Agriculture, forestry, and human settlements drive a regional gradient in forest fragmentation in the Great Lakes Basin, USA

Background/Question/Methods

From the Upper Peninsula of Michigan, southward through central Michigan into Northern Ohio, a strong gradient exists in numerous metrics related to forest cover, land use, and human domination.  A wave of settlement and industrial logging swept through Michigan and northern Ohio in the late 19^th to early 20^th centuries, leaving behind a heterogeneous landscape matrix of variable ownership patterns (including Federal and State Forests and large and small privately owned forests), patches of agriculture (often followed by abandonment and reversion to forest), and altered forest composition and age structures. Today, forest management and the expansion of low-density exurban settlements are creating additional layers of fragmentation at characteristic scales.  It is increasingly recognized that landscape structure and function must be conceptualized within the context of a coupled human-natural system.  Interdisciplinary approaches are needed to understand the causes of the complex patterns of forest cover, fragmentation, and age structures present in the region.  A wealth of digital geospatial data is rapidly becoming available, making it possible to use data-driven approaches across disciplines to investigate drivers of forest fragmentation across a range of scales.

Results/Conclusions

Along the regional gradient from Upper Michigan to Northern Ohio, we assembled a range of social and ecological datasets and converted these to spatial grid cells at 5’ (ca. 50 km2) resolution.  We used a framework of anthropogenic biomes to explore relationships among social-ecological variables and forest fragmentation.   Anthropogenic biomes varied from remote and populated woodlands in the north, to residential woodlands and croplands in the mid-section, to residential and populated croplands in the south.  Human appropriation of NPP (HANPP) varied from 0-10% in the north to 70-80% in the south.  From north to south, human-caused forest fragmentation increased steadily and tree canopy cover decreased steadily, while human population density increased and distance to urban land decreased.  Tree size class distributions provided insight into forest history and land use legacies at different locations along this regional gradient.  In a large forested expanse in the north, size class distributions reflected second-growth forest, with no stems larger than 45 cm diam.  In residential neighborhoods in the south, trees were present in size classes up to 85 cm, reflecting large, isolated trees that remain from the agricultural period prior to residential development.