Tree Population Dynamics at a Desert Springs Complex: From Seed to Landscape
AuthorKaram, Sarah L.
AdvisorWeisberg, Peter J.
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The cumulative effects of the abiotic and biotic environment on plant distributions are inherently complex due to spatio-temporal variation. The overall objective of this dissertation was to examine the population processes and environmental conditions that control tree distributions at a desert springs complex. In chapter 1, I examined the efficacy and spatial pattern of coyote dispersal of Prosopis seeds. Analyses showed that digested seeds remained viable and were deposited in environments suitable for germination, but feces were not a suitable seed bed for seedling emergence. Consequently, seed dispersal by coyotes may act as a demographic bottleneck. In chapter 2, I examined the effects of water availability and salinity on seed germination, seedling emergence, and seedling survival using a series of controlled laboratory and field experiments to identify the fundamental niches of three dominant tree species. Experimental results showed that the effects of water availability and salinity were strong, but varied among species, ontogenetic stages, and experimental conditions. Physiological tolerances to these abiotic factors were lowest during early seedling survival, suggesting another demographic bottleneck. In chapter 3, I examined the effects of plant interactions along water and salinity gradients on seedling establishment in the context of the stress gradient hypothesis. Results of the field experiment showed that both herbaceous and woody vegetation ameliorated environmental stress, but the net outcome of plant interactions depended on species, ontogenetic stage, and the stress gradient. Facilitative effects were common during seedling emergence and early seedling survival, but net interactions shifted to competition and became more consistent along stress gradients as seedlings aged. In chapter 4, I examined spatial variation in the interactions between trees across Ash Meadows. I developed a map of individual trees using object-based classification of multispectral and LiDAR spatial data and examined spatial interactions between trees using local point pattern analysis. Results provided strong evidence that the importance of competition between trees varied spatially in association with water availability and temporally over tree lifetime. All together, this research illustrates the spatial and temporal complexity inherent in plant-environment interactions depending on species, ontogenetic stage, and the abiotic and biotic environment.