Upland forests, riparian floodplain forests and stream ecosystems are considered to be the structural and functioning components of forested watersheds. The interactions and connections among these components represent an important linkage between terrestrial and aquatic environments in terms of energy flow and mass biogeochemical cycling. We investigated the tree species composition, density and basal area of floodplain and upland forests in the Thorn Creek watershed located in northeastern Illinois. The number, size and basal area of snags (standing-dead wood) and coarse woody debris (down-dead wood) as well as the advancement of their decomposition, and their orientation and position were compared between the two forest types.

Results/Conclusions:

The results showed that white oak (Quercus alba), red oak (Q. rubra) and basswood (Tilia americana) were the most important tree species in the upland forests, while Sugar maple (Acer saccharinum), white ash (Fraxinus americana) and hawthorn (Crataegus viridis L. ) made the greatest contribution to the floodplain communities. On average, stand density of living trees (DBH > 6.0 cm) was higher in uplands (681.3 tree/ha) than in floodplains (620.0 tree/ha), but basal area of the living trees was larger in floodplains (33.3 ± 16.1 m2/ha) than in uplands (30.4 ± 7.5 m2/ha). The number of snags was nearly significantly higher in uplands (60.0 snags/ha) than in floodplains (42.5 snags/ha) (P = 0.08). No significant difference was found in basal area of snags between uplands (3.2 m2/ha) and floodplains (3.3 m2/ha). The number and volume of coarse woody debris in floodplains were 267.5 pieces/ha and 30.6 m3/ha, respectively, which were slightly higher than those in uplands (252.5 pieces/ha and 29.2 m3/ha, respectively). More than 65% of coarse woody debris was found in an advanced stage of decomposition in both floodplain and upland forests. The proportion of snags in an advanced decomposition state was less than 15% in both of the study sites. The structural differences between the two forest types are expected to affect the functional processes in the watershed ecosystem.