Characteristics of invasive Pyrus calleryana stands in the Cincinnati, OH, region

Background/Question/Methods

Callery or Bradford pear (Pyrus calleryana) has recently emerged as an invasive tree in the Ohio River Valley. This popular ornamental tree, which is native to China, is beginning to spread into natural areas, and there is great concern over its potential impact. However, little is known about its ecology. In 2016, I surveyed stands with a large P. calleryana component in Hamilton County, Ohio, and in Boone, Kenton, and Campbell Counties in northern Kentucky. Stands were surveyed by centering 2 x 5 m plots every 10 m along transects that were placed through stands; number of plots/stand varied from 3 to 30. The diameter at stump height (dsh; 25 cm) was measured for every living woody stem. For the 23 stands that were sampled, total densities and basal areas of all species were determined. Size distributions were fit with negative exponential curves and two-factor Weibull distributions. NMDS was performed on stand basal areas, and NMDS scores for each size were correlated against basal areas of species found in the stands.

Results/Conclusions

Densities were ~2,000-134,300 stems ha^-1 (median = 11,000 stem ha^-1), while basal areas were 0.6-25.4 m² ha^-1 (median = 7.6 m² ha^-1). Median dsh ranged from 0.3 to 4.0 cm. The most common woody associates of P. callyerana in these stands were Lonicera maackii, an invasive shrub, followed by the small native conifer, Juniperus virginiana. Size structures in most stands were best described by two-factor Weibull distributions, with a negative shape parameter that indicated monotonically decreasing numbers at larger size classes; four stands were best described by negative exponential curves. The first and second axes of the NMDS ordination were significantly correlated with P. calleryana and L. maackii basal areas, respectively. The associated species and size class distributions indicate that pear is colonizing open areas, and new seedlings are still being established. Previously-collected physiological data indicates that P. calleryana is shade-intolerant, and so size distributions are expected to become more unimodal as these stands age.