Extensive logging and fire suppression in forests of the United States have altered plant communities and increased woody plant densities over the past century. Mechanical thinning has been widely implemented to reduce tree densities and restore historical plant community composition and structure. The reduction in tree densities from thinning may reduce competition and augment the productivity of remaining trees. Few studies have examined the ways in which thinning affects mature tree growth across taxa. The objective of this study was to explore the effects of thinning on broadleaf and coniferous tree growth. We studied managed stands of temperate mixed coniferous and broadleaf forest at Fort Indiantown Gap National Guard Training Center, a military training area in the Appalachian Mountains of southeastern Pennsylvania. Twenty-six deciduous broadleaf trees and six evergreen coniferous trees were monitored within and outside of thinned treatment areas. Marginal change in basal area (hereafter, ‘growth’) was recorded monthly from 2015 to 2017, during ‘growing season’ (June, July, August) and ‘non-growing season’ (May, September, October) periods. We used model selection and generalized linear mixed models to examine factors influencing growth, including thinning treatment, growing season, and tree type (broadleaf or coniferous).
Results/Conclusions
Conifer growth was less than broadleaf tree growth in unthinned areas, and more than broadleaf tree growth in thinned areas. Broadleaf tree growth was greater in the growing season than in the non-growing season. We did not observe significant effects of thinning on broadleaf tree growth, or of growing season on conifer growth. These relationships suggest that factors driving growth differ by tree type: conifers may be more sensitive to thinning than broadleaf trees, and broadleaf trees may be more sensitive to seasonality than conifers. The greater sensitivity of conifers to thinning may be due to a greater ability to exploit decreased competition and increased light availability in thinned areas. Greater sensitivity to seasonality among broadleaf trees may result from leaf senescence in September and October and leaf re-growth in May. Our results suggest that the relationship between thinning and tree growth is dependent on taxon and season, among other factors, and help clarify the contexts in which thinning treatments are most likely to result in desired ecological and silvicultural outcomes.