Résumé
Do contrasting biotic contexts in nutrient-poor grasslands affect the predictability of invasion by exploitative species following fertilization? French Alps. We examined community responses after 2 years of nutrient addition for two nutrient-poor European calcareous grasslands, a mesoxeric community dominated by the short bunchgrass Bromus erectus and a mesic community dominated by the tall rhizomatous grass Brachypodium rupestre. We also performed reciprocal transplantations of these two dominant slow-growing species and Arrhenatherum elatius, a tall fast-growing grass that dominates nutrient-rich communities and is likely to invade nutrient-poor communities after fertilization. Transplants were grown with or without neighbors, in order to measure their individual responses (without neighbors) and competition intensity (by comparing performances with and without neighbors using the Relative Neighbor Effect index - RNE) during one growing season in all three communities. In the Bromus community, fertilization induced a strong increase in fast-growing grasses (including A. elatius). Competition intensity was low for the three transplanted grasses, but strongly increased with resource addition, to reach values observed in the Arrhenatherum community. In the Brachypodium community, no change in competition intensity with fertilization was detected, because of the high mortality of the two "non-resident" species, irrespective of the presence of neighbors. Community responses to nutrient improvement are context-dependent and vary as a function of the biotic environment. Soil processes are proposed as the main drivers of community resistance to the invasion of fast-growing species in the mesic, nutrient-poor grassland dominated by the large conservative competitor B. rupestre.