PARENT SESSION
Oral Session #16: Disturbance in diverse systems: Flooding and other. Presiding: T. Stohlgren.
Monday, August 6, 2001. 1:00 PM to 5:00 PM. Hall of Ideas G.

Floodplain forest response to large-scale flood disturbance.

Cosgriff, Robert ¹, ¹

ABSTRACT- Periodic flooding is the most common disturbance event on floodplains and often determines the distribution of forest species and communities. In 1993, a long duration, high intensity flood inundated floodplain forests along Reach 26 of the Upper Mississippi River System throughout the growing season. Our study describes the impact of the 1993 flood on forest communities, community size classes, individual species, and seedling regeneration. Tree mortality was species, forest community and size specific. Mixed forest communities suffered greater mortality than maple-ash and oak. Celtis occidentalis and Quercus palustris were largely flood intolerant while Fraxinus pennsylvanica and Acer saccharinum were more tolerant. Larger diameter trees had greater survivorship than did smaller diameter trees. Seedling regeneration was forest community and species dependent. Maple-ash forest communities had an average seedling density of 365,700 seedlings/hectare while mixed and oak forests were significantly less (144,012 and 95,545, respectively). Acer saccharinum, F. pennsylvanica, and Ulmus americana were the dominant seedlings in all three forest communities. Early and late sere tree species are not likely to be dominant members of the floodplain forests of this region for considerable time. River regulation and high intensity flooding will result in a homogenous species composition and low species diversity. The lack of late sere species will also greatly reduce the availability of hard- and soft-mast and impact the number and diversity of fauna previously occupying these forest communities.

KEY WORDS: flood disturbance, floodplain forest, Upper Mississippi River, flooding