PREDICTING SEED/SEEDLING DYNAMICS: THE NON-TAROT CARD APPROACH.

Robert Cosgriff1, Yao Yin2, Theresa Blackburn3, Thad Cook4, Heidi Langrehr5, and 
Megan Moore6
1Great Rivers Field Station, Illinois Natural History Survey, 8450 Montclair 
Avenue, Brighton, IL 62012  2USGS, Upper Midwest Environmental Sciences Center, 
575 Lester Avenue, Onalaska, WI 54650  3Iowa Department of Natural Resources, 
Mississippi River Monitoring Station, 206 Rose Street, Bellevue, Iowa 52031  
4Illinois River Biological Station, Illinois Natural History Survey, 704 North 
Schrader Avenue, Havana, IL 62644  5Wisconsin Department of Natural Resources, 
Onalaska Field Station, 575 Lester Avenue, Onalaska, WI 54650  6Minnesota 
Department of Natural Resources, Lake City Field Station, 1801 South Oak Street, 
Lake City, MN 55041.

Predicting future floodplain forest composition is an arduous task that requires 
an understanding of how the annual flood pulse, various forms and degrees of 
herbivory, and inter/intraspecific competition effects seed germination and 
seedling survivorship.  A seed/seedling dynamics study was conducted from 1996-
2001 on five key pools of the Upper Mississippi River System (UMRS).  Twenty-
four species were identified on the five pools in which Populus deltoides, 
Betula nigra, Acer saccharinum, Fraxinus pennsylvanica, Ulmus americana, and 
Acer negundo were the most common species encountered.  River pool, forest 
community, species, and month were significant predictors of seed density.  Year 
was not a significant predictor of seed density.  This is due to the dominance 
of light-seeded species, which typically do not have masting years.  

River pool and germination year were significant predictors of seedling density, 
whereas forest community, germination month, and species were not.  It is likely 
that the dominance of a few light-seeded species at all of the forest sites 
played an integral role in decreasing the effect of forest community and species 
in determining seedling density.  

Although B. nigra produced the greatest number of seeds and had high 
reproductive power, this species had poor representation in the seedling cohorts 
of the sites studied (germination ranged from 0.0-0.15%).  Likewise, P. 
deltoides had no seedling germination in any of the sites studied.  The percent 
germination of A. saccharinum varied from 0.35% in the wet forests of the La 
Grange Pool to 218.2% in the wet forests of Pool 13.  Greater than 100% 
germination rates for A. saccharinum seeds may be explained by the additional 
deposition of seeds following annual flooding.  Fraxinus pennsylvanica and U. 
americana seed germination ranged from 0.0-25.3% and 0.92-85.0%, respectively.  
Hard-mast species (Quercus and Carya spp.) had much lower germination rates than 
what was expected (0.0-25.3%).  This is likely related to high mortality in seed 
crops due to predation.  

Due to low numbers of seedlings in all but five species, survivorship analysis 
was conducted for F. pennsylvanica, A. saccharinum, U. americana, Rhamnus 
cathartica, and Zanthoxylum americanum.  Seedling survival was strongly related 
to species, river reach, and day of mortality.  Fraxinus pennsylvanica, Z. 
americanum, and U. americana had greater long-term survivorship than did A. 
saccharinum and R. cathartica.  Survivorship within a species also varied 
according to forest community.  Fraxinus pennsylvanica had a greater chance of 
surviving on dry forests than wet (Chi-square=32.74, df=1, p<0.001) whereas A. 
saccharinum had a greater chance of survivorship on wet forests (Chi-
square=41.76, df=1, p<0.0001).  All river reaches other than Reach 4 and the La 
Grange Reach showed significant differences in seedling survivorship curves.  
Likewise, there were significant differences in survival curves between forest 
communities.  Dry forests had greater seedling survivorship than wet forests.  
Chance of survivorship increased as seedling age increased.  

The ability of Fraxinus pennsylvanica to survive long-term on the floodplains of 
the UMRS, coupled with abundant conspecific adults, adequate seed production, 
and adequate germination rates, indicate that this species will dominate the dry 
forests and be a major component of the wet forests.  For the same reasons, A. 
saccharinum will dominate the wet forests.  Quercus and Carya spp. need special 
attention as these species are failing to regenerate into the seedling and 
sapling cohorts.  Planting might be the only solution to preserving Quercus-
Carya forests long-term.
	
Keywords: floodplain forest, seed dynamics, seedling dynamics, Upper Mississippi 
River System