Jordan C. Argrett

Premise: Root hemiparasites form haustoria in belowground roots of hosts to extract nutrients and water from the surrounding hosts. Through parasitic symbiosis, these plants disproportionately affect plant communities relative to their abundance by reducing host size, altering nutrient flow, and eventually increasing local biodiversity. Their prevalence and potential as keystone species position hemiparasites as important drivers of community structure. Methods: In a 2‐year study at the Rocky Mountain Biological Laboratory, we selected paired presence and absence plots for three species of hemiparasitic Castilleja across seven study sites to explore the association of Castilleja with differences in the structure of montane plant communities. Our objectives were to (1) observe the relationship between three hemiparasitic Castilleja species and plant community diversity and composition and (2) assess the host range and preference for each species in situ using direct and indirect sampling. Percentage cover of all species was recorded to assess the effect of Castilleja presence on plant community diversity and composition. To assess host range and preference and to support nearest‐neighbor and indicator‐species analyses, we excavated Castilleja individuals to observe haustorial connections. Results: Across sites, species, and years, the presence of Castilleja was consistently correlated with increased plant biodiversity and site‐specific differences in community composition. Excavations suggested that Castilleja species are generalist hemiparasites with some preference for common, dominant hosts. Conclusions: Our results identify a positive relationship between Castilleja presence and local plant community diversity and support the interpretation that these species act as generalist hemiparasites.