The aim of this study was to assess the impact of isolation on forest bird communities in agricultural landscapes in The Netherlands. We studied the avifauna of 235 small (0.1-39 ha) woodlots composed of mature deciduous trees in [1984][1985]. These woodlots were selected in the eastern and central/southern part of the country within 22 regions showing great differences in landscape structure, i.e., degree of isolation. Multiple regression analysis indicated that woodlot size was the best single predictor of species number and probability of occurrence of most species. It turned out that the isolation variables, area of wood, number of woods, interpatch distance, and proximity and density of connecting elements, explained small but significant parts of the residual variances in species number. No single species was significantly affected by the density of connecting elements. Biogeographical differences between two groups of regions were emphasized. Evidence of four woodland species suggested that regional abundance affected the probability of occurrence in small isolates.

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The dispersal capacity of six perennial grassland species with different seed aerodynamic attributes was assessed in a wind tunnel. The selected species have difficulty in recolonizing restored ecosystems because of a poor dispersal of seeds. The variation in dispersal distances of seeds within and between species was assessed by releasing seeds at varying wind speeds and release heights, and expressed as 1st percentile, mode, and 99th percentile values. Dispersal distances of long-range dispersed seeds (99th percentile values) increased exponentially with wind speed. At wind speeds of 14 m/s, predicted maximum distances are 10–15 m for small and relatively heavy spherical seeds and 20–30 m for large and relatively light cylindrical or disk-like seeds. In the study area, wind gusts > 10 m/s at plant height occur at least annually, and plants of the selected species live up to several decades. This suggests a great potential for long-range dispersal during the lifetime of a plant. Plants may gain wider dispersal of seeds by increasing the release height (e.g., taller infructescences) and by requiring stronger winds to release seeds (e.g., dispersal in autumn and winter). Keywords: dispersal, wind tunnel, seeds, perennial, wind gust.

A b s t r a c tThe vegetation of a 150 ha coastal dune area in the S W Netherlands was carefully analyzed and mapped at a scale of 1:2500 in 1959 and 1980. A b o u t 600 relev6s were treated numerically. 127 vegetation types were distinguished, the overlap in community composition between the two years was very small. An intermediate level between this community level and the level of the structural-physiognomic formation was adopted. 56 so-called subformations, in which floristic and structural characters are combined, have been recognized and the overlap was then considerably larger, at least in number of types. Ordination results suggest that in each subset: grasslands, dune slacks, woodlands + scrubs, the first axis reflects floristic differentiation along with progression. The ordination of the entire material shows moisture as the main factor underlying the first axis, zonation as the second and nutrient status as the third factor.Change in vegetation was detected through a network overlay with 2 300 points for each of which the transition between 1959 and 1980 was scored.The pattern of change between 1959 and 1980 is apparently multiple pathway in type. This would be one of the few examples of such a pattern to be established on the medium term level of change, which is considered as succession sensu strictu. On an areal basis the change in vegetation is extremely large; even on the subformation level only 15% of the area was found to be unchanged, and on the community level only 4%.The overall trend is progression with a strong increase in the area of tall scrubs and woodlands. Retrogression also occurs, partly as a result of disturbances such as fire, partly due to local death of a dominant w o o d y species. This latter p h e n o m e n o n is interpreted against the background of the starting point of the succession; a sudden release of the inner dune area from overgrazing by cattle and a subsequent rapid dune development (outer dunes) on the beach in front of the inner dunes.Both species and community diversity increased, which is related to both differentiation under progression and to retrogression.