Stable Carbon Isotopes As Indicators of Plant Water Use Efficiency

Stable carbon isotopes have been utilized to better understand how environmental variables influence the efficiency of photosynthesis, specifically what factors limit the uptake and absorption of CO2 during photosynthesis. An understanding of the controls over both gas exchange and stomatal conductance can provide an explanation for the possible environmental influences on plant WUE. The δ13C of extractive-free wood was used as an index of plant water use efficiency at Mica Creek Experimental Watershed, Shoshone County, ID. The δ13C values of tree rings were used to determine the effects of clear cut and partial cut harvesting practices, the effect of elevation, and species differences in intrinsic water use efficiency (WUE) among coniferous species including: Thuja plicata, Larix occidentalis, Picea engelmannii, Pseudotsuga menziesii, Abies lasiocarpa, and Abies grandis. We found significant effects of harvest treatments (p=0.0197), elevation (p= 0.0268), and species (p<0.001) on tree δ13C. The significantly more enriched isotopic signatures observed in Thuja plicata (δ13C = -23.37 ±0.17‰), indicate that it is a more water use efficient species compared to Larix occidentalis (δ13C = -25.66 ±0.43‰), and Abies grandis (δ13C = -25.83 ±0.15‰). There was also an overall trend of δ13C enrichment with elevation. The isotopic composition of tree rings has been estimated to increase by 0.003 ‰ per meter of elevation gain, which may be related to a decrease in soil moisture with elevation. Finally, the mean δ13C values observed on partial cut (δ13C = -24.73 ±0.10‰) and clear cut treatments (δ13C = -24.45 ±0.29‰) were significantly more enriched than the mean value for the control treatment (δ13C = -25.25 ±0.19‰). The more enriched isotopic signatures observed on the harvested treatments indicate that the trees are more water use efficient, which may be a result of increased photosynthetic capacity with an increase in the availability of water, foliar nitrogen, and light to individual trees on the harvested treatments. The reduction of stand density through harvesting may reduce the transpirational water losses on a stand level, thus increasing the water availability for individual trees.