Divergent Responses of Fir and Pine Trees to Increasing CO₂ Levels in the Face of Climate Change

Abstract Understanding the response of forests to the increases in atmospheric CO 2 ( c a ) is fundamental to implementing innovative management strategies and for assessing impacts on the global carbon and water cycles. Here, we explored correlations between ecophysiological traits and climate variability that influence changes in stable isotope carbon and oxygen (δ 13 C and δ 18 O) of tree‐rings. We present these relationships between species of the contrasting genera Abies and Pinus , along a latitudinal transect encompassing different biogeographical regions in North America. We also tested if the rate of intrinsic water‐use efficiency per unit of c a (d W /d c a ) during two periods (1890–1965 vs. 1966–2016), for fir and pine were different and indicated acclimation to c a increases. We hypothesize that, spatially and temporally, the divergent responses among species to carbon and oxygen isotopes and d W /d c a are influenced by the site conditions and the historical increases in c a . From our results, we show that fir and pine species will behave physiologically different as global warming progresses. Firs are more responsive to atmosphere vapor pressure deficit along different geographical zones. The survival of forests species under climate change will rely on the response to water stress and species' traits that influence the regulation of d W . Finally, we want to highlight the concept of “progressive resource limitation” of soil water and nutrients, previously proposed by other authors, that likely indicate fir species that inhabit moister sites will benefit more from increased c a than pine, but this positive effect is likely transitory as global warming increases.