Plant species diversity promotes erosion resistance on sea dikes

Background/Question/Methods

To date, nearly all biodiversity experiments in grassland communities focussed on the effects of diversity on biomass production. The diversity effects on crucial ecosystem services such as erosion resistance received much less attention. On dikes along the Dutch coast we found consistent positive correlations between species diversity and erosion resistance, but such correlations might be indirect due to variation in soil fertility that affects simultaneously species diversity and biomass allocation to roots. We attempted to answer the question whether diversity has direct positive effects on erosion resistance. We hypothesized such effects are due to (1) an increased complementarity of rooting patterns, or (2) an increased insurance effect. In 2010 we constructed a low dike. On the south slope we laid out 100 plots measuring 1 m². We planted these plots with 8 species that frequently co-occur in hay meadows. The experiment contained 4 diversity treatments: monocultures of each species, random combinations of 2 or 4 species and the combination of all 8 species. The diversity treatments were allocated randomly to each of the 100 plots. From May 2012 to April 2013 we measured erosion from the individual plots using belowground containers that collected precipitation water and eroded materials.

Results/Conclusions

In 2010, 2011 and 2012 we measured strong positive plant species diversity effects on aboveground biomass production. Further analysis revealed that these effects were not due to complementarity effects, but should be attributed to selection effects. Diversity had significant negative effects on the amounts of eroded mineral materials. Part of this pattern could be explained by the effects of the cold winter 2011/2012 on the Plantago and Rumex plants. At higher frequencies of these two species there was a greater risk of bare spots in the vegetation providing more opportunities for erosion. A stepwise multiple regression revealed that the frequencies of Plantago and Rumex had significant negative effects, but also diversity had significant effects if the effects of the other species were included. We conclude that plant species diversity has positive effects on erosion resistance and that this effect should be explained by the insurance hypothesis.