1.
NUTRITIONAL MANAGEMENT OF CONIFEROUS FORESTS
Robert B. Harrison
Associate Professor of Forest Soils
College of Forest Resources
University of Washington
Seattle, WA 98195
2.
INTRODUCTION
2.1. What is nutrient management?
The role of nutrient management in coniferous forest health
and productivity has evolved from being little considered 40
years ago to presently occupying an important role in the
management of production coniferous forests. Much of the
recognition of the importance of nutrients in coniferous
forest productivity was established by studies on forest
fertilization, with the primary goal of increasing the rate
of volume growth of commercial forests (Gessel et al., 1951;
Coile, 1952; Walker et al., 1955; Steinbrenner, 1968).
Worldwide research on conifers initially showed widespread
response to two nutrients, nitrogen and phosphorus. Further
research and changing conditions have shown limitation from
a variety of other nutrients, notably potassium, sulfur,
calcium, magnesium, boron, copper, manganese and zinc (Kelly
and Johnson, 1982; Blake et al., 1990; Harrison et al.,
1994; Ende and Evers, 1997; Kavvadias and Miller, 1999a,b).
Generally, these deficiencies are more regional in nature or
not present in a variety of forest types. The application of
fertilizers to forests is now a widespread practice in most
regions where commercial forests are intensively managed
(Raupach, 1967).
Increasing forest production from a given area of land is an
important, and sometimes primary reason for a consideration
of nutrient management. However, the role of nutrients in
the enhancement or maintenance of other forest benefits has
received increasing interest. Higher percentages of forest
lands are now managed for biodiversity, water quality, soil
conservation, wildlife habitat, and special commodity
production. In addition, nutrient management can play an
important role in mitigating the impact of atmospheric
deposition and in restoring degraded soils. It will be
increasingly important to recognize the role of nutrient
management in enhancing the value of coniferous forests for
products other than wood and fiber in the future.
Perhaps the most important properties differentiating forest
nutrient management from agricultural nutrient management is
the central role of nutrient cycling in the retention and
supply of most nutrients in forest ecosystems. Studies of
nutrient cycling and the effects of atmospheric deposition
have shown that some nutrients are naturally depleted from
the soil over time, and that high levels of elemental input
can sometimes lead to accelerated loss of essential
nutrients. Nutrient cycling studies have also shown how
forest nutrients can sometimes be renewed over time through
soil weathering and atmospheric inputs. Forest nutrients
pools can sometimes change greatly after relatively short
occupations of the site by different species, showing either
large increases, or depletion or change into relatively
insoluble forms.
Nutrient management offers tools to follow and control the
amounts and forms of nutrients in coniferous forests, either
through manipulation of species, stand density and
composition, timing of treatments, removals and amendments.
In the future, some forms of forest certification for
sustainability may require forests to be managed renewably
without direct application of fertilizers to replace
nutrients that may be lost through forest management.
However, despite lowered use of fertilizers in some
coniferous forests of the world, notably nitrogen in Europe,
forest fertilization will likely remain an important
component of nutrient management in coniferous forests.
2.2. Rationale for nutrient management of coniferous
forests
Nutrient management can be used in two basic ways in forest
management. First, it can be used as a tool to maintain the
nutrient reserves on a particular site to not deplete
nutrient stocks. The removal on nutrients by harvest or
other removals is balanced against the inputs of nutrients
from atmospheric deposition, fertilization and other inputs.
Nutrient management can also be used to increase the
productivity of sites where nutrient availability limits
growth, or where other previously limiting site factors are
improved (i.e. after drainage).
Forest fertilization can sometimes be effectively used to
increase the yield and value of established or new forests.
In this way, nutrient addition is often looked at as a way
of increasing the speed of development of a forest stand.
Adding nutrients that are limiting to plant growth can
increase the growth of individual stands, but addition of
nutrients can also be used to rehabilitate disturbed sites,
such as eroded or otherwise degraded soils. Thus, nutrient
management can be used for a variety of purposes, to
maintain forest stands at their present rate of development
and productivity, or to increase the rate of development for
productivity increases or to decrease the period of time
until they develop other desirable characteristics (i.e.
old-growth nature).
2.3. Early work on nutrient management
Extensive early research work in many of the world’s
important coniferous forests has clearly shown that nutrient
deficiencies exist in some specific forest areas and
therefore trees growing on those sites are not able to make
full use of the other growth factors of the sites. In
addition to reduced growth rates they may also be suffering
decline in vigor and eventual death from nutrient
deficiencies.
Research activity has not only been able to describe these
nutrient deficiencies but has also shown how corrective
action can be taken to eliminate them. In order to avoid
excessive nutrient removals, forest management practices can
sometimes be changed to decrease nutrient loss. In other
cases, forest fertilization is designed to add elements
either lacking, or in low supply. To be successful,
fertilization must be very specific to the needs of a given
forest area and to the economics of forest management. In
most parts of the world, forest fertilization is generally
well-developed where the following three basic conditions
exist:
1) Forests respond to fertilization with significant
increases in growth rates,
2) high demand in that region or export practices makes the
price of raw wood high, and
3) the infrastructure for buying, moving and applying
fertilizers exists.
In the future, increasingly stringent environmental
considerations and alternative forest management goals may
restrict the ability of forest land managers to apply
fertilizers, even where these three conditions make
application profitable and attractive.
An extensive literature has been developed to cover this
subject and many reports of special symposia and conferences
exist. The development of the information base for good
programs of nutrient management in all the major coniferous
forest areas has been an important contribution of forest
soil research and the development of the relation of forest
soils to tree growth.
The other important aspect of nutrient management on forest
land which is now emerging very strongly is sustaining and
improving forest productivity. We have previously referred
to the great emphasis now being put on "sustained
productivity" in the management of forest land. For many
areas, researchers have developed nutrient cycles, which
include the sizes of discrete forms or locations of
nutrients (termed nutrient pools) and the relative movement
of nutrients from one pool to another (termed nutrient
fluxes). There is justified concern that one form of
management or harvest may remove more of a particular
element than the area can replace or sustain over a
particular time period. If the harvest is repeated at
frequent intervals then nutrient deficiencies may occur over
time. Harvest at long intervals, and only partial crop
removal constitutes less of a potential problem.
3.
EFFECTS OF NUTRIENTS ON FORESTS AND DIAGNOSIS OF
DEFICIENCIES
3.1. Nutrient requirements of coniferous forests
Coniferous forests vary significantly in their nutrient
requirements, and in the particular mechanisms by which they
acquire nutrients. A tree’s specific nutritional needs can
be acquired over its lifetime by direct uptake of the
nutrient, recycling and internal translocation of the
nutrient, and also by processes of nutrient conservation
(Cole and Gessel, 1992; Schaedle, 1991).
All coniferous tree species are perennial, and perennial
species show significant differences over annual species in
meeting nutritional requirements. First, uptake can take
place year round, depending on climate. The nutritional
needs of coniferous trees do not need to be met through
direct uptake, but can also be met in part by internal
translocation of nutrients. Since the stem, roots and
branches of coniferous trees are relatively permanent, they
do not have to be replaced annually. Multi-year retention of
foliage by conifers also results in less uptake to form
foliage each year, particularly since foliage is high in
most nutrients (Table 3.1.1). Harvesting losses are also
less in forests compared to agricultural crops because of
the low nutrient contents of harvested parts (generally the
bole) and the long time between harvesting cycles.
Table 3.1.1. Nutrient content of some tissues of
Douglas-fir trees (Walker and Gessel 1991).
TREE COMPONENT
NUTRIENT CONTENT (%)
FOLIAGE
BARK
CONES
BRANCHES
BOLE
N
P
K
CA
MG
1.40
0.29
0.65
0.36
0.08
0.21
0.07
0.13
0.07
0.01
0.85
0.31
1.26
0.21
0.05
0.45
0.42
0.06
0.51
0.10
0.11
0.13
0.10
0.05
0.05
The amounts of nutrients required by coniferous forests vary
depending on a variety of factors, but particularly
depending on the growth rate of the trees (Rennie, 1955).
Figure 3.1.1 shows the growth rate and nitrogen uptake vs.
age for Douglas-fir stands in the Pacific Northwest, USA.
There is a strong correlation of growth rate and N uptake.
15
40
GROWTH
30
10
20
5
10
GROWTH (m3/ha/yr)
NITROGEN (kg/ha/yr)
NITROGEN
0
0
0
20
40
60
80
100
120
AGE (yr)
Fig. 3.1.1.
Relationship between growth as periodic annual
increment (Curtis et al. 1982) and N uptake (Turner 1975) by
site IV Douglas-fir during stand development.
3.2. Diagnostic methods
There are a number of methods available to the forest land
manager to diagnose nutrient limitation or imbalances in
forests. Perhaps the best indication of a deficiency is an
increase in the growth rate of the stand following addition
of the limiting nutrient. Regional studies of fertilizer
response have been carried out for several decades in
several major coniferous areas in the world, including the
Southeast and Northwest United States, Canada, South
America, Australia and New Zealand, and Europe.
Unfortunately, such studies have not been carried out over
extended periods of time in many important coniferous
regions on a wide variety of soils and forest types. In many
cases, less time-consuming methods of assessing the
potential for nutrient deficiencies are desirable. In
addition, relying solely on volume growth following nutrient
additions may not consider the changes in tree physiology
that may lead to increased incidence of disease, damage from
ice, or other negative effects not directly related to
growth rate. Fortunately, in some cases, nutrient
deficiencies can be evaluated over relatively short periods
of time using visual, chemical or a combination of these
methods.
3.2.1.
Visual deficiency symptoms
Nutritional stress can sometimes be diagnosed by foliage
deficiency symptoms, which are abnormalities in anatomical
or morphological characteristics related to a particular
nutrient deficiency (Carter, 1991). There are a number of
difficulties in using visual deficiency symptoms as a
diagnostic tool and the user has to eliminate these other
possibilities. There can be many reasons for abnormal
appearance of foliage other than a nutritional deficiency
including physiological stress due to disease, insects
attacks, pesticide application, atmospheric pollutants such
as ozone, temperature extremes, herbicides and moisture
deficits.
Typically, a series of color pictures (or sometimes
drawings) are produced that depict common visual deficiency
symptoms. For instance, Bengtson (1968), Morrison (1974),
Kolari (1979), Will (1985), Ballard and Carter (1986), van
den Driessche (1991) and Walker and Gessel (1991) produced
color depictions of nutrient deficiency symptoms in
important conifers. Visual deficiency symptoms in conifers
can range from necrotic spots and regions on the needles to
banded coloring to a general chlorotic appearance on much of
the foliage. The appearance of foliar deficiency symptoms
can also vary by position of the foliage in the crown,
depending on the relative mobility of the nutrient. Due to
variability in observations, evidence of nutrient deficiency
by visual symptoms will usually need to be evaluated by
other methods as well, such as by foliage testing.
3.2.2.
Foliage testing
Foliage testing looks at the concentration or amounts of
nutrients in needles, and is thus an indication of the
amount of nutrient moved to the foliage, which is the
“factory” of tree productivity. The general relationship
between foliar concentration and growth rate follows the
general pattern shown in Figure 3.2.2-1.
maximum growth
"luxury" consumption
relative growth
toxicity
critical point (90% of max)
hidden hunger
visual deficiency
death
Ro bHa rris on :g rowth v s. nu trie nt co nc
foliar nutrient concentration
Figure 3.2.2-1. Relationship between foliar nutrient
concentration and relative growth or yield (modified from
Tisdale et al., 1993).
Visual deficiency symptoms generally appear at the lowest
range of this relationship, with growth rates greatly
decreased. However, some nutrient deficiencies may not
express themselves as lowered growth rate, but rather as
poor control over other growth processes, such as
differentiation of cells or allocation of growth to the
meristem regions. For instance, boron deficiency is often
expressed as excessive branching and dieback of the leader.
As nutrient concentration in the foliage increases above
visual deficiency levels, relative growth is still
depressed, but it becomes increasingly difficult to evaluate
a deficiency by visual symptoms alone. For instance,
chlorosis or other severe visual symptoms may not be present
or obvious in the region of “hidden hunger”, which is
characterized primarily by decreased growth rates. In this
region, differences would be obvious only if the growth rate
of a stand with low relative nutrient availability were
compared with another, similar stand. As the concentration
of the nutrient increases further in the foliage, the
nutrient no longer becomes limiting, and growth rates no
longer increase. By convention, the “critical point” is the
foliar concentration at which 90% of the maximum growth rate
is achieved. As nutrient concentrations are increased above
the critical level, there are much smaller increases in
yield relative to the amount of nutrient applied. This is
due to the “law of diminishing returns” where the nutrient
is used less efficiently. At even higher concentrations, no
growth increases occur; this is termed the region of “luxury
consumption”. At even higher concentrations, the nutrient
may interfere with necessary processes and result in lowered
productivity or death due to toxicity effects.
In theory, if the levels at which deficiencies occur in
conifers are established, a simple foliar analysis can
establish whether or not a particular nutrient is limiting.
However, the absolute shape of Figure 3.2.2-1 can vary with
the particular conditions of each forest stand, including
soil and site, species and provenance, stocking and stand
age. Some local expertise often needs to be developed for
best use of foliar analysis. In a review of diagnosis and
interpretation of forest stand nutrient status, Carter
(1991) considered crown position, foliage age, collection
technique, size of sample and between-tree variation to be
important sources of error in foliage analysis, and
suggested that generally between 10 and 30 trees per stand
should be sampled. There are a variety of sources available
for data on critical levels of conifers for macro, secondary
and micronutrients, including Carter (1991), Blake et al.
(1990), Pritchett and Fisher (1987), Walker and Gessel
(1991).
One problem with the use of critical levels in evaluating
the response of forests to fertilization is the interaction
of nutrients. When a single nutrient is amended, the
requirement for and efficiency with which another nutrient
is utilized may be affected. Thus, that nutrient can
initiate a deficiency for a second, or additional nutrients.
When the other nutrient is added, the level of response of
the first nutrient may be heightened. The potential
interaction of two nutrients and the effect on observed
critical levels is shown in Figure 3.2.2-2. For instance,
Teng and Timmer (1995) found P deficiency in white spruce
(Picea glauca (Moench) Voss) was initiated by N
fertilization only. Jokela et al. (1991) found subacute Mn
deficiency in slash pine (Pinus elliottii Engl. var.
elliottii) in Florida and hypothesized that intensive
culture and macronutrient fertilization might increase Mn
deficiency where Mn is not applied.
Fertilizer nutrients could also interact with native soil
elements, potentially either increasing or decreasing
availability. Burton et al. (1990) found that heavy
applications of sewage biosolids induced nitrification and
nitrate and cation leaching. Van Miegroet and Cole (1985)
found acidification and leaching of nutrient cations
associated with nitrogen fixation in a red alder stand.
Cole (1979) reported significant losses of K, Ca and Mg
after fertilization with urea without any observation of
nitrate leaching, apparently due to NH4+ replacement of
exchangeable cations on the soil CEC.
critical point
N+P
maximum growth N + P
relative growth
maximum growth N only
critical point
N only
Ro bH arris on :grow th vs . n utrie nt co nc
foliar nutrient concentration
Figure 3.2.2-2. Potential effect of nutrient interaction of
N and P on relative growth of coniferous forest species.
Table 3.2.2-1. Nutrient deficiency levels for several
western conifer species established from seedlings grown in
solution cultures (Walker and Gessel 1991).
Nutrient
N
P
K
Ca
Mg
S
Douglas-fir
1.25
0.16
0.60
0.25
0.17
0.35
Western
Hemlock
1.80
0.25
1.10
0.18
Western
Red cedar
1.50
0.13
0.60
0.20
0.12
0.40
Sitka
Spruce
1.80
0.09
0.40
0.06
0.06
0.15
Abies
spp.
1.15
0.15
0.50
0.12
0.07
A foliar analysis method that has gained considerable
popularity because it does not require deficiency levels to
be established prior to using it (thus saving the several
years minimum required to establish deficiency levels by
tree growth), is the foliar graphical analysis method
described by Timmer and Morrow (1984). This method utilizes
changes in the nutrient content and average size of foliage
to determine the potential for response to fertilization.
Two assumptions are required for the graphical method.
First, the size of the unit of foliage (needle or leaf) will
increase when a deficient nutrient is increased in
availability and concentration within the needle or leaf.
Second, when this observation is seen, an increase in growth
rate of other plant parts (i.e. volume growth), will also be
observed, though perhaps requiring a longer time period to
be measured. This method does not work well with tree
species that have highly variable foliage size (i.e. Tsuga
heterophylla), or for species that respond to increased
nutrient availability by increasing the number of needles
and not the average size of foliage.
The use of the graphical method of determining the potential
for a nutrient amendment response is shown in Figure 3.2.22. There are several possibilities for observations
following addition of a potentially-limiting nutrient.
Response "A" shows dilution, where the average weight of
needles increases due to some effect other than
fertilization with the nutrient being evaluated, but the
amount of nutrient allocated to each needle is relatively
constant. This decreases the average concentration of the
nutrient, and is termed "dilution". Response "B" shows an
increase in average needle size also due to some other
factor than the nutrient being evaluated; however, the
allocation of nutrient to each needle doesn't increase more
than that increase in weight, resulting in a constant
concentration. This will result in the nutrient
concentration remaining relatively constant, and an
interpretation that the nutrient supply is sufficient to
meet the requirements of the tree.
Response "C" is what would be expected of a deficient stand.
In this case, an increase in the availability of the
nutrient results in an allocation increase in excess of the
increase in needle weight, which would be consistent with an
increase in the nutrient in the foliage increasing the net
biomass production of the tree, with some of that increase
going to higher foliage production. This increase in foliage
biomass should also result in increased volume growth as
well. Response "D" in consistent with an observation of
luxury consumption as shown in Figure 3.2.2-1, where
increases in availability of a nutrient result in
concentration increases, but no increase in growth of
foliage. As nutrient concentrations increase even more, the
nutrient can become toxic, and the growth of the needles can
be decreased, as shown with response "E". Finally, response
"F" can be observed when some factor, usually one that
interferes with uptake of the nutrient, results in decreased
concentration in the needles and decreased needle weight.
elemental concentration (%)
lines represent unit needle weight (mg weight/needle)
E excess
D luxury consumption
C deficiency
B sufficient
control
A dilution
F excess
element content (µg/needle)
Figure 3.2.2-3. Use and interpretation of the graphical
analysis method for determining needle response to
fertilization (modified from Timmer and Morrow, 1984).
The use of this method has gained popularity in the early
interpretation of nutrient deficiency. However, since
increased needle growth does not always lead to increased
volume growth, the evaluations of potential nutrient
deficiency will couple this method with some more conclusive
method, such as field trials.
3.2.3.
Soil testing
In theory, since soil is the primary source of most
nutrients taken up by conifers, an evaluation of soil could
show the potential for nutrient deficiency. Soil tests for
nutrient availability have been used successfully with
agricultural crops for several decades, and at present, an
extensive network of analytical labs and fertilization
recommendation software drives the use of fertilizers for
many agricultural crops (ref**). Though this approach might
work equally well with evaluation of potential for nutrient
deficiencies with conifers in theory, in reality, soil
testing has not worked very well for evaluation of nutrient
deficiencies in coniferous forests. There are several
reasons for this. First, unlike the annual growth of most
agricultural crops, growth of forests is over multiple
years, and the availability of nutrients can change over
time. Second, forests regulate the movement and loss of
nutrients within the forest ecosystem and within individual
trees. This "nutrient cycling" can greatly affect the
availability of nutrients to trees over time, since some
nutrients may be utilized multiple times within the growth
cycle (rotation) of a forest stand (Cole and Gessel, 1992).
One additional factor that makes correlating soil testing
with tree growth difficult is the time from planting to
harvest of trees, which ranges from 15-20 years to 100+
years depending on the species, where it is growth, and the
desired products. Since many agricultural crops are
harvested annually, it takes only one year to correlate
yield with a soil evaluation. It typically takes several
decades to correlate forest growth with a single soil
evaluation. At the present time, soil testing is not highly
utilized as a means of evaluating the potential for nutrient
deficiency in coniferous forests. It is an area of
considerable past and present research, however.
3.2.4.
Pot trials
Pot trials utilize samples of soil collected from the area
where a forest is planted or is to be planted to evaluate
the potential for nutrient deficiency. The soil is usually
mixed, divided into plant pots, fertilized with one or more
nutrients (at one or more rates), and seeds or seedlings are
planted into each pot. In this way, it is easy to replicate
the study and evaluate multiple nutrients and rates. Pot
studies have been shown to be a successful method to assess
severe nutrient deficiencies in Douglas-fir and Western
hemlock in the Pacific Northwest (Walker and Gessel, 1991).
However, there are several potential problems with the
utilization of this method.
First, the nutrient requirements of trees change with age,
and the ability of trees to acquire nutrients from their
environment can change also (Cole and Gessel, 1992). In
addition, pot studies are typically conducted under
controlled conditions (i.e. in a greenhouse), and the
availability of nutrients from soil and the requirements of
seedlings may not be indicative of field conditions. The
disturbance of the soil profile may change (probably
increase) the availability of some nutrients due to
increased mineralization rates. Finally, the morphology of
rooting of a seedling in a pot is different than that of an
open-grown seedling. Seedlings are often able to concentrate
their growth in regions of good soil physical and chemical
properties, whereas a seedling in a pot is very limited in
its ability to distribute its root system. These limitations
should be kept in mind before utilizing pot studies to
evaluate nutrient limitations for conifers.
3.2.5.
Stand response
Perhaps the only conclusive means of evaluating nutrient
limitations is to utilize field studies. This has been done
for most of the commercially important conifer regions of
the world, including the southern pine region of the
southeastern U.S., the Douglas-fir region of the Pacific
Northwest. The establishment of nutrition cooperatives
patterned after genetic improvement cooperatives have not
only enhanced our understanding of the potential for
regional response to limiting nutrients, but have also shown
how nutrients not initially limiting can limit response to
fertilization through nutrient interactions. For instance,
Jokela et al. (1991) found that N fertilization of a slash
pine plantation in Florida initiated Mn deficiency, and Mn
addition was required to achieve the maximum benefit of
added N.
3.3. Effects on Nutrients on Trees and Stands
3.3.1.
Response Process
The response of trees and forest stands to nutrients is
exhibited in several ways. Nutrients may be partially taken
up by the plant canopy either by direct contact with added
nutrients, or volatilization of nutrients (i.e. ammonium)
following application. Usually, nutrients are taken up from
the soil by plant roots and translocated throughout the
plant from there. Thus, trees must compete for nutrients
with other sinks, including inorganic retention mechanisms
(precipitation and adsorption), biological uptake (microbial
and competing plants), and loss mechanisms such as
volatilization and leaching. Once the nutrient is acquired,
it must be transported to the plant tissue to be utilized,
and in the case of a growth response, that means foliage.
Additional nutrient availability may result in increased
growth of trees either by increasing the amount of foliage
available for photosynthesis, or increasing the net
efficiency of photosynthetic tissue. For conifers this can
mean adding new foliage, or increasing the efficiency of
retained or new foliage. Typically, initial responses of
trees to increased nutrient availability are seen in
increased production of branches and foliage.
3.3.2.
Growth and Yield Response
Foliage is the “factory” of trees, and increased net
photosynthesis can eventually lead to increased growth rates
of woody components, particularly branches and the bole.
Typically, root systems do not show as much an increase in
growth, since with higher nutrient availability in soil,
nutrient uptake may not require as much allocation of growth
to the root system. Trees vary in their relative allocation
of growth to different tissues with higher nutrient
availability, but increased shoot/root ratios are commonly
seen in all plants. Since root systems of trees are
typically not measured in “growth and yield” studies, any
increase in aboveground growth, particularly the bole, will
be considered an increase in growth and yield, despite
losses in growth of other plant tissues.
3.3.3.
Non-Volumetric Response
There are numerous other non-volumetric responses to
increased nutrient availability in a forest. Often, nutrient
contents in plant tissue increase, and there may be changes
in form of the trees, particularly a tendency to hold
branches. Color may change, particularly of the foliage.
Resistance to insects and disease may also change variably.
Nutritional content of plant parts may increase, with
increases in browsing by animals, and cambial feeding (i.e.
by bear). Both reduced and increased flowering and cone
production have been noted.
3.4. Effects on Other Ecosystem Components and Processes
Nutrient additions to coniferous forests can have effects on
ecosystem components and processes other than the trees
occupying the forest. When a receives a nutrient input, the
organisms most able to acquire and utilize that nutrient may
gain a competitive advantage. Thus, nutrient addition may
shift the species distribution as well as the total biomass
of a forest ecosystem.
The addition of one or more nutrients can directly or
indirectly affect the availability of other nutrients. One
mechanism for adverse effects of forest fertilization is due
to nutrient imbalances (Davey, 1968). For instance, Teng
and Timmer (1995) found P deficiency in white spruce (Picea
glauca (Moench) Voss) was initiated by N fertilization only.
Jokela et al. (1991) found subacute Mn deficiency in slash
pine (Pinus elliottii Engl. var. elliottii) in Florida and
hypothesized that intensive culture and macronutrient
fertilization might increase Mn deficiency where Mn is not
applied.
Fertilizer nutrients could also interact with native soil
elements, potentially either increasing or decreasing
availability. Burton et al. (1990) found that heavy
applications of sewage biosolids induced nitrification and
nitrate and cation leaching. Van Miegroet and Cole (1985)
found acidification and leaching of nutrient cations
associated with nitrogen fixation in a red alder stand.
Cole (1979) reported significant losses of K, Ca and Mg
after fertilization with urea without any observation of
nitrate leaching, apparently due to NH4+ replacement of
exchangeable cations on the soil CEC. Nutrient additions
can also affect competing biota. Fertilizing with N alone
can increase the susceptibility of a forest stand to insect
or disease attack (Ballard, 1979).
Categorizing the effects of nutrient additions and other
management practices according to their effect on
productivity is virtually impossible (Burger and Powers,
1991; Grier et al., 1989), and is outside the scope of this
paper. Instead, we take three case studies from Washington
State where treatments designed to increase forest
productivity by addition of nutrients instead resulted in
reduced growth rates. Our primary objective is to show
potential mechanisms, unanticipated at the time of
treatment, that might have led to reduced growth.
4.
MAINTAINING AND ENHANCING FOREST NUTRIENTS
4.1. Nutrient Budgets in Forest Management
It is important to include a consideration of nutrient
management in any forest management scenario. The forest
manager may evaluate nutrient losses vs. nutrient inputs and
nutrient pools in order to assess whether or not a
particular system of forest management will lead to the
depletion of a particular nutrient. If nutrient losses
greatly exceed inputs and nutrient pools are not extremely
large in comparison to those losses, there may need to be
changes made in the management to conserve critical
nutrients.
For instance, Figure 4.1-1 shows the nitrogen cycle for a
60-year-old Douglas-fir plantation in Washington State, USA.
The total N removal for a 60-year rotation with total
aboveground removal would be 320 kg/ha/y. The total N
belowground is 2984 kg/ha, and the total site N 3310 kg/ha;
thus the removal of all biomass would represent less than
10% of the total N pool on-site. If the harvest scenario is
changed to bole-only removal, the estimate of N loss over
one rotation would be much less. Clearly, not all of the
nitrogen would be available to the subsequent stand, and
this particular stand does respond to N addition.
trees
320 kg/ha
tree uptake
39 kg/ha/y
Soil
2809 kg/ha
return to
forest floor
16 kg/ha/y
understory
6 kg/ha
forest floor
175 kg/ha
leached from
forest floor
5 kg/ha/y
leached from soil
0.6 kg/ha/y
Figure 4.1-1. Nitrogen pools and fluxes in a 60-year-old
Douglas-fir stand in Washington State, USA.
There are several limitations of this method of assessing
the potential long-term effects of forest management. First,
not all inputs and losses may have been quantified
accurately, or they may change in the future. Second, all of
the N in the belowground pool may not be available for
future forest growth. For instance, Pacific Northwest
Douglas-fir forests typically contain most of the ecosystem
N in the belowground pool; however, only a small percentage
of it mineralizes and becomes available annually.
If nutrient budgets show a shortfall of a particular
nutrient, there are a variety of ways to management for that
nutrient. As mentioned earlier, an alternate management
system may avoid depletion of the nutrient in the first
place, either by reducing nutrient removals, or by
increasing the input of nutrient (i.e. addition of Nfixers). Necessary nutrients may also be directly added
through forest fertilization.
4.2. Forest Fertilization
Forest fertilization involves the direct application of
nutrients to the forest. Fertilizer source material varies
primarily depending on the nutrient. Some fertilizer
nutrients are taken primarily from ore deposits (i.e.
phosphorus, potassium); whereas, other nutrients are
produced through a manufacturing process (i.e. nitrogen).
However, present production of fertilizer nutrients
generally relies on extractive industries.
4.2.1. Application methods for forest fertilization
There are several requirements for application of fertilizer
nutrients to coniferous forests. The application method must
be economical, avoid injury to seedlings or trees, avoid
environmental problems, and deliver the required nutrients
to the trees where and when the nutrients are required.
The most popular methods of applying forest nutrients are 1)
by hand or ground-based machines at planting, or 2) by
aerial application over young stands or into established
forests. In many cases, particularly in dense stands or
steep or difficult terrain, aerial application is the method
of choice. Reasons not to use aerial application in mass
forest fertilization generally are based on avoiding
fertilizer drift and possibility of application to unwanted
areas such as adjacent properties or stream buffers. In
Canada, the need to provide jobs has sometimes been a reason
to hand-apply fertilizers.
4.2.2. Application of organic wastes, wastewaters and
composts
Coniferous forests offer a number of attractive reasons why
organic wastes can be utilized effectively as soil
amendments to increase forest productivity. Many coniferous
forests are limited in productivity due to nutrient
deficiencies (Weetman et al., 1992; Powers and Edmonds,
1992; Brockley et al., 1992). In addition, some coniferous
soils are low in productivity due to poor soil physical
properties (Henry, 1990). Public health concerns associated
with contamination of food-producing soils on amended sites
are potentially lessened by forest applications (Burd, 1990)
and forest soils may offer higher soil permeability and
biological activity due to the continuous forest cover on
the land compared to agricultural soils.
There are a number of materials that may potentially be
utilized as soil amendments for coniferous forests. These
include, but are not limited to:
a)
b)
c)
d)
municipal biosolids,
pulp and paper sludges and forest products wastes,
composts from a variety of source materials, and
fuel and smelter residues.
Most research to date has been carried out on the
utilization of municipal biosolids (Cole et al., 1990), with
less work carried out on utilization of other waste
materials. Organic wastes derived from living organisms are
likely to be excellent sources of nutrients. For instance,
Figure 1 shows a comparison of the chemical content of
surface horizon sampled from a Douglas-fir forest in Western
Washington compared to a sample of Seattle municipal
biosolids.
It can be seen that biosolids are rich in many of the
nutrient elements commonly limiting forest productivity,
including N and P. In addition, biosolids have a high
concentration of C, which can improve soil physical
properties and nutrient retention capacity.
Trace metals
(too small to show)
Mg Na
K
S
Al
Trace metals
(too small to show)
Fe
P, N
Na S
Mg
Al
Fe
K
P
N
H
Ca
Ca
O
O
Si
Si
C
C
soil
biosolids
Figure 4.2.3-1. Comparison of total elemental concentration
of a common Pacific Northwest soil (Alderwood soil series),
with City of Seattle biosolids.
Application of any material into most forest systems have
difficulties not found in agricultural systems. Trees
themselves are barriers to movement through forests, and
forests are often located on steep topography. Waste
materials can be applied at any time during the life of a
forest stand, but the major potential application times are
1) prior to regeneration of a stand, 2) into a young forest,
into an intermediate aged forest, and 4) into a mature
forest. Forest applications can also be timed with other
entrances into the forest, such as thinning operations.
Applications made prior to stand establishment, though they
offer advantages in terms of trafficablity, offer provide
unique problems. Active weed control programs are often
necessary due to the heavy growth of weeds that typically
occurs following fertilization. In addition, animal damage
is often intense due to the higher populations of small
mammals (Nickelson, 1993) and higher palatability of trees
on sites fertilized with biosolids.
Applications to young forest stands eliminate many of the
problems associated with weed competition and herbivory;
however, some of the application methods that could be
utilized in clearcuts cannot be utilized in existing stands
due to the potential for physical damage to trees.
Typically, applications are made with application vehicles
either spraying liquid or semi-dry materials. Application
trails approximately 150-200 m apart through the forest
stands are constructed prior to equipment entering the
forest stands.
4.3. Alternative Additions: Use of N-fixing Plants
The addition of nitrogen to coniferous forests by
cultivation of nitrogen-fixing plants has been carried out
purposefully for some time, perhaps the first documented
account was in Lithuania in 1894 with the use of lupines to
increase growth of Scots pine (Mikola et al., 1983). Though
the use of N-fixing plants has commonly been seen to
increase N pools in forests (Binkley et al., 1992; Van
Miegroet and Cole, 1985), increases in growth of coniferous
species associated with N-fixers has not been well
documented, partly because of difficulty in setting up
studies and partly because of known competition of N-fixing
species such as red alder with conifers such as Douglas-fir.
5.
OVERVIEW OF NUTRIENT MANAGEMENT PROGRAMS AND PRACTICES
It is impossible to review nutrient management in detail in
all areas of the world in which coniferous forests exist.
However, this chapter looks at some of the data available.
5.1. Europe
Nutrient management of Europe’s coniferous forests has
changed significantly during the last few decades,
particularly following identification of the “new type”
forest damage described in the literature since the mid1970’s (Kandler and Miller. 1991; Ende and Evers, 1997;
Innes, 1993). A good example of what has happened in large
parts of Europe can be seen in the example of Finland. One
eighth of Finland's forest land has been fertilized at least
once, with a peak of just less than 250,000 ha fertilized
during the year 1975, mostly with urea-N. In peak years,
forestry application of fertilizers accounted for 10 percent
of all fertilizer application in Finland (Finnish Forestry
Association, 1996). Due to concerns over nitrogen use, the
annual rate of fertilizer application decreased after 1975,
until it is insignificant in from 1991 to the present time.
Presently, there is a considerable emphasis on balancing
availability of a variety of nutrients, particularly Mg and
sometimes Ca, Zn and K, the availability of which has been
shown to be decreased by high rates of N applications
(Harrison et al., 1994), through the mechanism of
accelerated cation leaching. Though the mechanism of excess
N addition causing such deficiencies is certainly valid, and
evidence of higher rates of cation leaching due to enhanced
atmospheric deposition is common (Evers and Hüttl, 1991) the
new type damage often occurs in soils that also respond to N
fertilization in the same way that did before the new type
damage was first observed (Pettersson, 1994). Research on
the use of “vitality” fertilization has yielded variable
results in different parts of Europe. The need for use of
nutrients other than N to offset potentially detrimental
effects of excess N and other factors was discussed by Evers
and Hüttl (1991) for Germany. However, in one study in
southern Sweden, little or no response to P, K, Ca, Mg or
micronutrients was seen (Nohrstedt et al., 1993), despite a
growth response of 20-30% for N fertilization.
5.2. North America
The role of nutrient management has also been shifting in
North America, but not in the same way as in Europe. The
current changes of forest management practices toward New
Forestry (Franklin et al., 1989) have de-emphasized maximum
forest productivity in terms of volume yield on some of the
forestland in the Northwestern Douglas-fir region in the
name of maintaining long-term site quality. This is
particularly true of federal lands.
Utilization of fertilizers on private lands have increased
during the same time period, with the goal of increasing
yields of production land. Approximately 50-55,000 ha of
land per year are presently fertilized in Washington and
Oregon State, with a dosage typically of 224 kg/ha after 710 years from planting (Chappell et al., 1992).Data from the
Pacific Northwest Stand Management Cooperative shows a
general response of Douglas-fir to Nitrogen. For instance,
Figure 5.2-1 shows the percent volume response of sites with
>8 years growth data according to fertilizer rate. The sites
with the highest site quality responded with the lowest
percentage responses.
Figure 5.2-1. Percentage volume response for Douglas-fir at
five N application rates according to site quality.
In Canada, a silviculture prescription must contain a
requirement for fertilization if the district manager
determines that it is necessary to obtain a free growing
stand. This requirement will only be removed if the holder
of the silviculture prescription can demonstrate that
fertilization is unlikely to significantly improve stand
vigor. This demonstration would be done using evaluation of
site and stand characteristics and, documentation of stand
nutrient status by foliar analysis and fertilizer screening
trials.
Only four elements are currently operationally applied
through use of inorganic fertilizers on forests in British
Columbia. These are N (nitrogen), S (sulfur), P
(phosphorus), and B (boron). Nitrogen is applied in the
largest amount with S, P, and B being added to ensure the
benefits of the nitrogen fertilizer are not limited by
deficiencies of other elements. Supplements of nitrogen
frequently improve growth of coastal Douglas-fir, lodgepole
pine, and sometimes western hemlock.
In the interior of British Columbia, S deficiencies may
limit the responsiveness of lodgepole pine to N
fertilization over fairly extensive portions of the Subboreal Spruce and Sub-boreal Pine–Spruce biogeoclimatic
zones within the central and northern portions of the
interior plateau.
5.3. Other areas
Australia had approximately 959,000 ha of pine plantations
in 1993 (ABARE, 1995), while New Zealand presently has about
1.47 million hectares of conifer plantation. Forests are
actively fertilized in both countries, particularly with
phosphorus.
South America has little natural conifer forests, with the
exception of the Aurecaria forests of Brazil, Paraguay and
Argentina. However, conifers, particularly pine species,
have been extensively planted in almost all regions of South
America, particularly in the past. As a percentage of forest
plantation area, hardwood species such as Eucalyptus spp.
are becoming more popular due to their higher productivity.
However, total planted hectares of pine are being maintained
in many areas, particularly in subtropical and temperate
areas.
6.
SUMMARY
Nutrient management plays a critical role in the management
of the world's coniferous forests. The productivity of many
of these forests are limited by nutrient availability,
primarily nitrogen in the boreal and cooler regions and by
phosphorus in the more highly weathered soils of the
subtropical and tropical regions. However, some coniferous
forests show S, Ca, Mg, B and Zn deficiencies as well,
though these are less widespread. In situations where these
forests will produce high volumes of wood, a consideration
of the long-term role of nutrients is critical, either from
the standpoint of nutrient conservation, or the requirement
for fertilization. The approach of nutrient cycling offers a
valid tool for examining the relationship between nutrient
pools, inputs from atmospheric, mineral weathering,
fertilization and other sources, and outputs through
leaching, volatilization, harvest and other mechanisms.
7.
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