Dynamics of woody plant encroachment in Texas savannas : density dependence, environmental heterogeneity, and spatial patterns

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Title: Dynamics of woody plant encroachment in Texas savannas : density dependence, environmental heterogeneity, and spatial patterns
Author: González, Ana Verónica
Abstract: Woody plant encroachment , that is , a substantial increase in the abundance of woody plants in a grassland or savanna , occurs in many parts of the world . It often has large effects on plant and animal populations and communities and on ecosystem properties and processes . However , little is known about the dynamics of woody plant encroachment and how these are affected by soils , by topography , and by the spatial pattern of the vegetation . Encroachment in turn can affect the spatial pattern of the vegetation . Using data from historical aerial photographs , I measured changes in woody plant cover and constructed , parameterized and compared a set of dynamic models of woody plant encroachment in central Texas savannas . These models predicted final woody cover from initial woody cover and the initial spatial configuration of woody plants . Then I incorporated soil and topography into these models to determine their effects . Finally , I examined the effects of encroachment on the spatial pattern of the vegetation . Incorporating negative density dependence in our models improved their fit , demonstrating that encroachment is density -dependent . A function that predicted the formation of new woody patches from a density -independent seed supply also improved the models' performance . The improvement in the models that resulted from incorporating the total length of woody -herbaceous edges confirmed that encroachment in this system occurs in part by the outward expansion of woody patches . The spatial pattern of the vegetation changed during woody plant encroachment . Spatial pattern (measured as degree of fragmentation ) often had a non -linear relationship with cover . Furthermore , the spatial heterogeneity in fragmentation , that is , plot -to -plot variation in the degree of fragmentation , also changed during encroachment . Topography and soil type had , in general , little effect the dynamics of woody plant encroachment . Therefore , a relatively simple model of woody plant encroachment provided good predictions of woody cover at the end of the time periods . Other systems experiencing woody plant encroachment , forest succession , or invasion by non -native plants could be modeled using the same approach .
URI: http : / /hdl .handle .net /2152 /ETD -UT -2010 -08 -1887
Date: 2010-11-10

Citation

Dynamics of woody plant encroachment in Texas savannas : density dependence, environmental heterogeneity, and spatial patterns. Doctoral dissertation, University of Texas at Austin. Available electronically from http : / /hdl .handle .net /2152 /ETD -UT -2010 -08 -1887 .

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