Impacts of a parasitic dwarf mistletoe on the water relations of two host conifers with different drought tolerances

Background/Question/Methods

Dwarf mistletoes (Genus Arceuthobium) are hemi-parasitic angiosperms that infect conifers. Although dwarf mistletoes are considered among the most damaging of native forest pathogens, their physiological impacts on host trees remain little studied. While most host species experience eventual decline in response to infection, both the timing and the extent of the decline can vary widely. Eastern dwarf mistletoe (A. pussilum) infects black (Picea mariana), white (P. glauca), and red spruce (P. rubens) in northeastern and north central North America. While white spruce is severely impacted by eastern dwarf mistletoe infection and declines rapidly, red spruce appears to be able shed parasitized branches, which may minimize the overall impacts of the parasite on this species. White spruce is a fast growing, early successional species that is relatively more tolerant of drought. In contrast, red spruce is a slow growing, late successional species that requires higher environmental moisture availability. Since dwarf mistletoe infections are known to perturb host water relations, we hypothesized that differences in the impacts of infection on host water relations and/or differences in host life history strategies contribute the observed effects of eastern dwarf mistletoe on these two species. 

Results/Conclusions

Dwarf mistletoe infection reduced the capacity of infected stems to conduct water and increased overall water stress (i.e., more negative xylem water potentials) in both host species. However, the magnitude of the reduction in stem conductivities was greater in red than in white spruce suggesting a greater impact of mistletoe infection on water transport in this species. The level of water stress measured in red spruce was consistent with levels known to result in stomatal closure in this species possibly limiting gas exchange and the potential for photosynthetic carbon gain. In comparison white spruce, the more drought tolerant species, has been shown to maintain stomatal conductance and gas exchange under more severe water stress than was brought on by infection in our study. Overall, we found that red spruce which appears to be more tolerant of infection, experiences a greater impact on host water relations in response to dwarf mistletoe parasitism. We hypothesize that this species may better tolerate infection possibly by self-pruning brought on by the negative carbon balance caused by stomatal closure in parasitized branches. Thus, greater drought sensitivity may lead to greater whole-tree tolerance of dwarf mistletoe infection in red spruce.