Enhanced use of summer rain for Rocky Mountain conifers during the last interglacial warm period
Recent droughts have highlighted concerns of how rising summer temperatures will increase tree mortality rates across the western United States. However, there remains uncertainty about how summer rain might respond to warming and whether this will ameliorate the moisture stress associated with warming. We analyzed subfossil wood samples from Colorado dating to the last interglacial to assess the response of two common conifers to a previous warm period. The trees experienced comparable growth rates and water use efficiency during the interglacial relative to modern despite evidence from model simulations of a 30% increase in evaporative demand during the peak of the growing season. High-resolution isotopic analysis of the wood samples show an enrichment in the late season cellulosic d18O relative to modern samples, which we find was associated with increased reliance on summer rain. The data are consistent with model simulations showing the interglacial was associated with wetter summers across the western US. We propose enhanced summer rain during this period compensated for drought stress imposed by higher evaporative demand. The tree ring data from the subfossil wood are compared against a multi year field campaign measuring sap velocity and xylem water isotopes during years of historically high and low summer rains to illustrate the response of these trees to changing summer rain inputs.