Competition and facilitation determine dwarf mistletoe infection dynamics

Abstract

1. Interspecific interactions have a fundamental role in plant population dynamics, as they may set the conditions for species coexistence. Parasitic plants, like dwarf mistletoes, offer the opportunity to study competition for resources that are different from those consumed by most plants, allowing for a better understanding of the interaction. 2. We explored how interspecific interactions between two dwarf mistletoe species (Arceuthobium), co-infecting the same host species (even sharing the same individual tree of Pinus hartwegii) affect their infection dynamics at two different stages of population development (colonization of new hosts and subsequent growth), and if heterogeneity in resource availability (host density and size structure) affects these interactions. For that purpose, we integrated these processes into a spatially explicit model of density-dependent population growth. 3. We found that self-regulation (density-dependence) was strong for both species; however, the intensity and sign of interspecific interactions changed depending on host size and demographic process. Population growth in Arceuthobium globosum was reduced by competition, except for smaller hosts where A. globosum growth was facilitated by Arceuthobium vaginatum. Arceuthobium vaginatum was facilitated by A. globosum regardless of host size. Colonization of new hosts by A. globosum was enhanced by previous infection by the other species, showing intraguild facilitation. 4. Demographic importance of interactions depended on stand structure: in homogeneous, low-density forests, facilitation predominates, increasing the population sizes of both species, whereas the opposite occurs in heterogeneous and dense forests. Both species achieved stable coexistence, fulfilling the invasibility criterion because each mistletoe species can invade a forest that is already infected by the other species. 5. Synthesis. Despite the fundamentally different mechanisms underlying the interactions between mistletoes compared with non-parasitic plants, our results reveal that their behaviour at the population level is similar. Stabilizing mechanisms, like strong self-limiting population growth, allow dwarf mistletoe coexistence. Interactions shift as populations develop, and they depend largely on environmental factors such as forest structure. Intraguild mutualism is shown as a relevant process for colonization of new spaces, highlighting the complexity of competitive/facilitative interactions between parasitic plants, a formerly unexplored subject. Interactions can only be fully understood when integrating all their components at the population level. Analysing these interactions may contribute to the understanding of plant-plant interactions in general, and convey interesting implications for forest management.

Infrapopulation size and mate availability influence reproductive success of a parasitic plant

“…These results, analogous to pollen addition studies that fail to find pollen limitation, provide evidence that parasites in large infrapopulations are limited by resources rather than mating opportunities (Ashman et al., ). As the fitness estimates here are measured per flower, the negative effect on fruit set of sharing a host will be magnified when competition for resources also limits flower production or plant growth rate, as has been demonstrated in other mistletoes (Queijeiro‐Bolaños et al., ). However, more mating opportunities (greater pollen receipt) may represent a fitness benefit to females even in the absence of greater offspring production when females can selectively provision offspring (Marshall & Folsom, ; Sakai, ).…”

“…However, parasite reproduction is not always limited by host resources, especially when parasites are small relative to hosts (Hughes et al., ; Weinersmith et al., ). It has been hypothesized that high levels of parasitic plant aggregation within hosts should have detrimental effects on fitness (Watson & Rawsthorne, ), but competition within hosts has largely been untested in these systems (but see Queijeiro‐Bolaños, González, Martorell, & Cano‐Santana, ).…”

Dwarf mistletoes as a relevant component in temperate forest: an integral view

“…Besides, they have a complex infection dynamics where self-regulation is strong, limiting its own population growth and allowing a stable coexist among the two species; however, both species can experience competition or facilitation with each other, depending on the size of the tree and the homogeneity of the forest, where a low-density homogenous stand may promote a facilitation process between the two species. 17 This has management implications, as it suggests that system simplification can enhance dwarf mistletoes parasitism. Dwarf mistletoe infection depends on several factors, both biotic and abiotic, where altitude and slope define its distribution, 17,18 and host availability affects their prevalence (incidence and severity) as well as an aggregated distribution pattern.…”

“…17 This has management implications, as it suggests that system simplification can enhance dwarf mistletoes parasitism. Dwarf mistletoe infection depends on several factors, both biotic and abiotic, where altitude and slope define its distribution, 17,18 and host availability affects their prevalence (incidence and severity) as well as an aggregated distribution pattern. 19 Mostly, the prevalence is strongly affected by stand structure and hosts' size, as there is a clear preference for hosts with greater diameter and crown cover as well as there is a positive correlation with pines density, being more frequent in denser stands that are in regeneration process.…”

“…18 Fire incidence seems to be a natural way to control dwarf mistletoes, and population sizes of these two species fluctuate in response to fires. 17,21 The above shows that dwarf mistletoe ecology is complex, and many factors should be considered. In our studies on the effect on pines growth, there is no significant difference between the relative diameter growth rate between the non-infected and infected trees.…”