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| Stable isotopes and carbon cycle processes in forests |
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James Ehleringer
University of Utah, Salt Lake City, Utah, USA
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Scaling and partitioning are frequently two difficult challenges facing ecology today. With regard to ecosystem carbon balance studies, ecologists and atmospheric scientists are often interested in asking the scaling question of how do the fluxes of carbon dioxide scale across the landscape, region, and continent. Yet at the same time, physiological ecologists and ecosystem ecologists are also often interested in dissecting the net ecosystem exchange between the biosphere and the atmosphere through a better understanding of the balance between photosynthesis and respiration within a forest. In both of these multiple-scale ecological questions, stable isotope analyses of carbon dioxide can play a central role. In this talk, we both develop a theory and present field evidence to address scaling and partitioning in these areas. We first show that the isotopic signal that ecosystems impart to the atmosphere does not remain constant over time neither at temporal nor spatial scales. The relative balances of different biological activities and plant responses to stress result in predictable, dynamic changes in the isotopic exchange between the biosphere and atmosphere. We then examine how forests and grassland ecosystems differ fundamentally in their isotopic gas exchange and also how gas exchange parameters such as the ratio of intercellular to ambient carbon dioxide vary with canopy position. Lastly, we examine how carbon and oxygen isotopes are used to partition fluxes within an ecosystem in order to calculate shifts in the balance of photosynthesis and respiration.
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