Root hemiparasitic plants are associated with more even communities across North America

Jasna Hodzic and Jonathan D. Bakker, School of Environmental and Forest Sciences, University of Washington, Seattle, WA, Ian Pearse, United States Geological Survey, Fort Collins, CO

Background/Question/Methods
Root hemiparasitic plants are distinctive because they photosynthesize yet can also parasitize host plants. In experiments, this functional group has been shown to profoundly affect their communities by reducing the growth and abundance of dominant host plants, thereby facilitating diversity. We assessed whether the presence and abundance of naturally occurring hemiparasites are associated with increased evenness and richness of their communities. Our data were collected in 129 national parks across the U.S. More than 10% of the plots (2,431 of 21,127) contained hemiparasites. We calculated richness and evenness based on all strata of plants in each plot. We assessed the effect of hemiparasite presence by pairing plots with and without hemiparasites, selecting pairs in the same ecoregion and as close to one another as possible. We evaluated the effects of hemiparasite abundance by focusing on plots that contained hemiparasites, and fitting linear mixed models with park, ecoregion and year as random effects. Finally, we ran the same presence and abundance models on null datasets derived by sampling non hemiparasitic species with the same rank abundance as hemiparasites. These null models allowed us to test whether the patterns related to hemiparasites are a function of their distributional patterns.
Results/Conclusions
Hemiparasites had stronger and more consistent effects on evenness than on richness. Hemiparasite presence (p < 0.010) and abundance (p < 0.001) were both positively related to community evenness. However, hemiparasite presence models did not differ from the null distribution (p = 0.063) though abundance models did (p < 0.001). Patterns with respect to richness were more complicated: while hemiparasite presence was positively correlated with richness (p < 0.001), hemiparasite abundance was negatively correlated with richness (p < 0.001). Additionally, neither richness presence nor abundance models differed significantly from the null (p = 0.930 and p = 0.074, respectively). Our results suggest that the impacts of root hemiparasitic plants on evenness are substantial though density dependent, but that their effects on diversity are less pronounced. This is the largest scale analysis of this type – spanning multiple types of ecosystems and many genera of North American hemiparasites. These species can serve as ecosystem engineers that impact community structure. Given this role, their conservation should be prioritized and their potential use in ecosystem management investigated.