Increasing occurrence of natural and human-caused disturbances threatens sustainability of forest ecosystems, their ability to regenerate and maintain biological diversity. Simultaneously, forest tree species penetrate into abandoned agricul-tural lands, the areas of which have noticeably increased during the last decades. The aim of this study was to evaluate and compare the development of regenerating forest tree communities in terms of their growth, competition and species composition in ecosystems affected by various environmental stressors such as windthrows, insect pests, fungal diseases, traditional clearcuttings and agricultural land-use legacy. Lithuania, being located in the transition between the boreal and continental biogeographical regions of Europe, has been considered a favourable research area as both conifer and broadleaved tree stands as well as mixed ones are common here. Research methods used included inventory of vegetation and assessment of quantitative characteristics and sanitary condition of six regenerating forest tree species – Scots pine, Norway spruce, silver birch, pedunculate oak, common ash and black alder – in 7–8 circular research plots (50.24 m² each) per study site with total 17 study sites. Results of the study demonstrated that forest regeneration takes place in all disturbed ecosystems, although high variation in space and time was present. Species composition of the regenerating forest ecosystems depends on those prevailing before disturbance, although, pioneer species out-compete the others. The most intensive forest regeneration takes place in those forest ecosystems which were subjected to direct human impact, i.e., clear cuttings, and less intensive regeneration, particularly, that with the target species, takes place on sites cleared of windthrows and where sanitary fellings were applied. Although non-target species depending on habitat type usually prevail on cutting sites, numbers of saplings of target species are sufficient for the regeneration of high productivity forest ecosystems and development of the new ones given the competing species will be controlled by silvicultural measures. The pioneer herbaceous species established on areas affected by disturbances affect regeneration and species composition of the future forest communities.

Keywords: ecosystem disturbance; regeneration; abandoned agricultural lands; clearcutting; sanitary fellings; windthrows.