Juniperus spp. (Cupressaceae), keystone plants across the Intermountain West, are increasing in density and distribution and prompting large-scale removal efforts. Because this control is disproportionately focused on young trees at the expansion edge, this research aims to investigate the effects of tree density and tree ontogeny as determinants of species and interaction diversity on juniper to better understand ecological consequences of large-scale, targeted tree removal. More specifically, this research aims to address the following questions: 1) Do interaction and species diversity respond similarly to increases in juniper canopy? 2) Do old-growth juniper trees support a greater abundance and richness of interactions then immature juniper trees? Using ecological plots, I quantified species and interaction diversity per unit area across a gradient of canopy cover in Nevada and Arizona. Understory plant diversity was quantified within each plot, and was used as a covariate in statistical analyses.
Results/Conclusions
As previously reported, we observed a negative relationship between canopy cover and understory plant diversity, however that relationship differed among habitat types (e.g. shrub-steppe or grassland-savannah). Both species diversity and interaction diversity decreased with increases in canopy cover, but the strength of that relationship differed between the two measurements of diversity. After controlling for biomass, old-growth juniper trees supported fewer species and interactions between caterpillars and parasitoids. In the end, juniper provides an essential resource to a diverse assemblage of highly specialized caterpillar and parasitoid species and current efforts to control these habitats may have detrimental consequences for this ubiquitous insect community.