Reduction in lumen area increases the amount of δ18O exchange with source water during cellulose synthesis

Fine-scale sampling of secondary growth, isotopic composition and wood anatomical features has enabled the study of intra-seasonal tree growth dynamics in response to environmental and ecophysiological processes, and this fine-scale sampling approach has led to a re-examination of our fundamental understanding of how environmental factors are recorded in δ¹⁸O of cellulose (δ¹⁸O_cell). High resolution xylem anatomical analyses, such as wood density, lumen area (LA), cell wall thickness (CWT), and blue intensity have also been used to understand tree response to climate. However, linking wood anatomical traits with their isotopic signature has not yet been explored, but can provide new insights on the interpretation of the δ¹⁸O_cell through time. In this study, we test the response of wood anatomical features in Pinus ponderosa and Pseudotsuga menziesii, including cell-wall thickness (CWT) and lumen area (LA), along with the oxygen isotopic composition of α-cellulose (δ¹⁸O_cell) to shifts in relative humidity (RH) in two treatments: one from high to low RH and the second one from Low to high RH. We observed a significant decrease in LA and a small increase in CWT within the experimental growing season in both RH treatments. The measured δ¹⁸O_cell along the tree ring was also responsive to RH variations in both treatments. However, estimated δ¹⁸O_cell did not agree with measured δ¹⁸O_cell when the proportion of exchangeable oxygen during cellulose synthesis (P_ex) was kept constant. We found that modeled δ¹⁸O_cell agreed with measured δ¹⁸O_cell only when P_ex increased through the ring formation; we also found that P_ex linearly decreased with an increase in standardized LA. Based on this varying P_ex within an annual ring, we propose a targeted sampling strategy for different hydroclimate signals: earlier season cellulose (larger LA) is a better recorder of relative humidity while late season cellulose (smaller LA) is a better recorder of source water.