Shrub-oak seedling spatial associations change in response to the functional composition of neighbouring shrubs in coastal dune forest communities

Abstract

The abundance and distribution of tree regeneration is driven by several processes acting at both the seedling establishment and survival stages. Plant– plant interactions highly contribute to regeneration dynamics through seed dispersal and competition or facilitation processes. The aim of the present study was to assess the relative importance of different functional groups of shrubs for the early establishment of oak seedlings in the coastal forest communities of south-western France. We applied spatial and non-spatial analysis methods in eleven 315-m2 regeneration plots. Within each plot, oak seedlings (height 50 cm and DBH <4 cm) were mapped to ±1 cm. The spatial distribution patterns of the oak seedlings (Quercus robur, Quercus suber and Quercus ilex) were analysed for each species while the association patterns were detected between the seedlings of the three oaks and four functional groups of shrubs (Fagaceae, Ericaceae, Fabaceae and spiny shrubs) known to have contrasting effects on other species. Oak seedling species were very often spatially aggregated at short distances (<1 m). Oak seedlings were also positively associated with shrubs of the same species within the Fagaceae group, whereas we detected negative associations with Ericaceae and no dominant significant pattern for Fabaceae and spiny shrubs. The aggregated distribution of seedlings of the three oak species was due to short-distance seed dispersal, which also explained their positive spatial associations with Fagaceae shrubs. Thus, in order to enhance oak recruitment in these ecosystems, forest managers should focus on the control of Ericaceae shrubs, while preserving Fagaceae shrubs.