Pairing up for pear: Beginning a project on an emerging invasive woody species (Pyrus calleryana)

Background/Question/Methods

Callery or Bradford pear (Pyrus calleryana) has recently emerged as an invasive tree in the Ohio River Valley. Little is currently known about its ecology and physiology. A project to address gaps in knowledge was initiated between an undergraduate and a faculty member (first and second author, respectively). Data were gathered on density and basal area of this species in natural areas. Gas exchange measurements were initiated to estimate maximum values of photosynthesis (A_max) and quantum efficiency. We also gathered data for allometric equations by destructively sampling eight trees of different sizes. Trees were felled and cut into 1-m sections, with small sections cut from each 1-m section. After counting all live & dead branches, one was selected from each section, along with attached leaves and fruits. Samples were dried to constant weight in the lab, and weight of each component was calculated. Logarithms of these weights were regressed against logarithms of stump diameter (25 cm), while height was regressed against diameter. The student author contributed significantly to the estimation of tree components. Germination trials were begun on seeds collected from a wild population.

Results/Conclusions

In wild stands dominated by pear, density fell into the range 824-4200 ha^-1, while basal area was in the range 6.47-7.95 m² ha^-1. A_max was reached at light values ≥ 1000 μmol m^-2 s^-1. Values of A_max were relatively high for a tree, in the range 6-16 μmol m^-2 s^-1, and highest values were reached at T~20°C. Mean quantum efficiency was 0.174. Regressions of allometric equations all yielded P < 0.001, with r² values usually above 0.90. Germination trials have been plagued by fungal outbreaks, but preliminary results indicate that Callery pear needs stratification of at least one week at T = 4°C. While successful germination rates may be low due to fungal susceptibility, pear’s high fruit production assures suitable seed input for successful colonization. Photosynthetic measurements indicate that Callery pear is a shade-intolerant tree adapted to colonize disturbed areas, while density and basal area measurements show that it can dominate these areas once established. Allometric equations will be used in the future to estimate biomass. Integration of undergraduates in a research program can yield both educational results and research findings.