Bi olog y 3 7 0 0 : E cos y s t e m and Communi t y E colog y
L a b 4 Me thods for De t e rmining the Amoun t of Pla n t Bi oma ss
in Grassland and For e s t Communi ti es
Primary production is an important ecosystem characteristic that affects many other ecosystem
properties. The conversion of light energy to chemical energy during photosynthesis and the
subsequent production of plant biomass provides the energy source for all other trophic levels
in an ecosystem. In this exercise you will be introduced to some methods for determining the
amount of above- and below-ground plant biomass.
De fini tion of T erms
Plant biomass is the weight of living plant material contained above and below a unit surface
area of ground at a given point in time. The weight refers to the “dry weight” value after the
plant tissue has been dried to a constant weight at 60°C (this usually takes 24-48 hours in a
forced-air drying oven). The units regularly used for reporting plant biomass data are: g m-2.
Plant production is the biomass or weight of plant matter produced by a plant community or an
individual plant species, per unit ground area per unit time. The units regularly used for
presenting plant production data are: g m-2 yr-1.
In many instances it is more useful to present biomass and production data in terms of the
amount of carbon that has been produced. This is because it is carbon that is assimilated during
photosynthesis (the carbon originates as carbon dioxide taken up from the atmosphere). In
many plant species, biomass consists of a variable amount of other elements in addition to
carbon. So different plant species may have very different carbon contents, despite similar
biomass values. In many investigations the objective is to relate plant biomass and production
values to photosynthetic activity and so it is more useful to present biomass and production
values in terms of the weight of carbon in the plant or community. This requires an additional
analysis of the concentration of carbon in plant tissues. Plant tissue usually contains about 50 %
carbon, but this value can vary significantly among plant species and among individual tissue
types (eg. roots or leaves) within a plant. Units for biomass and production are then expressed
as: g C m-2 and g C m-2 yr-1, respectively.
Plant production data is often expressed as Gross Primary Production (GPP) or Net Primary
Production (NPP). Gross primary production is the total amount of organic matter assimilated
during photosynthesis, including carbon that gets given off subsequently in respiration. In order
to calculate GPP it is necessary to have information on the amount of carbon lost during
respiration. More often plant production data is presented as net primary production, which is
the total amount of organic matter assimilated minus that lost due to total plant respiration.
Net primary production includes the live biomass accumulated over a period of time, plus any
tissue that was produced during that same time interval that was lost because of: (i)
consumption by other organisms, and (ii) shedding of plant parts after senescence (plant litter
production).
NPP = Live Biomass Accumulation + Consumption Loss + Plant Litter Production
Based on the above equation, you should note that measurements of live biomass production
(or accumulation) should be related to NPP, but will not be identical to NPP values. In other
words, it is necessary to also measure consumption losses and plant litter production, in
addition to live biomass production, in order to accurately calculate NPP.
Measur emen t of Ab o v egr ound Plan t Bi oma ss
Measurements of aboveground biomass will be made in a grassland and an adjacent cottonwood
forest in Cottonwood Park. The field sampling will take place during the Lab 3 exercise. Dry
weights will be measured in the lab room during Lab 4 and allometric calculations will be done
during the Lab 4 time period in a computer lab.
Grassland Vegetation
In low stature grassland communities or in the understory of a forest, biomass and production
data are gathered by clipping vegetation at ground-level in randomly placed quadrats. The size
of the quadrats used in grassland sites is usually 20 cm x 50 cm, or 50 cm x 50 cm. Optimum
quadrat size is a function of the spatial variation in biomass over the study area and the time
required for harvesting and processing samples. The method for replicate placements of
quadrats will vary depending on the objective of the investigation. The method of placements
that we will use in this field class is described below.
Sample areas of sufficient size and homogeneity will be selected in a grassland area of
Cottonwood Park. The areas selected for sampling will be approximately 30 m by 30 m. The
corners of these areas or blocks will be marked with flagging tape. Each group of 4 people will
harvest 3 replicate samples of plant biomass in 20 cm x 50 cm quadrats in the grassland site.
The position of the quadrat within the treatment block will be determined using a random
numbers table as was done previously. After randomly choosing a sampling site, set up the 20
cm x 50 cm quadrat and harvest the plant biomass (plant material that was alive this past
growing season) that is rooted inside the frame and place it in marked paper bags. Bags should
be marked with your name, the date, sample type (eg. aboveground biomass), the site (eg.
grassland Cottonwood Park.), quadrat replicate number. Then repeat the quadrat selection
procedure until you have sampled 3 replicates. Upon return to the university lab, the paper bags
should be placed in a drying oven at 60°C. After drying, the plant tissue needs to be weighed on
a balance (this will be done next week). Above-ground plant biomass data should be expressed
in units of g m-2.
Forest Tree Vegetation (Cottonwood Trees)
Cottonwood Park has the following cottonwood tree species:
Plains or Western Cottonwood ( Populus deltoides ),
Balsam Poplar ( Populus balsamifera ),
Narrow-leaf Cottonwood ( Populus angustifolia )
and hybrids among the species listed above.
It is not practical to cut down large trees (and unethical, particularly in a city park) for biomass
and production measurements. Therefore, trees require a different set of methods for biomass
and production data collection. Forest ecologists regularly use allometric equations that relate a
simple, non-destructive measurement such as tree diameter (DBH) to stem, branch and foliage
mass, foliage area, and tree height. These equations are developed from measurements made
on selected tree species harvested as part of forestry practices or research studies. It is optimal
to use allometric equations developed specifically for a tree species of interest, right at the
study location of interest, but this does require some destructive sampling. In many cases
allometric equations for biomass estimates can be applied to closely related species growing in
slightly different environments without much loss of accuracy.
In this exercise we will use allometric equations developed for a Populus species related to
cottonwood trees to estimate tree aboveground biomass and tree height (Wang 2006).
Log 10 Total Aboveground mass (g) = 1.826 + 2.558 (Log10 DBH (cm))
Tree Height (m) = 1.3 + 22.241(1 – e-0.065 DBH (cm) ) 0.805
where e is the base of the natural logarithm.
The allometric equation for total tree leaf area (shown below) was developed by Gower et al.
(1997) for aspen trees ( Populus tremuloides, growing in Prince Albert National Park in northern
Saskatchewan) with DBH values ranging between 11.3 and 29.8 cm.
Log 10 Total Leaf Area (m2 ) = -2.001 + 2.611 (Log 10 DBH (cm))
Measur emen t of Roo t B ioma ss in a Grassland Communi t y
Measurements of plant root biomass will be made in the grassland site. Root biomass data are
gathered by collecting soil cores from near the placement of a quadrat for aboveground biomass
harvest. The vegetation and litter should be removed from an area on the ground before a soil
corer is inserted 15 cm into the soil. After removing the corer, the extracted soil should be
placed in a marked plastic bag. The bag should be marked with your name, the date, sample
type (eg. Grassland root biomass). Each group of four students should collect one soil sample
from the grassland.
The soil samples should be processed to determine root biomass using the following procedure.
Individual samples should be placed in a 100 ml beaker and soaked in water for 30 minutes, so
that soil aggregates can be easily broken down. Next the soil samples should be washed through
two sieves (1.41 mm and 600 µm diameter pore size). The roots collected on the two sieves
should be rinsed to remove any excess soil and placed into pre-weighed glass dishes and dried
at 60°C for 24-48 hours. After drying, the samples need to be re-weighed.
The root biomass data should be expressed as g m-2 for a 15 cm depth interval. The following
calculation procedure should be followed for each soil core sample. Different groups will use soil
corers of different size. The example below assumes a soil core volume (V ) of 73.63 cm3.
V = π r2 h
where
π = 3.14159
r = radius of soil corer (1.25 cm)
h = depth that the soil corer is pushed into the soil (15 cm)
Volume of a 1 m2 area of ground surface that is 15 cm deep is 150,000 cm3. So the soil core
sample represents 73.63/150,000 of a 1 m2 area that is 15 cm deep. So the dry weight of the
roots collected from a soil core sample should be multiplied by 2037.21 (150,000/73.63) in
order to calculate the corresponding weight of root biomass in a 1 m2 area that is 15 cm deep.
Measur emen t of Roo t B ioma ss in a For e s t Communi t y
The below-ground stumps of trees and the woody, coarse roots of trees cannot be easily
sampled using soil cores, as is done for the herbaceous, grassland vegetation (as described
above). Allometric equations are normally used to estimate the coarse root (roots greater than
5 mm in diameter) biomass of trees. Fine root biomass (roots less than 5 mm in diameter) can
be estimated based on soil core sampling.
In this lab exercise we will use the following allometric equations to estimate tree root biomass
( Wang 2006). Total tree root biomass is the sum of stump and coarse root biomass. This
calculation ignores a relatively small component of fine root biomass, but it is a good estimate
of the majority of tree root biomass.
Log 10 Below-ground Tree Stump mass (g) = -0.107 +2.502 (Log 10 DBH (cm))
Log 10 Below-ground Coarse Root mass (g) = 0.992 +2.563 (Log 10 DBH (cm))
Estimates of the tree mass (total aboveground biomass and total root biomass (stump plus
coarse root) and total leaf area of a tree should be made based on measurements of tree DBH
on all trees located within 10 m x 10 m quadrats sampled within a study area in the forest site.
We will use tree DBH data sampled at forest transect position 5 by the class during Lab 3 as a
data set for calculations in this exercise.
Ultimately the tree biomass measurements should be expressed as g m-2, based on the sampling
of trees in 10 m x 10 m quadrats. Tree leaf area values should be expressed as a leaf area index
(LAI, (m2 of leaf area per m2 ground area)), a unit-less parameter.
Re fe r enc e s
Bond-Lamberty, B., C. Wang and S.T. Gower. 2002. Aboveground and belowground biomass and
sapwood area allometric equations for six boreal tree species of northern Manitoba.
Canadian Journal of Forest Research 32: 1441-1450
Gower, S.T., J.G. Vogel, J.M. Norman, C.J. Kucharik, S.J. Steele and T.K. Stow. 1997. Carbon
distribution and aboveground net primary production in aspen, jack pine, and black
spruce stands in Saskatchewan and Manitoba, Canada. Journal of Geophysical Research
102: 29,029-29,041
Steele, S.J., S.T. Gower, J.G. Vogel and J.M. Norman. 1997. Root mass, net primary production
and turnover in aspen, jack pine, and black spruce forests in Saskatchewan and Manitoba,
Canada. Tree Physiology 17: 577-587
Wang, C. 2006. Biomass allometric equations for 10 co-occurring tree species in Chinese
temperate forests. Forest Ecology and Management 222: 9-16
Wilson, S.D. 1993. Belowground competition in forest and prairie. Oikos 68: 146-150
L a b Exer cise
Prepare a table that compares and contrasts the values for aboveground biomass and root
biomass for the grassland and forest communities. This table should show the average ±
standard error values based on replicate sampling. Calculations of biomass should be expressed
on a per meter square ground area basis.
Compare the aboveground biomass and root biomass values (calculate a root/shoot ratio) for
the grassland and forest sites. What does this data suggest about the magnitude of above- and
below-ground competition in these two communities?
In addition, the tree height and LAI value for the forest should be calculated.
How do the estimated tree heights (based on calculations using the allometric equation)
compare with tree heights measured using the clinometer in Lab 3?