Among the expected impacts of elevated atmospheric CO2 on tree growth is an increase in the efficiency of water use by trees as part of the photosynthetic process. This increase in water-use efficiency (WUE) comes about through the effect of CO2 on stomatal aperature and resulting water loss through transpiration. With higher atmospheric concentrations of CO2 available for photosynthesis, the stomates of trees do not have to open as wide to maintain a sufficient concentration of internal leaf CO2 for a given rate of carbon fixation. Since transpiration rate is related in a major way to the degree of stomatal opening, less water is lost through transpiration per unit carbon fixed. This interaction between external CO2 concentration, stomatal aperature, and transpiration rate is known to exist experimentally, and it ought to result in increased growth (i.e. wider ring widths) in trees photosynthesizing in moisture-limited environments. Yet, this latter effect has been extremely hard to document in tree rings through ring-width analyses alone. Some success has been made in detecting a WUE signal in the d13C ratios of tree rings from arid-site conifers in western North America. However, this is an expensive and time-consuming process, which necessarily limits its application across space and time.
Pavla Fenwick -ova!, beside what remains of once the worlds oldest living thing Prometheous, a Bristlecone pine tree cut down in1964.
We propose to test the feasibility of detecting a WUE signal in tree-ring widths through a statistical sampling design that should maximize our chances of success. Our research results will place well-needed constraints on the likely detection of a WUE signal in tree rings from additional arid and semi-arid ecosystems. In addition, process-based models for predicting changes in the growth and development of arid/semiarid shrub and forest ecosystems to altered climate and atmospheric chemistry will benefit from our analyses. For example, the objectives of the current phase of the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP) are to compare time-dependent ecological responses of biogeochemical and coupled biogeochemical-biogeographical models to historical and projected transient climate, atmospheric chemistry, and aerosol forcings across the conterminous US (see http://www.cgd.ucar.edu:80/vemap/).
The Great Basin project compliments the VEMAP activity by providing insight into the regional response of arid and semiarid ecosystems to climate variability, increased atmospheric CO2 concentrations, and nitrogen deposition across relevant temporal and spatial domains. Unfortunately, the VEMAP simulations for the historical and future transient scenarios are well into the analysis phase, and there is no opportunity to dovetail the VEMAP modeling activity with a field project. Yet profound questions still exist concerning WUE and tree growth in natural environments.
Ancient Bristlecone pine high on Mt. Wheeler, Nevada.