Are ecosystem carbon inputs and outputs coupled at short time scales? A case study from adjacent pine and hardwood forests using impulse-response analysis.

A number of recent studies have attributed a large proportion of soil respiration (R(soil)) to recently photoassimilated carbon (C). Time lags (tau(PR)) associated with these pulses of photosynthesis and responses of R(soil) have been found on time scales of hours to weeks for different ecosystems, but most studies find evidence for tau(PR) on the order of 1-5 d. We showed that such time scales are commensurate with CO(2) diffusion time scales from the roots to the soil surface, and may thus be independent from photosynthetic pulses. To further quantify the role of physical (i.e. edaphic) and biological (i.e. vegetative) controls on such lags, we investigated tau(PR) at adjacent planted pine (PP) and hardwood (HW) forest ecosystems over six and four measurement years, respectively, using both autocorrelation analysis on automated soil surface flux measurements and their lagged cross-correlations with drivers for and surrogates of photosynthesis. Evidence for tau(PR) on the order of 1-3 d was identified in both ecosystems and using both analyses, but this lag could not be attributed to recently photoassimilated C because the same analysis yielded comparable lags at HW during leaf-off periods. Future efforts to model ecosystem C inputs and outputs in a pulse-response framework must combine measurements of transport in the physical and biological components of terrestrial ecosystems.