| The water and carbon cycles are tightly coupled at the leaf level through a term called plant water use efficiency (WUE), which is the ratio of carbon assimilation (A) to transpiration (E). At larger spatial scales, however, WUE is important in determining the exchange of carbon and water between the biosphere and atmosphere. In this thesis, I examined the relationship between the water and carbon cycles at three spatial scales: the leaf, whole tree, and ecosystem scales. At the leaf scale, I used stable carbon isotope analysis (delta13C) of recently fixed sugars to examine seasonal changes in WUE of three conifer species in the subalpine forest in the Colorado Rocky Mountains. The three species included: Pinus contorta (lodgepole pine), Picea Engelmanni (Engelmann spruce), and Abies lasiocarpa (subalpine fir). The delta 13C of recently fixed sugars revealed two trends: (1) seasonal changes in plant WUE were mainly driven by cold, air and soil temperatures, and (2) fir had the lowest WUE, while pine and spruce had similar WUE. At the whole tree scale, I used a combination of delta13C and transpiration (E) measurements to estimate whole tree carbon assimilation rates (A Tree). I found that pine and spruce had higher ATree than fir. Finally, at the ecosystem level, I used two approaches to examine ecosystem coupling of the water and carbon cycles. I first scaled ATree to ecosystem GPP over two growing seasons; I then used nine-years of eddy flux observations to show that longer growing seasons resulted in less annual CO 2 uptake. Stable hydrogen isotopes (deltaD) analysis of snow versus rain, coupled with SIPNET demonstrated that annual forest net ecosystem productivity was highly dependent on snow water, which decreased in abundance during years with a longer growing season. In the past fifty years, snowpack in this region has also decreased, concomitant with earlier spring, potentially diminishing the strength of the terrestrial sink. These results collectively demonstrate the importance of linking the water and carbon cycles from the leaf level to the ecosystem level in order to understand how the subalpine ecosystem will respond to future climate changes. |
