Photosynthesis and shoot allocation of amur honeysuckle and two native shrubs in deep shade

Abstract

Amur honeysuckle (Lonicera maackii (Rupr.) Maxim) is a notorious invader of forests and forest edges throughout eastern North America. The species is di cult to exterminate and reestablishes readily. A perpetual presence of Amur honeysuckle in most forests is largely a forgone conclusion. Forests throughout the eastern U.S. are changing due to canopy closure after historical disturbance and long-term re prevention. Fire intolerant species, especially sugar maple (Acer saccharum Marshall.), are coming to dominate forest canopies, creating shady understories and forest dynamics that pivot on shade tolerance. Two shade tolerant understory shrubs, pawpaw (Asimina triloba Adans.) and spicebush (Lindera benzoin Thunb.) compete with honeysuckle in maple understories. Based on eld observations we hypothesized that these native species were more shade tolerant than invasive honeysuckle. We examined shade tolerance in honeysuckle, pawpaw, and spice bush by comparing photosynthetic light responses, leaf characteristics, and allocation to leaf area versus stem mass (leaf area ratio, LAR) in annual shoots. Honeysuckle had comparable light responses measured on a leaf area basis, but mass-based measurements show it is less e cient paying back carbon invested in leaf tissue. This arises because honeysuckle is unable to reduce speci c leaf mass to the level of pawpaw and spicebush. LAR is distinctly higher in pawpaw and spicebush, suggesting an advantage in heavy shade. Since the great majority of photosynthesis in these species occurs under a closed canopy, we suggest honeysuckle will diminish in mapledominated stands. Informal observations of greater height growth in pawpaw and spicebush in the eld support this.