Browsing by Subject "Woody plants"
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Item Dynamics of woody plant encroachment in Texas savannas : density dependence, environmental heterogeneity, and spatial patterns(2010-08) González, Ana Verónica; Fowler, Norma L.; Schwinning, Susan; Linder, Craig R.; Leibold, Mathew A.; Meyers, Lauren A.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.Item Factors determining post-wildfire plant community recovery trajectories in Central Texas(2017-08) Booth, Emily Mary; Fowler, Norma L.; Hawkes, Christine; Dunton, Kenneth; Jha, Shalene; Roux, StanleyWildland fires are becoming more frequent and more severe in the United States, due in part to climate change and in part to long-term fire suppression and the subsequent build-up of fuels. Following wildfires of greater severity than what were historically present in an area, plant community recovery trajectories may diverge from the pre-disturbance plant community. The Lost Pines region of central Texas supported the westernmost stands of loblolly pine (Pinus taeda) in the United States. In 2011, a wildfire burned most of Bastrop State Park (BSP), located in the Lost Pines. Pre-fire, BSP was a mostly closed-canopy forest dominated by loblolly pine and several species of oak (Quercus spp.), with sparse herbaceous vegetation and a dense mid-canopy of yaupon (Ilex vomitoria). Most plants in BSP were either killed or top-killed in the wildfire. We studied pre- and post-fire plant community dynamics to understand and predict post-fire plant community recovery trajectories. Top-killed oak species sprouted vigorously in more severely-burned plots (Chapter 1, Chapter 2); yaupon sprouted in all burn severity classes (Chapter 3). Loblolly pine, which can only recruit from seed, established more slowly than sprouting species, in part due to the transitory inhibitory effect of an erosion control product (Chapter 3). In the first year after the fire, it appeared that oak sprouts might out-compete loblolly pine seedling recruitment. However, in 2015, a large loblolly pine recruitment event occurred following a year of unusually high precipitation (Chapter 1, Chapter 2). These results indicate recovery trajectories towards continued survival of the loblolly pine population in BSP, although with a potentially greater abundance of oak species than what was present pre-fire. Furthermore, yaupon is likely to re-form dense thickets such as those present pre-fire without measures to prevent woody plant encroachment. Immediately post-fire, the herbaceous plant community increased in abundance, richness and diversity, likely due to greater canopy openness (Chapter 4). Very few invasive species were present either pre- or post-fire (Chapter 5). Alternate trajectories towards open-canopy savanna with a diverse understory community and lower mid-story tree abundance could be maintained by management actions such as prescribed fire or mechanical thinning.