Effect of hemlock dwarf mistletoe on the physiology of host western hemlock using tree rings and C and O stable isotopes

Background/Question/Methods

Dwarf mistletoe (Arceuthobium spp.) is an obligate, hemiparasitic plant that is considered to be extremely destructive to commercially valuable forests because it reduces wood quality and induces host tree death. However, the mechanism of tree mortality and the physiological effects of dwarf mistletoe on its host remain unresolved, as few studies have analyzed tree-scale physiological effects of the infection on its host. This study uses tree-ring stable isotope ratio analyses to investigate the effect of the hemlock dwarf mistletoe (Arceuthobium tsugense) infection on radial growth, δ¹³C, and δ¹⁸O of uninfected and infected old-growth (~250 years old) western hemlock (Tsuga heterophylla) in southwestern Washington. Because hemlock dwarf mistletoe reduces overall photosynthetic capacity (A) by causing dieback of entire branches and diverting nitrogen from remaining foliage, we hypothesized that infected trees would display decreased radial growth. We expected δ¹³C values in growth rings of infected trees to be more negative than those of uninfected trees because prior work suggests infected trees lack stomatal adjustment in response to reduced A. We do not expect tree-ring δ¹⁸O values to differ between infected and uninfected trees because of their similar stomatal conductance (g_s).

Results/Conclusions

Preliminary data show that from 1870 to present, the decline in growth rate in infected trees was approximately twice as large as that of uninfected trees. From 1865 to 1905, the mean δ¹³C value of infected trees was 0.46 ‰ less negative than that of uninfected trees. In contrast, from 1905 to present, the mean δ¹³C value of infected trees became 0.52 ‰ more negative than that of uninfected trees. These data suggest that the trees studied may have become infected around 1900. The shift in δ¹³C trajectories of infected and uninfected trees suggests that hemlock dwarf mistletoe infection may lead to reduced host water use efficiency by reducing the ratio of A to g_s. To identify the predominant mechanism responsible for the shift in δ¹³C in rings of infected trees, we are using δ¹⁸O values and a semi-quantitative model to determine the relative contributions of changes in A and g_s to reduced δ¹³C. Studies integrating leaf to whole-tree physiology will improve our understanding of this system from the tree to canopy scale.