Asiatic shrub honeysuckle invasion is at nearly epidemic proportions throughout southern Illinois and the Ohio River Valley. In Madison County, IL (adjacent St. Louis, MO) honeysuckle populations in disturbed forests appear to have surged in the last decade. This may be a case of pulsed recruitment, or the consequence of demographic inertia from a maturing founder population. In addition, it is not clear how strongly recruitment depends on canopy openings or proximity to forest edge. Two studies were conducted in established honeysuckle populations on the campus of Southern Illinois University Edwardsville. In the first study we wished to determine whether recruitment to a closed canopy forest understory exhibited pulsed or continuous characteristics. In addition, we wished to determine if recruitment was still ongoing in this largely saturated habitat. Dominant stems from shrubs in a 20 x 45 m mapped plot were harvested for growth ring analysis. (Previous studies had shown that age estimates from dominant stems were within 2 years of estimates taken from rootstocks.) In a second study we evaluated whether recruitment into a closed secondary forest had emanated from forest edge shrubs as opposed to seeds deposited more randomly by flying birds. Shrubs were censused and harvested along 7, 45 m transects perpendicular to a west facing edge. Understory light conditions in both studies were estimated from hemispherical canopy photographs.
Results/Conclusions
Analysis of 350 stem sections taken from the mapped closed forest stand showed that recruitment exhibited a pulsed pattern, with the majority of recruits less than 10 years old. The oldest individuals dated 15-20 years old, representing just 3 stems of the total sample. Recruitment is ongoing in this population, with about 1/3 of the sample consisting of shrubs < 35 cm height and < 5 years old. The density of these individuals varied greatly in space (0-11 individuals per 25 m2 subplot). Canopy openness openness ranged 3.7-13.6%, and was not correlated with seedling density. In the forest edge study, we hypothesized a gradient of decreasing density, size, and age with distance from the forest edge. None was present. In addition, there was no relationship between current canopy density and honeysuckle occurrence. Taken in aggregate, these studies suggest that honeysuckle seed sources have recently reached sufficient levels in the landscape to largely erase seed source limitations on recruitment. Clearly forest understories in the study sites are sufficiently well lit to permit honeysuckle colonization anywhere seeds are dispersed.
