Relative humidity recorded in tree rings: A study along a precipitation gradient in the Olympic Mountains, Washington, USA

Humidity is one of the fundamental variables controlling the energy balance of the climate, and thus its reconstruction represents a significant aspect of paleoclimate studies. We here report a study of oxygen and hydrogen isotopic compositions of tree-ring cellulose along a precipitation gradient in the Olympic Mountains of Washington, USA, to examine whether and how the relative humidity is recorded in the cellulose. According to the physically-based model described in this paper, the relationship between δD and δ18O of cellulose should have the same slope as that in the relationship between δD and δ18O in the source water (∼8.8 for the local meteoric water) if there is no systematic variation in the mean relative humidity among study sites and no physiologic differences among trees. However, our isotopic analyses of cellulose yielded a slope of 17.4, significantly greater than the slope of the Local Meteoric Water Line. We show that to produce such a slope, a positive covariation between the relative humidity and the δD and δ18O in the source water is required across the precipitation gradient. This work suggests that the δD vs. δ18O relationship in tree rings can be a useful tool for paleohumidity reconstruction.