EFFECT OF THINNING ON EVAPORATION OF SCOTS PINE FOREST

This paper presents the results of a study carried out on a Scots pine stand, aged over 70 years, growing on ICP Forests land. The stand underwent late thinning, this being the last operation performed prior to complete felling, which will take place when the trees reach an age of approximately 100 years. The thinning, which reduced the basal area by 20%, caused a decrease in LAI by approximately 30% on average and an increase in the gap fraction by almost 50%. The thinning of the tree canopy increased by more than 40% the quantity of light reaching the lower levels of the tree stand and the soil. Comparison of the results for actual evapotranspiration and tree transpiration with potential evapotranspiration (T ETP-1, EVT ETP-1) revealed an 8% increase in tree transpiration, while the actual evapotranspiration of the ecosystem increased by 14% after thinning. This increase was partly the result of the higher tree transpiration, but another factor may have been the increased evapotranspiration of forest floor vegetation and the soil, resulting from the greater quantity of light reaching the forest floor. The ratio of the pines' transpiration to the actual evapotranspiration of the ecosystem (T EVT-1) was practically unchanged, from 0.60 before thinning to 0.61 after. Modelling was also carried out (based on a Plant Water model) for average meteorological conditions to determine the effect of thinning on the rate at which the trees depleted supplies of soil water. This effect was greatest in September, when plant available water was present for 9 days longer than prior to thinning. In the months with the highest levels of transpiration (from May to August) the period in which plant available water was present lengthened by 2-3 days.