Spatial and temporal effects of land use on temperate grassland and forest ecosystems

In this thesis, I investigate the effect of land-use intensity on the spatial distribution of plant species (in grassland and forest), as well as on changes in species abundance over time within grasslands. This thesis contains five chapters. Chapter one gives a general introduction to the topic. Chapters two to four address three research questions, presenting the corresponding results and discussions in the format of scientific publications. In a final chapter, I resume the main findings and conclude the thesis. In chapter two, I looked at the direct and indirect effects of land-use intensity on the species-area relationship in temperate grasslands. The effect of land-use intensity on species richness has been largely studied, but not its effect on plant distribution at small spatial scale. To measure the plants distribution in space, I used the species-area relationship. I analysed in 147 agricultural grasslands how the land-use intensity directly or indirectly (via species richness) affected the plant distribution using structural equation models. This modelling approach is ideally suited to distinguish between direct effects of a variable from variables acting indirectly via others. I also looked at effects of the different land-use components (mowing, grazing and fertilisation). In chapter three, I studied the effects of forest management on the species-area relationship in temperate forests. Forest management affects plant communities, but the direct and indirect consequences of forest management effects on the species-area relationship in forests are still little known. Using structural equation models, I looked in 149 forest plots at the impact of forest management and forest features on direct and indirect effects via species richness on the species-area relationship. We also included diameter at breast height as a proxy of forest age and tree cover as an indicator of light availability on the forest floor. In chapter four, I assessed whether plant species trends in grasslands over 14 years can be explained by land-use intensity. Times-series looked mainly at the species richness decline over time. However, few studies looked at the abundance changes over time, particularly in a context of land-use intensity gradients. I hypothesise that land-use intensity, biotic drivers and their interactions can explain species cover trends. I thus looked at the abundance changes over time of 232 species surveyed from 2008 to 2021; plant traits, functional groups and growth form were also taken into account. Finally, in chapter five I summarised the main results found, and compared the effects of land-use intensity and forest management on the species-area relationship in forests and grasslands. I also discuss the results of our analysis on species abundance changes over time. To conclude, I have identified possible future research ideas in fields of species-area relationship in a land-use gradient as well as for further study on species abundance trends over time.