The impacts of the parasitic plant eastern dwarf mistletoe (Arceuthobium pusillum) on host photosynthesis and branch biomass partitioning in white spruce (Picea glauca)
Dwarf mistletoes (genus Arceuthobium [Viscaceae]) are parasitic angiosperms that infect conifers. Collectively they are considered to be among the most damaging native forest pathogens, where infections are known to reduce host tree growth and increase mortality. Despite their widely documented negative ecological and economic impacts, the physiology of dwarf-mistletoe-induced host declines remains little studied. Along the Maine coast, eastern dwarf mistletoe (Arceuthobium pusillum) has contributed to the loss of white spruce (Picea glauca) in the region. In this host species, we have found few of the leaf-scale impacts of infection on host photosynthetic performance and biochemistry that have been linked to host declines in other plant parasite/host systems. However, we have not previously made measurements on the most heavily infected trees, where the impacts of infection may be more extreme, nor have we considered how parasite-induced increases in host branching (i.e. witches’ broom formation) and needle size reductions influence branch-level gas exchange. Here, we describe the needle-scale physiological impacts of eastern dwarf mistletoe infections across a continuum of infection severities and consider the broader impacts of parasite-induced perturbations to needle and limb growth on branch-scale host photosynthetic performance and biomass partitioning in white spruce.
Consistent with our previous work, at the needle scale, we found no impacts of eastern dwarf mistletoe infection on host photosynthetic capacities in lightly and moderately infected white spruce; however, in heavily infected trees, host photosynthetic capacities were reduced by 33%. At the primary branch scale, when photosynthetic rates during exposure to direct full sunlight were expressed relative to the total needle biomass, net photosynthetic rates were ~30% lower in parasitized branches. We found large differences in dry matter partitioning between infected and uninfected branches where the ratio of needle to woody biomass was lower in infected branches (39:1) than in uninfected branches (56:1). Thus at the branch level, mistletoe infections reduce host photosynthesis and alter the balance between the growth of needle and branch biomass. The decrease in host photosynthesis at the branch scale may be related to light capture inefficiencies within the dense witches’ brooms of infected branches resulting in increased self-shading. At the same time, mistletoe infection increases the amount and alters the patterns of host biomass production which may increase branch construction and maintenance costs ultimately to the detriment of the parasitized tree.