eLTER PLUS Deliverable D8.3 - Water use efficiency (WUE) of ecosystems and resilience to drought

The world faces many Grand Challenges including those posed by climate change, biodiversity loss, and water and soil pollution. These multiple stressors act simultaneously over a range of temporal and spatial scales, resulting in significant loss of ecosystem services that eventually affect societal well-being and humanity. While immediate impacts sometimes receive considerable attention, little is known about their long-term and systemic effects and cross-scale interactions. Closing these knowledge gaps requires an improved, transdisciplinary understanding of the multifaceted environmental system, in order to develop appropriate mitigation measures (Mirtl et al., 2018). The Whole System Approach in eLTER PLUS has a clear position to develop scientific, practical and conceptual innovations for addressing environmental resilience and sustainable social and ecological systems. The eLTER Whole System Approach encompasses the Earth system and related domains/disciplines from geosphere to hydrosphere, biosphere and socioeconosphere to atmosphere.

The eLTER-PLUS Project, Work Package 8 aimed to explore how long-term biodiversity, biogeochemistry, hydrology and socio-ecology data from the eLTER network can provide the necessary holistic understanding of the ecosystem response to environmental and societal pressures. WP8 focuses on assessing the capacity of data provision of eLTER Sites to evaluate environmental impacts using the Whole System Approach. In particular task 8.3 aimed to improve the knowledge of the impact of drought events on ecosystem water-use efficiency (WUE) and resilience. Data from 10 well-instrumented sites that cross climatic, geological and socio-ecological regions were used to test resilience indicators. The overall objective of Task 8.3 is to address the following scientific question: “What is the impact of drought events on ecosystem water-use efficiency (WUE) and resilience, and the relationships between WUE, soil structure, plant productivity and ecosystem resilience?”

Field data and integrated models were used to calculate the ecosystem WUE of sites to:

• Assess the impact of drought events on the WUE of ecosystems
• Analyze the resilience of the ecosystems with respect to their ability to recover single & multiple droughts
• Assess the relationship between WUE, soil structure, plant productivity and ecosystem resilience and develop mitigation measures for drought adverse impacts
• Assess the strengths and weaknesses of the RI-design to derive resilience indicators 

Our methodology followed a three-prong approach:

• Flux tower Eddy Covariance long term data were used to assess the impact of droughts on WUE. Ten well instrumented sites with good availability of long-term data as well as good spatial distribution covering the whole European continent were selected for this analysis.
• Two integrated models, 1D-ICZ and CLM5.0, that simulate the plant-soil-water system were used to assess the relationship between WUE and soil structure, plant productivity and ecosystem resilience. For this analysis, in addition to flux tower data, data regarding vegetation, soil stocks of C, N, P, K, lysimeter chemistry, atmospheric chemistry and fertilization loadings were utilized to calibrate the models. Three sites (Zöbelboden, Austria; Hyytiälä, Finland; Svartberget, Sweden) were selected with the best available data.
• Finally, two watershed scale models, SWAT and INCA were used to assess the impact of droughts at the basin scale. Additional hydrologic data and river and groundwater chemistry data were required for the calibration of the models. Two different sites, Koiliaris CZO in Greece and Svartberget in Sweden were used for the simulations of SWAT and INCA-N respectively.

The methodology aimed at assessing how forested ecosystems respond to drought across Europe using flux tower eddy covariance field data and then examine if 1D integrated models and watershed models can capture the ecosystem response. Given that very few sites have long time series of flux tower data, this study is assessing the capability of integrated and watershed scale models to reconstruct historical impacts of drought events in a historical reanalysis exercise.