Temporal changes in tree spatial patterns in uneven-aged interior Douglas-fir dominated stands managed under different thinning treatments

Abstract

Understanding the spatial patterns of trees and their interactions can reveal the ecological processes driving forest stand structure and stand development over time. We assessed temporal changes in tree spatial patterns in uneven-aged interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) dominated stands in central British Columbia, Canada. Data were available on 24 plots in three blocks over 21 years, 18 of which had received pre-commercial thinning (PCT) treatments of varying intensity. We first applied the Clark and Evans aggregation index and the L function, a transformation of Ripley’s K function, to describe the spatial pattern of live trees in thinned and unthinned plots over time. Second, we analysed the spatial correlations between live tree diameters using the mark correlation function and the mark variogram. Third, the spatial pattern of dead trees in the unthinned plots after 21 years was analysed. Lastly, we tested the spatial relationship between dead and surviving trees in the unthinned plots. In all three blocks, the spatial patterns of live trees in the unthinned plots were clustered through time. The moderate thinning treatments had random or regular spatial patterns that remained unchanged through time, because of reduced mortality rates and low levels of ingrowth. The heavier thinning initially retained a clustered pattern at small inter-tree distances; however, this changed to a random pattern with increasing distance 15 years post-thinning and remained as such until the end of the study period. Tree diameters were not spatially autocorrelated in the thinned plots, although there was positive spatial correlation of tree diameters in the unthinned plots, probably due to competitive growth inhibition among neighbouring trees. Dead trees were primarily smaller in size and were significantly clustered at all spatial scales. The lack of spatial relationship between dead and surviving trees indicated that mortality was a random process. Our study contributes to a better understanding of how spatial patterns of trees change over time in these stands, which could help in the design of silvicultural regimes that mimic natural processes.