Summary 1.We investigated effects of declining plant species richness ( S ) on resistance to extremes in grassland communities. 2. Synthesized model ecosystems of different S , grown outdoors in containers, were exposed to a stress peak combining heat and drought. The heat wave was induced experimentally by infrared irradiation in free air conditions. 3. Before the heat wave, the more species-rich communities produced more biomass as a result of a large and positive complementarity effect that outweighed a small negative selection effect. 4. Water use during the heat wave was likewise enhanced by S , which could not be attributed to dominance of 'water-wasting' species. Instead, water consumption at high S exceeded that expected from changes in community biomass and biomass composition. The observed enhancement of resource (water) acquisition under stress with increasing S therefore probably originated from complementarity. 5. Despite enhanced water use in the more diverse communities, plant survival was significantly less, affecting all species alike. Physiological stress, recorded as photochemical efficiency of photosystem II electron transport, was significantly greater. Before the heat wave, the changes in biomass composition that coincided with increasing S did not favour species that would later prove intrinsically sensitive or insensitive. 6. Complementarity in resource use for biomass production had a cost in terms of reduced survival under stress, despite the likelihood of complementarity in water acquisition during exposure. The greater loss of individuals from the more diverse grasslands suggests enhanced risk of local extinction.

Summary• An experiment with synthesized grassland communities was performed to identify plant traits that contribute to invasiveness and community traits that promote invasibility, and to study the relationship between, and the relative importance of, invasiveness and invasibility.• Eight perennial grass species were used both as invasible monocultures and as potential invaders in gaps in these monocultures. Invasion success in the establishment phase, and invader and monoculture traits were assessed.• Invasion success expressed as germination correlated significantly with germination time (invader trait), light penetration in the gaps and N acquisition by the edge plants (monoculture traits). Success expressed as leaf length correlated with seed mass, germination time (invader traits) and light penetration. Forty-six per cent of the variation in germination was explained by invader identity and 8% by monoculture identity, whereas, for leaf length, they explained 15% and 18%, respectively.• Regenerative traits (seed mass and germination time) correlated with invasiveness, and resource availability (light and nitrogen) with invasibility. The results suggest that species characteristics would largely determine the extent of an invasion event (number of seedlings), while the success of individual invaders (growth and survival) is determined by both species and ecosystem characteristics.

SummaryWe studied the effect of different ways to establish and to maintain unfertilised field margins on the development of potential weeds and seed dispersal into adjacent crops. Plant communities in field margins either developed spontaneously or were sown with different seed mixtures of grasses and forbs. Margins were mown twice a year and the cuttings were either removed or left in situ. Three years after establishment, the importance of the unsown rhizomatous species Elytrigia repens and Urtica dioica was significantly higher in the unsown community or when cuttings were not removed after mowing. Seed dispersal from the margin into adjacent crops was important in the unsown community during the first year after establishment. Between 82% and 99% of the seeds were disseminated within 4 m from the margin strip. Overall risk of contaminating the adjacent crop with weeds originating from the field margin strip was concentrated within a few metres of the crop edge. In order to minimise the dissemination of weed species and invasion by noxious vegetatively propagated weeds on nutrient-rich land, it is recommended that field margins are established by sowing and cuttings removed after each cut.

In naturally colonised species-rich grassland communities, we examined the properties of a plant's aboveground neighbourhood that affect its performance (aboveground biomass). To this end a range of neighbourhood parameters were measured: number, biomass and species richness of the neighbours, number and biomass of the conspecific neighbours, and light availability at the base of the target plant. We also determined at which neighbourhood size the strongest target plant-neighbour interactions occurred, and whether conspecific neighbours affected competitively stronger or weaker target species differently. Target plant performance varied with target identity, and was significantly affected by light availability and the number of neighbouring plants (neighbourhood density). Depending on the target species, there was also an effect of total neighbour biomass on plant performance. The target plants were most strongly affected by their neighbours within a 3-cm distance, which could account for 78% of the variance in target biomass. Number or biomass of the conspecific neighbours did not contribute to the explanation of target performance in any of the target species. Whereas in an 8-cm neighbourhood the amount of light penetration was the strongest predictor of target performance, the number of neighbours was more important in a 3-cm neighbourhood. These experimental results might be useful to extend existing neighbourhood competition models for one or two species to multispecies competition models.

Summary 1.We examined how the performance of three grass species, inserted as invaders in the gaps of synthesized communities, was affected by percentage light transmittance in these gaps and by the species richness and productivity of the surrounding neighbour plants. We also investigated whether these influences changed through time. 2. In both the first and the second season following invader establishment, realized growth (invader leaf length) was positively related to percentage light transmittance (which was the best predictor for invasibility), and negatively related to neighbour biomass, in all species examined. The richness-invasibility relationship became significant only in year 2, when increasing neighbourhood richness enhanced complementarity (demonstrated by means of I max , an index for assessing the degree of complementary resource use), which in turn negatively affected invader leaf length. In the first year such a relationship had not yet developed, probably because the communities were still young and plant interactions were still limited. However, even in year 2 the richness effect saturated at low richness, with no mixture over-yielding the best two-species mixture. 3. The data suggest that moderately productive, species-rich communities, preferably with some broad-leaved species, and with a nutrient input as low as possible, will offer the best defence against invasions.