Carbon flux across scales in a changing climate
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Ecological patterns are scale dependent. Understanding how and why ecological patterns vary across scales is a central problem in ecology. Stream metabolism and soil respiration, two important processes in the global carbon cycle, are particularly scale dependent. In this dissertation, I employed a dynamic modeling approach to address multiple aspects related to the issue of scale in stream metabolism and soil respiration. Specifically, in chapter 2, I used a dynamic model of dissolved oxygen to quantify the temperature sensitivity of whole-stream metabolism in streams from six biomes, ranging from the tropics to the Arctic. I found that warming leads to convergence in stream metabolic balance, realized as reduced inter-site variability of GPP/ER. The GPP/ER ratio in streams with higher temperature and higher current GPP/ER is predicted to decrease in response to warming, whereas in streams with lower temperature and lower current GPP/ER it is expected to increase, although by a smaller magnitude. In chapter 3, I compared reach-scale metabolism quantified using open channel method and habitat-scale metabolism quantified using chamber incubations. I found that the reach-to-habitat ratio of GPP and ER, standardized to the same light and temperature conditions, decreased with the variance of habitat-scale metabolism within a reach. By combining theoretical analyses and numeric simulations, I showed that the heterogeneity of habitat-scale metabolism within a reach, the negative correlations between light and GPP per light, and temperature used for habitat-scale incubations, could explain this pattern of mismatch between reach and habitat scale metabolism. In chapter 4, I demonstrated the importance of recognizing soil respiration as an aggregated process. I showed that aggregating over space influenced temperature sensitivity, but aggregation over time did no alter temperature sensitivity. I also demonstrated that recognizing soil respiration as the sum of contributions from distinct substrate pools could explain several often observed relationships between temperature sensitivity and temperature, and influenced interpretations of the mechanisms driving changes in temperature sensitivity of soil respiration. Collectively, these studies demonstrated scale dependency of soil respiration and stream metabolism, and highlighted the utility of dynamic modeling as a central approach to tackling the issue of scale.