In 1961, a study was initiated on four western larch stands on the Flathead and Kootenai National Forests to evaluate how thinning affects growth in high density stands. Today, researchers with the Rocky Mountain Research Station and University of Montana are using this existing long-term study to answer questions about overstory and understory carbon accumulation in western larch forests.
Objectives of this study were to examine how differing intensities of pre-commercial thinning affect the following:
The four western larch stands selected for the 1961 study were clear-cut in the early 1950s and regenerated naturally. They were thinned in 1962 to specific tree densities (200, 360, and 680 trees per acre) at varying thinning intervals (10-, 20-, and 40-years). These stands were remeasured in the summer of 2015. Stand growth and carbon sequestration were evaluated by estimating the carbon pools of live trees, understory vegetation, dead woody material, and the forest floor (decomposing plant material).
The results showed that thinning before stand age 10 years led to long-term constant yield across the tested stand densities. There was also constant volume growth across stand densities. The number of entries did not affect any tree- or stand-level attribute. We showed that early pre-commercial thinning controls whether wood volume and growth are concentrated on few large, stable trees or spread over many small, unstable trees.
Trees in dense stands compete with each for resources and some trees die; this is called self-thinning and the dead trees contribute to woody debris. Coarse woody debris existing from before the clear-cuts accounted for about 45% of total woody detritus biomass. We found that the size of woody debris pieces decreased as stand density increased. There was higher woody debris biomass in the high-density and unthinned treatments from self-thinning, as well as from tops breaking from snow and ice and branch self-pruning. Because our results were driven by self-thinning mortality, these results should hold for maturing even-aged stands in other cool temperate and boreal forests.
Aboveground carbon is similar across treatments, primarily due to the increased carbon of large trees grown at lower stand densities. Dead wood existing from before 1952 when the stands were clear-cut accounted for approximately 20–25% of aboveground carbon stores (carbon is only a portion of total biomass). Given enough time since early thinning, there is no trade-off between managing stands to promote individual tree growth and understory vegetation development versus maximizing stand-level accumulation of aboveground carbon over the long term. In this case, early precommercial thinning can simultaneously achieve climate change mitigation and adaptation objectives, provided treatments occur early in stand development before canopy closure when trees begin to compete with each other.
Schaedel, M.S., A.J. Larson, C.J. Weisbrod and R.E. Keane. 2017. Density-dependent woody detritus accumulation in an even-aged, single-species forest. Canadian Journal of Forest Research 47(9): 1215-1221.
Schaedel, M.S., A.J. Larson, D.L.R. Affleck, R.T. Belote, J.M. Goodburn and D.S. Page-Dumroese. 2017. Early forest thinning changes aboveground carbon distribution among pools, but not total amount. Forest Ecology and Management 389: 187-198.
Schaedel, M.S., A.J. Larson, D.L.R. Affleck, R.T. Belote, J.M. Goodburn, D.K. Wright and E.K. Sutherland. 2017. Long-term precommercial thinning effects on Larix occidentalis (western larch) tree and stand characteristics. Canadian Journal of Forest Research 47: 861-874.