Floodplain Forest Response to Large-Scale Flood Disturbance

Abstract

Flood disturbance is the principal factor influencing species composition and distribution in floodplain forests. It has been widely assumed that these floodplain species have developed the ability to survive different magnitudes of flooding. However, research on the effects of large-scale flooding is lacking. Floodplain forests on Pool 26 of the Upper Mississippi River System, suffered high mortality following a 1993 flood disturbance. The degree of mortality was dependent upon tree species, forest community and tree size. Mixed forests suffered greater mortality in trees and saplings than did the maple-ash and oak forests studied. Celtis occidentalis and Quercus palustris were largely flood intolerant while Fraxinus pennsylvanica and Acer saccharinum were more tolerant. Larger diameter trees had better flood survivorship than did smaller diameter trees. Tree seedlings and lianos were intolerant of the flood event. Seedling regeneration was forest community and species dependent. Maple-ash forests had a seedling density of 365,700 seedlings/hectare while mixed and oak forests were significantly less (144,012 and 95,545 seedlings/hectare, respectively). Acer saccharinum, F. pennsylvanica and Ulmus americana were the dominant seedlings of the three forests communities. High canopy, overstory and understory covers reduced seedling density. Openings in the canopy and silt deposition initiated a tremendous response in the herbaceous strata. Annual forbs, grasses and grass-like plants responded favorably to the flood event, whereas, biennial and perennial forbs were largely lacking. Bidens aritosa was the dominant herbaceous plant occurring in over 75% of the quadrats sampled and having an average cover of 36%.