Temperatures are rising across the globe, but temperatures at high northern latitudes are increasing at an even faster pace. This warming is correlated with tree species mortality at the temperate-boreal ecotone, particularly among boreal species growing near the southern end of their range. However, it is unclear if the observed decline is caused by the increase in temperature, higher incidences of drought stress, or the combination of both stresses. To address this question, the Boreal Forest Warming at an Ecotone in Danger (B4WarmED) experiment was established at two sites in northern Minnesota. At each site replicate plots of forest saplings are grown under control, heated, drought, and heated + drought conditions. To assess the impact of these treatments on plant hydraulics, we measured the maximum specific hydraulic conductivity and vulnerability of the xylem to embolism propagation in eight species (four conifer, four angiosperm) at both sites and under all conditions. Measurements were made on branches or trunk segments that had been growing under treatment conditions for three years. A Weibull function was fitted to the vulnerability curves and used to calculate the pressure at which 50% of the hydraulic conductivity was lost (P50).
The maximum specific hydraulic conductivity and P50 differed among species, as expected. On average under all treatments, the conifers had lower specific conductivities and more negative P50 values than the angiosperms. Similarly, the boreal species tended to have lower specific conductivities and more negative P50 values. Not all species responded to the treatments. Across all species the specific conductivity was lower in the heated + drought treatment, but not under the conditions of the two stresses separately. The P50 values did not differ from the ambient treatment for any species in the drought or heated + drought treatments. However, under the heated condition, two of the angiosperms (Betula papyrifera and Quercus rubra) exhibited less negative P50 values, while the wood of Abies balsamea became more resistant to embolism. Our results indicate that the ability of trees to adapt their hydraulic network to novel conditions of higher temperatures and drier conditions may be limited. This inability to acclimate may limit species’ capacity to tolerate future warming.