Distribution of Arceuthobium divaricatum and Phoradendron juniperinum (Viscaceae) on the South Rim of Grand Canyon, Arizona

Abstract

The distribution and effects of the mistletoes Arceuthobium divaricatum Engelm. on Pinus edulis Engelm. and Phoradendron juniperinum Engelm. on Juniperus osteosperma (Torr.) Little on the pinyon-juniper vegetation type on the south rim of Grand Canyon National Park were investigated. Fire was the most limiting factor in the distribution of the mistletoes. The incidence of infection increased as trunk diameter and height increased in both pinyon and juniper. Arceuthobium divaricatum had the more serious impact on its host in respect to both vigor and mortality. Both mistletoes appear to be in equilibrium with their hosts at this time. Two mistletoes of the pinyon-juniper ecosystem of the Grand Canyon are Arceuthobium divaricatum Engelm. and Phoradendron juniperinum Engelm. Research on A. divaricatum and P. juniper- inum has been extremely limited because the trees infected have not been economically important as timber resources. With rising in- terest in utilization of the resources of the pinyon-juniper vegetation type, especially in a recreational capacity, effective management of these stands will be paramount in national parks and recreation areas (Clary 1975). Information on the site preferences and environmental factors that predispose the host trees to mistletoe infection will fa- cilitate management of diseased stands. This study was conducted to determine the distribution and effects of dwarf mistletoe on pinyon (Pinus edulis Engelm.) and true mistletoe on juniper (Juniperus osteosperma (Torr.) Little) on the South Rim at Grand Canyon Na- tional Park, Arizona.