The Decurrent False Aster: A fugitive of the Illinois River Floodplain

by Marian Smith, Ph.D.

The decurrent false aster (Boltonia decurrens) once flowered profusely along the banks of the Illinois River Valley. In the early 20th century, it was abundant in wet prairies, shallow marshes and open shores of lakes of the Illinois River. The number of populations, which fluctuates annually, has declined over the past 100 years. Since the 1970s, it has been collected only from disturbed alluvial ground and open, muddy edges of floodplain forests. Populations are now restricted to a narrow band of floodplain along a 248 mile (400 kilometer) reach of the lower Illinois River system. The decurrent false aster was placed on the federal list of threatened species in 1988, and is considered threatened in Illinois and Missouri—the only states in which it occurs.

The life cycle of the decurrent false aster is intimately linked to the annual regime of flooding with which it evolved. It is a fugitive species, always becoming extinct in one site as it is replaced by later successional species, and surviving by establishing a population at another site as a new niche opens (Hutchinson 1951). The disturbance on which the decurrent false aster is dependent is a flood of appropriate timing during early winter followed by sufficient rainfall during the growing season later in the year. Under these conditions, it colonizes and rapidly dominates the vegetation; however, without subsequent disturbance, the population crashes within three to five years.

Seeds are dispersed to new sites by floating on floodwaters. The Illinois River flows slowly and meets a hydrological barrier at the point where it joins the Mississippi River. When both are in flood stage, the result is significant back- and lateral-flow, so seed movement is possible in any direction when dispersal is unimpeded by levees and navigation structures. If suitable habitat exists in areas connected to the ebb and flow of the river, new populations are established when seeds are deposited in areas cleared by the receding flood.

 

Frequent “great floods” have been recorded on the Illinois River since 1725, e.g., at Beardstown the flood stage of 14 feet (4 meters) was surpassed 15 times from 1844–1899 (ISM 2011). In spite of the intensity of such disturbances, a variety of ancient and endemic species, including the decurrent false aster, evolved to take advantage of the natural flood cycle and thrived in the Illinois river-floodplain system. Human activities, however, have caused major changes in the timing and severity of flooding.

Although settlers in the late 1800s changed the prairie landscape in Illinois by cultivating the fertile upland areas, the wetland prairies that formed the habitat for the decurrent false aster were safe from settlement, due to their frequent flooding. By 1939, a series of locks and dams was completed on the Illinois River with the primary goal of keeping the river navigable for barge traffic. The locks and dams created a series of navigation pools (the stretch of river impounded by each navigation structure) in which water levels are manipulated to maintain the necessary channel depth. A network of 54 levee districts constructed during the early 1900s constricted the river channel, raising water levels and increasing the velocity of flow. The combination of water level manipulation and channelization has drastically altered the historic hydrologic cycle, creating chaotic flood conditions that shorten or eliminate the summer growing season for floodplain plants. In addition, many areas that formerly provided habitat for the decurrent false aster are isolated from the river, permanently inundated, or experience flooding during the growing season.

One strategy for species recovery would be to restore the river-floodplain system to pre-1900 conditions. Economic and political considerations make this impossible; therefore, complete restoration of the decurrent false aster to its former status is unlikely, and conservation efforts must necessarily focus on mitigating the effects of levees and navigation dams. Although the combination of advances in managing navigation pools and a rising concern about the effects of navigation structures on floodplain species has resulted in experimentation with a pool drawdown strategy that would provide a 30-day summer growing season for floodplain plants, this is inadequate for the decurrent false aster, since it requires approximately 120 days to complete its life cycle, from seedling to production of seeds.

The most beneficial management strategy would be to reconnect the river channel to the floodplain wherever possible, while maintaining enough patches of accessible habitat to ensure that new niches are available to sustain the normal cycle of population extinction and reestablishment. System-wide co-operative agreements among agencies and private landowners would provide the best assurance that areas would exist where new populations could colonize as populations in other areas decline. Where the river is isolated by agricultural levees, management plans could include using floodgates during the winter and spring to allow floodwaters to flow into floodplain areas behind levees, with drawdown implemented by mid-June and lasting until seeds are set in October. When a well-timed flood regime is impossible to simulate, other disturbances, such as discing or plowing, that provide bare ground suitable for seed germination and seedling establishment may offer viable alternatives. To be successful, conservation efforts for this species must look beyond attempts to maintain static, “protected” populations and make strategic use of the environmental variability to which the decurrent false aster is adapted.

Dr. Marian Smith, Distinguished Professor of Research in the Biological Sciences, Emerita, Southern Illinois University at Edwardsville, is the recognized authority on the life history, management, and recovery of the decurrent false aster.

Editor’s note: In 2011, Dr. Smith was honored by the Service as a Recovery Champion for her work with the decurrent false aster.

References

Hutchinson, G. E. 1951. Copedodology for the ornithologist. Ecology 32: 571 – 577.

ISM. 2011. http://www.museum.state.il.us/RiverWeb/harvesting/history/river/riverlevels Accessed Aug 17, 2011.

Smith, M., H. Caswell and P. Mettler-Cherry. 2005. Stochastic flood and precipitation regimes and the population dynamics of a threatened floodplain plant. Ecological Applications 15:1036 –1052.

USFWS. 1988. Endangered and threatened wildlife and plants. Determination of threatened status for Boltonia decurrens (decurrent false aster). Federal Register 53§45858-45861.