LIMITED EVIDENCE FOR CO2-RELATED GROWTH ENHANCEMENT IN NORTHERN ROCKY MOUNTAIN PINUS CONTORTA POPULATIONS: TRENDS IN GROWTH AND INTRINSIC WATER-USE EFFICIENCY ACROSS CLIMATE GRADIENTS

Forests sequester large amounts of carbon annually; however, the degree to which enhanced forest productivity is due to more conducive climate or CO2 fertilization remains uncertain. Increasing atmospheric CO2 may enhance photosynthesis and/or decrease stomatal conductance thereby enhancing intrinsic water use efficiency (iWUE). While increasing iWUE has been observed in most trees globally, this increase does not necessarily translate into greater growth. This study aims to evaluate whether responses of radial growth and iWUE to increasing CO2 vary across climatic gradients. To investigate interactions between climate and CO2 and their impacts on tree physiology and growth, I used an environmental gradient approach as a natural global change experiment. I combined dendrochronology with carbon isotope analysis (δ13C) to assess the covariation of basal area increment (BAI) and iWUE for Pinus contorta over time. Stands were sampled at 18 sites spanning two climatically distinct 800 m elevation transects encompassing the majority of P. contorta’s regional elevational range. Trends in BAI and iWUE were analyzed for the past 65 years, and correlations with monthly temperature, precipitation and vapor pressure deficit (VPD) were assessed. Increases in iWUE were observed across all sites; however, concurrent BAI increases were only observed at the lowest elevation of the climatically warmer transect. All other sites experienced decreased or constant growth over the study period. The climatically warmer transect exhibited the strongest iWUE response, with the greatest increases (30-42%) observed at the low and mid elevations. In addition to being driven by rising CO2, strong increases in iWUE appear to be driven by long-term increases in summer VPD. Correlations between climate variables and growth indicate that late summer climate of the previous year is important for growth across sites. These results indicate that because late summer climate is a strong driver of P. contorta growth, greater increases in iWUE observed at low elevations of the warmer transect may effectively extend the growing season by ameliorating some of the negative effects of summer drought on growth