Illinois Natural History Survey - University of Illinois

Forest Regeneration and Understory Dynamics Following the 1993 Flood on the Illinois River

Floodplain forests along major rivers are very dynamic systems whose plant species are strongly adapted to disturbance. Tree regeneration in these forests mainly occurs when major floods kill large percentages of the existing stands. A rare opportunity to study floodplain forest tree regeneration and understory dynamics presented itself in 1993 when a devastating flood occurred on the Illinois River.

Within the Sanganois Wildlife Management Area near Beardstown, Illinois, four stands were identified in which virtually all trees had been killed. These stands were underwater continuously for six to eight months, including one entire growing season. In 1995, in the second growing season after floodwaters receded, I began to study changes in species composition of tree regeneration over time and compare the species composition of the new stands of trees to that of the forests that had died. I also wanted to study species changes in the understories as the new trees grew larger and produced more shade. Recent research has shown that man-made alterations to river hydrology in the Midwest over the last 60 or 70 years has caused floodplains to flood much more often during the growing season than they did under natural conditions. This change has resulted in a change in species composition in the associated floodplain forests. A large body of research has shown that, after forests are cleared, the understories are first dominated by aggressive, fast-growing, shade-intolerant herbaceous and shrub species. Over time, these are replaced by more conservative, slower-growing, shade-tolerant species typically found under dense forest canopies.

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Floodplain forest decimated by the Great Flood of 1993.

In each of the four stands, permanent transects were established and the vegetation was quantitatively sampled every year, starting in 1995. Bark and branching patterns of the dead trees were used to make species determinations. Tree mortality in the four stands varied from 92% to 100%. Average tree diameters ranged from 8 to 12 inches and each site had a few trees 20 to 30 inches in diameter. Tree densities averaged 219 stems per acre. Although other studies have shown that smaller trees are more likely to be killed by flooding than large trees, the average diameter of live and dead trees in this study was virtually identical. It seems that when conditions are so extreme that tree mortality exceeds 90%, difference in size becomes less important.

In all four stands, the overstories were strongly dominated by silver maple and green ash (60%, 30%), with much lower numbers of cottonwood, black willow, and elm. In each stand, a dense cover of herbaceous vegetation over a meter tall has developed. Panicled aster is dominant, along with cocklebur, bur cucumber, beggar's ticks and white morning glory. Although panicled aster is common in floodplain forests, all of these are early successional, pioneer species. Therefore, after five years, a shift to lower, more shade-tolerant forest herbs has not yet occurred. However, at each site a new stand of shrub/sapling stage trees has developed. At sites 1, 3, and 4, tree species four to eight feet tall now occur at a density of 810 stems per acre. Site 2, which appears to be considerably wetter than the other sites, has a much lower density (255 stems per acre) of shrub/sapling stage trees.

In 1995, the second growing season after the flood, tree regeneration was present in the seedling stage. As one would expect, at all sites the seedlings were dominated by silver maple and green ash, the same species that dominated the overstory. However, by 1998, a drastic change in tree regeneration species composition had occurred. The dominant species are now black willow, cottonwood, and green ash (30%, 20%, 15%). The silver maple, dominant in 1995, has almost disappeared in all but site 3, where it occurs at a much

uderstory.JPG

Rampant growth in uderstory follow-ing floodplain forest die-off after 1993 flood.

reduced level. It appears that the new forests developing at these sites along the Illinois River will indeed have different species compositions than forests that became established here 50 or 60 years ago. Black willow, and possibly cottonwood, are considered to be more tolerant of wetter conditions than silver maple. Green ash, on the other hand, is thought to be similar in flood tolerance to silver maple. It is not clear why conditions at these sites were not suitable for silver maple but acceptable for green ash.

Research will continue at these sites. As the young trees begin to produce increasing shade over the next few years, it is hoped that changes will occur in the herbaceous understory species composition. Elevations will be determined for each site in order to quantify differences in hydrology. Vegetation dynamics between sites and between years will be analyzed statistically in order to more accurately look for patterns.

Allen Plocher, Center for Wildlife Ecology



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