Drought Sensitivity of Terrestrial Ecosystems in Europe and North America

Drought frequency and severity are expected to in-crease globally under projected climate change, with unknown consequences for hydrological and terrestri-al ecosystems, while the terrestrial hydrological cycle is strongly linked to changing ecosystem processes. Current management tools, however, are based on the assumption of stationarity. To solve complex questions related to drought and ecosystems in a non-stationary world we need new hypotheses and new tools. Traditionally, the effects of drought on hydrolo-gy and vegetation are considered separately, despite the integral role of the terrestrial hydrological cycle in determining streamflow. This exciting PhD project proposes to answer the questions: (1) what are the driving meteorological conditions for streamflow drought and ecosystem impacts, (2) what is the rela-tionship between terrestrial and hydrological drought sensitivity, and (3) how does this relationship vary among vegetation types (grassland, forest, agricul-ture) and climates (precipitation/temperature gradi-ent)? Using a novel methodology, we propose to di-rectly compare hydrological drought sensitivity (measured by streamflow) and terrestrial drought sensitivity (measured by aboveground net primary productivity, ANPP) via synthetic analysis of a number of experimental catchments where this data is available. This PhD project will work in Europe and North America by analysing and comparing various sites. The prospective sites in Europe are the Birmingham Insti-tute of Forest Research site in the UK (BIFOR, www.birmingham. ac.uk/research/activity/ bi-for/index.aspx), the Krycklan catchment in Sweden (www.slu.se/Krycklan; Figure 1), the Alzette River ba-sin in Luxembourg, and the TERENO network in Ger-many (teodoor.icg.kfa-juelich.de). For the North American sites we will make use of the LTER network. The goal is to investigate a large number of sites in order to quantitatively assess tradeoffs and synergies between hydrological and terrestrial drought sensitivi-ty across a climatic and ecosystem gradient. In a unique collaborative action, the results of this PhD project will then be compared between the European and North-American dataset with the aim to predict complex relationships between the ecosystem and (lack of) water under future climate change and vege-tation changes (a non-stationary world).

The methodology for this project will be developed in conjunction with our international partners and will draw from the experience of the supervisory team. For all of the sites, drought indices and hydrological and ecological drought sensitivity metrics will be calculated from generally available datasets of meteorological conditions (precipitation and potential evapotranspiration), streamflow observations and terrestrial ecosystem variables (net primary production). Terrestrial and hydrological drought sensitivity will be assessed separately; the relationship between the drought indices and the hydrological and ecological drought sensitivity metrics indicates how much of the interannual variability is explained by drought severity. Hydrological and terrestrial sensitivity will then be considered in tandem to assess the relationship between streamflow and ecosystem productivity.