BackgroundA regional-scale sensitivity study has been carried out to investigate the climatic effects of forest cover change in Europe. Applying REMO (regional climate model of the Max Planck Institute for Meteorology), the projected temperature and precipitation tendencies have been analysed for summer, based on the results of the A2 IPCC-SRES emission scenario simulation. For the end of the 21st century it has been studied, whether the assumed forest cover increase could reduce the effects of the greenhouse gas concentration change.ResultsBased on the simulation results, biogeophysical effects of the hypothetic potential afforestation may lead to cooler and moister conditions during summer in most parts of the temperate zone. The largest relative effects of forest cover increase can be expected in northern Germany, Poland and Ukraine, which is 15–20% of the climate change signal for temperature and more than 50% for precipitation. In northern Germany and France, potential afforestation may enhance the effects of emission change, resulting in more severe heavy precipitation events. The probability of dry days and warm temperature extremes would decrease.ConclusionsLarge contiguous forest blocks can have distinctive biogeophysical effect on the climate on regional and local scale. In certain regions of the temperate zone, climate change signal due to greenhouse gas emission can be reduced by afforestation due to the dominant evaporative cooling effect during summer. Results of this case study with a hypothetical land cover change can contribute to the assessment of the role of forests in adapting to climate change. Thus they can build an important basis of the future forest policy.

Climate change particularly threatens the xeric limits of temperate-continental forests. In Hungary, annual temperatures have increased by 1.2 °C–1.8 °C in the last 30 years and the frequency of extreme droughts has grown. With the aim to gain stand-level prospects of sustainability, we have used local forest site variables to identify and project effects of recent and expected changes of climate. We have used a climatic descriptor (FAI index) to compare trends estimated from forest datasets with climatological projections; this is likely for the first time such a comparison has been made. Four independent approaches confirmed the near-linear decline of growth and vitality with increasing hot droughts in summer, using sessile oak as model species. The correlation between droughts and the expansion of pest and disease damages was also found to be significant. Projections of expected changes of main site factors predict a dramatic rise of future drought frequency and, consequently, a substantial shift of forest climate classes, especially at low elevation. Excess water-dependent lowland forests may lose supply from groundwater, which may change vegetation cover and soil development processes. The overall change of site conditions not only causes economic losses, but also challenges long-term sustainability of forest cover at the xeric limits.

Climatic effects of forest cover change have been investigated for Hungary. For the time period 2071-100 we have analyzed whether the climate change signal for summer precipitation and the probability of droughts can be reduced assuming maximal afforestation for the entire country (forests covering all vegetated areas). The biogeophysical effects of land cover change have been assessed using the results of an A1B IPCC-SRES emission scenario from REMO (regional climate model at the Max Planck Institute for Meteorology, Hamburg). The simulation results indicate that afforestation may reduce the projected climate change through higher evapotranspiration and precipitation as well as lower surface temperature for the entire summer period. The magnitude of the feedback of the forest cover increase on precipitation differs among regions. The strongest effects are visible in the northeastern part of the country. Here, half of the projected precipitation decrease can be relieved and the total number of drought events can be reduced, assuming maximal afforestation. Afforestation brings about the smallest climatic effect in the southwestern region, in the area that shows the strongest climate change. The results can help to identify areas where forest cover increase should most effectively support the alleviation of climate change effects.

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