Effects of thinning, weather and soil moisture on tree and stand transpiration in a Swedish forest

Abstract

The thesis investigates how thinning affects transpiration in a heterogeneous, but typical, mixed pine and spruce forest. Measurements were made during three years in a forest 30 km north of Uppsala, Sweden. The measured stand was divided into two plots, of which one was thinned by 25% of the basal area after the first year. The sapflow technique, by which transpiration was estimated, was evaluated. Sapflow was measured on five trees by the Cermak method and the Granier method. The methods diverged considerably. After comparison with eddy-correlation data, it was concluded that Granier measurements needed correction. The first year after thinning was very dry; transpiration of individual trees was modelled by means of weather and soil-moisture data. The response to soil moisture varied greatly; the number of days below the onset threshold for reduced transpiration varied between 21 and 111. The relationship between individual transpiration and growth, and three measures (diameter, needle mass and competition index), potentially useful as scaling-up factors, was also tested. Of the three, none was superior to the others. Thinning increased variation, and trees around the strip-roads from a thinning about ten years earlier were still affected. After thinning, transpiration on the thinned plot started at only 60% of that on the reference plot. The ensuing drought affected the thinned plot far less than the reference plot, and during the driest period, transpiration on the thinned plot was far higher than that on the reference plot. In the second year after thinning, the thinned plot transpired 20% more than the reference plot. Since neither an increase in LAI nor a reaction by the ground vegetation was detected, an increase in photosynthetic activity as a result of higher needle nitrogen concentration was a likely explanation. This hypothesis was supported by a the results of a simulation.

Keywords

Picea abies; Pinus sylvestris; forest management; MAESTRA; tissue heat balance

Published in

Acta Universitatis Agriculturae Sueciae. Silvestria
2001, number: 231ISBN: 91-576-6315-7Publisher: Department of Production Ecology, Swedish University of Agricultural Sciences

SLU Authors

• Lagergren, Fredrik

  • Department of Production Ecology, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Forest Science


Permanent link to this page (URI)

https://res.slu.se/id/publ/108002