Distribution pattern of the flora in a peri-urban forest: an effect of the city–forest ecotone

We studied the spatial variation of plant assemblages in a 4383 ha deciduous forest in the periphery of Brussels. All vascular plant species were recorded following a grid-map of 1 km2-cells totally or partly covered by the forest. A total of 414 plant species was observed. Species composition and plant functional groups in forest edges and the neighbouring forest interior are studied. The study quantifies plant functional groups (e.g. ancient forest plants, true forest species, rare species, geophytes, C, S or R-strategists) in the plots and tests for relationships with the geographical position in the forest according to a city-to-forest gradient. Our results led us to identify distinct characteristics of forest flora based on the distance to the actual city border. The study revealed significant outer edge effects on the flora. Plant composition of the forest interior and forest edge can be separated by ordination on species composition, suggesting that these two biotopes have a distinct species assemblage. χ2-test and Two-Way-Indicator-Species-Analysis detected significant edge-associated species that were recorded exclusively in the edge or were the most frequent in the forest edge. We found that forest stands close to urban areas support populations of indicators of disturbance (pioneer species, C and R-strategists and aliens) to plots which are distant from the city. However, our observations also suggest that species groups with high conservation value (e.g. ancient forest species, or rare species) may also be more represented at the edge than in the core of the forest. Moreover, no forest specialists which would occur only in the interior zone were found. These observations are in disagreement with the hypothesis that true forest plants and species groups of high conservation value would be more frequent in the forest than on the borders. These findings are applicable to the management of forests in an urban context.