After sigmatism: What we learned about spatiotemporal changes in grassland communities after 10 years

In this paper we summarize our findings on grassland communities after more than ten years of research on spatiotemporal patterns of vegetation, phenology, productivity and species richness. Grassland species from the Pyrenees and Cantabric mountains were organized in guilds, and coexistence was facilitated through vegetation stratification and temporal shifts in phenology between plant guilds. Graminoids were the most abundant group, developed earlier and grew taller than forbs. Most of the forbs were rare plants and acted as fugitives. Some of them were relatively abundant either in the seed rain or in the soil seed bank, although in general reproduction by seeds was scarce. Feedback processes and spatiotemporal changes of resources at different scales, including fertility and available space by gap opening, interacted to produce community structure in subalpine grasslands. At the landscape scale, fertility was the main factor structuring grassland communities, and although it varied gradually, two separated mesic and xeric community types resulted, as defined by plant species composition. Grazing was the main biotic disturbance and acted at several scales, increasing the space available for establishment and producing changes in composition. In montane grasslands there were changes in nitrogen fluxes in vole and mole mounds, resulting in an increase of ruderals, annuals and non-mycorrhizal plants, such as Caryophyllaceae. Only in subalpine ant hills, among the biotically formed mounds, was possible to find species not present in the surrounding pasture, such as Festuca gautieri and Vicia pyrenaica, usually found on unstable slopes. In conclusion, community organization is an extremely complex process, resulting from the action of different feedbacks and stochastic mechanisms operating at different spatial and temporal scales, and no simple mechanism explains the process.