Meta-analysis of standing crop reduction by Rhinanthus spp. and its effect on vegetation structure

We performed a quantitative literature review on the effect of the root hemiparasite Rhinanthus on vegetation standing crop. (1) Across all available experimental studies in mixed vegetation and in pots, above-ground biomass of co-occurring species is generally reduced, with on average 40% and 60% of the value in the controls respectively. Total above-ground biomass, as the sum of parasite biomass and biomass of co-occurring species, decreases in most cases. For field experiments this reduction amounts, on average, to 26% of the control value. This implies that there is no compensation by the parasites' biomass for the loss of biomass of co-occurring species due to parasite infection. This can be attributed to the low resource-use efficiency of hemiparasites. Meta-analysis confirmed these trends. (2) In pot experiments, the negative effect of the parasite on the above-ground biomass of the host increases with the number of Rhinanthus plants. In field experiments, we found no relationship between biomass reduction and Rhinanthus density. (3) Total above-ground biomass reduction in field experiments increases with standing crop of the vegetation. However, reduction in above-ground biomass of co-occurring species seems to decrease with standing crop. Functional and species diversity buffer the community against negative effects of Rhinanthus. (4) In field experiments, functional groups are affected differently by Rhinanthus spp. Grasses and legumes are mostly strongly reduced by the hemiparasites. Non-leguminous dicots mostly benefit from the presence of Rhinanthus. (5) In one out of four weeding experiments, Rhinanthus spp. has a significant (positive) effect on species number. However, the response of plant diversity to invasion of parasitic plants requires further research.