Long-distance seed dispersal is an important topic in ecology, but notoriously difficult to quantify. Previous modeling approaches have failed to simulate long-distance dispersal, and it has remained unclear which mechanisms determine long-distance dispersal and what their relative importance is. We simulated wind dispersal of grassland plant seeds with four mechanistic models of increasing complexity and realism to assess which processes and which attributes of plants and their environment determine dispersal distances. We compared simulation results of the models to each other and to data from field dispersal experiments. The more complex and realistic models predicted short-distance dispersal more accurately and were the only models able to simulate long-distance dispersal. The model comparisons showed that autocorrelated turbulent fluctuations in vertical wind velocity are the key mechanism for long-distance dispersal. Seed dispersal distances are longest under high wind velocity conditions, when mechanically produced turbulent air movements are large. Under very low wind velocity conditions seeds are dispersed farther when there is more surface heating, but never as far as during strong wind events. Model sensitivity analyses showed that mean horizontal wind velocity, seed release height, and vegetation height are crucial determinants of dispersal potential and dispersal distances. Between plant species (but not within a species), seed terminal velocity is an additional important determinant of long-distance dispersal. These results imply that seed release height is the most important plant-controlled dispersal parameter for grassland plants, and that the structure of the local vegetation greatly affects dispersal distances. Thus, management plans for grasslands should take into account that changes in vegetation structure, e.g., due to eutrophication, can reduce the seed dispersal ability of wind-dispersed plant species.

Summary Long-distance dispersal (LDD) is important in plants of dynamic and ephemeral habitats.For plants of dynamic wetland habitats, waterfowl are generally considered to be important LDD vectors. However, in comparison to the internal (endozoochorous) dispersal of terrestrial plants by birds, endozoochorous dispersal of wetland plants by waterfowl has received little attention. We quantified the capacity for endozoochorous dispersal of wetland plants by waterfowl and identified the mechanisms underlying successful dispersal, by comparing the dispersal capacities of a large number of wetland plant species. 2. We selected 23 common plant species from dynamic wetland habitats and measured their seed characteristics. We fed seeds of all species to mallards ( Anas platyrhynchos ), a common and highly omnivorous duck species, and quantified seed gut survival, gut passage speed and subsequent germination. We then used a simple model to calculate seed dispersal distances. 3. In total 21 of the 23 species can be dispersed by mallards, with intact seed retrieval and subsequent successful germination of up to 32% of the ingested seeds. The species that pass fastest through the digestive tract of the mallards are retrieved in the greatest numbers (up to 54%) and germinate best (up to 87%). These are the species with the smallest seeds. Seed coat thickness plays only a minor role in determining intact passage through the mallard gut, but determines if ingestion enhances or reduces germination in comparison to control seeds. 4. Model calculations estimate that whereas the largest seeds can hardly be dispersed by mallards, most seeds can be dispersed up to 780 km, and the smallest seeds up to 3000 km, by mallards during migration. 5. Synthesis . This study demonstrates the mechanism underlying successful endozoochorous dispersal of wetland plant seeds by mallards: small seed size promotes rapid, and hence intact and viable, passage through the mallard gut. Mallards can disperse wetland plant seeds of all but the largest-seeded species successfully in relatively large numbers (up to 32% of ingested seeds) over long distances (up to thousands of kilometres) and are therefore important dispersal vectors.

Plant species diversity in Eurasian wetlands and grasslands depends not only on productivity but also on the relative availability of nutrients, particularly of nitrogen and phosphorus. Here we show that the impacts of nitrogen:phosphorus stoichiometry on plant species richness can be explained by selected plant life-history traits, notably by plant investments in growth versus reproduction. In 599 Eurasian sites with herbaceous vegetation we examined the relationship between the local nutrient conditions and community-mean life-history traits. We found that compared with plants in nitrogen-limited communities, plants in phosphorus-limited communities invest little in sexual reproduction (for example, less investment in seed, shorter flowering period, longer lifespan) and have conservative leaf economy traits (that is, a low specific leaf area and a high leaf dry-matter content). Endangered species were more frequent in phosphorus-limited ecosystems and they too invested little in sexual reproduction. The results provide new insight into how plant adaptations to nutrient conditions can drive the distribution of plant species in natural ecosystems and can account for the vulnerability of endangered species.

In 1969 N and P fertilizer experiments were carried out on the Westerheide (The Netherlands) to investigate the growth of Calluna vulgaris. In 1981 the floristic composition of the experimental plots was analyzed. Repeated nitrogen treatment of 28 kg • ha 1 . yr I resulted in dramatic replacement of Calluna vulgaris by Festuca ovina as a dominant. Phosphorus treatments did not result in such a change. The amounts of nitrogen applied in this experiment are similar to those that will be available during the first years following the dying off of Calluna vulgaris as result of a heather beetle infestation. It is hypothesized that a heather beetle infestation alone may result in a similar replacement of Calluna vulgaris by grass species.

Summary1 Habitat fragmentation as a result of intensification of agricultural practices decreases the population size and increases the site productivity of remnant populations of many plant species native to nutrient-poor, species-rich grasslands. Little is known about how this affects the colonization capacity of populations, which is highly important for regional species survival. We studied the effects on four wind-dispersed forbs that represent two major dispersal strategies in grasslands: Cirsium dissectum and Hypochaeris radicata , which have plumed seeds and are adapted to long-distance dispersal by wind, and Centaurea jacea and Succisa pratensis , which have plumeless seeds and are adapted to only short-distance dispersal by wind. 2 Colonization capacity decreased with decreasing population size. This was due to lower seed germination ability in all species, and a lower seed production and a narrower range of seed dispersal distances in the species with plumed seeds. Inbreeding depression is the most likely cause of this. We found no evidence for a stronger selection for reduced dispersal in smaller populations. 3 Increasing site productivity changed the colonization capacity of all species. The capacity for colonization of nearby sites increased, due to higher seed production and seed germination ability, but the capacity for colonization of distant sites decreased, due to a lower long-distance dispersal ability. 4 Seed dispersal ability and germination ability were negatively correlated in the species with plumeless seeds, but not in the species with plumed seeds. The dispersal ability of individual plumed seeds remained constant under changes in population size and site productivity. This indicates a strong selection pressure for long-distance dispersal ability in these species. 5 When habitat fragmentation results in a simultaneous decrease in population size and increase in site productivity, both the local survival probability and the colonization capacity of remnant populations of wind-dispersed grassland forbs are likely to be severely reduced. This increases regional extinction risks of the species.