Plant colonization in heterogeneous landscapes: an 80-year perspective on restoration of broadleaved forest vegetation.
Abstract
Afforestation of agricultural land has become an important issue in Europe during the past two decades. In particular, broadleaf plantations have been promoted as multifunctional forests for biodiversity, timber production and recreation. The aim of this study was to analyse how the colonization of ground-layer plants in recently planted woodlands is affected by stand age, spatial isolation, former land-use type and canopy species. Colonization was studied in 50 plantations of pedunculate oak Quercus robur and 16 plantations of European sycamore Acer pseudoplatanus on former arable land and open pastures in the Torup-Skabersjö area of southern Sweden. Twelve oak plantations and 10 sycamore plantations on ancient woodland sites were used as reference areas. Woodland species richness increased continuously with stand age in plantations contiguous with ancient woodland. The oldest plantations (70-80 years) approached the species richness of ancient oak woodland. Species richness decreased with increasing distance from ancient woodlands, and there was no significant increase in species richness between stands aged 20 and 80 years in isolated plantations. Most species that regularly colonized isolated plantations had adhesive seeds or small, wind-dispersed diaspores. Ordination analysis showed clear differentiation between former land-use categories and spatiotemporal stand categories (recently isolated, recently contiguous and ancient). No consistent differences were found in vegetation patterns in oak and sycamore stands. Woodland species richness was similar on former arable land and open pastures. Pasture plantations were characterized by the presence of more acid-tolerant species, whereas the frequency of some acid-sensitive species was higher on former fields. Synthesis and applications. This study demonstrates that new woodland should build out from cores of ancient woodland for optimal ground vegetation development. A species-rich ground vegetation can be achieved by spontaneous colonization within 70-80 years when plantations are contiguous with ancient woods. Formerly cultivated soils are generally well suited for understorey plants of forests on base-rich loamy soils as long as closed canopy conditions are maintained. However, many typical forest species are not able to disperse across open fields and their establishment in isolated stands will require sowing or planting.