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Doctorat
États-Unis
2017
Plant Responses to Environmental Heterogeneity in Great Basin Sagebrush Steppe
Titre : Plant Responses to Environmental Heterogeneity in Great Basin Sagebrush Steppe.
Auteur : Barga, Sarah C.
Université de soutenance : University of Nevada, Reno
Grade : Doctor of Philosophy in Ecology, Evolution, and Conservation Biology 2017
Résumé
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.
Page publiée le 29 janvier 2018