Ecological forestry is an increasingly important tool for forest management and restoration efforts looking to incorporate the structures and spatial patterns created by natural disturbances into management practices. In northern Lower Michigan, jack pine (Pinus banksiana Lamb.) plantations are managed as habitat for the endangered Kirtland's Warbler (Dendroica kirtlandii Baird, KW) and have been criticized for creating homogenized stand structures, reducing natural jack pine regeneration, and excluding other threatened species. Historically, jack pine forests experienced stand-replacing wildfires on intervals of 26 to 69 years, and little is known about the impact these natural disturbances had on ground-flora composition and structure. If management plans are to incorporate ecological forestry principles, it is important to understand the disturbances and ecological processes that influence vegetation patterns and development. Ground-flora vegetation can indicate disturbance and environmental stress in an ecosystem and provide insight into ecological processes for forest managers.
Ground flora composition, structure, and diversity were examined at 12 wildfire-regenerated jack pine sites either < 8 years post-wildfire (young) or > 22 years post-wildfire (mature) in northern Lower Michigan. Percent cover of woody and herbaceous vegetation ( < 1 m tall) was collected within nested 1-m^2 quadrats, in conjunction with information of the overstory composition and structure, and fuel loadings. Semivariograms were also used to examine the spatial autocorrelation in ground-flora species richness and Shannon's Index of Diversity.
Young stands were dominated by blueberry (Vaccinium spp.) and sedges (Carex spp.), while mature stands were dominated by eastern teaberry (Gaultheria procumbens), mosses, and lichens. On average (± SD), mature stands had significantly higher species richness, with 13.4 ± 3.3 species per m^2 in young stands and 17.4 ± 4.8 species per m^2 in mature stands (P < 0.01). Gradient analyses suggest ground-flora composition and structure shift as stands develop over time, and these changes are related to changes in the overstory and fuel structure. Young stands showed strong positive correlation with seedling density and a negative correlation with overstory basal area, while the mature stands showed a positive correlation to overstory basal area. The mature stands also appeared to cluster into three groups along the first canonical axis, presumably along a productivity gradient (as indicated by the strong, positive relationship with live herbaceous cover). This suggests ground-flora composition and structure is variable within the wildfire-regenerated stands despite having similar environmental characteristics. Spatial analyses suggest that the variability of species richness and diversity is expressed at relatively small spatial scales, as the sill and range values were low for most of the sites. These results suggest that the ground flora developing in wildfire-regenerated jack pine forests can be variable and shift in terms of species dominance and functional groups over time. The changes in the ground-flora composition and diversity appear to be driven by stand structure and fuel loadings, factors influencing light availability and forest floor conditions. Restoration and management practices emulating natural disturbance patterns could enhance flora diversity and composition, as well as promote ecosystem services such as habitat for additional threatened bird species.