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Joshua Gray
September 2007
Introduction
The variable allocation of carbon acquired by photosynthesis to foliage, stems and
roots is known to depend on environmental conditions. This investigation sought to
explore the nature of this carbon allocation in a forested stand under variable
environmental conditions, specifically, the differences between allocation in years with
abundant precipitation and those with relatively small amounts of precipitation.
Methods
BIOME-BGC version 5.0 for Excel was used for the numerical simulations. The
study area was Missoula, MT (46º51’N, 114 º 0’W). Climatic data detailing the daily
precipitation, temperature, solar radiation and vapor pressure deficit was obtained for the
years 1950 through 1993. Tree specific parameters were typical of evergreen needle leaf
stands. The model was initialized with a 1m deep loamy soil (30% sand, 50% silt, 20%
clay). A model spinup was run for a maximum of 6000 years to allow the carbon stores to
equilibrate. The model was then run for 100 years, reusing the climatic record as
necessary. The following output variables appropriate to the exploration of carbon
allocation were selected for activation: gross primary production (GPP), net primary
production (NEP), net ecosystem exchange (NEE), soil carbon, fine root carbon, live and
dead stem carbon, leaf area index (LAI), and leaf carbon. Soil water potential,
evapotranspiration, and precipitation were also included as model output. The years
chosen for comparison were 1976 (24.1cm total precipitation) and 1980 (82.8cm total
precipitation).
Results
Total GPP was significantly higher for 1980 (the wetter year) than 1976: 0.93
kg/m2 compared to 0.61 kg/m2 (Figure 1). The intra-annual variation in GPP was
explained well by variations in soil water potential (Figure 3).
Figure 1
0.009
0.008
GPP (kg/m2)
0.007
0.006
0.005
0.004
0.003
0.002
0.001
1976
Year Day
1980
Aboveground carbon can be divided between foliage and stem. 1980 exhibited
both the highest maximum leaf area index (LAI) as well as the greatest variation in LAI
throughout the year. The amount of carbon in stems exhibited the same pattern. Although
1980 had higher maximum stem, leaf and fine root carbon, 1976 started adding carbon to
these stores earlier in the year and reached an annual maximum for these stores earlier
(Figure 2). This phenomenon might be explained by the less negative soil water
potentials observed early in the 1976 year (Figure 3).
361
346
331
316
301
286
271
256
241
226
211
196
181
166
151
136
121
106
91
76
61
46
31
16
1
0
Figure 2
Figure 3
Annual Variation in Soil Potential
0
100
200
300
0
Soil PSI
-0.5
-1
-1.5
-2
-2.5
-3
Year Day
1976 SoilPSI
1980 SoilPSI
A comparison of net primary productivity (NPP), which is gross primary
production minus plant respiration, and gross primary production (GPP) showed a near
constant ratio (Figure 4). The ratio was 0.50 for 1976 and 0.57 for 1980. These values are
higher than the values reported by Waring and Running (average of 0.46) for a variety of
forests worldwide.
Figure 4
This result indicates that about half of total carbon acquired through photosynthesis was
respired by the plant. This respiration is the sum of synthesis, growth and maintenance
respirations. The remaining half is available to be incorporated into new biomass. When
GPP is low NPP can sometimes be negative, indicating that the plant has respired more
carbon on a given day than it has fixed through photosynthesis.
Conclusions
This comparison of carbon allocation in wet and dry years allows us to conclude
that annual precipitation is an important factor in determining not just GPP and NPP, but
also the allocation of NPP to roots, stem and foliage biomass. Comparison of the annual
temperature and solar radiation for 1976 and 1980 indicated that these variables were not
dissimilar enough to exert a strong influence on the observed patterns of carbon
allocation. In general, the results indicate that higher annual precipitation will allow for
higher maximum LAI and stem carbon. Soil water potentials were lower during the
growing season in 1976 than the same time in 1980. The higher amount of available
water in 1980 during the growing season led to larger daily values of GPP, which the
forest patch utilized for the production of stem and leaf biomass. Fine root carbon was
also higher in the wetter year of 1980. This result is somewhat surprising considering that
water stress generally leads to a greater allocation of carbon to belowground, water
absorbing, biomass. When comparing 1976 and 1980 fine root carbon as a percentage of
total carbon in roots, stem and leaves we would expect to see the fine root carbon a
greater percentage of overall carbon during times of stress. However, this is not the case
for this simulation, with 1976 having a lower percentage of total carbon allocated to fine
roots during the most severely water stressed growing season. Perhaps there is some type
of momentum effect in allocation scheme carried over from the previous year. This is a
likely explanation given that the year preceding 1976 was relatively wet, and the year
preceding 1980 was relatively dry. Future investigations might explore if this
phenomenon is observed for relatively long dry and wet periods.