Maple-basswood forests in southeastern Minnesota have declined as land has been increasingly used for agriculture and urbanization since the late 1800’s. Restoring these forests and maintaining existing forests helps preserve high levels of biodiversity and facilitates ecosystem resilience to environmental changes. As part of the development of the Natural Lands at St. Olaf College, Northfield, Minnesota, a forest restoration project was started in 1990 on two former agricultural fields. In order to better understand growth patterns in young trees and to increase the effectiveness of restoration projects, over 1500 trees, including original and colonizing trees, have been measured for tree diameter and height and tagged in sixteen 0.1 ha transects. The specific objectives of the study were to 1) compare tree size and mortality patterns among species over time, 2) measure seedling and sapling density (in trees less than 2.5 cm diameter), 3) measure soil physical and chemical characteristics, and 4) make projections about future changes in the forest
Results/Conclusions
After 27 years trees have diverged in size and exhibited differential mortality rates among species. Tree species with the largest diameter were northern red oak, white oak, basswood, and white ash. White ash and white oak had the highest growth rates. Overall tree mortality was 30% for the first five years and then decreased to the current average of 1.28% per year. Since time of planting, sugar maple had the highest total mortality, 55%, with bur oak total mortality at 30% and white ash at 22%. The most common species was white ash, with black walnut, maples, and oaks being the next most common. The most common seedlings and saplings were white ash, box elder and the invasive amur maple. Overall there were 18 species and the Simpson species diversity index was significantly higher (P < 0.05) in Field 1 (D=0.87) than in Field 2 (D=0.76). Mean tree diameter was higher in Field 1, but unexpectedly Field 2 had higher levels of soil moisture and organic matter. Overall trees show steady growth and soil properties such as moisture and organic matter have increased over the period of the restoration, suggesting that this restoration project is on a healthy trajectory.