Impacts of hemiparasitic plants on the vegetation and biogeochemical cycling in two contrasting semi-natural grassland types

Hemiparasitic plants from the family Orobanchaceae play a key role in the conservation of species-rich semi-natural grasslands. They can alter plant community composition and diversity by both parasitism and litter effects. The drainage of resources by the hemiparasite suppresses preferred host species to the benefit of non-host species. On the other hand, hemiparasitic litter inputs enhance nutrient cycling with indirect positive effects on both host and non-host. When parasitism effects dominate, decreased host and total biomass are expected to result in an increased diversity of the non-host community. In contrast, when litter effects compensate for the biomass loss due to parasitism, minimal to no changes in diversity, minor changes in host biomass and an increase in total biomass are expected. The relative importance of the litter pathway is expected to increase with decreasing nutrient status of the ecosystem. Here we report on litter and net community effects of two native hemiparasitic plant species growing in vegetation types with a contrasting nutrient status: Rhinanthus angustifolius C.C. Gmel. favoring mesotrophic grasslands (3 sites) and Pedicularis sylvatica L. growing in oligotrophic heathgrasslands (3 sites). We first linked hemiparasitic litter nutrient returns to the net effect on biomass production; second, we assessed the net effect of both hemiparasites on species abundances and seedling recruitment; third, we studied the impact of hemiparasitic and non-parasitic litter on gross nitrogen (N) transformations in the soil; and forth, we traced N uptake from hemiparasitic litter by the vegetation. We compared nutrient returns from Rhinanthus and Pedicularis litter with nutrient stocks in the vegetation and in the soil and assessed nitrogen release rates from both hemiparasitic litter types. The net effect of the hemiparasites on aboveground biomass was studied by a hemiparasite removal experiment. We found that Pedicularis litter N, phosphorus (P) and potassium (K) returns to the soil were higher compared to Rhinanthus litter and even more so when expressed relative to nutrient amounts in the vegetation. Pedicularis litter also released more N to the soil over an eightmonth period. Rhinanthus had a negative effect on total, graminoid and forb biomass. In contrast, Pedicularis did not significantly affect aboveground biomass. These results support that litter effects are more important when nutrients are more limiting. In the same hemiparasite removal experiment, the effect of both hemiparasites on individual species abundances and seedling establishment after seed addition were studied. Rhinanthus had both positive and negative effects on individual species, while Pedicularis only had negative effects on some species. The high variation within growth forms suggested that they are not a good predictor of vulnerability to parasitism. Both Rhinanthus and Pedicularis increased seedling establishment of two of the sown species. Together, these results suggest that the effect on community composition is stronger for Rhinanthus compared to Pedicularis In a laboratory experiment, control soils, soils amended with hemiparasitic litter and soils amended with a non-parasitic litter mixture were incubated. Using 15N labeling of the ammonium and nitrate pools coupled to a numerical tracing model, we quantified gross N transformation rates. Rhinanthus and Pedicularis litter additions increased N cycling relative to non-parasitic litter additions. In contrast to expectations based on the higher N release rate from Pedicularis, Rhinanthus had the strongest effect on gross N transformations in the soil. After addition of 15N labeled Rhinanthus and Pedicularis litter to field plots, we traced 15N in the vegetation. The overall percentage of vegetation N derived from litter (NL) was not higher than 1%. In agreement with the higher effect of Rhinanthus litter on gross N transformation in the soil, the NL values were higher at Rhinanthus sites compared to Pedicularis sites. Interestingly, we found that NL values were positively related with leaf traits typical for a fast-growth strategy. Our results considerably improved our understanding of litter and net effects of Rhinanthus and Pedicularis on community composition in semi-natural grasslands. Most strikingly, short-term effects of Pedicularis litter on N cycling were smaller even though more N had returned to the soil compared to Rhinanthus litter.

Grantor dc:publisher

Ghent University. Faculty of Bioscience Engineering

Year dc:date

2013

Author dc:creator

• Demey, Andreas

Contributors dc:contributor

• Verheyen, Kris
• Boeckx, Pascal
• Hermy, Martin

• Earth and Environmental Sciences
• hemiparasitic plants
• semi-natural grassland
• species composition
• nitrogen cycle
• 15N tracing

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://biblio.ugent.be/publication/3195355
urn:isbn:9789059896024
https://biblio.ugent.be/publication/3195355/file/4336470

OAI identifier oai:identifier

oai:archive.ugent.be:3195355