Hemlock dwarf mistletoe (Arceuthobium tsugense subsp. tsugense) is an arboreal, hemiparasitic plant that principally parasitizes western hemlock (Tsuga heterophylla). Hemlock dwarf mistletoe exerts a profound influence on infected trees that can drastically change the structure of the tree crown due to reduced growth, top dieback, branch deformation and death, resulting in unique habitat structures, changed fire dynamics, and more severe drought impacts. Although dwarf mistletoe is important to tree crowns, most research has been from the ground or by felling trees. In this study, we climbed 16 western hemlock trees (age 97 – 321 years) across a gradient of infection (0 – 100% of branches infected) and measured occurrence of all dwarf mistletoe infections, branch and crown architecture, and dwarf mistletoe caused deformities. Sapwood area was measured at the top of buttressing or 1.37 m above the ground (f-diameter) and at base of live crown. To explore the impacts of increasing infection severity, over 25 different response variables were examined using linear and generalized linear models to estimate mean responses among the 16 western hemlocks. We developed three metrics of
severity as explanatory variables for the models: total infection incidence, proportion of all live branches infected, and proportion of all live, infected branches with 33 percent or more foliage distal to infection. Many effects of dwarf mistletoe on crown structure appear subtle, except for deformations. Increasing severity led to crowns experiencing an apparent compaction, where crown volume was reduced while deformity volume increased, and crown volumes became increasingly comprised of deformities. A strong effect of dwarf mistletoe intensification is the reduction of branch foliage and an increase in the proportional amount of foliage distal to infections, and therefore a likely reduction in carbohydrates available to tree growth. Sapwood areas at f-diameter and at base of live crown were unrelated to all metrics of infection severity. Branch length and diameters were also unaffected by increasing infection severity despite heavily infected branches supporting 1 to 70 infections. This suggests infected trees compensate for the water and nutrient loss through changes to the crown architecture such as reduced foliage instead of through conductive tissues. Our results suggest shifts in crown structure reflect a shift in function, from prioritizing biomass production and growth, to tree survival.