Relating changes in understorey diversity to environmental drivers in an ancient forest in northern Belgium

Background and aims - A renewed interest in the functional role and dynamics of forest understorey plant communities has lead to an increasing number of publications that present the results of understorey resurveys. However, studies looking at the possible causes of temporal changes in the understorey often lack data on soil and tree layer conditions for the old survey. We investigated how changes in the tree layer, soil, and understorey were related in two contrasting forest types. Methods - Full re-inventory of the tree layer (1997, 2010) and resurvey of the soil and understorey in 40 plots on a permanent grid (1993, 2011) in an ancient temperate deciduous forest. The 1.83 ha study area consisted of a part dominated by oak (Quercus robur) and beech (Fagus sylvatica) and a part dominated by ash (Fraxinus excelsior). Key results - Overall, the basal area and shade-casting ability of the tree layer had increased. In the oak-beech forest, the soil pH had decreased; below ash, the topsoil pH had increased. The understorey species richness had increased, and the change in species richness was correlated with the change in soil pH and basal area, the latter in the oak-beech forest only. The observed patterns in understorey dissimilarity were different for the dataset including or excluding the regeneration of woody species. Conclusions - The changes in soil and understorey differed between the two forest types, and there were some indications of recovery from soil acidification in the ash forest.