NEP of a Swiss subalpine forest is significantly driven not only by current but also by previous year's weather

Understanding the response of forest net ecosystem productivity (NEP) to environmental drivers under climate change is highly relevant for predictions of annual forest carbon (C) flux budgets. Modeling annual forest NEP with soil-vegetation-atmosphere transfer models (SVATs), however, remains challenging due to unknown delayed responses to weather of the previous year. In this study, we addressed the influence of previous year's weather on the interannual variability of NEP for a subalpine spruce forest in Switzerland. Analysis of long-term (1997-2011) eddy covariance measurements showed that the Norway spruce forest Davos Seehornwald was a consistent sink for atmospheric CO2, sequestering 210 +/- 88 gCm(-2) yr(-1) on average. Previous year's weather strongly affected interannual variability of NEP, increasing the explained variance in linear models to 53% compared to 20% without accounting for previous year's weather. Thus, our results highlight the need to consider previous year's weather in modeling annual C budgets of forests. Furthermore, soil temperature in the current year's spring played a major role controlling annual NEP, mainly by influencing gross primary productivity early in the year, with spring NEP accounting for 56% of annual NEP. Consequently, we expect an increase in net CO2 uptake with future climate warming, as long as no other resources become limiting.