Long-term Study of the Magney-Snively Old-Growth Northern Hardwood Forest, Duluth, Minnesota, USA

Abstract

Point-quarter measurements of trees and saplings made in a putatively old-growth Minnesota northern hardwood forest in 1961, 1980, and 2016 were analyzed by using plotless density-estimation methods, including a modified method that we developed. Stem densities of trees in 1980 and 2016 were higher than in 1961, and sapling stem density was much higher in 2016 than in each of the earlier years. Basal-area densities for both trees and trees-plus-saplings did not change significantly among years, but were higher for saplings in 2016 than in each of the other two years. Stem density and basal-area density of sugar maple increased in the tree stratum through time. The sugar maple tree population had a demographically stable size structure in each year. Sugar maple dominated the sapling stratum, increasing with time in relative densities and very strongly in absolute densities. Over the same time span yellow birch abundance declined in the tree stratum and mountain maple seemed to decline in the sapling stratum. Yellow birch mean individual size became larger with time. Basal-area densities estimated by the angle-gauge method in 2017 found standing-dead values to be about 8% of the live-stem values, with similar species compositions. Fallen large boles near the sample points in 2016 were more likely to be yellow birch than the large live trees were; in contrast, sugar maple was a smaller share of fallen large boles than of large live trees. Boles tended to have fallen toward the south, which did not match the directions in the record of strong wind gusts at Duluth International Airport. This old-growth ecosystem may hold at least 230 metric tons of organic carbon per hectare. Stem cores from red oaks showed little evidence of contamination from a steel mill that operated nearby prior to 1980, but have a somewhat elevated content of manganese. The Magney-Snively forest is an important complement to the other old-growth northern hardwoods remnants in the Lake Superior region, which differ from it by having a sizable representation of eastern hemlock.