Daily total ecosystem evapotranspiration (ET) during spring 2010 and 2011
Figure 4. Daily total ecosystem evapotranspiration (ET) during spring 2010 and 2011. Lines are 7 day running means and numbers following years show total spring ET. Arrows indicate management at grassland sites, i.e. grass cuts (a), (b) and start of grazing (c). The dashed lines confine the period of spring drought across all sites (DOY 102–132). Abbreviations in titles indicate the IGBP land-use class with grasslands (GRA), mixed forest (MF) and evergreen needleleaf forest (ENF).
Since the European summer heat wave of 2003, considerable attention has been paid to the impacts of exceptional weather events on terrestrial ecosystems. While our understanding of the effects of summer drought on ecosystem carbon and water vapour fluxes has recently advanced, the effects of spring drought remain unclear. In Switzerland, spring 2011 (March–May) was the warmest and among the driest since the beginning of meteorological measurements. This study synthesizes Swiss FluxNet data from three grassland and two forest ecosystems to investigate the effects of this spring drought. Across all sites, spring phenological development was 11 days earlier in 2011 compared to the mean of 2000–2011. Soil moisture related reductions of gross primary productivity (GPP) were found at the lowland grassland sites, where productivity did not recover following grass cuts. In contrast, spring GPP was enhanced at the montane grassland and both forests (mixed deciduous and evergreen). Evapotranspiration (ET) was reduced in forests, which also substantially increased their water-use efficiency (WUE) during spring drought, but not in grasslands. These contrasting responses to spring drought of grasslands compared to forests reflect different adaptive strategies between vegetation types, highly relevant to biosphere–atmosphere feedbacks in the climate system.