Current projections of climate change in the southwestern U.S. suggest increasing temperatures and reduced summer precipitation. High temperature and water deficits have major influence on ecosystem functioning by restricting plant growth and productivity. However, there are limited data on what influences plant sensitivity to temperature, and these dynamics are not often captured in ecosystem models. Understanding the sensitivities, linkages, and feedbacks among biotic processes and abiotic forces is especially important within Critical Zone Sciences, which seeks to integrate among disciplines. Here, we analyzed several potential drivers of photosynthetic temperature sensitivity, including differences in soil parent material, aspect, and seasonality within a suite of species. Each of these variables captures a different physical driver: (i) soil parent material influences water holding capacity of the soil; (ii) aspect influences how incoming energy drives evaporative loss of soil water, creating warmer and drier environments on south/east faces; and (iii) seasonality captures temporal patterns of soil moisture recharge. Our research was conducted within two V shaped zero-order catchment basins of the Santa Catalina Critical Zone Observatory in southern Arizona, one with schist bedrock and the other with granite. We used leaf-level gas exchange measurements on 24 trees across a range of temperatures to quantify this plant temperature sensitivity during the dry pre-monsoon and wet monsoon seasons. Preliminary results show that maximum photosynthetic rate was 51% higher during the monsoon than pre-monsoon season. Optimal photosynthetic temperature decreased 25% while the span of functional temperatures (Ω50) was 21% higher following the onset of monsoon rains. During the rainy season, soil parent material became an important factor. The greater water holding capacity of schist soils yielded greater maximum photosynthesis and reduced tree sensitivity to higher temperatures. This variability in the temperature sensitivity and the responsiveness of that sensitivity to increases in available soil moisture is important as we consider the structure and composition of our future forests. More detailed analysis will allow us to examine the relative influence of each abiotic variable in driving this photosynthetic response, which impacts a suite of downstream critical zone processes.
Yang J., Barron-Gafford G., Minor R., Heard M. (2013): Examining the Physical Drivers of Photosynthetic Temperature Sensitivity Within a Sub-alpine Mixed Conifer Forest. Abstract EP13C-0877 presented at 2013 Fall Meeting, AGU, San Francisco, CA, 9-13 Dec..