Catastrophic disturbance in the presettlement forests of the Upper Peninsula of Michigan

The General Land Office (GLO) survey notes (1840-1856) were used to examine the interaction among natural disturbance, vegetation type, and topography in the presettlement forests of the Luce District, an ecological unit of approximately 902 000 ha in the Upper Peninsula of Michigan, U.S.A. The surveyors recorded 104 fire and 126 windthrow incidences covering 3.1 and 2.8% of the total length of the surveyed lines, respectively. The rotation periods over the entire landscape were 480 years for fire and 541 years for windthrow, but these varied with vegetation type and topographic position. Fire occurred more frequently on southerly aspects and at elevations where pinelands were concentrated. The density of windthrow events increased with elevation and slope, with the highest occurrence on westerly aspects. Based on the estimated rotation periods, we calculated that 7.5, 24.4, and 68.1% of the presettlement forest were in the stand initiation, stem exclusion, and old forest (including both understory reinitiation and old growth) stages, respectively. Pinelands and mixed conifers were the major components in both the stand initiation (34.5 and 31.1%) and the stem exclusion stage (20.9 and 39.8%), while mixed conifers (39.3%) and northern hardwoods (34.7%) were the major old-forest cover types. The diverse mosaic of various successional stages generated by natural disturbance suggests a "shifting-mosaic" landscape in this region.