Heterogeneous Isotope Effects Decouple Conifer Leaf and Branch Sugar δO and δC

Isotope ratios of tree-ring cellulose are a prominent tool to reconstruct paleoclimate and plant responses to environmental variation. Current models for cellulose isotope ratios assume a transfer of the environmental signals recorded in bulk leaf water to carbohydrates and ultimately into stem cellulose. However, the isotopic signal of carbohydrates exported from leaf to branch may deviate from mean leaf values if spatial heterogeneity in isotope ratios exists in the leaf. We tested whether the isotopic heterogeneity previously observed along the length of a ponderosa pine (Pinus ponderosa) leaf water was preserved in photosynthetic products. We observed an increase in both sugar and bulk tissue δO values along the needle, but the increase in carbohydrate δO values was dampened relative to the trend observed in leaf water. In contrast, δC values of both sugar and bulk organic matter were invariant along the needle. Phloem-exported sugar measured in the branch below the needles did not match whole-needle values of δO or δC. Instead, there was a near-constant offset observed between the branch and needle sugar δC values, while branch δO values were most similar to δO values observed for sugar at the base of the needle. The observed offset between the branch and needle sugar δO values likely arises from partial isotope oxygen exchange between sugars and water during phloem loading and transport. An improved understanding of the conditions producing differential δC and δO isotope effects between branch phloem and needle sugars could improve tree-ring-based climate reconstructions.