The role of carbon dioxide and water vapor in biosphere-atmosphere exchange

The exchange of mass, energy, and momentum between the biosphere and the atmosphere continues to be a fundamental and practical research problem in hydrology, meteorology, and ecology. Recent interest in the role of the biosphere to function as a carbon sink has moved the question of biosphere-atmosphere exchange into a central research topic in the climate community. My dissertation work focuses on understanding and depicting much of the primary hydrologic, eco-physiological, radiative, and turbulent transport processes regulating biosphere-atmosphere exchange. In particular, my focus is linking hydrologic and ecological processes to arrive at a mathematical framework for describing sources and sinks of carbon in ecosystems.; My first paper, which appeared in Advances in Water Resources, considered the movement of water from soil to the rooting system. This subsurface zone is the water reservoir for much of the biosphere's function. Here, we seek “low-dimensional” descriptors of the root-water uptake along with Richards' equation for soil water redistribution. After describing the root-water-soil system, the next logical progression is above-ground processes. In a sequence of two papers, published in Boundary Layer Meteorology and Journal of Geophysical Research, I developed a multilevel canopy model that couples turbulent transport with radiative transfer schemes, eco-physiological principles to arrive at a mathematical description of the biosphere-atmosphere exchange. Particularly, these models seek to address the following problem: How can one deconvolve the effects of biophysical controls on biosphere-atmosphere fluxes? ; While addressing this problem, I found that night-time carbon fluxes are both difficult to measure and model. This uncertainty possessed a fundamental challenge when considering the carbon balance at annual time scales. This challenge motivated my fourth paper, which considered the use of “inverse methods” to estimate night-time CO₂ respiration. Based on the respiration estimates from the proposed inverse method, “closure” of the carbon budget at Duke Forest to within 15% was attained. We estimated gross primary production (GPP) as about 1800 g m−2 yr−1 and net ecosystem exchange (NEE) as 605 g m−2 yr −1 for this pine plantation. The method was the first to integrate the array of ecological, eco-physiological, and micrometeorological measurements collected at Duke Forest for the purpose of understanding the carbon budget at annual time scale. These results were recently accepted by Global Change Biology.; A final application of the proposed modeling approach deals with the question of the responses of forest ecosystem to external perturbation, namely, the effect of fertilization on carbon uptake from the atmosphere for a young southeastern pine forest. Using a combination of field experiments and model results, I have demonstrated that many feedback mechanisms, e.g., increased plant respiration, can coincide to limit the enhancement of carbon uptake following fertilization.; To summarize, the proposed modeling approach is the first model that resolves the entire canopy microclimate considering the dynamical interactions between biosphere and atmosphere if the canopy attributes and mean meteorological conditions above the canopy are given. For future works, this framework can be readily implemented to investigate the responses of biosphere to the elevated atmosphere CO₂ concentration.