eastern white pine tree may be more tolerant to shade when it
is younger in age, while a black spruce (Picea mariana) tree may acquire shade tolerance as it
ages (Martin and Gower 1996).
Shade-tolerant trees include sugar maple (Acer saccharum), basswood (Tilia americana),
black spruce (Picea mariana), eastern hemlock (Tsuga canadensis), European beech, American
beech (Fagus grandifolia), redwood, grand fir (Abies grandis) (Figure 20), Sitka spruce (Picea
sitchensis), and other commercially-important tree species with crowns in the upper reaches of a
canopy, along with dogwood (Cornus florida), big-leaf maple (Acer macrophyllum), yew, holly
(Ilex opaca), and other tree species with crowns typically found below the main tree canopy.
30
Shade-intolerant trees include commercially-important species such as hickories (Carya spp.), a
number of pines (e.g., loblolly pine), quaking aspen, paper birch (Betula papyrifera), black
cherry (Prunus serotina), western larch (Larix occidentalis), and black walnut (Juglans nigra),
among others, along with willows (Salix spp.) and sassafras (Sassafras albidum). The
intermediate shade tolerance class includes important commercial tree species such as the ashes
(Fraxinus spp.), many of the oaks and elms (Ulmus spp.), Douglas-fir (Pseudotsuga menziesii),
eastern white pine, western white pine (Pinus monticola), and sugar pine (Pinus lambertiana),
among others.
Two of the key determinants in forest stand structure and forest dynamics are the
tolerance of trees to shade and the competition for limited light resources (Zavala et al. 2007). If
we were to imagine the natural establishment (primary succession) of a forest after a harvest,
fire, or wind-driven disturbance, if colonized by shade-intolerant tree species, the diameter
distribution of trees would eventually become somewhat of a normal (bell-shaped) or a flattened
form of a bi-modal distribution representative of an even-aged forest (Figure 21), where most of
the trees are about the same size, relatively speaking (diameter and height). In this case, there are
very few smaller trees because these would not be able to survive in limited light conditions. If a
forest were colonized mainly by shade-tolerant tree species, the diameter distribution of trees
would eventually have a downward-sloping, reverse J-shape that is representative of unevenaged forest conditions. Here, the establishment and growth of smaller, younger trees would
continue through time as conditions allow. In forests with trees of mixed tolerance to shade,
secondary successional processes would favor the decline of shade-intolerant trees or trees of
intermediate tolerance due to mortality and a decrease in recruitment (lower regeneration
success). In addition, when shade-tolerant trees share a canopy with trees in these other groups,
31
the sensitivity to shading and the resulting stress and reduced growth rates of intermediate
tolerance and intolerant trees generally results in a decline in the number of these per unit area
(Yoshida and Kamitani 1999). As a result, natural succession processes are very strongly related
to the shade tolerance levels of trees occupying a site. Shade-intolerant tree species may occupy
a site first after a disturbance creates an opportunity for tree establishment, yet they will likely be
followed and replaced by intermediate and shade-tolerant tree species as time progresses (Martin
and Gower 1996).
Some silvicultural systems take advantage of variations in shade tolerance. For example,
in Germany, the long-rotation management of two shade-intolerant tree species, pedunculate oak
(Quercus robur) and sessile oak (Quercus petraea), may require the incorporation of a shadetolerant species to suppress the development of epicormic branches. The most frequently
recommended admixture species for this purpose is European beech although European
hornbeam (Carpinus betulus) has also been used on high quality sites (von Lüpke 1998). In the
management of central United States hardwood forests, some oak species are more shade
intolerant than others. The presence of oak seedlings in the understory of a forest is important for
the natural regeneration of these forests, as this condition favors the successful re-establishment
of an area with this group of tree species (Larson and Johnson 1998).
IX. TREE NUTRITION
Plant development and biomass production depend on the availability of nutrients, such
as inorganic nitrogen, in the soil. When considering the growth of trees, nitrogen is usually a
limiting factor, thus nitrogen assimilation and metabolism by trees is important. For wood supply
32
concerns, the manipulation of nitrogen metabolism in trees may impact wood production
potential (Gallardo et al. 2003). The ability to use nutrients contained in soils likely varies by tree
species, tree age, the current nutritional status, and the physiological development of the plant.
As an example, Table 1 illustrates the distribution of nutrients such as nitrogen, phosphorous,
potassium, calcium, and magnesium by tree component for a 16 year old loblolly pine (Pinus
taeda) plantation (Wells et al. 1975). Of the four nutrients listed in Table 1, nitrogen is the most
commonly found nutrient throughout a pine tree, particularly in the needles, tree bole, and root
system. Calcium is the next most abundant nutrient and the largest concentrations of this nutrient
are generally found in the tree bole, bark, and root system. Substantial amounts of potassium are
also prevalent throughout a pine tree, and the largest concentrations of potassium are located in
the tree bole, root system, and needles. Phosphorous and magnesium are mostly found in the tree
bole and root system of a pine tree as well. Deficiencies in one or more of these main elements
can adversely affect plant growth (Kramer and Kozlowski 1979). For example, nitrogen is
essential for the development of amino acids, and a lack of nitrogen can lead to the failure of a
plant to produce chlorophyll and cytochrome, which can lead to leaf chlorosis (chlorotic leaves
are yellow or pale in color). A deficiency in magnesium can also lead to leaf chlorosis. A
deficiency of phosphorous can lead to the stunting of height growth in a tree, and a deficiency in
potassium can cause problems in water vapor transfer and the absorption of carbon dioxide
necessary for photosynthesis (Kramer and Kozlowski 1979). Further, a deficiency of calcium in
a tree can affect cell wall development and root tip development.
Other essential micronutrients, if deficient, can adversely affect plant growth; these
include boron, iron, manganese, zinc, copper, and molybdenum (Kramer and Kozlowski 1979).
For example, a deficiency of boron can affect the development of apical buds (affecting height
33
growth), root tips, and mycorrhiza, and impacts on these can also affect the cycling of other
nutrients such as carbon (Lehto et al. 2010). Manganese and zinc deficiencies can lead to leaf
malformation, copper deficiency can affect the function of plant enzymes, and molybdenum
deficiency can affect nitrogen fixation by leguminous trees. In isolation of other influences on
growth and survival, trees and plants maintaining an appropriate balance of these nutrients are
capable of acceptable growth rates and successful reproduction processes.
X. TREE RESPONSES TO SIGNALS
Broadly speaking, signals within plants, such as changes in hormone levels or changes in
water pressure, help initiate the formation of stems, leaves (and needles), flowers, and fruit, and
are an important aspect of tree growth. For example, communication of water deficit information
between roots and shoots is thought to occur through hydraulic signals (Chavez et al. 2011).
Further, changes in hormone concentrations seem to be correlated with changes in metabolic
activity in the growth of plant tissue around seeds (Krugman et al. 1974). For both angiosperms
and gymnosperms, chemical changes during seed germination are essentially the same. During
germination, stored food material in the cotyledons is mobilized and transferred to the embryo,
and oxygen, water, and minerals are acquired. The products of metabolic activity provide the
energy and the carbon fragments for the development of new plant material. In this process,
starches, sugars, and fats are converted to soluble sugars, some proteins are reduced to form
amino acids, and cellular growth is facilitated with the assistance of ribonucleic acid (RNA) and
deoxyribonucleic acid (DNA). RNA and DNA are long chains or sequences of nucleotides that
34
encode genetic information about a plant or animal. Some RNA and proteins actively sense and
communicate cellular-level responses, particularly in the construction of other proteins.
Root-to-shoot communication can play a vital part in a tree's defense against drought
(Mansfield 1998). Partial stomatal closure and reduced rates of expansion of new leaves have
been observed when water availability in the soil is reduced, long before the leaves are stressed
due to reduced water availability in the plant. It is suggested that the sensing of dry soil around
the root system and the communication of information to the shoot is facilitated in part by a plant
hormone, abscisic acid. Thus, stomatal responses to drought and salinity are thought to be
controlled through a signal provided by changes in levels of abscisic acid (Chavez et al. 2011).
Auxins seem to be one of the main classes of plant hormones (phytohormones) associated with
the regulation of cambial activity (Lachaud et al. 1999). During growth, a cambial cell must be
able to receive hormonal signals, nutrients, and various metabolites. These activities may be
facilitated by changes in the pH gradient within and between cells. Gibberellins, auxins, and
cytokinins are all classes of hormones that are also influential during the reproduction phases of
a plant. The timing of plant flowering, for example, may be manipulated using external
application of gibberellins (Krugman et al. 1974). While these are but a few examples, one
should recognize that signals within plants (e.g., changes in hormone levels) can influence the
onset of metabolic processes or the development of stems, leaves (and needles), flowers, and
fruit.
XI. SUMMARY
Trees situated in a forested environment may be competing with other trees and plants for
35
nutrients, light, and water. Trees are complex organisms that absorb energy, CO2, water, and
nutrients in their quest to survive. They transpire, respire, and flower, and if they are not
successful competitors, they may eventually die. The role of a tree in the ecological framework
of a given forest varies depending on the health of the tree. The growth of a tree is obviously
directed upward and outward, and growth mechanisms of tree cells are important to understand,
since photosynthesis not only allows trees to survive and thrive, but the process also provides
oxygen for humans to breathe. Tree rooting behavior is important to understand as well, since it
facilitates the uptake of nutrients from the soil, and helps trees withstand severe wind events.
Tree reproduction processes vary between angiosperms and gymnosperms, and simply
understanding the timing of pollen release and flower production may help one estimate the
extent of potential cone or fruit crops. Tree shade tolerance is an important issue in the
management of forests since advanced reproduction of a set of tree species may be necessary,
and since the interactions among tree species and the response of trees to the resources available
along environmental gradients may be influential in management decisions. Although many of
the topics in this chapter seemed to have delved deep into the technical processes of tree anatomy
and physiology, we barely scratched the surface of the science of forest biology. The processes
by which trees live and grow are essential biological concepts for forest and natural resource
managers. This knowledge is an essential tool set for a professional, and we encourage you to
explore further into the topics that have piqued your interest.
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QUESTIONS
1. Tree root development. During a recent family gathering, a distant family member described
how a large oak tree in their yard fell during a recent wind storm, pulling with it much of the
root system. They noticed that the tree had no taproot and wondered why. In a short
paragraph, describe how you would contribute to the conversation with information
regarding the depth and radius of root systems, as well as the presence of taproots and other
types of rooting systems.
2. Plant signals. In a short paragraph of five sentences or less, describe some of the mechanisms
within plants that act as signals to change plant development of functional behavior.
3. Seed development. Choose a tree species that is of interest to you. Develop a short, five-slide
Powerpoint presentation that describes the salient aspects of plant flowering and seed
development of this species. For readers in North America, the Silvics of North America
(http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm) might be of value.
4. Material transport. In a short paragraph, describe the different functions of xylem and phloem,
and how they might contribute to material transport within a tree. Through a search of the
Internet, locate examples that refer to specific tree species, and locate information not
provided in this chapter. Share your findings with a small group of others and discuss the
45
similarities and differences of your collective findings.
5. Photosynthesis. The photosynthetic process is intimately related to a hot topic of natural
resource management, global climate change. In a one-page memorandum to your instructor,
describe the main inputs and outputs of the photosynthetic process, and how these are related
to the climate change issue.
List of Figures
Figure 1. Elk damage on conifers in Lahemaa National Park, Estonia (Photo courtesy of
Athanasius Soter, through Wikimedia Commons).
Figure 2. A conceptual model of the apical or shoot meristems at the tip of a tree leader.
Figure 3. A conceptual model of the cambium and the production of secondary conducting
tissues (adapted from Lachaud et al. 1999; image of tree courtesy of Bill Cook, Michigan
State University, through Bugwood.org).
Figure 4. Growth rings associated with a southern United States pine tree. (Photo courtesy of Tor
Schultz, Mississippi State University)
Figure 5. A 10X, transverse view of yellow poplar (Liriodendron tulipifera) The bright line of
terminal parenchyma at the annual ring boundary is characteristic of this diffuse porous
hardwood (Photo courtesy of Tor Schultz, Mississippi State University).
Figure 6. A conceptual model of a woody cell wall showing the primary wall and the three
secondary wall layers (Photo courtesy of Pete Bettinger).
Figure 7. A conceptual model of a transverse section of earlywood tracheids.
Figure 8. A conceptual model of a shoot apical meristem.
46
Figure 9. The photosynthetic process (image of sun, stem, and leaf courtesy of Stepa, through
Wikimedia Commons).
Figure 10. Cross-sectional conceptual models of a deciduous tree leaf.
Figure 11. Cross-sectional conceptual models of coniferous tree needle.
Figure 12. Douglas-fir seedling development, 2, 5, 8, and 22 days after emergence (adapted from
Owsten and Stein 1974).
Figure 13. A single carpel female reproductive organ of an angiosperm (Photo courtesy of
Mariana Ruiz, through Wikimedia Commons).
Figure 14. Longitudinal sections through the pistil of an angiosperm (left) and the ovule of a
gymnosperm (right) (adapted from Krugman et al. 1974).
Figure 15. Loblolly pine (Pinus taeda) cones, Starkville, Mississippi, United States (Photo
courtesy of H. Alexis Londo).
Figure 16. Epigeal germination (A) of pin cherry (Prunus pensylvanica) and hypogeal
germination (B) of Allegheny plum (Prunus alleghaniensis) (adapted from Krugman et al.
1974).
Figure 17. Seeds (acorns) of willow oak (Quercus phellos) (Photo courtesy of the U.S.
Department of Agriculture, through Wikimedia Commons).
Figure 18. Beech seedlings that have become established through seed on an old spruce trunk
(Photo courtesy of Don Manfredo, through Wikimedia Commons).
Figure 19. A simple continuum of a few shade tolerant trees of North America.
Figure 20. Grand fir regeneration in shaded conditions, Idaho, United States (Photo courtesy of
Chris Schnepf, University of Idaho, Bugwood.org).
Figure 21. Diameter distributions representative of even-aged and uneven-aged forests.
47
Table 1. Distribution of nutrients (kg / hectare) by tree component for a 16 year old loblolly pine
(Pinus taeda) plantation (Wells et al. 1975).
Live
Dead
Tree
Nutrient
Needles
branches
branches
bole
Bark
Roots
Nitrogen
82
34
26
79
36
64
Phosphorus
10
5
2
11
4
17
Potassium
48
24
4
65
24
61
Calcium
17
28
30
74
38
52
8
6
3
23
7
22
Magnesium