Tree growth declines and mortality were associated with a parasitic plant during warm and dry climatic conditions in a temperate coniferous forest ecosystem.

Insects and pathogens are widely recognized as contributing to increased tree vulnerability to the projected future increasing frequency of hot and dry conditions, but the role of parasitic plants is poorly understood even though they are common throughout temperate coniferous forests in the western United States. We investigated the influence of western hemlock dwarf mistletoe (Arceuthobium tsugense) on large (≥45.7 cm diameter) western hemlock (Tsuga heterophylla) growth and mortality in a 500 year old coniferous forest at the Wind River Experimental Forest, Washington State, United States. We used five repeated measurements from a long-term tree record for 1,395 T. heterophylla individuals. Data were collected across a time gradient (1991-2014) capturing temperature increases and precipitation decreases. The dwarf mistletoe rating (DMR), a measure of infection intensity, varied among individuals. Our results indicated that warmer and drier conditions amplified dwarf mistletoe effects on T. heterophylla tree growth and mortality. We found that heavy infection (i.e., high DMR) resulted in reduced growth during all four measurement intervals, but during warm and dry intervals (a) growth declined across the entire population regardless of DMR level, and (b) both moderate and heavy infections resulted in greater growth declines compared to light infection levels. Mortality rates increased from cooler-wetter to warmer-drier measurement intervals, in part reflecting increasing mortality with decreasing tree growth. Mortality rates were positively related to DMR, but only during the warm and dry measurement intervals. These results imply that parasitic plants like dwarf mistletoe can amplify the impact of climatic stressors of trees, contributing to the vulnerability of forest landscapes to climate-induced productivity losses and mortality events. Published 2019. This article is a U.S. Government work and is in the public domain in the USA.