Creation, colonization, and invasion of grassland gaps

Plant invasions can change ecosystem functioning and threaten ecosystem biodiversity. The aim of our study was to elucidate some of the mechanisms underlying plant invasions and related topics, such as gap creation and colonization. Because an increasing number of studies suggest that the mechanisms governing invasion (by exotic species) and colonization (by native species) are similar, we chose to use native grass species in our experiments.; A first experiment demonstrated that species with a larger leaf area per plant survived longer under a climatic extreme. Also species in which the decreasing stomatal conductance and photosynthetic rate during the stress responded less to daily fluctuations in the abiotic environment of the leaf survived longer. These results suggest that resistance to extremes is governed by other mechanisms than resistance to moderate drought. Subsequent colonization of the communities of the previous experiment showed that the zones of influence of neighbour number, biomass, and richness on target plant performance were quite similar, whereas the influence zones of parameters concerning only intraspecific interaction were larger. Although neighbourhood size increased with target size, even in the larger target species performance was related most to neighbour features within 3 to 5 cm. An experiment on invasion in synthesized grass monocultures revealed that invader germination correlated significantly with germination time, light penetration, and nitrogen acquisition by the edge plants. Invader leaf length correlated with seed mass, germination time, and light availability. There was no relationship between invasiveness and invasibility, and invasion success depended on both invader and ecosystem characteristics. Invasion in synthesized grass mixtures demonstrated that invader leaf length was positively related to light availability and negatively to neighbour biomass. A negative relationship between species richness and invasibility was found only in year 2, when increasing neighbourhood richness enhanced complementarity (demonstrated by means of Imax, an index for assessing the degree of complementary resource use), which in turn negatively affected invader growth. In year 1, such a relationship had not yet developed, probably because the communities were still young and plant interactions were limited. However, the richness effect in year 2 saturated at low diversity.