Subordinate plant species enhance community insurance to drought in semi-natural grasslands

Background/Question/Methods

According to the insurance hypothesis, more diverse plant communities have higher probability to be more resistant to drought. While many experiments have been carried out to determine the effects of plant diversity on plant community insurance, the results are still contradictory. Here, we conducted a drought experiment where we tested if independently of diversity, the presence of drought-resistant subordinates species increases plant community insurance. In semi-natural grassland, we combined a removal experiment of subordinate plant species with a summer drought event using rainout shelter during two months. For true control of shelter effects, we established roofs for both control and drought plots, with water collected from the roof added back to the control plot. Plant community composition was determined after the drought and based on community biomass measurements, we estimated also resistance, recovery and resilience of plant community for each combination of treatments (removal x drought). Moreover, to assess the drought impacts on water use efficiency of plants, we analysed carbon isotope ratios (δ13C values) in plant leaves of two dominant and two subordinate species collected at the end of the drought period.

Results/Conclusions

Our results showed that subordinate species seem to be more resistant to drought and increased community resistance by enhancing their aboveground biomass during the drought period. These outcomes are also linked with a decreasing competitiveness of dominant species whose biomass decreased during drought. The significant increase of δ¹³C values in plant biomass under drought indicated a better WUE for all plant species but interestingly, the WUE was significantly higher in plots where subordinates were removed.  Recovery and resilience were not affected by the summer drought event but the absence of subordinate species significantly reduced overall aboveground biomass in both watered and drought plots. Our results provide further evidence for the diversity-stability hypothesis underlining the role of subordinate species during drought events and, more generally, for the functioning of grassland ecosystems.