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Accueil du site → Doctorat → États-Unis → 1996 → Multi-scale vegetation responses to long-term herbivory and weather variation on the Edwards Plateau, Texas

Texas A&M University (1996)

Multi-scale vegetation responses to long-term herbivory and weather variation on the Edwards Plateau, Texas

Fuhlendorf, Samuel Dean

Titre : Multi-scale vegetation responses to long-term herbivory and weather variation on the Edwards Plateau, Texas

Auteur : Fuhlendorf, Samuel Dean

Université de soutenance : Texas A&M University

Grade : Doctor of Philosophy (PhD) 1996

Vegetation dynamics (1949-1993) of heavy, moderate and ungrazed treatments were analyzed across spatial scales that included (1) the sum of treatments (landscape), (2) treatment units (community), (3) sites (aggregated quadrats) and (4) point sample (individual quadrats). Compositional shifts of graminoid species (basal area) were dependent upon grazing intensity. Species response was individualistic, however, three species grazing functional/response groups were identified (short-grass, grazing tolerant ; short/mid-grass, opportunistic ; mid-grass, grazing sensitive). Within each grazing treatment, dynamics of perennial graminoids were dependent upon local soil depth. Dynamics of composition were driven by grazing intensity, local soil variation and episodic weather events (drought and temperature extremes). Plant density of perennial graminoids increased with grazing, while basal area of individuals decreased. However, total basal area within treatments was highly dependent upon precipitation and not dependent upon grazing intensity treatments. The relationship between basal area and precipitation accentuated grazing influences on compositional shifts by increasing grazing intensity (if pre-drought stocking rate is maintained) during dry years through a reduction in primary production. The strongest relationship with production was a three-year running precipitation average. Over 45 years, graminoid diversity increased when all treatments were considered collectively (landscape level). At the community level (treatments), removal and moderation of grazing resulted in dramatic increases in perennial graminoid diversity. These increases peaked after about 25 years and appear to be decreasing in the ungrazed treatment, reflecting competitive exclusion by dominant mid-grasses and woody plants and supporting the intermediate disturbance hypothesis. Point sample diversity (quadrats) had a weaker relationship with grazing opposite of larger scales. Landscape patterns of graminoid diversity were driven by influences on patch heterogeneity, community patterns were driven by competitive responses to herbivory, and point sample diversity patterns were driven by a relationship between plant size/density and quadrat size. Analysis of large scale units across the grazing gradient indicated synchronous movement driven primarily by precipitation. Small scale dynamics were stochastic and units had divergent trajectories. Heavy grazed small scale units were less variable than other treatments. At the large scale, between unit variation was highest for moderate grazed treatments due to feedbacks initiated by site differences in primary productivity.

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