Early establishment of fast-growing tree species on forest land and forested arable land in southern Sweden: Implications for forest diversification

Climate change and pest outbreaks are increasingly threatening conifer-dominated forests in Northern Europe, highlighting the need for greater species diversity to improve resilience. This study assessed early establishment success of six tree species: European aspen (Populus tremula), hybrid aspen (P. tremula & times; P. tremuloides), silver birch (Betula pendula), Norway spruce (Picea abies), Scots pine (Pinus sylvestris), and hybrid poplar (P. trichocarpa & times; P. maximowiczii), across seven sites in southern Sweden. Sites were categorized as either forest land (continuous forest cover >100 years) or forested arable land (former arable land afforested with Norway spruce for 40-70 years). Over three years, we monitored survival, height, and diameter growth. All experimental sites were fenced to exclude browsing. Wood ash was applied to a subset of hybrid poplars to assess its effect on establishment in acidic soils. Our results showed that hybrid aspen, birch, and European aspen had high survival and growth on forest land. On forested arable land, untreated Norway spruce, Scots pine, and hybrid poplar showed low survival, likely due to competition from dense vegetation. However, ash-treated poplar improved survival to approximately 80% and showed strong growth on forested arable sites. Principal Component Analysis indicated overall higher establishment success on forest land for most species, whereas hybrid poplar performed similarly on forest and forested arable land when wood ash was applied. These findings underscore the importance of matching species to site conditions during early establishment and provide empirical evidence to inform species selection for forest regeneration under similar site conditions in southern Sweden.