Plant Responses to Environmental Heterogeneity in Great Basin Sagebrush Steppe

Abstract

Plant populations experience both spatial and temporal environmental heterogeneity, and their strategies for coping with environmental heterogeneity are shaped by their inability to move in response to unfavorable conditions. In addition, human induced land-use change, including changes in grazing regimes and shorter fire-return intervals, has become increasingly common as a source of environmental heterogeneity experienced by plant populations. This research focuses on how native Great Basin plants respond to environmental heterogeneity, studying three stages of plant life-histories: seed germination, seed banks, and mature plants. My dissertation sought to: 1) identify relationships between climate variability and population-level variation in germination strategies of arid land forbs, 2) use occurrence records from herbaria to compare the climate niches for a group of arid land forbs, and 3) investigate the relationship between disturbance history and seed bank dynamics in sagebrush steppe communities.

The second chapter examines the similarities and differences between the climate niches and the geographic distributions of a set of co-occurring understory forbs found in sagebrush steppe systems. We used distribution models of the potential habitat for our species to estimate the range size, niche breadth, and geographic overlaps between our species. Next, we used model results to identify climate variables most predictive of the distributions of the individual species. Lastly, we compared the mean and variability for precipitation and temperature across known occurrence locations for each species to assess similarities and differences in climate characteristics where these species grow. We found that species varied in their predicted area of occupancy, niche breadth, and the climate characteristics most predictive of their suitable habitat. Only two of the ten species shared a comparable climate niche. This work demonstrated that herbarium records can be used to estimate climate preferences and potential habitat for understudied species.

The third chapter investigates seed bank dynamics in a Great Basin sagebrush steppe system, comparing sites that differ in their disturbance history. We asked whether shrub cover, ground cover, climate, or disturbance history (fire and grazing) were predictive of the seed densities in the soil, the diversity of native and introduced species, the presence of rare species, and similarity between the above and below-ground species composition. We found that common measures of fire history and grazing use may be overly coarse for predicting the effects of disturbance on seed bank dynamics. We also found that shrub cover was highly predictive of the seed bank dynamics in this system. Shrub cover of early seral shrub species was predictive of patterns consistent with moderate disturbance or recovery from disturbance within the above and below-ground plant community, while increasing cover of later seral species, such as Artemisia tridentata, produced patterns indicating a longer time since disturbance.

The fourth chapter asks how mean climate and climate variation at individual sites and across a species’ range affects the specialist-generalist spectrum of germination strategies exhibited by ten arid land forbs. We investigated these relationships using climate data for the western United States, occurrence records from herbaria, and germination trials with field-collected seeds. We found that nine out of ten species exhibited population-level variation in germination, and that generalist strategies were associated with higher spatial variation in actual evapotranspiration at a local scale and higher variation in available water in the spring and annual precipitation at a range-wide scale.