Lay summary
The project Forest Ecosystem Responses to Climatic Drivers aims at understanding time lags between climatic drivers and the respective ecosystem responses in terms of net ecosystem productivity (NEP) at the two Swiss Fluxnet sites Davos and Lägeren. The project has a highly interdisciplinary character and brings together detailed knowledge from plant physiology, forest ecology and meteorology to disentangle the effects on NEP of (i) actual physical drivers, and (ii) biotic conditions determined by past and recent climatic conditions. Understanding the natural processes determining the carbon balance of forest ecosystems is of great global interest for estimating country-specific carbon budgets within the United Nations Framework Convention on Climate Change (UNFCCC). The topic is a current hotspot at which precise questions from politics meet the incomplete knowledge of various environmental science disciplines.

We hypothesize that the current year NEP is significantly driven by climatic drivers of the recent past (month to years) and not only by present conditions as typically assumed. Furthermore we hypothesize that observed time lags between climatic drivers and NEP are due to storage dynamics of carbon and water in trees.
Our specific aims are to (i) identify climatic drivers of NEP at two contrasting forest sites, (ii) to quantify the impact of the climatic drivers on observed time lags of ecosystem responses, and (iii) to assess the underlying physiological mechanisms explaining such time lags and therefore flux partitioning.

In order to address these aims, we plan:
o to measure microclimate profiles, eddy covariance net ecosystem CO2 and H2Ovapor exchange, continuous stem radius changes, and [CO2] in tree stems at two Swiss Fluxnet sites Davos and Lägeren.
o to analyse statistical patterns of historic time series which quantify the time-dependent weight of climatic drivers on ecosystem responses.
o to compare the results of two forest types from the Swiss Fluxnet sites Davos and Lägeren.
o to test the applicability of physiological concepts to explain the observed time lags and thus flux partitioning of CO2 and H2Ovapor fluxes at the ecosystem level.
Our long-term forest ecosystem research sites (Davos since 1997; Lägeren since 2005) are predestined locations to address topics that depend on temporally and spatially highly resolved field data at different integration levels with long-term records.