Graduate Thesis Or Dissertation


Snag longevity, bird use of cavities, and conifer response across three silvicultural treatments in the Oregon Coast Range Public Deposited

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  • In the interest of meeting multiple forest management goals that include maintenance of wildlife, particularly cavity-nesting birds, uneven-aged silvicultural treatments are used increasingly in the Pacific Northwest. However, questions remain regarding the responses of cavity-nesting birds and residual green trees to different harvest intensities and patterns. To study these issues, between 1989 and 1991 the Oregon State University College of Forestry Integrated Research Project (CFIRP) applied 3 silvicultural treatments to 30 mature (85-125 year old) Douglas-fir stands in the Oregon Coast Range. Silvicultural treatments consisted of group-selection cuts (18 stands with 33% of the timber volume extracted from 0.2-0.6 ha patches), two-story regeneration harvests (6 stands with 75% scattered removal of the timber volume resulting in 20-3 0 distributed residual trees/ha), and clearcuts (6 stands that retained 1.2 mature green trees/ha). In addition, 939 conifers 53 cm diameter at breast height [DBH]) were topped to create snags in clustered and scattered arrangements. In the current study, CFIRP stands were utilized to 1) test for differences in cavity-nesting bird use of snags across silvicultural treatments and snag arrangements 10 years after harvest, 2) compare 10 year with 5 year nesting levels, 3) evaluate associations between snag characteristics and cavity nest site location, 4) quantify snag fall, 5) assess silvicultural treatment effects on residual tree growth and condition, and 6) quantify tree mortality. Snags and topped conifers that remain alive were observed for nesting and foraging use during the 2001 breeding season. Eight species of birds nested in created snags and a mean of 5.1 total cavities/snag were found one decade after creation. However, no active nesting was observed in topped trees that remained alive. Higher cavity-nesting bird levels, species richness, and species diversity occurred in open-canopy stands (two-story and clearcut treatments) compared to closed-canopy stands (group-selection treatment). Bird species composition was most similar between two-story and clearcut stands, and least similar between clearcut and group-selections stands. No difference was found in nesting or foraging levels between clustered and scattered snags. Active nests in created snags were most commonly located in the top 25% of bole, cavities on average faced northeast, and created snags with and without dead branches received equal nesting use. Compared with 6 years prior, the mean number of cavities per created snag increased 3.3- to 6-fold, and 4 additional avian species were observed nesting. One topped conifer fell in the decade since creation. Residual green trees (n = 848) were measured for growth and condition from November, 2001 to February, 2002. Across silvicultural treatments, residual green tree basal area, DBH, and height growth, and crown width and crown fullness did not differ among silvicultural treatments 10-12 years following harvest. Overall, 45% of trees experienced greater basal area growth in the decade following harvest than in the decade prior to harvest. Among silvicultural treatments, mean live crown ratio (live crown length/total tree height) of residual trees was significantly greater (0.74) in clearcuts and the percentage of trees with epicormic branching (35%) was significantly higher in two-story stands. Over the last decade residual tree mortality resulted in 134 standing dead trees (snags) and 185 blowdowns. Two-story stands experienced the highest recruitment of snags (0.76 per ha) and blowdowns (1.12 per ha). Results from this study suggest that topped, large conifers provide snags that offer valuable nesting and foraging habitat for cavity-nesting birds during the first decade after treatment, if the tree dies. Snags in both clustered and scattered arrangements appear to receive equal use by cavity-nesting birds. Also, snags created by topping may have the potential to stand for several decades in the Oregon Coast Range. Furthermore, although silvicultural treatments in this study did not appear to affect residual tree basal area growth during the first 10 years after treatment, partial harvests can promote increased diversity in stand structural complexity, which includes longer tree crowns, epicormic branching, and new snag recruitment, that also can benefit cavity-nesting bird populations through increased nesting and foraging opportunities.
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