9/10 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.