The effect of drought on carbon exchange in a deciduous forest

One potential source of interannual variability in net ecosystem exchange (NEE) of carbon in deciduous forests is drought, a fairly common occurrence in the forests of the southeastern United States. In deciduous forest species, drought is known to reduce carbon fixation, either by reductions in stomatal conductance or changes in photosynthetic capacity. However, a counteracting effect of drought on NEE is the potential for reductions in respiration, especially from the roots and soil. Low and high frequency wetting and drying events of the leaf litter layer also may cause fluctuations in heterotrophic respiration.

A multiyear study of NEE measured by the eddy covariance technique above the forest is examined to elucidate the effects of drought on carbon exchange rates. The processes involved in controlling the response of NEE to soil water content were investigated. Soil respiration estimated by a separate eddy covariance system in the subcanopy and temporal patterns of photosynthetic capacity from a leaf-level gas exchange system were used to investigate how several of the components of NEE are affected by drought. Soil respiration estimates are also compared against a model based on chamber measurements. This model separates heterotophic and autotrophic respiration and respiration from the mineral soil horizons from that in the leaf litter. Drought induced reductions in photosynthesis were also separated into stomatal and non-stomatal contributions.

The decreases in both leaf-level photosynthesis and soil respiration partially compensate each other in this forest. As a result, the effect of drought on the interannual variability of NEE was reduced. Drought is a common occurrence in the Southeast, but the interannual variability in the length of growing season is probably more important for determining interannual variability of NEE. The effect of climate change on NEE may be determined more by changes in the length of growing season than changes in the occurrence or duration of drought.