Long-term trends in size-class distribution, recruitment, and mortality of trees in a hemlock-hardwood forest in northern Michigan

The old-growth forest in Sylvania Wilderness, northern Michigan, is characterized by its striking mosaic of hemlock- and hardwood-dominated patches. To better understand the patterns and mechanisms of the stand dynamics, all trees ≥5 cm DBH were monitored in four permanent plots, totaling 27 ha in size, established between 1987 and 1990. Although plot locations were selected to capture different spatial patterns of canopy trees, demographic trends were remarkably similar between plots, as well as between censuses from 1987-1990, 1993-1995, and 2006. Major demographic trends include: (1) The density of hemlock >35 cm DBH increased, while the density of hemlock <35 cm decreased. The density of sugar maple >15 cm DBH increased, while the density of sugar maple <15 cm DBH decreased. Yellow Birch density declined across all size classes.  (2) Species’ rank for mortality rate was consistent across size classes, with yellow birch>sugar maple>hemlock.  (3) The ratio of new recruits (trees whose diameter increased to ≥5 cm DBH during the study period) to canopy trees was much higher for sugar maple than for yellow birch or hemlock.  Sugar maple’s percentage of total composition increased across all size classes, with the highest increase, 8.4%, occurring for stems between 15 and 35 cm DBH. Sapling and seedling data also show disproportionately more sugar maple than yellow birch or hemlock, suggesting that the observed increase in sugar maple dominance may continue in the future.