HR: 0830h
AN: B11D-0713    [PDF]
TI: Climatic versus biotic constraints on carbon uptake in ponderosa pine forests
AU: * Schwarz, P A
EM: paul.schwarz@oregonstate.edu
AF: Department of Forest Science, Oregon State University, Richardson Hall, Corvallis, OR 97331 United States
AU: Williams, M
AF: School of Geosciences, University of Edinburgh, Darwin Building, Edinburgh, EH9 3JU United Kingdom
AU: Law, B E
AF: Department of Forest Science, Oregon State University, Richardson Hall, Corvallis, OR 97331 United States
AU: Irvine, J
AF: Department of Forest Science, Oregon State University, Richardson Hall, Corvallis, OR 97331 United States
AU: Kurpius, M R
AF: Department of Atmospheric Sciences, Oregon State University, Oceanography Administration Building, Corvallis, OR 97331 United States
AB: An assessment of the trade-offs between climatic versus biotic controls on carbon uptake in forests is critical to developing a clearer understanding of disturbance on carbon cycling in terrestrial ecosystems. We used a detailed, process-based simulation model to investigate the effects of interannual variation of precipitation on carbon assimilation in ponderosa pine (Pinus ponderosa var. Laws.) forests in the Cascade Mountains of central Oregon, USA, characterized by cool, wet winters and hot, dry summers that result in seasonal drought stress. We selected three ponderosa pine stands representing a range of ages since the previous stand-initiating disturbance, clearcut logging. In 2002, the young stand (YS) was about 25 years old and had a mean total leaf area index (LAI) of about 1.1. The mature stand (MS) was around 90 years old with a mean LAI of about 3.0, and the old stand (OS) was never logged and had a mean LAI of about 2.3. Simulations of carbon and water fluxes at each site were performed using the Soil-Plant-Atmosphere (SPA) model (Williams et al. 2001) using stand and meteorological data. Comparisons between simulations and eddy flux measurements at each site indicated generally good agreement, with relative differences between the model and measurements across the three sites of about 25% for tree transpiration, 30% for total latent energy fluxes, and 25% for gross carbon uptake, GPP. Four years of climate data (1999-2002) for the sites showed three- to four-fold variation in winter precipitation, nearly five-fold variation in summer precipitation, and over 50% variation in total annual precipitation. Based on the simulations, mean annual carbon uptake across the four years was 850 gC m${-2}$ y${-1}$ at the YS, 1260 gC m${-2}$ y${-1}$ at the OS, and 1530 gC m${-2}$ y${-1}$ at the MS. Despite the substantial interannual variation in precipitation, the interannual variation in GPP at each of the three sites was only about 5%, suggesting that variation in precipitation across the four years was not a major constraint on GPP, whereas soil CO2 efflux measurements at the OS and YS showed large interannual variation that could be attributed to soil moisture availability in the deeper soil horizons (OS) and the quantity of summer precipitation (YS), suggesting that interannual variation in net ecosystem production (NEP) is largely due to the respiration responses to moisture availability. Additional simulations, eliminating summer drought conditions, indicated that annual GPP could increase by up to 20% at the sites if soil moisture was not limiting. Moreover, the large differences in annual GPP among the three sites, suggest that stand development following disturbance probably plays the largest role in limiting carbon uptake in these forests. Finally, sensitivity tests involving maximum rooting depth at each site, suggests that this factor may be crucial to enabling these semi-arid forests to minimize the effects of summer drought.
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2003 Fall Meeting