Long-term effects of grazing on the plant diversity and ecosystem functioning of a Mediterranean grassland

Background/Question/Methods

Mediterranean grasslands are diverse ecosystems subjected to a long history of grazing. Changes in grazing regimes and increased climatic variability in the Mediterranean Basin are potentially affecting the diversity of grasslands and the ecosystems services they provide. We compared plant species richness and diversity and litter decomposition rates in grazed and ungrazed plots in a Mediterranean grassland.

We established five blocks of fenced x unfenced plots of 25m x 25m each in the grassland component of an evergreen oak woodland of Southern Portugal. We recorded plant species composition in spring for each of the plots, during an 8 year long period, using the pin-point method. Cover of each plant species and of bare ground was recorded by randomly positioning the pin-point 32 times in each of the plots. The cotton-strip method was used to assess decomposition rates in grazed (unfenced) and ungrazed (fenced) plots. Temperature at soil surface and water moisture were measured at 10 cm soil depth

We computed number of plant species and the Shannon-Wiener diversity index for each sampling period and used generalized linear models to compare plant richness and diversity and decomposition rates between fenced and unfenced plots.

Results/Conclusions

There were no differences between fenced and unfenced plots in the number of plant species. Plant species identity was different between fenced and unfenced plots. The Shannon-Wiener index tended to increase in grazed plots across time. Micro-environmental conditions differed between fenced and unfenced plots. Unfenced plots had higher proportions of bare ground, higher surface soil temperature and higher soil moisture at 10 cm depth than unfenced plots. Decomposition rates were also higher in unfenced plots as compared to fenced plots.

Results suggest that grazing exclusion changes the plant diversity and the ecosystem functioning of Mediterranean grasslands. Grazing exclusion will, in the long term, lead to lower grassland diversity and induce micro-environmental changes in soil moisture and temperature that will slow down decomposition rates and nutrient cycling.