Shared drought responses among conifer species in the middle Siberian taiga are uncoupled from their contrasting water-use efficiency trajectories.

A shift from temperature-limited to water-limited tree performance is occurring at around 60°N latitude across the circumboreal biome, in concord with current warming trends. This shift is likely to induce extensive vegetation changes and forest die-back, and also to exacerbate biotic outbreaks and wildfires, affecting the global carbon budget. We used carbon isotope discrimination (ΔC) in tree rings to analyze the long-term physiological responses of five representative species that coexist in the middle taiga of Western Siberia, including dark-needled, drought-susceptible (Abies sibirica, Picea obovata, Pinus sibirica) and light-needled, drought-resistant (Larix sibirica, Pinus sylvestris) conifers. We hypothesized that droughts are differentially imprinted in dark and light conifers, with stronger ΔC-responsiveness in the latter reflecting a more conservative water use. We found similar ΔC-climate relationships related to the moisture regime of the summer season across species, indicating shared drought responses; however, divergent intrinsic water-use efficiency (WUE) trajectories from 1950 to 2013 were observed for pines (increasing by ca. 10%) and other conifers (increasing by ca. 25%). These contrasting patterns suggested the passive and active stomatal regulation of gas exchange in these trees, respectively, and led us to discard our initial hypothesis. Discriminant analysis shed light on the climate characteristics responsible for such differential behavior, with years having lower temperatures from May through August (3 °C colder on average) being responsible for reduced pine WUE. This finding may be related to the higher plasticity of phenology of pines and the greater susceptibility of fir and spruce to cold damage and heat shock during the early growing season (late April-May). Together with recent negative growth trends and increasing ring-width vs. ΔC coupling, these results indicate the greater susceptibility of spruce and fir, compared with pines and larch, in boreal ecosystems when transitioning from a temperature- to a moisture-sensitive regime.