The beginning of the end? High mortality rates seen in invasive open-grown Amur honeysuckle stands in northern Kentucky

Background/Question/Methods

Amur honeysuckle (Lonicera maackii) is recognized as an invasive plant with major impacts on ecosystems in the Ohio River Valley. Recent work by Boyce et al. (2014) found recent honeysuckle mortality levels in one open-grown stand to be well above those reported in the 1990s. This coincided with several years in the 2010s of the widespread appearance on Amur honeysuckle of honeysuckle leaf blight (Insolibasidium deformans), a native pathogen that affects most members of the genus Lonicera. While there have been reports of leaf blight outbreaks and areas with high mortality, more measurements are needed to determine the extent and degree of mortality for Amur honeysuckle. Three open-grown (no tree cover) stands, all located on or near the campus of Northern Kentucky University in Campbell County, Kentucky, were surveyed. A 2 x 5 m plot was located every 10 m along one or more transects in each stand, and diameter at stump height (25 cm) was measured for every woody plant, both alive and dead. Total live and dead density and basal area for was determined, and the fraction dead was calculated for all species. 

Results/Conclusions

For Amur honeysuckle density, the fraction dead was 20-36 %, while for basal area, it was 16-24 %. These fall below the numbers reported by Boyce et al. (62 and 37 % for density and basal area, respectively), but they are well above the 3.2 % reported for dead density fraction in the 1980s. While all of these stands were dominated by Amur honeysuckle (live densities and basal areas were 16,000-51,800 ha^-1 and 13-18 m² ha^-1, respectively), four other species (Acer saccharum, Cornus drummondii, Juniperus virginiana, and Pyrus calleryana) that were present in significant amounts usually had much smaller dead fractions. These results indicate increased mortality in open-grown stands of Amur honeysuckle, above both historical levels and the levels of other species growing in the same habitat. While this study did not examine a causal link between leaf blight and honeysuckle mortality, the timing of both blight and honeysuckle mortality is suggestive, perhaps indicating a means to lessen the impacts of Amur honeysuckle. Thus, leaf blight effects on Amur honeysuckle should be further studied.