Marian Smith

STOCHASTIC FLOOD AND PRECIPITATION REGIMES AND THE POPULATION DYNAMICS OF A THREATENED FLOODPLAIN PLANT

Boltonia decurrens is an endangered plant restricted to the Illinois River Valley. Its complex life cycle has evolved in response to the dynamics of the historic flood regime, which has changed dramatically in the last century due to the construction of navigation dams and agricultural levees. To explore the effects of these changes, we de- veloped deterministic and stochastic matrix population models of the demography of Bol- tonia. We used periodic matrix models to incorporate intra-annual seasonal variation. We estimated parameters as a function of the timing of spring flood recession (early or late) and of growing season precipitation (high or low). Late floods and/or low precipitation reduce population growth (X). Early floods and high precipitation lead to explosive pop- ulation growth. Elasticity analysis shows that changes in floods and precipitation alter the life history pathways responsible for population growth, from annual to biennial and even- tually clonal pathways. We constructed and analyzed a stochastic model in which flood timing and precipitation vary independently, and we computed the stochastic growth rate (log Xs) and the variance growth rate (U2&) as functions of the frequency of late floods and low precipitation. Using historical data on floods and rainfall over the last 100 years, we found that log Xs has declined as a result of hydrological changes accompanying the reg- ulation of the river. Stochastic elasticity analysis showed that over that time the contribution of annual life history pathways to log Xs has declined as the contributions of biennial and clonal pathways have increased. Over the same time period, Ur2 has increased, in agreement with observations of large fluctuations in local B. decurrens populations. Undoubtedly, many plant and animal species evolved in concert with dynamic habitats and are now threatened by anthropogenic changes in those dynamics. The data and analyses used in this study can be applied to management and development strategies to preserve other dynamic systems.

Effect of the flood of 1993 on Boltonia decurrens, a rare floodplain plant

Boltonia decurrens (Torrey and Gray) Wood (Asteraceae), a perennial species confined primarily to a 400-km stretch of the Illinois River floodplain, is threatened with extinction due to the destruction of its natural habitat. Construction of a system of levees along the Illinois River has altered flood patterns during the last 100 years, converting wet prairies and natural marshes into cropland. Remaining shore habitats have been modified by altered flooding regimes. The flood of 1993 exceeded 100-year records on the Mississippi River, but was much less severe on the Illinois River. A gradient of flood severity on the Illinois River, from the area of confluence with the Mississippi River to a site 267.5 km upriver, provided an opportunity to study the effects of flooding on the vegetation at three B. decurrens sites which experienced different flood regimes. A comparison of pre- and post-flood data (1991 and 1994) revealed that the flood altered site vegetation. Species richness declined in all three study areas by an average of 34% and species diversity declined at two sites (11% and 37%). Populations of B. decurrens increased in size at all three locations (5-, 10- and 400-fold) following the flood, with the greatest increase occurring at the two sites which had the most severe flooding. The results suggest that the removal of competing species by flood waters may be an important factor in maintaining populations of B. decurrens in the floodplain.

Effect of soil texture and microtopography on germination and seedling growth inBoltonia decurrens (Asteraceae), a threatened floodplain species

Boltonia decurrens, a perennial species endemic to the Illinois River Valley, is threatened with extinction. Field observations indicate thatB. decurrens is confined to areas with sandy soils formed from alluvial deposits, but there is little understanding of the factors controlling its distribution. Plans for reintroduction of the species to suitable habitats within its historical range are hampered by a lack of understanding of the site characteristics necessary for germination and seedling growth. This study examined the effect of soil texture and soil surface features on germination and growth ofB. decurrens under greenhouse conditions. We found that both parameters are greater on sand, compared to clay, and on bare soil with either a smooth or lumpy surface, compared to litter-covered surfaces. These conditions are identical to those in whichB. decurrens has persisted along the Illinois River and may be important in determining its distribution pattern.

HABITAT CHARACTERIZATION AND GEOSPATIAL METAPOPULATION DYNAMICS OF THREATENED FLOODPLAIN SPECIES BOLTONIA DECURRENS USING A GIS

The flood pulse of the Illinois River (Illinois, USA) has been distorted and the floodplain extensively modified as a result of human activity. This activity has changed the historic flood regime from moderate, late winter-early spring flood pulses, followed by a summer drawdown period, to a chaotic hydrology with floods occurring throughout the year. Boltonia decurrens (Torrey and Gray, Wood), federally listed as a threatened species, is endemic to the Illinois River floodplain. The species occurs as a large metapopulation; however, flood regime and habitat alteration have substantially reduced the size and number of populations within the metapopulation. We developed a geographic information system (GIS) to analyze habitat and geospatial population dynamics for B. decurrens. We analyzed floodplain land cover, identified specific wetland patches supporting B. decurrens, quantified habitat availability, habitat pattern, and analyzed the distribution of populations and individuals by wetland type and flood regime for the census period 1984–2001. Our results show that floodplains of the navigation pools analyzed were dominated by row crop agriculture and forested wetlands with five land-cover types accounting for > 90% of floodplain land-cover. Distribution analyses indicate that more populations and larger populations of B. decurrens occur on nonforested wetlands than forested wetlands. Habitat has been reduced for the species, and remaining habitat is highly fragmented with reduced connectivity. Patch occupancy rates were ≤ 1% for each pool and for all pools combined. Colonization and extinction rates fluctuated widely during this study, and there were no significant differences in the means of these rates for each individual pool and for all pools combined. Although during this study, there was relative stability between colonization and extinction rates, low patch occupancy combined with habitat reduction and reduced connectivity leaves the species vulnerable to extinction.