The ecophysiological interpretation of carbon (δ13C) and oxygen (δ18O) isotopes in cellulose at a seasonal scale hinges upon identifying rates of tree-ring development as they vary throughout the growing season. Recent insights in xylogenesis have improved our knowledge of the timing of the different phases of cell development but have not yet been incorporated into isotopic analysis of tree rings. To that end, we have developed novel approaches for using the isotope composition of tree-ring subdivisions to study seasonal dynamics in tree-climate relations. We examined the δ13C and δ18O, across thirty years, in earlywood (EW), false latewood (FLW, a narrow band of latewood-like cells within the EW), summerwood (SW, a band of EW-like cells following FLW), and latewood (LW) in ponderosa pine (Pinus ponderosa) from a montane forest in the Southwestern U.S. By combining xylogenesis observations of tree-rings with isotopic analyses of serial sub-sections of tree rings, we aimed to determine proper alignment between isotope fractionation processes and associated climate drivers, and to understand the seasonal hydroclimate (precipitation versus atmospheric humidity) controls on isotope fractionation.
Results/Conclusions
We observed a consistent pattern of intra-seasonal δ13C and δ18O with the greatest depletion in the heavier isotopes occurring in the FLW and SW subdivisions, and the greatest enrichment in the heavier isotopes occurring at the earliest and latest ring boundaries in the EW and LW subdivisions. This pattern reflects relatively high intrinsic water-use efficiencies and high rates of evaporative fractionation during EW formation as compared to the FLW and LW subdivisions. This isotope distribution pattern is counter-intuitive, given the typical pattern of lower (higher) water stress and lower (higher) precipitation δ18O values during the cool (warm) season. However, use of a mechanistic isotope-climate model revealed that this pattern can be explained by the existence of higher evaporative demand during the late cool season (spring) and seasonal phenological lags, lasting several weeks, between the initial formation of tracheids and the production of cellulosic secondary cell-walls during maturation. These patterns highlight the role of summer monsoon in carbon-water coupling of the southwest montane forests.
Our results reveal new potential in the use of tree ring isotopes to reconstruct past tree-climate relations, if lags in cambial phenology are reconciled with isotope ratio observations.