Forests and agroecosystems play an important role in biosphere-atmosphere gas exchange, affect and are affected by climate change, while providing many valuable ecosystem services. Plant ecophysiology is often at the core of these fluxes and services, thus its relevance has greatly increased over the last decades. Combining long-term observations of net ecosystem fluxes with stable isotope applications to trace water and experimental drought manipulations has provided unprecedented insights into ecosystem resilience today and in the future.
Within the Swiss FluxNet, a network of ecosystem flux measurement sites in Switzerland (http://www.swissfluxnet.ch/), CO2 and H2O vapor fluxes of two forests (mixed deciduous forest at 800 m; evergreen forest above 1600 m), three grasslands (along an elevational management intensity gradient) and one cropland (4-yr rotation in the lowlands) have been intensively studied since more than a decade, with the spruce forest (since 1997) and the cropland (since 2004) being one of the longest and the longest dataset globally.These time series allow detailed ecophysiological studies, while also serving as platforms for complementary research.
Results/Conclusions
Both forests have been carbon sinks ever since measurements started, despite large climatic variations over time. In contrast, the cropland has been losing C over the last decade, with pronounced differences among crop species but counteracted by manure applications. Environmental conditions as well as management events have clear impacts on net ecosystem fluxes of grasslands, reinforcing (increased growth under favourable conditions) but also balancing out each other (increased grazing after enhanced growth).
Forests acclimated fast to a spring drought, much in contrast to temperate grasslands. Similar responses were seen during the hot and dry summer in 2018. Productivity declined at the lower elevation sites, but not at sites above 1000 m. In contrast to grasses, trees showed a very pronounced sensitivity of stomatal aperture that led to a decline in canopy conductance in response to increased vapor pressure deficit. Moreover, trees showed a large plasticity in water uptake depths, from the top soil (non-limiting conditions) or from deeper soil layers (stress conditions) for deciduous trees, but not for spruce. Both beech and spruce relied on water which precipitated during the current growing season (about 50%), but also on water originating from the previous winter (about 40%). In contrast, grassland species did not change their water uptake depth with drought stress, but always stayed very shallow. Nevertheless, their response to simulated extreme drought was highly resilient.