Carbon isotope analyses of plant materials, such as sugars, starches, bulk leaf tissue, and wood have been used to provide insight into topics ranging from plant water use efficiency (WUE) to global carbon budget dynamics.  More recently, studies have focused on understanding the environmental variables that influences changes in plant WUE on shorter time scales, such as daily or weekly.  Carbon isotope analysis (expressed as δ¹³C) of recently fixed sugars (leaf or phloem) has the potential to reveal these short-term changes.  In this study, we examined the environmental variables that influence the δ¹³C values of needle and phloem sugar in order to track changes in the seasonal patterns of plant WUE from a cold, subalpine forest.  In 2006, 2007, and 2008, and at the Niwot Ridge AmeriFlux site, we collected needle and phloem sugars from Pinus contorta (lodgepole pine), Picea engelmannii (Englemann spruce), and Abies lasiocarpa (subalpine fir) at two-week intervals throughout the growing season, as well as throughout the winter. 
Results/Conclusions

We found three important results.  First, we found difference among species; subalpine fir had the most negative δ¹³C phloem and needle sugars, suggesting that fir had the lowest WUE.  Second, we found a seasonal pattern, where the most enriched phloem and needle sugar δ¹³C during April (needle sugars: -26.34 ± 0.23, -25.41 ± 0.18, -25.52 ± 0.20 for fir, pine, and spruce, respectively), followed by a progressive depletion in May and June; the most depleted phloem and sugar δ¹³C occurred in early August (needle sugars: -28.55 ± 0.39, -27.71 ± 0.15, -27.72 ± 0.20 for fir, pine, and spruce, respectively).  Third, we found this early season ¹³C enrichment was due to the effects of cool spring temperatures, where stomatal conductance was reduced.  In fact, of the four environmental variables we explored (air temperature, soil temperature, soil moisture, and vapor pressure deficit), air temperature had the greatest effect on needle and phloem δ¹³C.  These results suggest that in the subalpine forest, temperature, particularly cool temperatures during early spring, control the δ¹³C of recently fixed sugars and overall plant WUE.