Climate change impact on a groundwater‐influenced hillslope ecosystem

Abstract

This study investigates the effect of climate change on a groundwater‐influenced ecosystem on a hill slope consisting of two vegetation types, one adapted to wet and one adapted to dry soil conditions. The individual effects of changes in precipitation, temperature, and atmospheric CO2 concentration are compared to the combined effect of these factors. Change in atmospheric conditions is based on the Netherlands. Projected climate change is obtained from an ensemble of nested global and regional climate models (GCMs and RCMs), representing the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios A2 scenario for 2100. For each GCM‐RCM combination, change factors were determined and transferred to a stochastic weather generator. All projections show higher temperatures and less annual precipitation. Simulations were performed using an ecohydrological model, consisting of a dynamic soil‐plant‐atmosphere‐continuum model that is fully coupled to a variably saturated hydrological model, using the stochastic weather data as input. Model results show that increasing atmospheric CO2 concentration results in higher biomasses because of higher water use efficiency and a decrease in evaporation downslope where vegetation growth is light limited. The change in precipitation regime (drier summers, wetter winters) causes a decreased biomass of especially the dry‐adapted species and increased upslope groundwater recharge, resulting in groundwater rise and an upward shift of wet‐adapted vegetation. Temperature rise results in decreased biomass because respiration increases stronger than carbon assimilation, while increased transpiration causes drier soils and a prolonged period of water‐limited growth. The combined effect is dominated by the increase in temperature and change in precipitation regime, causing decreased biomass throughout. Surprisingly, the effect on groundwater level depends on the degree by which precipitation distribution changes within the year, showing a drop at a small change and a rise when change is larger. This study thus shows that climate change effects on hydrology and vegetation are far from straightforward and call for fully coupled ecohydrological models and upslope‐downslope interaction.