Dracula and Robin Hood: Hemiparasites play both roles

Although hemiparasitic plants have been found to strongly influence community structure, their impact on ecosystem functioning is not well studied. Hemiparasites may exert both positive (Robin Hood-like) and negative (Dracula-like) effects, and this may depend on the level of organization (host, community, ecosystem) addressed. In the dry meadow alpine tundra of Niwot Ridge, Colorado, we examined effects of hemiparasitic Castilleja occidentalis on community and ecosystem processes. At the community level, plots containing Castilleja only slightly differed in composition and did not differ in diversity compared to adjacent plots without the hemiparasites. Given the weak effects at the community level, we were surprised to find more substantial effects on ecosystem processes. Although soil pools of inorganic N did not differ between plots with Castilleja and the surrounding tundra, soil moisture was lower and aboveground biomass was almost twofold greater. The difference in biomass was due to the production of Castilleja; biomass of the non-Castilleja portion of the community did not differ between the two communities. Foliar N of non-Castilleja vegetation was higher in association with Castilleja. We speculate that Castilleja facilitates nutrient cycling in the alpine tundra by accelerating decomposition of the co-occurring species. Accelerated nutrient cycling may alleviate the direct cost of parasitism to the co-occurring species. While hemiparasites may be more like Dracula in their direct effects on hosts and soil moisture, their effects on nitrogen cycling may be more analogous to Robin Hood, robbing nitrogen from the “rich” and giving to the “poor” co-occurring species.