Susceptibility to parasitism by the mistletoe Phoradendron quadrangulare (Kunth) Griseb on its host Guazuma ulmifolia Lam may increase with host size

Abstract

Host-parasite interactions and host susceptibility are key traits in understanding trophic energy transfer, nutrient movement and general macro-ecoevolutionary dynamics of mistletoe systems and plant-plant interactions. This research investigates host susceptibility and size-dependent interactions of the mistletoe Phoradendron quadrangulare, a widely distributed species, on Guazuma ulmifolia. We studied the interplay between mistletoe load and host tree size, while also exploring the allometric relationship between host branch size and mistletoe size. A field surveys on 67 trees revealed varying mistletoe loads, with most trees showing no occurrence of P. quadrangulare. Parasitized trees had significantly larger diameters at breast height (DBH) than non-parasitized trees. The susceptibility of host trees to mistletoe parasitism increased with increasing DBH, indicating a positive relationship between host size and mistletoe prevalence. Furthermore, mistletoe stem diameter was found to be influenced by the diameter of the host branch suggesting that larger host trees provide more substrate for larger-sized parasites and surface area for mistletoe colonization, potentially contributing to the parasite's survival and prevalence. This study also highlights the importance of host size in mistletoe presence and performance and provides insights into the broader eco-evolutionary dynamics and conservation strategies needed to conserve mistletoes, an often-underappreciated keystone taxa.