This thesis investigates the variability of ecosystem processes—gross primary production (GPP), evapotranspiration (ET), and water-use efficiency (WUE)—in Europe under climate change, with a focus on their trends and responses to droughts from 1995 to 2018. The work uses three complementary data sources: satellite-based remote sensing, in situ eddy covariance observations (ICOS), and land surface model simulations with CLM5. The thesis is structured around three main studies. Study 1 (Chapter 3): Water-use efficiency is the amount of carbon assimilated per water used by an ecosystem and a key indicator of ecosystem functioning, but its variability in response to climate change and droughts is not thoroughly understood. Here, we investigated trends, drought response and drivers of three water-use efficiency indices from 1995 – 2018 in Europe with remote sensing data that considered long-term environmental effects. Inherent water-use efficiency decreased by -4.2% in Central Europe, exhibiting threatened ecosystem functioning. In European grasslands it increased by +24.2%, by regulated transpiration and increased carbon assimilation. Further, modulation of wateruse efficiency drought response by hydro-climate and the importance of adaptive canopy conductance on ecosystem function is highlighted. These results imply that decoupling carbon assimilation from canopy conductance and efficient water management strategies could make the difference between threatened and wellcoping ecosystems with ongoing climate change, and provide important insights for land surface model development. Study 2 (Chapter 4): Evapotranspiration (ET) and gross primary production (GPP) are critical fluxes contributing to the energy, water, and carbon exchanges between the atmosphere and the land surface. Land surface models such as the Community Land Model v5 (CLM5) quantify these fluxes, estimate the state of carbon budgets and water resources, and contribute to a better understanding of climate change's impact on ecosystems. Past studies have shown the ability of CLM5 to model ET and GPP magnitudes well but emphasized systematic underestimations and lower variability than in the observations. CLM5's predictions of water and energy fluxes are evaluated using observations from eddy covariance stations from the Integrated Carbon Observation System (ICOS), remote sensing, and reanalysis data sets. We assess simulated ET and GPP from the grid scale (CLM5grid) and the plant functional type (PFT) scale (CLM5PFT). CLM5PFT exhibited a low systematic error in simulating the ET at the ICOS sites (average bias of −4.68 %). GPP was underestimated by CLM5PFT, especially indeciduous forests (bias of −43.76 %). The results showed an underestimation of the spatiotemporal variability in the simulated ET and GPP distribution moments across PFTs for both CLM setups compared to reanalysis data and remote-sensing products. These findings provide essential insights for improving land surface models, highlighting the need to enhance the CLM5's ability to capture the spatiotemporal variability in ET and GPP simulations across PFTs. Study 3 (Chapter 5): Droughts significantly impact European ecosystems, but these effects vary due to the complex interactions among various hydrological compartments and across different temporal scales, making a comprehensive understanding challenging. 24 years (1995-2018) of high-resolution (3km) Community Land Model version 5 (CLM5) simulations over Europe are analyzed, applying a three-dimensional clustering algorithm to identify and characterize spatiotemporal drought events based on standardized indices of precipitation, vapor pressure deficit, soil moisture, runoff, and groundwater across multiple aggregation periods. The aggregation period strongly modulates drought duration, severity, and propagation speed, with more extended periods yielding more severe and persistent events. Notably, the severity of atmospheric droughts has increased over the study period (0.22±0.11 ×107 km2 days year-1), while groundwater droughts have become less severe (-0.28±0.38 ×107 km2 days year-1). Ecosystem responses, including transpiration, gross primary production, and water stress, show complex spatial patterns linked to plant functional types and aridity gradients, with compound droughts in soil and atmosphere causing the most widespread negative impacts. This event-based, multi-compartment study advances the understanding of drought impacts on ecosystem processes andinforms holistic drought risk assessments and water management. Together, these studies provide a robust assessment of how European ecosystems function and respond under increasingly frequent and severe drought conditions, offering insights for improving land surface models and informing future ecosystem management under climate stress.
Christian Poppe (Wed,) studied this question.