The fire ecology of mature ponderosa pine (Pinus ponderosa) stands infected with western dwarf mistletoe (Arceuthobium campylopodum)

Dwarf mistletoes (Arceuthobium spp.) are native flowering plants that parasitize a range of conifer species throughout western North America. Dwarf mistletoes are considered destructive forest pathogens, but recent research indicates that mistletoes play a key role in structuring plant communities by increasing structural diversity and providing wildlife habitat. The primary objective of my research is to quantify the structural and functional differences between infected and uninfected forest communities.;I investigated the effects of mistletoe (Arceuthobium campylopodum ) on host community dynamics and the interaction between mistletoe infection and fire in old-growth ponderosa pine (Pinus ponderosa) forests. Fifty study plots were established in Crater Lake National Park and Lava Cast Forest, Oregon. I compared fuel composition, broom formation, and flammability between infected and uninfected stands. I also compared mortality rates following prescribed fire treatments and variation in climate response between infected and uninfected trees.;My results indicate that mistletoe increases fine fuel accumulations. It is a weak predictor of fine fuels compared to stand structure measures such as basal area. I also found evidence that infected branches do not persist lower in the crown or live longer than uninfected branches, but it is difficult to accurately age old-growth branches. Prescribed fire results show that plots infected with mistletoe burned at higher temperatures for shorter periods than uninfected plots. There is no difference in fuel reduction between the two groups. Post-fire mortality rate was 35%, but there was no difference in mortality between uninfected and infected plots. Burn chamber tests indicate that infected branches lost a greater proportion of their mass during burns, primarily as needle combustion. Results from the climate response study show that mature trees infected with dwarf mistletoe have higher radial growth rates, exhibit greater sensitivity, and respond more strongly to climate variation. The results of this work indicate that the influence of dwarf mistletoe on ponderosa pine fire ecology is complex and often indirect. My results suggest that managers should consider mistletoe abundance when modeling fire behavior in unmanaged ponderosa pine stands, but do not need to consider different prescriptions for fire treatments in intensively managed stands.