Trajectories of aboveground tree biomass accumulation and the development of forest complexity were investigated and compared between two long-term forest chronosequences of stands at the University of Michigan Biological Station (UMBS) categorized as moderately or severely disturbed. The stands experienced either cut or cut and burn disturbances over the course of the past century. Diameter at breast height, crown class, tree location, and species identification were used to characterize the stands and assess biomass accumulation. Saplings (DBH < 8.0 cm) were also counted by species, noted as living or dead, and categorized by DBH. Aboveground biomass was determined for each plot using allometric equations based on DBH measurements, and biomass accumulation over time, canopy species frequency, species diversity, and spatial and crown class distributions were compared between the chronosequences. Although there was no difference between biomass or biodiversity accumulation curves between the moderate and severe disturbance chronosequences, differences in species composition and crown class distributions indicate that disturbance severity does influence successional patterns. The results of this study demonstrate the continued importance of these forests as a part of the North American carbon sink and suggest that the successional stage of forest development would be an important factor to consider to increase the accuracy of models predicting the future capacity of this sink.
This study was completed in June-August 2014 at the University of Michigan Biological Station (UMBS) in northern Lower Michigan (45°35′ N, 84°43′ W; Figure 1). The mean annual temperature of the region is 5.5° C, with a mean annual precipitation of 817 mm. Soils were well-drained Haplorthods in all study sites. The mixed hardwood forests of around the Biological Station are dominated by bigtooth aspen (Populus grandidentata), red maple (Acer rubrum), red oak (Quercus rubra), eastern white pine (Pinus strobus), white birch (Betula papyrifera), trembling aspen (Populus tremuloides), American beech (Fagus gandifolia), and red pine (Pinus resinosa). The entire region experienced a cut and burn disturbance in 1911, and select stands experienced additional disturbances in later years. Data were collected from two parallel chronosequences of stands categorized as either moderately or severely disturbed (Table 1).
In two or three circular 0.1 ha plots within each stand (with the exception of the severe disturbance stand burned in 1998, which contains two rectangular plots of 0.14 and 0.06 ha), every tree with a diameter at breast height (BH = 1.37 m) 8.0 cm or greater was identified by species, designated by crown class (dominant, codominant, overtopped, or broken), categorized as living or dead, and measured for DBH. The distance and azimuth from plot center was also recorded for each tree. The rectangular plots that experienced the 1998 burn were divided into quadrats, and the distance and azimuth measurements for each quadrat were taken from the easternmost post on the north side of the quadrat. Saplings (DBH < 8.0 cm) were counted by species, noted as living or dead, and categorized as DHB < 2.0 cm, DHB = 2.0-3.99 cm, DHB = 4.0-5.99 cm, or DHB = 6.0-7.99 cm.
Work funded through the National Science Foundation REU grant to the University of Michigan Biological Station.