Assessing Water and Nutrient Use by Leafy Mistletoe (Phoradendron spp.) and Some of Its Urban Host Trees

Some urban tree species are suffering from severe leafy (broadfleaf) mistletoe (Phoradendron spp.) infections. These infestations are mostly considered an aesthetic nuisance and their water and nutrient tapping from the host trees is often disregarded, with the parasite being left without control or management. In this preliminary study we are collecting baseline information on some water and nutrient use parameters by mistletoe plants and three Texas native tree hosts, cedar elm (Ulmus crassifolia), hackberry/ sugarberry (Celtis laevigata) and bois d’arc (Maclura pomifera). Data collected to date indicates that mistletoe leaves are actively transpiring (stomatal conductances, g_s, of 18-20 mmol m^-2 s^-1), on both leaf sides (abaxial and adaxial), through the winter months, when the hosts are dormant (leafless). In the spring and summer the mistletoe leaves had g_s averages, on both sides of the leaves, of ~45 mmol m^-2 s^-1 on the bois d’arc and hackberry hosts and ~100 mmol m^-2 s^-1 on the cedar elm host. The g_s activity in host trees was similar to those in the semi-parasite leaves but largely confined to the abaxial (lower) leaf surfaces. The water potential values of mistletoe leaves were lower (more negative) than those observed in the twigs of the host trees. Based on preliminary data, nutrient concentrations in mistletoe leaves appear to be within the normal ranges reported for landscape trees and shrubs, but higher leaf chlorophyll indexes were found in mistletoe leaves.