Interactions among forest composition, structure, fuel loadings and fire history: A case study of red pine-dominated forests of Seney National Wildlife Refuge, Upper Michigan

Red pine (Pinus resinosa Ait.) has been a historically important species in the eastern United States and Canada. Prior to European settlement, fire played a major role in determining the composition and structure of red pine-dominated forests. However, fire suppression efforts have prevented natural regeneration of red pine and the development of structurally diverse red pine-dominated forests across its natural range. To better understand how past forest history affects the current state of red pine-dominated forests in Upper Michigan, we quantified the role of forest history on forest structure and fuel loadings on eighty 500m² plots distributed across Seney National Wildlife Refuge (SNWR). The Seney Wilderness Area of SNWR has experienced few direct human effects, and has escaped the impact of fire suppression policies. Using principal components analysis, we quantified the variation in stand composition, structure, and diversity and related this variability to current fuel loadings, fire history, and harvesting history. The first principal component represented the structural and compositional variation across our dataset associated with the harvesting history of the 50 sampled stands. Stands with a history of cutting clearly differentiated themselves by high abundance of jack pine (Pinus banksiana Lam.) and lower structural diversity of the overstory and of the understory vegetation. The second principal component revealed a negative correlation between red pine overstory abundance and compositional diversity of the stands, as determined by the Shannon Diversity Index. The third principal component differentiated complex, multi-cohort stands with old (250-300 years) trees in the overstory, which we believe resulted from repeated fires, from younger, less structurally complex stands. Stands which experienced repeated fires showed reduced amounts of fine woody debris and shallower duff depth. No relationship was found between descriptors of fire history and the amount of coarse woody debris (CWD) or litter. CWD increased in multi-cohort stands with high variations in tree diameter, whereas litter depth was higher in both jack pine-dominated, harvested stands and structurally diverse red pine stands and lower in the middle of this gradient. We suggest that fire has a role in restoration programs and sustainable management of red pine-dominated forest ecosystems of this region.