Do hemiparasites shape alpine plant community structure? An observational and experimental test with Castilleja in the Rocky Mountains, Colorado

Background/Question/Methods

For decades, plant community ecologists have largely focused on how competition and herbivory influence community structure and perhaps mediate ecosystem processes. Indeed, much of ecological theory, and what we know about how aboveground communities influence belowground processes, is derived from this perspective. But the focus on competition and herbivory has largely ignored the ubiquity and perhaps importance of parasitic plants. Globally, there are >3000 parasitic plant species from more than a dozen plant families in a variety of ecosystem types. Though much is known about how parasitic plants influence host performance, their roles in shaping community- and ecosystem-level processes remain unexamined relative to the influence of competitors and herbivores. In an observational study along an elevational gradient (2450-3260 m) near Gothic, CO we asked whether the presence of Castilleja influenced plant community structure. Additionally, at a single site at 3000m elevation, we experimentally removed Castilleja, removed comparable amounts of biomass of other species, and left some plots intact to ask whether the presence of Castilleja influenced species density, diversity, and evenness in an alpine meadow ecosystem. 

Results/Conclusions

Perhaps not surprisingly, we found no association between the presence of Castilleja and any aspect of plant community structure (P > 0.40 for richness, diversity, and evenness) in the observational study along the elevational gradient. The experimental removal study, in contrast, revealed that, over the course of one growing season, Castilleja influences alpine plant community structure. Species evenness was 30% lower  (P < 0.001) and diversity was 28% lower (P = 0.004) in Castilleja removal plots than in control or random biomass removal plots. However, species density did not differ (P = 0.47) among treatments. These results from the experiment indicate that the presence of a hemiparasitic species promotes diversity and evenness in alpine plant communities, and likely influences a variety of ecosystem functions. We suggest that hemiparasitism can have effects on the structure and dynamics of alpine plant communities, in much the same way that competition by dominant species or herbivory might.  But, at least based on the observational component of our work, these effects might not be consistent among communities.