Model computations of the impact of climatic change on the windthrow risk of trees

The more humid, warmer weather pattern predicted for the future is expected to increase the windthrow risk of trees through reduced toe anchorage due to a decrease in soil freezing between late autumn and early spring, i.e during the most windy months of the year. In this context, the present study aimed at calculating how a potential increase of up to 4 degrees C in mean annual temperature might modify the duration of soil frost and the depth of frozen soil in forests and consequently increase the risk of windthrow. The risk was evaluated by combining the simulated critical windspeeds needed to uproot Scots pines (Pinus sylvestris L.) under unfrozen soil conditions with the possible change in the frequency of these winds during the unfrozen period. The evaluation of the impacts of elevated temperature on the frequency of these winds at times of unfrozen and frozen soil conditions was based on monthly wind speed statistics for the years 1961-1990 (Meteorological Yearbooks of Finland, 1961-1990). Frost simulations in a Scots pine stand growing on a moraine sandy soil (height 20 m, stand density 800 stems ha(-1)) showed that the duration of soil frost will decrease from 4-5 months to 2-3 months per year in southern Finland and from 5-6 months to 4-5 months in northern Finland given a temperature elevation of 4 degrees C. In addition, it could decrease substantially more in the deeper soil layers (40-60 cm) than near the surface (0-20 cm), particularly in southern Finland. Consequently, tree anchorage may lose much of the additional support gained at present from the frozen soil in winter, making Scots pines more liable to windthrow during winter and spring storms. Critical wind-speed simulations showed mean winds of 11-15 m s(-1) to be enough to uproot Scots pines under unfrozen soil conditions, i.e. especially slender trees with a high height to breast height diameter ratio (taper of 1:120 and 1:100). In the future, as many as 80% of these mean winds of 11-15 m s(-1) would occur during months when the soil is unfrozen in southern Finland, whereas the corresponding proportion at present is about 55%. In northern Finland, the percentage is 40% today and is expected to be 50% in the future. Thus! as the strongest winds usually occur between late autumn and early spring, climate change could increase the loss of standing timber through windthrow, especially in southern Finland.