First Advisor

Andrés Holz

Term of Graduation

Spring 2025

Date of Publication

2-25-2025

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Geography

Department

Geography

Language

English

Subjects

Climate change, Ecology, Forest

Physical Description

1 online resource (vi, 92 pages)

Abstract

Climate change and its disruption of historical drought and wildfire patterns pose a substantial threat to forest health in the western United States. This threat is compounded by past management practices like fire suppression. The largest, oldest, and most ecologically valuable trees are particularly vulnerable, making their conservation a key objective of climate-adaptive forest management. Restoration treatments such as mechanical thinning have been shown to enhance tree growth and drought resilience in dry conifer forests, but long-term monitoring is necessary to understand their sustained effects. This study used dendroecological tree-ring analysis to investigate the effects of thinning on the growth, vigor, and resilience of large old trees. Four species were sampled in 2022 from the Ashland watershed in Southwestern Oregon, which underwent variable-intensity thinning from 2011-2013 as part of the Ashland Forest Resiliency Stewardship project. Results revealed that legacy tree growth generally declined over the past decade due to extreme drought, but thinning mitigated drought impacts for some species by improving resistance to growth decline or slowing reductions. Treatment responses were species-specific, with Pinus ponderosa showing improved drought resistance, Pinus lambertiana exhibiting temporary growth release followed by decline, and Pseudotsuga menziesii var. menziesii and Arbutus menziesii displaying inconclusive or masked responses. Thinning effects also varied by topoclimatic conditions, with moderately dry sites showing the most significant improvements for P. menziesii and P. ponderosa, while P. lambertiana benefited most at wet sites. Overall, findings suggest that thinning can enhance vigor and drought resilience in some legacy tree species, but its efficacy depends on species traits, site conditions, and treatment intensity.

Rights

In Copyright. URI: http://rightsstatements.org/vocab/InC/1.0/ This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).

Comments

Funded by the Oregon Watershed Enhancement Board.

Persistent Identifier

https://archives.pdx.edu/ds/psu/43274

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