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Lodgepole pine (Pinus contorta) forests are widely distributed throughout North America and are subject to mountain pine beetle (Dendroctonus ponderosae) epidemics, which have caused mortality over millions of hectares of mature trees in recent decades. Mountain pine beetle is known to influence stand structure, and has the ability to impact many forest processes. Dwarf mistletoe (Arceuthobium americanum) also influences stand structure and occurs frequently in post-mountain pine beetle epidemic lodgepole pine forests. Few studies have incorporated both disturbances simultaneously although they co-occur frequently on the landscape. The aim of this study is to investigate the stand structure of lodgepole pine forests 21–28 years after a mountain pine beetle epidemic with varying levels of dwarf mistletoe infection in the Deschutes National Forest in central Oregon. We compared stand density, stand basal area, canopy volume, proportion of the stand in dominant/codominant, intermediate, and suppressed cohorts, average height and average diameter of each cohort, across the range of dwarf mistletoe ratings to address differences in stand structure. We found strong evidence of a decrease in canopy volume, suppressed cohort height, and dominant/codominant cohort diameter with increasing stand-level dwarf mistletoe rating. There was strong evidence that as dwarf mistletoe rating increases, proportion of the stand in the dominant/codominant cohort decreases while proportion of the stand in the suppressed cohort increases. Structural differences associated with variable dwarf mistletoe severity create heterogeneity in this forest type and may have a significant influence on stand productivity and the resistance and resilience of these stands to future biotic and abiotic disturbances. Our findings show that it is imperative to incorporate dwarf mistletoe when studying stand productivity and ecosystem recovery processes in lodgepole pine forests because of its potential to influence stand structure.

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.

Large vegetation disturbance rates have been reported in the "Zoquiapan y Anexas" Protected Natural Area in Central Mexico. Arceuthobium globosum and A. vaginatum coexist within this area and have a deleterious impact on Pinus hartwegii. This study seeks to understand the relationship between this disturbance and the two dwarf mistletoe species prevalent in this zone. Twenty-four plots measuring 60 × 55 m containing P. hartwegii trees were selected. Within these plots, the physical features of the land, the density of host and non-host trees, the prevalence of each mistletoe species, and six disturbance indicators were recorded. We found that A. vaginatum infests up to 47 % of P. hartwegii trees and that its prevalence is affected positively by the slope, non-host tree density, and the proportion of stump and dead trees, but is negatively affected by the prevalence of A. globosum, fi re incidence, waste deposit, and the distance to the nearest disturbance. Arceuthobium globosum infests up to 37 % of the trees and is affected positively by altitude, the density of non-host trees, waste deposit and the distance to the nearest disturbance, but is negatively affected by the prevalence of A. vaginatum and the proportion of dead trees. The prevalence of both mistletoe species within the study area is governed by the physical environment, the anthropogenic disturbance and the negative interaction between these mistletoe species. Disturbance has an important impact on mistletoe populations, modifying their prevalence; so it is primordial to understand this relationship in order to propose control methods.Key words: disturbance indicators, interactions, logging, parasitic plants, Pinus hartwegii. RESUMENAltas tasas de disturbio han sido reportadas en el Área Natural Protegida "Zoquiapan y Anexas" en el Centro de México. Arceuthobium globosum y A. vaginatum coexisten en esta área y tienen un impacto negativo sobre Pinus hartwegii. Este estudio busca entender la relación entre el disturbio y las dos especies de muérdago enano que prevalecen en esta zona. Se seleccionaron 24 parcelas de 60 × 55 m con presencia de P. hartwegii. Dentro de estas parcelas, las características físicas del terreno, la densidad de árboles hospederos y no hospederos, la prevalencia de cada especie de muérdago y seis indicadores de disturbio fueron registrados. Se encontró que A. vaginatum infesta hasta 47 % de los árboles de P. hartwegii y que su prevalencia está afectada positivamente por la pendiente, la densidad de árboles no hospederos, y la proporción de árboles muertos y de tocones; pero es negativamente afectado por la presencia de A. globosum, el área quemada, los tiraderos de basura y la distancia al disturbio más cercano. Arceuthobium globosum infesta hasta 37 % de los árboles y es afectado positivamente por la altitud, la densidad de árboles no hospederos, los tiraderos de basura y la distancia a los disturbios; pero es negativamente afectado por la presencia de A. vaginatum y la proporción de árboles muertos. La prevalencia de a...

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