Graduation Date

Fall 2023

Document Type

Thesis

Program

Master of Science degree with a major in Biology

Committee Chair Name

Erik Jules

Committee Chair Affiliation

HSU Faculty or Staff

Second Committee Member Name

Kerry Byrne

Second Committee Member Affiliation

HSU Faculty or Staff

Third Committee Member Name

Catalina Cuellar-Gempeler

Third Committee Member Affiliation

HSU Faculty or Staff

Fourth Committee Member Name

Matthew Reilly

Fourth Committee Member Affiliation

Community Member or Outside Professional

Subject Categories

Biology

Abstract

The conversion of natural forested lands to managed forests has reduced the amount of older, structurally diverse forests worldwide. In conifer forests of the Pacific Northwest (USA) – where the understory plant communities comprise only 1% of forest biomass but represent 90% of the plant species richness – the long-term impacts of timber harvesting are not fully understood. I used a chronosequence of forests in southwestern Oregon that ranged from 25 to 445 years of age to compare changes in plant communities in logged (i.e., managed) stands with that of stands in late succession and old growth conditions. The chronosequence consisted of 13 50 m2 permanent plots with similar elevation, aspect, slope, and forest type that were previously sampled in 2003 by Jules et al. (2008). In 2021, I resurveyed the herbaceous understory in each plot to evaluate if the relationship between stand attributes (e.g., canopy closure) and understory vegetation had changed over the 18-year period. I found that canopy cover increased markedly over the 18-yr period (34%), with most of the increase occurring as young, recently clearcut sites entered a phase of canopy closure. These same stands lost an average of 10 understory species between sample years (2003 and 2021), and the community composition of these younger stands became more similar to closed canopy stands of both years. In addition, I observed a decline in evenness and diversity in the mid-age and older stands due to an increase in dominance of shade tolerant plants (e.g., Achlys triphylla, Adenocaulon bicolor, and Chimaphila umbellata). These results support the non-linear relationship between stand age and richness also observed in 2003, such that richness was highest in the youngest stands, reached a low in mid-aged stands, then increased in the oldest stands. Overall, these results demonstrate a legacy effect of the historic phase of clearcut logging on federal forest lands and that a large portion of forests on the landscape have now entered this period of high canopy cover and low richness and diversity. Because of this, it will be critical to discuss the potential methods managers can use to recover and maintain diverse plant communities.

Citation Style

Ecology

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