18 YEARS OF CHANGE IN AN ILLINOIS STREAMSIDE DECIDUOUS FOREST

The vegetation structure of the streamside forest along Hickory Creek in northeastern Illinois was recorded five times during the period 197 7 to 1995. In 1977, lowland parts of the forest subject to frequent fl ooding were dominated by Fraxinus pennsylvanica, Crataegus mollis and Ulmus americana, gradually progressing vertically to an unflooded upla nd forest dominated by Quercus rubra, Crataegus mollis and Acer saccha rum. Over the 18 years of recording, the lowland, flooded section of t he forest showed replacement of Crataegus mollis by Ulmus americana. I n the unflooded upland, Crataegus mollis was generally replaced by Ace r saccharum. Mean tree diameter growth over the period was 2.78 mm yr( -1) and mortality 3.10% yr(-1). In general species with low rates of g rowth had high rates of mortality. Of the more common species, annual rates of mortality in Crataegus mollis, Fraxinus pennsylvanica, Ulmus americana and Acer saccharum were 7.0, 3.3, 1.8 and 0.5, respectively. Total tree (<4.0 cm dbh) density decreased from 950 to 753 trees ha(- 1), stand basal area increased from 28.99 to 33.49 m(2) ha(-1), while the biomass increased from 227 to 271 tons ha(-1). Tree species richne ss and diversity decreased, and dominance increased over the period of recording. Biomass in the most heavily stocked study plots tended to follow the ''-3/2 thinning line'', indicating that density dependent s aturation of the sites is beginning to occur. The overall size-class s tructure of the forest followed the negative exponential model at each sampling, indicating constant rates of mortality with age, but specie s specific population patterns were apparent. Significant exponential curves for Ulmus americana were also recorded for all sampling years. In Crataegus mollis and Fraxinus spp., however, the number of saplings recruited into the smaller size classes diminished as time progressed and the exponential model was no longer appropriate in later sampling s. Acer saccharum populations tended to be skewed in favor of the smal l size classes. Species of Quercus showed a predominance of individual s in the larger size classes. The observed patterns of species change, biomass saturation and negative exponential fits of populations of lo w-light tolerant species in the Hickory Creek streamside forest sugges t that the facilitation model of succession applies, and that the fore st is currently in an advanced phase with Fraxinus pennsylvanica domin ating stream-margin regions and Acer saccharum beginning to dominate u pland, unflooded areas.