Conifer regeneration, understory vegetation and artificially topped conifer responses to alternative silvicultural treatments Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8623j204k

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  • Historically, between 40-60% of the Coast Range of Oregon was comprised of structurally diverse, old forests initiated by disturbances of various spatial scales ranging from thousands of acres (large fires) to the size of a single tree (windthrow). The predominant regeneration method of the past several decades, however, has been clearcutting of units that are at least 8 ha in size followed by burning and/or herbicide application and planting of Douglas-fir (Pseudotsuga menziesii) seedlings at high densities. Some question the ability of this regeneration method to provide many of the structural characteristics that existed historically in Pacific Northwest forests. In order to address these concerns, alternative silvicultural practices have been proposed in which green trees and snags are maintained after harvest so that species reliant upon these structures are able to persist through the artificial disturbance. Our research assessed conifer regeneration, understory vegetation, and artificial snag dynamics 16 to 18 years after treatment in clearcuts and two alternative silvicultural regimes: twostory-75% of volume removed resulting in 20 to 30 green trees/ha and group selection- 33% of volume removed in 0.2 to 1.0 ha circular, square, or strip-shaped gaps. All harvested areas were planted with Douglas-fir seedlings and competing vegetation was controlled using herbicide. Uncut controls were included in the study and monitored. Concurrent with harvest, 804 mature Douglas-fir trees were topped both with and without retention of live branches in order to create snags and living character trees. Conifer regeneration growth and survival were greatest in the clearcut treatments, intermediate in the two-story treatment and least in the gaps of the group selection treatment. Gap size was positively correlated with regeneration growth but had no significant effect on survival. Understory vegetation communities were generally resilient to disturbance and silvicultural regime had no effect on either total plant cover or tall shrub cover. More disturbed areas had greater species diversity which was driven largely by greater abundance of exotic ruderals. Young stand development may have had a larger impact on vegetation communities than silvicultural treatment. Twenty-four percent of artificially topped conifers with live branch retention remained living 16 to 18 years after treatment. Only 4% of artificially topped conifers with no live branch retention had broken 16 to 18 years after treatment. DBH of artificially topped conifers was negatively correlated with probability of falling.
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