Threshold light levels for growth of Lonicera maackii seedlings in forest understories
Amur honeysuckle (Lonicera maackii) is a widespread invasive shrub in the northeastern U.S. It is a tremendous problem in the lower Midwest within 300 km of the Ohio River. Considerable attention has focused on the panoply of negative effects it exerts on native species, natural communities, and ecosystem processes, as well as various strategies for extermination. The factors that contribute to honeysuckle recruitment and dominance have received much less attention. In general, the species recruits best to open forest and forest edge, with seemingly less success in grasslands and closed canopy habitats. To be successful in forest understories, an invader must be shade tolerant as measured on the basis plant success. Positive seedling height growth is a seemingly reasonable measure of plant success, therefore we hypothesized that light regimes permitting no or negative height growth defined less invasible habitat. Studies were conducted in successional forests on the campus of Southern Illinois University Edwardsville, ca. 16 km northeast of St. Louis, MO. We planted 232 small (ca. 6 cm) honeysuckle seedlings premeasured for height into five forest sites. Seedlings were clustered in 8 groups of 5 within sites and monitored through the summer. Dead or disturbed seedlings were replanted. Final growth measurements were completed in late autumn (November 7). Canopy photos were made over each cluster in late summer, and analyzed for total PAR and canopy openness.
About 36% of seedlings remained from planting to final measurements in autumn. Most of the missing seedlings were lost because of digging animals, as opposed to dehydration or disease. As expected, relative growth rate (RGR) and proportional growth were significantly correlated with total PAR and canopy openness. More helpfully, if PAR and canopy openness are regressed on growth measures, the intercept of the regression (growth = 0) gives a threshold estimate of PAR or canopy cover before growth becomes positive. This was roughly PAR = 4 mol m-2 d-1, or 12% canopy openness. Stands can be readily screened based on canopy openness for their favorability to honeysuckle growth.