Insights on the identifcation and characterization of interactions in grassland ecosystems

Abstract

Grasslands are one of the most valuable and biodiverse cultural landscapes in Europe. Among all European grasslands, mountain grasslands are especially susceptible to the climate and land use changes that global change involves. In this thesis, we address one of the aspects most neglected when considering the effects of global change on biodiversity and other ecosystem functions: the interactions between factors, carrying out five studies that intend to identify and characterize interactive ecological processes occurring in grasslands. The first two studies were focussed on plant diversity, using two databases (the PASTUS database, compiling data from grasslands of the Pyrenees, and a database gathering information about the grasslands of England). We found that climate can enhance negative effects of fragmentation and lack of connectivity of plant taxonomical diversity in the Pyrenees (measured as species richness in 100 m2), and that livestock management may modify the impact of patch disaggregation and fragmentation. Similarly, we found that soil abiotic properties (pH, and nitrogen (N) and phosphorus (P) content) but not climate were the main environmental factors explaining variation in grassland plant functional diversity (measured by six different indices) of English grasslands, with a significant contribution of soil nutrient stoichiometry (N/P ratio). Furthermore, the relationship between taxonomic and functional diversity can often be mediated by soil abiotic factors like pH clay content or N/P ratio. The following pair of studies used the PASTUS database to identify factors affecting soil organic carbon (SOC) stocks in grasslands. In the first one we found that temperature seasonality (difference between mean summer temperature and mean annual temperature; TSIS) was the most important geophysical driver of SOC in our study, interacting with topography and management. Concerning biochemical SOC drivers, we found unexpected interactive effects between grazer type, soil nutrients and herbage quality. In the second study about SOC, we found that legume effects on grassland SOC vary depending on legume proportion consistently across broad spatial scales. SOC increased with legume proportion up to between 7-17%, then decreased. These effects were strengthened when grasses and forbs were co-dominant. Grazing intensity modulated grass proportion effects on SOC, being maximum at relatively high intensities. The last study addressed a phenomenon that drives heterogeneity in grasslands at detailed scales: the fairy rings, which promote circular greening belts in the vegetation. We characterized the structure of soil microbial communities of six fairy rings by phospholipid fatty acid (PLFA) profiling in a montane grassland in the Pyrenees, studying the correspondence between soil properties and changes in soil bacterial and fungal PLFA patterns. The most relevant result was that gram-negative bacteria where associated with the zones outside the ring, with the highest exchangeable K content, whereas the relative abundance of gram-positive bacteria did increase inside the ring-affected zones. Observing these studies and previous works overall also allowed the extraction of some interesting conclusions. For instance, TSIS was highly relevant for both plant diversity and soil organic carbon, always in interaction with other factors. Additionally, not only climate and habitat fragmentation interacted to drive plant diversity and structure in grasslands, but grazing and but grazing and soil nutrients (which can be affected by management), might play an important role in those interaction processes, and should be considered in further studies. Combining our results with information from the literature, we can assume that the four properties of plant communities studied in this thesis, plant taxonomic and functional diversity, plant guild proportions and herbage quality, are related with SOC, and in the last two, this relationship was mediated by grazing species. Considering that according with our results and previous evidence, grazing species drives these features of plant communities, we posit that grazing species could affect SOC modifying multiple characteristics on plant communities, like functional and taxonomic diversity or herbage quality, and through other processes like their different trampling and deposition spatial patterns. A detailed observation of the literature about local spatial heterogeneity in general and fairy rings in particular suggests that studies about fairy rings comprising several locations with varied climate, topography or management scenarios could yield some informative results.