Influences of Terrain Complexity on the Temporal Sensitivity of Terrestrial Carbon Fluxes to Climate

Exchanges of carbon, water, and energy between terrestrial ecosystems and the atmosphere not only influence Earth's climate, but they are also sensitive to climate change. Terrain complexity (e.g. slope, aspect, upslope accumulated area and other variables) have the potential to mediate the sensitivity of net ecosystem carbon exchange (NEE), gross primary productivity (GPP) and ecosystem respiration (RE) to climate variability by influencing soil water availability, which in turn mediates the sensitivity of terrestrial ecosystem fluxes to temperature and precipitation. Here we use 178 site-years of data from 31 different sites across the Ameriflux network to investigate the following questions: (1) How sensitive are NEE, GPP and RE to temperature and precipitation across a wide range of temporal scales, climate, vegetation and terrain conditions? (2) Where and when does terrain complexity mediate the sensitivity of NEE, GPP and RE to climate variability? and (3) Does terrain complexity provide a basic template to help to predict the sensitivities of NEE, GPP and RE to climate despite biophysical differences across sites? Our results suggest that temperature sensitivity of NEE, GPP and RE across sites was strongest at the daily timescale whereas sensitivity to precipitation was greater at the annual timescale. The magnitude of such sensitivities varied greatly among sites, suggesting the existence of locally imposed controls, including terrain complexity. Overall, NEE, GPP and RE sensitivities to temperature and precipitation were found to be influenced by slope and aspect across sites; however, other terrain variables such as upslope accumulated area help explain the climate sensitivity observed at certain sites. Altogether, our results suggest that terrain complexity does contribute to the sensitivity of terrestrial ecosystem fluxes to climate variability; therefore, our understanding of terrestrial carbon cycling may be improved by considering terrain complexity in addition to ecological characteristics as indicators of how terrestrial landscapes respond to climate variability and climate change.