Browsing by Author "Whelan, Andrew"
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Item Carbon Dynamics of Pinus palustris Ecosystems Following Drought(MDPI, 2016-04-29) Starr, Gregory; Staudhammer, Christina L.; Wiesner, Susanne; Kunwor, Sujit; Loescher, Henry W.; Baron, Andres F.; Whelan, Andrew; Mitchell, Robert J.; Boring, Lindsay; University of Alabama Tuscaloosa; University of Colorado System; University of Colorado BoulderDrought can affect forest structure and function at various spatial and temporal scales. Forest response and recovery from drought may be a result of position within landscape. Longleaf pine forests in the United States have been observed to reduce their carbon sequestration capacity during drought. We collected eddy covariance data at the ends of an edaphic longleaf pine gradient (xeric and mesic sites) over seven years; two years of normal rainfall were followed by 2.5 years of drought, then 2.5 years of normal or slightly above-average rainfall. Drought played a significant role in reducing the physiological capacity of the sites and was compounded when prescribed fire occurred during the same periods. The mesic site has a 40% greater basal area then the xeric site, which accounts for its larger sequestration capacity; however, both sites show the same range of variance in fluxes over the course of the study. Following drought, both sites became carbon sinks. However, the xeric site had a longer carry-over effect and never returned to pre-drought function. Although this study encompassed seven years, we argue that longer studies with greater spatial variance must be undertaken to develop a more comprehensive understanding of forest response to changing climate.Item Cyclic Occurrence of Fire and Its Role in Carbon Dynamics along an Edaphic Moisture Gradient in Longleaf Pine Ecosystems(PLOS, 2013-01-13) Whelan, Andrew; Mitchell, Robert; Staudhammer, Christina; Starr, Gregory; University of Alabama TuscaloosaFire regulates the structure and function of savanna ecosystems, yet we lack understanding of how cyclic fire affects savanna carbon dynamics. Furthermore, it is largely unknown how predicted changes in climate may impact the interaction between fire and carbon cycling in these ecosystems. This study utilizes a novel combination of prescribed fire, eddy covariance (EC) and statistical techniques to investigate carbon dynamics in frequently burned longleaf pine savannas along a gradient of soil moisture availability (mesic, intermediate and xeric). This research approach allowed us to investigate the complex interactions between carbon exchange and cyclic fire along the ecological amplitude of longleaf pine. Over three years of EC measurement of net ecosystem exchange (NEE) show that the mesic site was a net carbon sink (NEE = -2.48 tonnes C ha 21), while intermediate and xeric sites were net carbon sources (NEE = 1.57 and 1.46 tonnes C ha(-1), respectively), but when carbon losses due to fuel consumption were taken into account, all three sites were carbon sources (10.78, 7.95 and 9.69 tonnes C ha(-1) at the mesic, intermediate and xeric sites, respectively). Nonetheless, rates of NEE returned to pre-fire levels 1-2 months following fire. Consumption of leaf area by prescribed fire was associated with reduction in NEE post-fire, and the system quickly recovered its carbon uptake capacity 30-60 days post fire. While losses due to fire affected carbon balances on short time scales (instantaneous to a few months), drought conditions over the final two years of the study were a more important driver of net carbon loss on yearly to multi-year time scales. However, longer-term observations over greater environmental variability and additional fire cycles would help to more precisely examine interactions between fire and climate and make future predictions about carbon dynamics in these systems.Item Effects of drought and prescribed fire on energy exchange in longleaf pine ecosystems(Ecological Society of America, 2015-07-31) Whelan, Andrew; Starr, Gregory; Staudhammer, Christina L.; Loescher, Henry W.; Mitchell, Robert J.; University of Alabama Tuscaloosa; University of Colorado System; University of Colorado BoulderThe structure and function of longleaf pine savanna ecosystems is regulated by cyclic fire, yet there is a lack of understanding about how the frequency of fire influences longleaf pine ecosystem energy dynamics. There are further uncertainties in how predicted changes in temperature and precipitation may affect the interaction between fire and energy exchange in these ecosystems. We investigated energy dynamics in three frequently burned longleaf pine ecosystems along a gradient of soil moisture availability using eddy covariance techniques. We analyzed sensible energy (H), latent energy (LE) and soil heat flux (G) over time since fire, using micrometeorological variables as covariates. Based on statistical tests of autocorrelation, data were analyzed as 30-day averages with general linear models. Over three years of measurement, we found that sensible energy, latent energy and soil heat flux recovered to pre-fire rates within one month following prescribed fire. Changes in water availability associated with drought over the study period had a stronger influence on energy dynamics than did fire. When precipitation was near long-term averages, annual evapotranspiration (ET) was 743, 816 and 666 mm y(-1) at the mesic, intermediate and xeric sites, respectively. During extreme drought, annual ET decreased 4 and 7% at the xeric and intermediate sites, to 754 and 642 mm y(-1), respectively, and decreased 20% at the mesic site, to 594 mm y(-1). Similarly, Bowen ratios were up to two times higher during drought years versus those with average precipitation. These frequently burned longleaf pine ecosystems are known to be well adapted to fire. The more xeric the site, the more resilient they were to drought, suggesting adaptations of this ecosystem maintained higher levels of physiological activity. This three-year study begins to illuminate longleaf pine ecosystem energy dynamics, however long-term observations over a greater range of environmental conditions are necessary to increase our knowledge of the complex interactions between fire, climate and energy dynamics in these ecosystems.