Competition between congeneric species is thought to be a major force shaping their population dynamics. When two plant species are closely related, they are expected to exploit the resources similarly, leading to a competitive interaction. Parasitic plants, such as dwarf mistletoes, take their resources from another plant’s stem, and it is frequent to find two or more species sharing the same host. The aim of this study was to determine if competition affects the population dynamics of two dwarf mistletoe species, Arceuthobium globosum and A. vaginatum, parasitizing Pinus hartwegii in a temperate forest in Central Mexico. We used the Dwarf Mistletoe Rating system (DMR) as a qualitative measure of mistletoe abundance on 270 pines every six months for three years. In addition, we took measures of the stem diameter at breast height (dbh). The pines were either not infected, infected with A. globosum, A. vaginatum or both species. Based on the Gompertz growth equation, we modeled the growth in DMR and local extinction processes of each dwarf mistletoe species considering the effect of the other species and the host’s dbh.
Results/Conclusions
The growth and local extinction of A. globosum and A. vaginatum was, among other factors, determined by their interspecific interactions. Arceuthobium globosum was negatively affected by A. vaginatum; although the intraspecific competition was stronger than the interspecific one. On the other hand, A. vaginatum was facilitated by A. globosum in small trees, but inhibited in trees with larger dbh. As in A. globosum, the intraspecific competition was stronger than the interspecific one; however, in larger trees A. vaginatum can be excluded if there are large densities of A. globosum. For both species there was a positive effect of the tree diameter on DMR growth rates. Even though the larger trees are more heavily parasitized, smaller trees have fewer defenses and they can be easily weakened by the effect of both mistletoes species, allowing a faster growth of these parasites. These results indicate that interspecific interactions have an important effect on population dynamics of parasitic plants.