Individual tree growth responses to coinciding thinning and drought events in mixed stands of Norway spruce and Scots pine

Abstract

Both thinning and drought can strongly affect individual tree diameter growth. Effects of thinning on individual tree growth responses to drought have been studied extensively, but the interaction between the thinning response and the drought response, and their dynamics is often overlooked.Diameter growth responses were studied using a dataset containing individual tree data from six mixed Norway spruce - Scots pine stands thinned between one year before and two years after a drought event in 2006. In four sample plots per stand, all trees with a diameter at breast height larger than 5 cm were analyzed for diameter increment, covering the full range of social status for both species. Growth reductions during the drought event were quantified using a new method accounting for the strong growth trends following the thinning. A new metric was developed to describe the growth response in first phase of the thinning response during which the diameter growth gradually increases, considering both the duration and the dynamics of the growth response.In the majority of the stands, thinning had no effect on the growth reduction during the drought event. The growth reduction during the drought event did not significantly delay thinning response. For spruce, the growth reduction during the drought event was not independent of the growth rate across the full range of social status. Neither the growth reduction during the drought event nor growth response in the first phase of the thinning response was significantly affected by the neighborhood species composition.