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

Abstract

Root hemiparasitic plants both compete with and extract resources from host plants. By reducing the abundance of dominant plants and releasing subordinates from competitive exclusion, they can have an outsized impact on plant communities. Most research on the ecological role of hemiparasites is manipulative and focuses on a small number of hemiparasitic taxa. Here, we ask whether patterns in natural plant communities match the expectation that hemiparasites affect the structure of plant communities. Our data were collected on 129 national park units spanning the continental United States. The most common hemiparasite genera were Pedicularis, Castilleja, Krameria, and Comandra. We used null models and linear mixed models to determine whether hemiparasites were associated with changes in community richness and evenness. Hemiparasite presence did not affect community metrics.Hemiparasite abundance was positively associated with increasing evenness of herbaceous species, but not with species richness. The associations that we observed on a continental scale are consistent with evidence that the impacts of root hemiparasitic plants on evenness can be substantial and abundance dependent but that effects on richness are less pronounced. Hemiparasites mediate competitive exclusion in communities to facilitate species coexistence and merit consideration of inclusion in ecological theories of coexistence.

Reversing expansion of Calamagrostis epigejos in Carpathian mountain grasslands by mowing and application of hemiparasitic Rhinanthus alectorolophus

“…Mowing twice was generally responsible for these community effects, while the further effect of R. alectorolophus was not significant. Still, R. alectorolophus had a strong significant effect on community evenness, implying reduced intensity of competition as reported by previous studies with Rhinanthus and other root hemiparasites (Demey et al., 2015; Heer et al., 2018; Hodžić et al., 2022). The increase of target grassland species’ proportion (but not absolute cover values) in the community was probably related to the increase of evenness, that is, the decline of dominant(s).…”

“…Rhinanthus and other root hemiparasites (Demey et al, 2015;Heer et al, 2018;Hodžić et al, 2022). The increase of target grassland species' proportion (but not absolute cover values) in the community was probably related to the increase of evenness, that is, the decline of dominant(s).…”

Common, showy, and perennial species dominate a restoration species pool

“…In contrast to our prediction, warm and cool season grass species exhibited similar commercial availability, with about a third of all species belonging to each group being available. In open grasslands, hemiparasitic species can be ecosystem engineers suppressing dominant species (usually grasses) and increasing community evenness, are often missing from restored communities, and may improve restoration outcomes when reintroduced (Barak et al 2017; Chaudron et al 2021; Hodžić et al 2022). Since half of the hemiparasites in the regional pool were available in the restoration pool, this suggests that the lack of hemiparasites in restored grasslands may be due to difficulty of establishment (e.g.…”