Browsing by Subject "Savanna"
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Item Biotic and abiotic controls on carbon dynamics in a Central Texas encroaching savanna(2014-12) Thijs, Ann; Hawkes, Christine V.; Litvak, Marcy E.Anthropogenic activities are responsible for increases in atmospheric CO₂ and climate change. These increases are partly counterbalanced by natural processes, such as carbon uptake in land surfaces. These processes are themselves subject to climate change, creating a coupled carbon-climate system. I investigated the carbon sink that woody encroachment represents, using a Central Texas savanna as study site, and studied how climatic factors influence this carbon sink. Woody plant encroachment, a worldwide structural change in grassland and savanna ecosystems, alters many ecosystem properties, but the net effect on the carbon balance is uncertain. Woody encroachment represents one of the key uncertainties in the US carbon balance, and demands a more detailed understanding. To come to a process-based understanding of the encroachment effect on carbon dynamics, I analyzed patterns of carbon exchange using eddy-covariance technology. I expected the imbalance between carbon uptake and release processes associated with the encroaching trees specifically, to be responsible for the carbon sink. I also expected that the sink would vary in time, due to strong links between carbon fluxes and soil water in this semi-arid ecosystem. I further studied the ecophysiology of the dominant species, as well as soil respiration processes under different vegetation types, and scaled these findings in space and time. I found that the ecosystem was a significant carbon sink of 405 g C m⁻² yr⁻¹. The encroaching trees increased photosynthesis by 180% and decreased soil respiration by 14%, compared to the grassland, resulting in a strong carbon sink due to the encroachment process. The encroaching process also altered carbon dynamics in relation to climatic drivers. The evergreen species Ashe juniper effectively lengthened the growing season and widened the temperature range over which the ecosystem acts as a carbon sink. The drought resistance of the encroaching trees reduced the sensitivity of this savanna to drought. I conclude that encroachment in Central Texas savannas increased the carbon sink strength by increasing the carbon inputs into the ecosystem. Woody encroachment also reduced the sensitivity to climatic drivers. These two effects constitute a direct effect, as well as a negative feedback to the coupled carbon-climate system.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.Item Interactions and influences of savanna fire across multiple extents(2019-02-06) Marden, Alexander William; Crews, Kelley A.The spatiotemporal interactions and feedbacks of fire and vegetation in savanna systems are a key component of savanna structure, function, and can give insight into woody plant encroachment. The increased accessibility to remote sensing data and software via Google Earth Engine for fire analysis facilitates integrated savanna assessments for local-to-regional analyses and management. This study assesses spatial and temporal fire dynamics in the Botswana Kalahari at different extents and resolutions to test the hypothesis that fire and vegetation patterns interact via distinct processes at different extents and resolutions in savanna systems. MODIS burn products (MCD64A1 500meters) and in situ vegetation and fire measurements were used to test the affects of variables at different extents and resolutions on savanna fire and vegetation patterns. Spatiotemporal analysis at the extent of the Botswana Kalahari explored 1) the spatial autocorrelation of fire return time using a bivariate Moran's I analysis to observe how large patterns of fuel dynamics affect neighboring areas across time and space and 2) how variability of variables over small spaces influence larger patterns of fire occurrence using Geographically Weighted Regression or GWR. In situ grass and woody vegetation characteristics were measured and combined with MODIS fire history data to observe how vegetation at the square meter scale is influenced by regional affects of fire occurrence, grazing, and human influence using OLS regression. Fire intensity and post-fire vegetation mortality assessments were performed on sites that burned approximately two months after initial measurements were recorded to observe vegetation affects on fire intensity and the initial affects of fire on vegetation. Woody plant populations were projected across a number of fires under divergent fuel conditions to observe how fine resolution fire and vegetation patterns influence larger patterns. In the Botswana Kalahari study area, fire occurrence over time was heavily affected by fire presence in neighboring 500m pixels (first and second order), an indication that spatiotemporal patterns of fire are affected by patterns in neighboring areas. The temporal and spatial patterns observed suggest that temporally dynamic neighboring fuel conditions impact fire in a given location. Large scale patterns were observed between variables and fire occurrence using an OLS regression, and spatial variability of local coefficients was observed using a GWR model. Seasonality of precipitation, boreholes, and EVI had negative significant coefficients. Soil moisture, drought severity, and herbaceous cover had positive significant coefficients, but when examined locally using GWR there were high amounts of spatial variation -- every variable ranged from positive to negative significant local coefficients except for seasonality of precipitation. Explanatory power of the variables was significantly improved by the GWR model. The variation in the local coefficients present from the GWR maps indicate that larger patterns of fire presence are influenced by locally specific contexts, while the relative consistency of coefficient sign and high coverage of significance of EVI and herbaceous cover variables signals that fuel conditions are important across the area. The In situ OLS model showed that fire along with regional patterns of herbivory and human influence were highly impactful on grass biomass. Past fire presence was significant and highly correlated with grass biomass. Woody plant canopy cover and regionally specific anthropogenic influence via a road/river with an associated grazing intensity gradient had negative associations with grass biomass. The In situ pre and post-burn vegetation measurements show that distinct fuel conditions caused fire intensity to vary dramatically over a small area on the same day, and the projected woody plant population model suggests that continued disparate fuel conditions would cause significantly different vegetation conditions. Post burn vegetation measurements indicated that the plot with high grass biomass and low woody biomass had high fire intensity while the plot with low grass biomass and high woody biomass had a low intensity fire. Long term modeling of the two fuel scenarios projected high amounts of woody plant growth with high recruitment and little mortality in the low fuel scenario, while the high fuel scenario projected a stable woody plant population. These findings support the idea that fine resolution variability of fire presence affects larger patterns of vegetation uniformity/non-uniformity. Taken together, the findings suggest that different specific factors interact between extents and resolutions of fire patterns -- land use at large extents affect fire presence in a given area, and fine resolution vegetation conditions affect directionality and spread of fire that influence larger patterns of fire. However, fuel connectivity is a common variable that connects these interactions, and since fuel conditions are affected by past fire presence and intensity, fire history is a common factor that is important across scales. With these interactions in mind, the observed vegetation heterogeneity in the study is likely caused in part by the diversity of land uses in the surrounding areas.