Ecological consequences of topographic gradients in mixed-oak forests of southern Ohio

Topographic gradients and their associated microhabitat conditions have been shown to greatly influence forest ecosystem structure and function. However, the influence of these gradients on a wide number of ecosystem processes has yet to be elucidated in much of the topographically dissected landscape of unglaciated eastern North America. The goal of this investigation was to identify how topographic gradients affect a wide range of structural and functional attributes in mixed-oak forests of southeastern Ohio (USA). Specific objectives of this study were to (1) utilize dendrochronological techniques to determine how white oak (Quercus alba L.) productivity, growth trends, and response to climate varies across edaphic and topographic gradients; (2) identify the influence of topographic gradients on coarse woody debris (CWD; dead and downed boles and branches ≥10 cm in diameter) identity and distributions and compare CWD with tree and sapling distributions; (3) experimentally evaluate (through the use of wooden blocks prepared from commercial lumber) woody debris decay and nutrient dynamics (C/N) across topographic gradients for ten tree species common to mixed-oak forests; and (4) determine the influence of topographic gradients and other environmental parameters on macrofungal (basidiomycetes and ascomycetes) and slime mold (myxomycetes) communities on CWD. Results from the individual studies revealed a strong link between topographic gradients (slope aspect, position, and steepness) and the studied ecosystem parameters. Specifically, (1) white oak growing in mesic sites exhibits different growth patterns and responses to climatic variability than trees growing in intermediate and xeric sites; (2) CWD identity and loads vary considerably throughout the landscape in response to changes in topography and woody vegetation; (3) woody debris decay and nutrient dynamics are influenced by woody debris taxon and topography; and (4) macrofungal and myxomycete community richness and composition are influenced by differences in topography and CWD characteristics. Overall, topographic variability and its impact on microsite conditions have profound consequences on mixed-oak forest function, structure, composition, and diversity. Through their influence on productivity rates, forest structural characteristics, decay dynamics, and habitat suitability and utilization, topographic gradients result in a complex interplay of forest attributes and processes across the landscape.