Climatological controls on net primary productivity in a southern Appalachian forest

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Long-term climatological records from the Coweeta LTER station have shown two trends that are expected to have important influences on future net primary productivity (NPP): (1) temperature has increased over the past several decades and (2) the frequency of large and small storms has increased, with no change in mean annual precipitation. Changes in these meteorological drivers, both individually and interactively, can simultaneously increase and decrease components of the carbon budget with uncertain effects on NPP. For example, warmer temperatures can lead to phenological shifts that lengthen the growing season and increase gross primary productivity (GPP). On the other hand, carbon loss through ecosystem respiration (RE) is highly sensitive to temperature, so warmer conditions may decrease NPP. Similarly, during periods of low precipitation, soil water content is depleted, leading to reductions in both GPP and RE. Furthermore, temperature and water availability combine to determine the gradient between soil water availability and the atmospheric demand for water (vapor pressure deficit), which under certain conditions, can lead to irreversible constraints on GPP. We examine these covarying mechanisms using a combination of data from an eddy covariance tower, a subcanopy eddy covariance system, and soil CO₂ efflux chambers. Higher air temperature in February and March of 2012 corresponded with leaf expansion occurring approximately two-weeks earlier than in 2013 and 2014. However, high mid-summer temperatures in 2012 resulted in high vapor pressure deficit and high respiration, effectively nullifying springtime carbon gains. Overall, the combined effects of intra-annual variability in meteorological drivers are expected to have a large influence on interannual variability in NPP.