Graduation Date

Fall 2020

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

David Imper

Third Committee Member Affiliation

Community Member or Outside Professional

Fourth Committee Member Name

Jeffrey White

Fourth Committee Member Affiliation

HSU Faculty or Staff

Subject Categories

Biology

Abstract

Early successional wetland habitat is being lost in temperate regions worldwide as a result of changes in disturbance regimes that allow for the establishment and dominance of woody species. In particular, this phenomenon is pronounced in fens, which harbor high numbers of rare herbaceous species that require early successional habitat. I investigated the relationship between woody vegetation encroachment and herbaceous species diversity within a Northern California coastal fen that has been undergoing encroachment by woody vegetation for ca. 80 years by recording species richness and cover data from 338 permanent plots throughout the fen. I also investigated the effect of a woody vegetation removal treatment on herbaceous species richness, non-native and special status botanical species occurrences by comparing the same plots before and after treatment. Before treatment, lower species richness was associated with higher woody vegetation cover and height along with higher litter cover and no special status botanical species were observed in areas with complete canopy closure. In addition, I found a significant reduction in herbaceous species richness with 65% woody vegetation cover resulting in, on average, a 50% loss of herbaceous species. Following woody vegetation removal, herbaceous species richness increased across the fen with the greatest increases within areas that experienced more than 50% woody vegetation cover removal that were nearer the edge of encroaching vegetation. In addition, special status botanical species occurrences increased by 43% and non-native species occurrences increased by 71% after treatment. The results of this study suggest that the re-introduction of disturbance, specifically the removal of woody vegetation and litter accumulation, is likely essential to maintain herbaceous species diversity and persistence of special status species populations in coastal fens. Furthermore, disturbance can have mixed effects on sensitive vegetation communities, with the potential for promoting non-native species invasion that may require follow-up treatments to prevent unintended degradation of sensitive vegetation communities. Lastly, I developed a monitoring plan for the continued study of the fen to document changes in vegetation cover and composition for five years following the treatment. Results from the continued monitoring of the site should direct additional treatment and study.

Citation Style

APA

Available for download on Friday, December 10, 2021

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