Is it getting hot in here? Adjustment of hydraulic parameters in six boreal and temperate tree species after three years of warming

Background/Question/Methods

Climate change induced temperature elevation is of particular interest at ecotones where species distributions intersect their range limits.  At the temperate-boreal transition zone in the northern hemisphere, there is the potential for species range shifts because of the rate at which temperatures are increasing. However, we currently know little about how the species growing in that zone will respond physiologically to long-term conditions of higher temperatures. To address this unknown, a large-scale free-air warming experiment, B4WarmED, has been heating a subset of plots at two sites in northern Minnesota to +3.4˚C above ambient since 2009.

To better understand how increased temperatures affect hydraulics at this ecotone, we measured various hydraulic parameters in three-year old branches from three boreal and three temperate species within the B4warmED experiment. We measured maximum hydraulic conductivity, hydraulic capacitance, and xylem conduit lumen diameters. We hypothesized that temperate species near the northern end of their range would be better able (than boreal species) to increase lumen diameters-which would increase hydraulic conductivity-when grown under elevated temperatures. We also predicted that the angiosperm species (two temperate and one boreal), which have more complex xylem, would adjust their xylem parameters more than the conifers.

Results/Conclusions

Across all species, the average conduit lumen diameter was wider in individuals growing at higher temperatures than in control individuals. In support of our hypotheses, the increase in vessel diameter under elevated temperature vs. ambient was larger in angiosperm species than conifers, and larger in temperate species than boreal species. Driven by these changes in conduit diameter, the specific hydraulic conductivity was higher in elevated than ambient temperatures for angiosperm and temperate species, but not in conifers or boreal species. In contrast, the wood density in boreal species and conifers was significantly higher at elevated temperatures than ambient. The hydraulic capacitance did not change under elevated temperature.

In conifers, greater wood density at elevated vs. ambient temperatures was associated with lower values of hydraulic conductivity. This trend may be related to an increased need for resistance to embolism propagation. The patterns we observed suggest that angiosperms and temperate species growing at the temperate-boreal ecotone may be more able to acclimate their hydraulic parameters to increases in temperature than conifers or boreal species. The inability of boreal species, in particular, to adjust their hydraulic network may result in contraction of their range northward.