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Predicting the probability of severe droughts and changes in growth potential under changing climate
Tapio Linkosalo, Raisa Mäkipää, Annikki Mäkelä

Severe droughts during the growing period are currently rare in the boreal climate zone. In the future rainfall events may be more sparsely distributed but more intense. Increase in temperature may lead to higher evapotranspiration. As a consequence, periods of drought may become more frequent. It has been suggested that this might reduce the biomass production of boreal forests. The soil water balance depends on soil properties, evaporation, plant transpiration, climatic conditions and micrometeorology, and their interactions. Therefore the probability of drought cannot be simply derived from scenarios of future climate, but a model taking into account the weather, soil and plant interactions is required.

In this study we estimated the number of drought days and growth potential coupling a model of photosynthetic production (Mäkelä et al. 2008), with another for soil water conditions. In our simulations we tallied the number of draught days and estimated the growth based on a simple, daily-based summary model (Härkönen et al. 2010).

We first calculated the results under current climate conditions in a 10km*10km grid covering the whole of Finland. We then repeated this calculation with a simple simulation of climate change, increasing the temperature (by 3 or 5ºC) and modifying the amount of precipitation (±10% from current).

There are few drought days in current climatic conditions, and the drought risk concentrates on the coastal areas. The results of the simulation indicate that the increase in drought risk under changing climate is rather modest, with the risk being greatest in soils with low water retention capacity. The benefit of increased temperature to the growth potential is larger than the reduction due to drought. The increase was smaller on sites where the water retention capacity is low, and if decrease of precipitation was simulated.

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