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 regionally uncertain. Increasing atmospheric CO2 may enhance photosynthesis and/or decrease stomatal conductance thereby increasing intrinsic water use efficiency (iWUE). While increasing iWUE has been observed in most trees globally, this increase does not necessarily translate into increased 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, we used differing gradients in atmospheric lapse rates as a natural global change experiment. To examine the physiological and growth responses of Pinus contorta populations across climatic gradients, we combined conventional dendrochronology with carbon isotope analysis (δ13C) to assess basal area increment (BAI) and iWUE over time. Stands were sampled at 18 sites spanning two climatically distinct 800 m elevation transects encompassing the majority of P. contorta’s regional altitudinal 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.
Significant increases in iWUE were observed across all sites; however, concurrent increases in BAI were only observed at the lowest elevation of the climatically wetter transect. All other sites experienced decreased or constant growth over the study period. Contrary to expectations, the wetter transect generally 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 at the low and mid elevations of the wetter transect 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, but that long-term climate trends are likely not directly responsible for the observed growth trends. These results indicate that because late summer climate is a strong driver of P. contorta growth, greater increases in iWUE observed at the low elevation of the wetter transect may effectively extend the growing season by ameliorating some of the negative effects of summer drought on growth. Our results challenge the expectation that increases in CO2 and temperature will work in concert to enhance tree growth primarily at higher elevations.