Floodplain Hardwoods
Floodplain hardwood stands occur along river floodplains, primarily in
the southern 2/3rds of the state. Floodplain hardwood forests are dominated
by eastern cottonwood, green ash, river birch, swamp white oak, and silver
maple. Associated species include hackberry, bur oak, and black willow.
Eastern cottonwood and black willow stands are considered pioneer communities, but nearly all floodplain tree species are unable to withstand heavy shade. Small silver maple seedlings may be found in the shade, but they will not persist more than a few years without being released to abundant sunlight. Green ash and swamp white oak are the most shade tolerant species of the floodplain trees, but they also will not thrive beneath heavy shade and generally require access to an open sky above them. Box elder trees may dominate highly degraded floodplain forests, and the understory may be dominated by reed canary grass.
Floodplain tree species typically grow best on deep, nutrient rich alluvial soils with good internal drainage, but most floodplain trees can and do withstand prolonged seasonal flooding. Soils on floodplain sites are typically quite complex producing highly variable microsites across a stand. For more details on how floodplain trees and forests grow and develop, see the USDA Manual of Silvics.
The highly variable nature of the floodplain forest makes the development of a single management prescription problematic for consistent reliable results. All management prescriptions should be developed with careful consideration of the specific factors present in each individual stand by an experienced professional forester. Most floodplain hardwood stands can benefit from thinning harvests during stand development. Thinning harvests (the removal of some of the trees to promote the growth of the remaining trees) should remove the poorest quality trees and create growing space for the highest quality trees, but thinning should not reduce residual overstory cover below 80 percent. Thinning harvests should be avoided in about the last 20-to-30 years prior to any regeneration efforts so that dense overstory shade can be used to control the development of understory competition that might challenge regeneration efforts. Green ash and swamp white oak stands can commonly be regenerated using a group selection method or a shelterwood system where the overstory is removed slowly over multiple, 2-to-4, harvest entries. The other floodplain forest species can often be regenerated with a more traditional shelterwood system.
In all floodplain stands, regeneration efforts should be approached with great care. Care should be taken to ensure that harvesting of large overstory trees does not cause the local water table to rise so much that a site becomes so wet that it impedes the development of new tree seedlings. When overstory shade is removed, understory communities of reed canary grass, cattails, or sedges can explode, which can also prevent new tree seedlings from becoming established. Additionally, partial cutting on floodplain soils can increase the risk of wind damage to the remaining overstory trees. Deer browse and ice flow scouring can both also significantly damage tree seedlings.
The Thompsons have about 320 acres of degraded river-bottom floodplain forest dominated by box elder on their woodlands in Buffalo and Trempealeau counties. The Thompsons are managing these floodplain woodlands by systematically removing the box elder, by encouraging natural ash seedlings, and by planting walnut seedlings, all in an effort to restore their floodplains to a more natural and valuable condition. While they have had some challenges controlling deer browse of their seedlings, the Thompsons have successfully converted 4 acres per year every since this project has begun.
For more details on managing floodplain hardwood stands, see the DNR Silviculture and Forest Aesthetics Handbook.