Reduction in lumen area is associated with the δ18 O exchange between sugars and source water during cellulose synthesis.

High temporal resolution measurements of wood anatomy and the isotopic composition in tree-rings have the potential to enhance our interpretation of climate variability, but the sources of variation within the growing season are still not well understood. Here 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 (δ18 Ocell ) to shifts in relative humidity (RH) in two treatments, one from high-low RH and the second one form low-high RH. We observed a significant decrease in LA and a small increase in CWT within the experimental growing season in both treatments. The measured δ18 Ocell along the ring was responsive to RH variations in both treatments. However, estimated δ18 Ocell did not agree with measured δ18 Ocell when the proportion of exchangeable oxygen during cellulose synthesis (Pex ) was kept constant. We found that Pex increased throughout the ring as LA decreased. Based on this varying Pex within an annual ring, we propose a targeted sampling strategy for different hydroclimate signals: earlier season cellulose is a better recorder of RH while late-season cellulose is a better recorder of the source water.