Skip to Main Content
The effects of shade, fertilizer, and pruning on eastern hemlock trees and hemlock woolly adelgidAuthor(s): Thomas McAvoy; Ryan Mays; Nels Johnson; Scott Salom
Source: Forests. 8(5): 156
Publication Series: Scientific Journal (JRNL)
Station: Pacific Southwest Research Station
View PDF (1.0 MB)
DescriptionHemlock woolly adelgid (HWA), Adelges tsugae Annand, an invasive insect native to the Pacific Northwest and Asia, is responsible for widespread health decline and mortality of native hemlocks (Tsuga spp.) in the eastern United States. Shading and fertilizer has been found to affect the survival and health of both HWA and hemlocks. These abiotic factors have been studied separately but not in combination. In this three year study, eastern hemlock trees (1–2 m tall) were treated with pruning, fertilizer, and shade to determine their effects on hemlock tree health and HWA survival and density. Shade cloths were erected over individual trees, granulated fertilizer was applied, and trees were pruned annually. The total number of HWA were counted during the sistens and progrediens adult stages on the low, mid, and high branches on the north, east, south, and west sides of each tree for three years. Survival of aestivating sistens was recorded in artificially, naturally, and unshaded hemlocks. The mean of percent tips alive, branches alive, and foliage density was used to calculate a hemlock health index (scale of 0–100). Shade cloth reduced solar radiation to the trees to levels similar to a naturally-forested hemlock canopy, but did not alter temperature. Trees exposed to shade alone and shade plus fertilizer maintained the greatest HWA density. On unshaded trees, branches on the west side of the tree had lower HWA densities and branches high on the tree had the lowest HWA densities. Pruning plus fertilizer and shading plus fertilizer reduced tree health. Shaded trees had reduced branchlet new growth length. Survival of summer aestivating sistens was nearly twice the survival under artificially- and naturally-shaded trees compared to unshaded trees. There was an inverse density-dependent survival response for aestivating HWA under artificially-shaded and unshaded trees but not naturally-shaded trees. Unshaded hemlock trees had lower HWA densities due to increased mortality of summer aestivating sistens. Unshaded trees had better health and longer new growth branchlets due to increased exposure to solar radiation and lower HWA densities. Silvicultural thinning of hemlocks in forest stands could increase direct sunlight reaching the trees and help decrease HWA densities and improve hemlock health.
- You may send email to firstname.lastname@example.org to request a hard copy of this publication.
- (Please specify exactly which publication you are requesting and your mailing address.)
- We recommend that you also print this page and attach it to the printout of the article, to retain the full citation information.
- This article was written and prepared by U.S. Government employees on official time, and is therefore in the public domain.
CitationMcAvoy, Thomas; Mays, Ryan; Johnson, Nels; Salom, Scott. 2017. The effects of shade, fertilizer, and pruning on eastern hemlock trees and hemlock woolly adelgid. Forests. 8(5): 156. https://doi.org/10.3390/f8050156.
KeywordsAdelges tsugae, aestivation, density-dependent, fertilizer, hemlock woolly adelgid, prune, shade, solar radiation
- Evaluation of aestival diapause in hemlock woolly adelgid
- Maintenance of eastern hemlock forests: Factors associated with hemlock vulnerability to hemlock woolly adelgid
- The role of silvicultural thinning in eastern forests threatened by hemlock woolly adelgid (Adelges tsugae)
XML: View XML