Invasive woody plant species are widely recognized as problematic for native plants. Callery pear (also known as Bradford pear; Pyrus calleryana) is a popular ornamental tree that is native to China. In much of the US, including the Ohio River valley, it is spreading into natural areas, and there is great concern that it may become a problematic invasive species. Amur honeysuckle (Lonicera maackii) was introduced into the US from eastern Asia as an ornamental shrub. It is recognized as one of the most important invasive plants in the Ohio River valley. Both of these species often establish in disturbed areas, along with the native eastern redcedar (Juniperus virginiana), and they may all be found growing together. Photosynthetic light response is an important predictor of plant life history; while it has been measured for honeysuckle and redcedar, little is known for pear. In 2015, we measured electron transport rate (ETR) vs. PAR using chlorophyll fluorescence measurements in these three species at a disturbed roadside site in northern Kentucky; ETR can be used as a proxy for CO2 assimilation rates (A).
ETRmax (response at high levels) and α (response to low light) were regressed against day of year (DOY) and temperature in all three species. In pear, α decreased with both factors. ETRmax declined with DOY for honeysuckle. For redcedar, the response of α was complex. ETRmax increased with temperature and appeared to be higher than for the invasive species, which may reflect its adaptations for hot, dry habitats. The invasive species’ decline of ETRmax with DOY may reflect their deciduousness. Redcedar had the highest ETRmax,, consistent with its shade intolerance, while honeysuckle had the lowest, consonant with its ability to invade closed-canopy forests. Pear’s intermediate ETRmax may therefore indicate that it is unable to invade under a closed canopy.