FOUR YEARS OF MONITORING WOODY SPECIES SEEDS AND SEEDLINGS IN TWO
FOREST STANDS ON THE UPPER MISSISSIPPI RIVER FLOODPLAIN NEAR ALTON,
ILLINOIS.

Robert Cosgriff1 and Yao Yin2.  
1Illinois Natural History Survey, Great Rivers Field Station, Brighton, IL 62012. 2U.S.
Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, WI 54603.

Floodplain forests of the Upper Mississippi River System (UMRS) are characterized as having
unusually high species richness.  This has been partially attributed to the highly variable biotic
and abiotic environment that these tree species experience at the seed and seedling stage. 
Knowledge of how these species respond to and survive in a floodplain regime is generally
lacking.

Woody species seed production and seedling survivorship were monitored from 1996 to 2000 in
an oak and a maple-ash stand on Pool 26 of the UMRS.  The objective was to obtain information
on the seasonal and annual variation in woody species regeneration.  Within each stand, fifteen
subplots were randomly established.  At each subplot, a seedfall trap was erected (plastic bucket)
and a 1x1-m quadrat permanently installed.  All seeds captured within the seedfall trap were
collected and identified to the species level.  Within the quadrat, all tree seedlings were given a
unique identification tag, aged and identified to the species level.  Once a month, each subplot
was revisited, seeds counted and seedlings inventoried.  The presentation is based on a
preliminary analysis of the data collected between August 27, 1996 and December 16, 2000.

Variation in seed production was related to species, forest stand, season, and year.  Boxelder,
pecan, burr oak, pin oak, and American elm seeds were more numerous in the oak stand,
whereas, green ash and eastern cottonwood seeds were more numerous in the maple-ash stand. 
There was no noticeable difference in the number of silver maple seeds captured between forest
stands.  There was a distinct pattern in the timing of seed dispersal by species related to season
and flow regime of the river.  Boxelder dispersed seeds well in advance of spring flooding
(February-March), American elm dispersed seeds during spring flood (April), silver maple
dispersed seeds immediately following the spring flood (May-June), and eastern cottonwood in
mid-summer (July).  Green ash had a unique bimodal dispersal pattern of once following the
spring flood (June-July) and again in early fall (October).  Hard-mast species (pin oak, burr oak,
and pecan) generally had a fall dispersal pattern (October-November).  In 1998, all species,
except green ash, had distinctly lower seed production and consequently seedling recruitment. 
Low seed and seedling production in 1998 corresponds with the longest and highest intensity
flood of all the years studied.  Newly dispersed seeds appeared capable of germinating
immediately following dispersal.  While most seedlings died within two years of germinating,
survivorship increased as seedling age increased.  Silver maple seedlings experienced 85%
mortality within the first two years following seed germination.  Annual mortality dropped to
approximately 5% by age seven.  Green ash and American elm had annual mortality of
approximately 48% during the first year following germination.  Annual mortality increased to
near 75% during the second year and gradually decreased to 4% by age seven.  Silver maple
produced the most seeds and seedlings annually, followed by green ash, boxelder, and American
elm.  However, green ash dominated the age three and older age classes.  

Floodplain forests provide valuable habitat for many species of birds, invertebrates, herps, fish,
and mammals, provide organic input into the aquatic system, support a commercial timber
industry, and reduce bank erosion.  A further understanding of seedling dynamics is crucial for
developing floodplain forest management strategies to satisfy the many uses of these important
ecosystems.

Keywords: seedling dynamics, Upper Mississippi River System, floodplain forests, flood
disturbance, seed dispersal