SENSITIVITY OF FORGRO TO CLIMATIC-CHANGE SCENARIOS - A CASE-STUDY ON BETULA-PUBESCENS, FAGUS-SYLVATICA AND QUERCUS-ROBUR IN THE NETHERLANDS

The impacts of the climate change predictions of four general circulat ion models (GFDL, GISS, OSU and UKMO) on net primary production (NPP) of Betula pubescens, Fagus sylvatica and Quercus robur in The Netherla nds were analysed using the process-based model FORGRO. FORGRO is a mo del suitable to simulate growth of managed mono-species stands. For th e GCMs mentioned, both transient and equilibrium 2 x CO2 scenarios of temperature and precipitation change were evaluated and compared with responses under current climate. It was found that the NPP increases i n the transient scenarios, but remains the same or declines in the 2 x CO2 scenarios. This is because respiration increases more with rising temperature than photosynthesis. During the transient scenarios this effect gradually increases, while in the 2 x CO2 scenario this effect is operating over the entire simulation period. If water limitation is taken into account, then the NPP of the reference scenario is reduced . In both the transient and 2 x CO2 scenarios this water limitation is annulated, resulting in a stronger response of NPP compared to the si tuation without water limitation. This enhancement of the response is most pronounced in the transient scenario due to the gradual effect of temperature on respiration. Similar results were obtained with a vers ion of FORGRO in which the photosynthesis module of HYBRID (PGEN) is i ncorporated, although the response in FORGRO-PGEN is usually higher th an that of FORGRO. This is because the response of photosynthesis to C O2 rises with increasing temperature as defined in the PGEN-model, but not according to FORGRO.