PS 27-118
Changes in tree growth patterns and species composition over 23 years in a maple-basswood forest restoration
Less than 10 percent of the maple-basswood forest type endemic to south-central Minnesota USA remains after land conversion for agriculture, urban development, and logging. Consequently many species native to this region have lost their habitats and experienced population declines, ultimately threatening the structure of entire communities. Awareness of human-induced damage to forest ecosystems has made restoration efforts more prevalent and raised unanswered questions about the design and maintenance of successful restoration projects. A forest restoration was started on former agricultural lands at St. Olaf College, Northfield, Minnesota in 1990 to better understand patterns of growth in young trees and to increase the effectiveness of restoration projects. Over 1000 tree seedlings were tagged in sixteen 0.1 ha transects. Specific objectives of the study were to 1) track tree growth and mortality patterns by measuring tree diameter and height of all previously measured trees, 2) examine natural regeneration patterns by measuring new trees over 2.5 cm diameter, 3) document changes in soil nutrient characteristics over time, and 4) provide information useful to future restoration projects.
Results/Conclusions
Over 23 years tree sizes have diverged with differential success among species. Of the future canopy trees northern red oak, American basswood, white oak and white ash had the largest diameters while sugar maple and black walnut had the smallest. Overall mortality was 33.2% with variation from 55% in sugar maple to 17.7% in basswood. Mortality decreased to 3.4 % in the last three years as trees have become larger. The more frequently established new trees were white ash and box elder, both fast-growing, shade-intolerant species found in surrounding forested areas. The combination of original and new trees showed white ash, black walnut and bur oak were the more common species (all more than 10% of the population) in the restoration. Soil organic matter and nutrient levels were lower than in surrounding forests but have increased over the period of the restoration. The small size and high mortality of sugar maples suggest that in future restorations, sugar maple should not be planted until some shade is available. Although the canopy is starting to close over in places, the slow growth of sugar maple means that the restored forest will require many years to resemble a mature fully restored maple-basswood forest.