Effect of climate change on temperate forest ecosystems

Abstract

In temperate climates groundwater can have a strong effect on vegetation, because it can influence the spatio-temporal distribution of soil moisture and therefore water and oxygen stress of vegetation. Current IPCC climate projections based on CO2 emission scenarios show a global temperature rise and change in precipitation regime, which will affect hydrological and vegetation systems. This thesis provides a quantitative framework for studying eco-hydrology in groundwater influenced temperate ecosystems. Models with increased complexity have been developed, to investigate the interactions and dynamics of these ecosystems and determine their response to climate change. The final model combines a dynamic bio-physiologically based vegetation model to simulate vegetation dynamics and competition and a physically-based variably-saturated hydrological model. In this model transpiration and stomatal conductance depend on atmospheric forcing and soil moisture content. Carbon assimilation depends on environmental conditions, stomatal conductance and biochemical processes. Light competition is driven by vegetation height and water competition is driven by root water uptake and the vegetations response to water and oxygen stress. Modeled and measured atmospheric H2O and CO2 fluxes compare well on a diurnal and a yearly timescale. Long term simulations reproduce realistic densities of wet and dry adapted vegetation along a wet to dry gradient. The results clearly show the importance of simulating groundwater on modeled vegetation dynamics. This involves both the presence of groundwater and the upslope-downslope interactions through lateral groundwater connection. The influence of groundwater is apparent for a large range of groundwater depths, by both capillary rise and water logging. The results also show the influence of vegetation dynamics, composition, patterns and biomass on the water balance of a forest ecosystem and therefore groundwater dynamics. Water and oxygen stress can occur simultaneously within a slope and at the same location throughout a year. Light competition is important as the light capturing advantage of established vegetation most often overrules water and oxygen competition. The response to climate change is investigated by forcing the coupled model with an ensemble of nested global and regional climate models, representing the IPCC SRES A2 scenario for 2100. A stochastic weather generator is calibrated to each GCM-RCM combination. All projections show higher temperatures and less precipitation. Results show that increased atmospheric CO2 concentration results in higher biomasses due to higher water use efficiency and less evaporation downslope where vegetation growth is light limited. Change in precipitation regime (drier summer, wetter winter) causes decreased biomass and increased upslope groundwater recharge, resulting in groundwater rise and an upward shift of wet adapted vegetation. Temperature rise results in a lower biomass, because respiration increases stronger than carbon assimilation, while increased transpiration causes drier soils and prolonged periods of water limited growth. The combined effect of CO2 and temperature rise change in precipitation regime is dominated by temperature rise and change in precipitation regime, causing a lower biomass. The effect on groundwater level depends on the degree by which precipitation distribution changes, showing a drop at a small difference between summer and winter precipitation and a rise at a large difference. This study shows that quantifying and understanding the response of temperate forest ecosystems to climate change requires combined physically-based hydrological and bio-physically-based vegetation models