Changes in size inequality and growth dominance in thinned and unthinned uneven-aged interior Douglas-fir dominated stands

Abstract

This study sought to gain insight into the impact of thinning treatments on stand structure dynamics in uneven-aged interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) dominated stands in central British Columbia (BC), Canada. We applied the Gini coefficient (GC) and the growth dominance coefficient (GDc) to determine how size inequality and growth dominance changed through time in both pre-commercially thinned (PCT) and unthinned stands across a moisture gradient. We used data from 24 plots in three blocks of a long-term study on the response to PCT, comprising five measurements over a 21-year period. The stand structures of each of the thinned plots were not identical before thinning. Therefore, the GC of the post-thinning measurements was expressed as a ratio relative to the pre-thinning treatment GC, referred to as the Gini coefficient ratio (GCr), to compare changes in size inequality through time across the PCT treatments. Three PCT treatments were applied, which differed in intensity and spatial pattern. All PCT treatments were applied in such a way that size inequality was reduced immediately after thinning. Size inequality did not decrease with increasing thinning intensity. There was a non-constant decrease in GCr in the unthinned plots, but an increase in the more heavily thinned plots over time. GDc decreased with increasing thinning intensity, where smaller trees were relatively more productive as the thinning intensity increased. Over time, thinned plots on the site with limited soil moisture and the moistest site shifted from negative to zero to positive growth dominance, while the GDc in the thinned plots with moderate soil moisture remained negative. These results suggest that site characteristics impact the response to PCT in these stands, and consequently should be considered in the design of the thinning treatments.