Plant colonization of oak plantations - the interactive effects of local environment and land-use history

Sammanfattning: In Europe, only small fragments of deciduous woodlands remain intact and species dependent on this ecosystem have declined. In southern Sweden, a shifting trend during the 20th century, with more afforestations on former arable land, resulted in new oak stands. These new stand are either on former arable fields or on continuously forested land. This thesis has its emphasis on colonization patterns of vascular plants in oak plantations and the interactive effects of local environment and different land-use history. All together 194 species were found in 44 sample plots (12- 80 years). They where groupt according to habitat preferences; true forest species, forest fringe species, semi-natural habitat species i.e. open forest, pastures and meadows, ruderal species and weeds of agricultural fields and indifferent species. Difference between land-uses did not reside in total species number but rather in the species richness of groups with different habitat preference. Many typical forest species were absent from the plantations on former arable sites, especially spring ephemeroids, while weedy species were more abundant on former arable sites. The single most important abiotic environmental factor for colonization was age of the plantation. Stand age appeared to influence all groups. Forest species and forest fringe species increase with age. Weedy and ruderal species are more sensitive to increased vegetation cover and accumulation of a deep litter layer and by that decrease with age. Species predominantly occurring in semi-natural habitats decreased as the environment become increasingly unsuitable with increasing total vegetation cover and accumulation of a litter layer on the forest floor. In situ incubation showed lower NO3- and higher NH4+ mineralization rates in continuously forested land than in former arable fields, mostly at 0-5 cm soil depth. The seasonal variation in N mineralization rates was considerable and the land-uses showed different N mineralization rates for all seasons except the autumn. Lower pH and P, and higher C content, in continuously forested sites were the most obvious differences in soil parameters. Differences in vegetation between sites are related to soil pH and though many true forest species are sensitive to acid conditions these results indicate that oak stands on former fields are potential refuges for acid-sensitive forest plants. Differences between land-use histories were also visible in the spontaneous regeneration after small-scale disturbance. Regeneration in plantations on continuously forested sites was more similar to established vegetation than regeneration in former arable sites. Acer pseudoplatanus was very abundant on continuously forested sites while lacking from sites on former arable land. Quercus robur, had better regeneration on continuously forested sites than on former arable land. In contrast, Fraxinus excelsior, Sambucus nigra and Ulmus glabra were more abundant on former arable. F. excelsior and U. glabra were regarded as indicator species for former arable sites in indicator species analysis. Anemone nemorosa, Deschampsia cespitosa, Oxalis acetosella and Stellaria holostea nearly only occurred on continuously forested sites, while Urtica dioica and Geum urbanum had higher cover in former arable sites. Richness of forest and forest fringe species was higher in plantations adjacent to existing strongly suggests that colonization in oak plantations by forest species are controlled by dispersal limitation. There was the very strong post-dispersal seed predation pressure on seeds of Crataegus monogyna, Frangula alnus, Sambucus racemosa, Sorbus aucuparia and Viburnum opulus and Carex sylvatica, especially in isolated plantations on former arable land. Colonization of recent forest by woody forest species appears to be habitat limited. In conclusion, my results indicate that stand age and distance from existing forest are more important to the number of forest species in new plantations than habitat quality parameters.