Background/Question/Methods Asiatic shrub honeysuckles (Lonicera spp.) are notorious forest and forest edge invaders throughout northeastern North America. Their wide ecological amplitude, copious seed production, and effective seed dispersal by birds are essential aspects of their success. It is repeatedly asserted that a period of leaf display longer than native competitors enhances their success. This purportedly occurs in spring for L. x bella in WI and in fall for L. maackii in MA. The value of this trait necessarily depends on the seasonal photosynthetic performance of the shrubs, weather pattern, and canopy density. These can vary considerably place to place and year to year. Given that the L. x bella work consisted of spot measurements during a lightly replicated season-long study, and the L. maackii work came from model results, my interest was to determine how photosynthetic response varied in L. maackii in the hotter, longer growing season of southern IL. Using detached branches taken from shrubs in the field, I monitored growing season photosynthetic light response curves under natural conditions of temperature, humidity, and CO2. Measurements were taken at intervals from early April leaf-out, to hard frost in early November. Results/Conclusions Leaf characteristics changed substantially through the year. Leaf area per leaf in April was about half that in midsummer, while area based total chlorophyll doubled from May to August, and chl a:b ratio was highest in May and October, and least in July and August. These patterns indicate a leaf acclimation response to the closing canopy and reduced light. Photosynthetic light response curves were especially variable between replicate shrubs in April, reflecting spatial heterogeneity in canopy closure, but became much more consistent later in the year. Maximum photosynthetic rate (+26%), light compensation point (+270%), and dark respiration were elevated (+210%) in April versus May-October. These results reinforce the case that early spring photosynthesis could be an advantage for L. maacki in southern IL. However, we as of yet have no data that take into account total leaf area and light interception throughout the season. Higher unit leaf area performance may be counterbalanced by lower light interception by smaller leaves in spring. Notably, several studies have shown that shade substantially limits reproductive output in L. maackii, indicating that early photosynthesis doesn’t greatly offset the effects of shading.
