Research Header

Southern Wisconsin Upland Forests

Rogers, D. A., T. P. Rooney, D. Olson, D. M. Waller. 2008. Shifts in southern Wisconsin forest canopy and understory richness, composition and heterogeneity. Ecology 89(9). 2482-2492.

Upland Forest

Although the replacement of oak (Quercus spp.) forests by more shade-tolerant species across Eastern North America is widely appreciated, little is known about its impact on understory species. We re-surveyed the under-and over-story species composition of 94 undeveloped stands in southern Wisconsin in 2002-2004 to assess shifts in canopy and understory richness, composition and heterogeneity relative to 50 years ago. Tree composition has shifted away from oaks (Quercus spp.) towards more mesic and shade-tolerant species (primarily Acer spp.). Most sites (80%) experienced declines in understory native species richness with mean species density declining 25% at the 1 m² scale and 23.1% at the 20 m² scale. Rates of native understory species loss are much greater in late-successional stands with conspicuous declines in remnant savanna species and those with narrow leaves. Initial overstory composition predicts changes in the understory better than changes in the overstory. Despite declines in absolute abundance, woody species have increased 15% in abundance relative to herbaceous species in the understory. Exotic species that occurred in 13 stands in 1949-1950 representing 5.5% of the flora now occur in 76 stands and account for 8.4% of the species present. Gains in exotic species richness and abundance are not closely linked to declines in native species richness or community heterogeneity. Although canopy succession has clearly influenced shifts in forest understory composition and diversity, these results suggest that understory dynamics are becoming decoupled from overstory dynamics as landscape effects start to play an increasing role.