Effects of Hydrology and Livestock Use on Modoc Plateau Vernal Pool Region Plant Communities
AuthorGosejohan, Meredith C.
AdvisorWeisberg, Peter J.
Natural Resources and Environmental Science
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Vernal pools are unique ecosystems that serve as ephemeral wetlands in Mediterranean climates and contain a great number of endemic plant species specially adapted to vernal pool hydrologic regimes. Vernal pool plant communities are distributed along inundation gradients, created by basin microtopography, depending on individual species habitat requirements. Vegetation in pool basins is dominated by native annual plant species that can tolerate the disturbance created by fluctuating hydrology. Another form of disturbance experienced in vernal pools is livestock grazing which has been part of the California landscape for over 150 years. My research examined the effects of these two disturbance processes on vernal pool plant communities and quantified how one endemic vernal pool specialist plant, Orcuttia tenuis, responded to gradients of hydrology and livestock use.In the first chapter, I examined hydrologic influences on vernal pool plants in two vernal pools located within the Modoc Plateau Vernal Pool Region. I utilized multivariate analyses to identify five vernal pool plant community types and determine how communities responded to maximum depth and inundation length. The five community groups were defined as edge, short-term inundated, long-term inundated, shallow tolerant, and deep tolerant depending on indicator species for each group and tolerance to hydrologic thresholds. My findings revealed a community group characterized by longer inundation periods than had previously been reported for California vernal pools. I also utilized generalized linear models and ordinal regression analyses to estimate favorable hydrologic conditions for O. tenuis occurrence, abundance, height, and spikelet density. My findings indicate likelihood of occurrence, abundance, height, and spikelet density increased with increasing maximum depth and inundation length with no upper threshold for this response.In the second chapter, I examined how livestock use and environmental metrics affect vernal pool plant community structure and O. tenuis. This study utilized data collected over a three year period from 2009 to 2011 from twenty O. tenuis populations occurring in vernal pools. I utilized multivariate analysis to examine vernal pool plant community structure along ecological gradients defined by the physical environment and livestock use. Responses of O. tenuis population characteristics to livestock use were modeled with quantile regression and generalized linear mixed effects models. Livestock use and grazing intensity were not significantly associated with vernal pool plant community structure; however, both variables had negative associations with O. tenuis population characteristics including presence and height. High intensity grazing had a stronger negative relationship with O. tenuis responses than low intensity grazing. These analyses at the level of individual 1m2 quadrats suggest that livestock use may not be beneficial for O. tenuis conservation and recovery, but further investigations are needed to explore O. tenuis responses to alternative grazing management scenarios at the population level.Overall, my research aims to clarify the complex processes occurring in Modoc Plateau vernal pools and has significant implications for the management and recovery of sensitive vernal pool plant species. The novel methodology utilized in this study, incorporating remote photography, field surveys, and topographic relief modeling, will be valuable in ecosystems experiencing similar climatic conditions or monitoring limitations. My results also highlight the importance of using continuous metrics of livestock grazing intensity in exclosure studies. The findings contribute to a better understanding of vernal pool ecology and offer insight into potential hydrologic restoration of vernal pool habitat.