Tree Growth Patterns, Mortality, and Colonization in a Restored Maple-Basswood Forest

With the increased interest in forest restoration, many projects include goals to increase biological diversity and enhance ecological services but provide little information about results of species composition over time, tests of ecological concepts, and resulting management guidelines. The aim of this project was to study maple-basswood forest restoration in former agricultural fields in what was part of the Big Woods landscape in southeastern Minnesota, USA. Tree seedlings were planted in two adjacent fields in 1990 at a density of 1024 trees/ha and measured every two–four years for twenty-three years. Colonizing trees were also measured in 2013. Quercus rubra (northern red oak), Tilia americana (basswood), Quercus alba (white oak), and Fraxinus americana (white ash) were the largest trees (mean DBH [diameter at breast height] of 9–12 cm) and Acer saccharum (sugar maple) and Juglans nigra (black walnut) were smaller with a mean DBH of 4.5–6 cm. The overall tree mortality of 35.8% since 1990 has been balanced by a nearly equal number of colonizing species from nearby forest fragments and internal seed production. Colonizing species, with F. americana and Acer negundo (boxelder) the most common, led to increases in species diversity. While A. saccharum and T. americana currently make up less than 10% of the individuals, they are expected to increase in frequency over time. The restored forest area can be characterized as an early successional maple-basswood forest in which priority effects and microhabitat variation support a diverse forest likely to survive future changes in climate and invasive species.