In the Western US, carbon uptake by subalpine forest ecosystems is highly controlled by spring snowmelt; years with reduced snowpacks exhibit lower rates of carbon uptake than years with larger snowpacks. However, summer monsoon rains in early August are still critical in ameliorating the dry soils that result from the midsummer drought. At the Niwot Ridge AneriFlux site in Colorado, seasonal changes in water use efficiency (WUE) were monitored by analyzing the carbon isotope of recently fixed leaf sugars of three dominant tree species: Pinus contorta (lodgepole pine), Picea engelmannii (Engelmann spruce), and Abies lasiocarpa (subalpine fir). Throughout the growing season, sun leaves were collected in the field and leaf sugars were extracted in the lab. The carbon isotopic compositions of bulk leaf tissue and leaf sugars were then analyzed for δ¹³C. From the bulk leaf results, fir exhibited more negative δ¹³C than pine and spruce (fir: -29.32‰ ± 0.24, pine: -27.94‰ ± 0.31, spruce: -27.08‰ ± 0.27), indicating that fir have lower WUE than pine and spruce. Leaf sugar δ¹³C followed a similar pattern, with fir exhibiting less negative δ¹³C than pine and spruce throughout the season. In all three species, δ¹³C values were more negative in April (as the snowpack began to melt) than in July (when soil moisture decreased), suggesting that all three species became more WUE later in the growing season. These results demonstrate that analyzing leaf sugars for carbon isotopic composition can be a powerful tool for tracking seasonal changes in WUE.