Impacts of eastern dwarf mistletoe on the stem hydraulics of red spruce and white spruce, two host species with different drought tolerances and responses to infection

Key message Red spruces are less severely impacted by the parasite eastern dwarf mistletoe than white spruce. Differences in stem vulnerabilities to cavitation do not seem to explain this pattern. Abstract Parasitic dwarf mistletoes are damaging forest pathogens, yet the physiological mechanisms by which infections contribute to host decline remain poorly understood. In this study, we sought to determine if differences in the degree of perturbation to stem hydraulics contribute to the more severe impacts of eastern dwarf mistletoe (Arceuthobium pusillum) infection on white spruce (Picea glauca) when compared to red spruce (P. rubens). Of these primary hosts, red spruce exhibits greater drought sensitivity. We hypothesized that the ecophysiology of red spruce may make it more vulnerable to the added water stress brought on by dwarf mistletoe infection and that increased water stress could result in emboli formation and the hydrological shedding of water-stressed branches, which could ultimately allow red spruce to better tolerate infection at the level of the whole tree. In support of our hypothesis, we found greater infection-induced reductions in stem hydraulic conductivities in red spruce than in white spruce. However, we also found that losses in hydraulic conductivity attributable to xylem cavitation were low in parasitized branches of both red spruce and white spruce and did not differ significantly by host species. Consistent with this, branch water potentials following a prolonged period without precipitation were considerably less than the tensions reported to cause 50 % cavitation-induced reductions in hydraulic conductivities in both hosts, suggesting ample hydraulic safety margins. Therefore, we conclude that a greater susceptibility to water stress-induced xylem failure is not the mechanism by which red spruce protects whole-tree resources from dwarf mistletoe by shedding infected branches.