Controls on Carbon Uptake and Storage in Southeastern Forests
Uptake and storage of carbon by forest ecosystems continues to be a major research topic needed for the quantification of global budgets in an increasing atmospheric carbon dioxide environment. However, there are considerable challenges in quantifying carbon budgets of forest across a wide range of spatial and temporal scales. Although general trends in the components of carbon budgets emerge when analyzed over large spatial or temporal scales, these relationships tend to weaken, or even reverse, at smaller spatial (e.g. stand level) and temporal scales. On the other hand, continuous measuring and monitoring is not a feasible or sensible approach for the range of global forests. There is growing need to identify the key variables that drive variability in these localized budgets at multiple time scales. These results will assist in upscaling stand-level observations into large-scale modeling approaches.
Forest carbon dynamics are closely-coupled with the hydrologic cycle, so an approach that attempts to bridge these dynamics must incorporate water availability and use. Water is necessary for trees to transport nutrients, maintain cellular function, and regulate stomatal conductance; however, water is also related to other biological processes, including microbial decomposition of soil carbon, and physiologically-important abiotic factors, such as atmospheric vapor pressure deficit. Thus, much of the key to understanding the variability in forest carbon cycles is identifying the sensitivity of the processes of the carbon cycle to water availability.
Therefore, my research takes the following approach: I begin by using sap flux sensors to measure tree-level transpiration over a four-year period and combine these values with other estimates of stand-level evaporation to generate an accurate estimate of total evapotranspiration, partitioned by component and tree species (Chapter 2). To assess the sensitivity of the water fluxes in the forest, I next establish a complete hydrologic budget for the forest stand over four years, including one severe and one mild drought (Chapter 3). I then focus on the flux of carbon from the soil and its variability over space and time. Using automated, high-frequency measurements of soil CO2 flux over a 10-year period and including 3 forest stands, I assess inter- and intra-stand variability as well as inter- and intra-annual variability in soil flux in relation to climatic factors and stand characteristics representing productivity (Chapter 4). In order to assess how soil CO2 flux may change over longer periods of time within the context of global change, I analyze how enrichment of [CO2] independent of and combined with soil nitrogen availability alter the balance of carbon in a stand (Chapter 5). Finally, building off these previous chapters, I examine the relationship between carbon uptake, allocation, and turnover in a mixed-species forest experiencing interannual variability in water availability (Chapter 6).
I conclude that (Chapter 2) sap flux sensors can successfully be used to estimate tree- to stand-level transpiration if one accounts for both nocturnal water movement through the tree stem and spatial variability of species composition and demography within a stand. (Chapter 3) Despite reductions in transpiration by some species during water-limited (i.e. drought) periods, the magnitude and duration of these reductions results in annual water use that is similar to a non-drought year. The consequence of this invariability in transpiration and evapotranspiration for the hydrologic cycle is that changes in annual precipitation translate directly to changes in water supplied to rivers and streams. (Chapter 4) Diurnal to seasonal variability in soil CO2 flux is driven by temperature, whereas interannual variability is most-strongly influenced by soil moisture. Furthermore, spatial variability of soil CO2 flux is directly related to forest productivity, and by proxy, leaf production, across biomes and, to a lesser extent, across stands within a region. However, within-stand variability may be inversely related to leaf production as a result of differential allocation of carbon between aboveground and belowground uses based on local resource availability. (Chapter 5) Although elevated atmospheric [CO2] enhances productivity, it may only result in a small increase in the flux of CO2 from soils. Instead, nitrogen availability explains much of the variability within a forest stand, regardless of [CO2], with increasing nitrogen availability resulting in lower allocation of carbon belowground and greater aboveground productivity. (Chapter 6) Interannual variability in water availability can affect gross primary productivity in mature forests but these effects may primarily affect the following growing season. The proportionate changes in gross primary productivity appears to show greater reductions with previous year's soil moisture than net primary productivity, leading to increased carbon use efficiency following drought. Variability in leaf biomass in this relatively stable, mature stand appears to drive the interannual variability in photosynthesis as well as the demand for carbon used for biomass production and metabolic activity.
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