Differential growth responses of mature trees and seedlings to a gradient of atmospheric nitrogen deposition

Background/Question/Methods

Temperate forest ecosystems around Chicago land are characterized by their dominance of oak-maple communities. These systems have been exposed to large inputs of atmospheric nitrogen (N) deposition for the last 4 decades. We have been studying various facets of ecosystem dynamics, including a possible shift in species composition along a gradient of N deposition in this region.

Results/Conclusions

The results reported here focus on the growth responses of seedlings and mature red oak trees at the two ends of a natural N deposition gradient. We used both dendrometric and tree rings to assess growth of mature trees, while whole plant biomass was used to assess seedling growth during the period between 2001 and 2006. Tree growth was inhibited by about 30% from the low deposition site at Cook County, IL to high deposition site in Porter County, IN. Along the same gradient, seedling growth was reduced by about 71%. Additional work indicated that seedling growth in sugar maple also decreased with increased N deposition, but the reduction was not as dramatic as in red oak seedlings. Taken together, the data suggests that N deposition is a considerably more effective agent of change in forest ecosystems through its effect on seedlings and the seedling establishment phase of development than by its effect on mature trees. During these early establishment phases, however, N deposition could affect species composition by having a differential effect on red oak and sugar maple.