Capturing season-specific precipitation signals in the northern Rocky Mountains, USA, using earlywood and latewood tree rings

Douglas-fir (Pseudotsuga menziesii Mirb. Franco) total width, earlywood, and latewood tree ring chronologies were developed from six lower forest border sites in the northern Rocky Mountain region of central Idaho and southwestern Montana, USA, to assess the potential for season-specific moisture reconstructions. These long-lived arid-site trees share strong between-tree and between-site coherence, and subannual tree ring chronologies reliably span the past seven centuries. Mapping spatiotemporal patterns in northern Rocky Mountain precipitation highlighted winter- and summer-dominated precipitation regimes that transition along a west to east gradient. When Douglas-fir tree rings were compared with instrumental climate records, season-specific correlations emerged between earlywood and latewood. Total width, earlywood, and latewood shared the most statistically significant monthly correlations with April-June precipitation, whereas variability in adjusted latewood was tuned to June-August precipitation. Principal component analysis indicated that the leading mode of common variance for earlywood and adjusted latewood explained 65% and 55% variance in the chronologies, respectively. Pearson's correlations between earlywood principal component one and the northern Rocky Mountain precipitation field showed that annual (July-June) and spring (April-June) precipitation exhibited the strongest pattern of significance in central Idaho and southwestern Montana valleys and the Snake River Plain. Summer precipitation (June-August) was correlated with adjusted latewood principal component one and was particularly pronounced along and east of the continental divide in southwestern Montana. These results indicate that Douglas-fir earlywood and adjusted latewood tree rings in the northern Rocky Mountains retain season-specific precipitation signals and may be helpful for studying historical precipitation within the winter-summer transition zone. Key Points Seasonal precipitation regimes are mapped with fine-scale precipitation data Earlywood and latewood tree rings have season-specific precipitation signals Summer precipitation signal is novel and unique to latewood chronologies