Responses and mechanisms of a mediterranean grassland ecosystem to nutrient addition

Doutoramento em Engenharia Florestal e dos Recursos Naturais - Instituto Superior de AgronomiaGlobal changes, resulting from anthropogenic activities, are increasing precipitation variability, drought and nutrient inputs into ecosystems. These global change drivers are expected to induce changes in grassland species richness and composition and functional structure and diversity which may in turn affect ecosystem functioning. This is particularly important for the Mediterranean basin, a climate change hotspot. Understanding how these changes affect grassland structure and functioning is critical to anticipate impacts of global change, improve management actions and develop land management strategies and restoration tools to mitigate grassland degradation. Through a pot greenhouse experiment, we applied three levels of extended autumn drought and two levels of nitrogen deposition to grassland communities. The severe drought originated a shorter growing season, and led to lower net ecosystem exchange and gross primary productivity, which translated into reduced productivity. Drought induced changes in functional group proportion and delayed plant phenology. Nitrogen addition did not affect productivity, diversity or phenology. However, nitrogen interacted with the severe drought treatment to attenuate the negative effects on total carbon fluxes. A 5-year nutrient addition field experiment was also conducted. Nitrogen, phosphorus and potassium were added in a factorial way to establish three treatments of one, two and three added nutrients, including controls. Grassland productivity was co-limited by multiple nutrients and precipitation. Nutrient addition decreased species richness and interacted with climatic variability to alter functional group composition. Resilience to disturbance was not affected by nutrient addition, as resistance that decreased with nutrient enrichment due to lower species richness was cancelled out by increased recovery due to the dominance of competitive graminoids. Community functional structure was affected by nutrient addition and precipitation and, for some traits, by their interaction and was the main determinant of productivity. Functional diversity was affected by precipitation, with drought negatively affecting community functional diversityN/