The influence of clear felling on temperature and vegetation in an esker area at Lammi, southern Finland

Abstract

The paper discusses the basic features of the microclimate of an esker of variable morphology, and in particular the effect of clear felling on temperature conditions and that of the resulting microclimate on the vegetation and on the damage sustained by the plants and trees. Daily temperature variations were measured at four sites from May to July 1987 using thermographs and minimum and maximum thermometers placed in meteorological screens. One of these sites was at the bottom of a kettle hole in a clear-felled area and another on the nearby high ground, while a similar pair of sites were established in an adjacent forested area. In addition, regional temperatures were measured with an electronic thermometer during temperature inversion at night and changes in tree growth were examined dendrochronologically.

The clear felling carried out in 1981 raised temperatures on the high ground and increased the difference between the extreme temperatures recorded in the kettle holes, which served as an accumulation basin for cold air, lowering the minimum temperatures in particular. The mean temperature at the highest point in the clearfelled area was 1.7(C higher than that at the corresponding point in the forested area, and about 3(C higher than in the kettle holes in both areas.Minimum temperatures in the kettle hole in the clear-felled area were an average of 3.3(C lower than in that in the forest area, and 6.6(C lower than at the highest point in the clear-felled area. The average temperature amplitude in the kettle hole of the clear-felled area, 15.2(C (max.28.2(C), was 69% greater than that of the highest point in the forest area and 39% greater than that of the forested kettle hole.

Local temperature differences of up to 10(C were possible between the open kettle hole and the highest points of the area on calm, clear nights, with night frost occuring in the former throughout the summer, even during warm spells. Night temperatures during the growing season were low, remaining below zero for up to 10 hours, slowering reforestation in the lower parts of the kettle hole and causing changes in the other elements of the vegetation.