Tree ring ¹⁴C and ¹³C content reveal reliance on fast cycling carbon for pine growth in semiarid montane forests

Summary Trees harbor large stores of nonstructural carbohydrates, some of which are quite old (> 10 yr), yet we know little of how these older stores may be used for woody growth. Crucially, the use of old carbohydrates during cellulose biosynthesis could confound climate reconstructions that rely on tree ring stable isotope ratios. We analyzed tree‐ring cellulose Δ 14 C and δ 13 C in earlywood of two pine species from montane forests in western North America using tree rings produced during the radiocarbon bomb pulse (1966–1980). Comparison of the Δ 14 C from ponderosa pine in Utah with estimates of atmospheric 14 CO 2 showed that the cellulose Δ 14 C values can be explained using only carbon fixed in the same growing season as ring construction. In the more arid Arizona pine, the cellulose Δ 14 C values indicate that up to 50% of the carbon used in tree‐ring construction can be from photosynthate assimilated the year before ring construction. Correlations between cellulose δ 13 C time series and aridity indices validated the results obtained from Δ 14 C values. The results reveal that in some semiarid coniferous forests, tree‐ring isotope composition could partially reflect the climate from at least one previous growing season, but that carbon sources older than 2 yr are likely seldom used.