Physiological competitiveness of common and glossy buckthorn compared with native woody shrubs in forest edge and understory habitats

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Common buckthorn (Rhamnus cathartica L.) and glossy buckthorn (Frangula alnus Mill.; or, Rhamnus frangula L.) are non-native, invasive plant species invading U.S. Midwest natural areas. Both are ubiquitous on the Calvin College campus in Southwest Michigan where they compete with native plants for light, water, and nutrients exceptionally well. We hypothesize that common buckthorn will assimilate carbon faster and use water more efficiently than glossy buckthorn and native co-habiting shrubs in relatively dry and bright edge and meadow habitats whereas glossy buckthorn will thrive in moist and thin forest canopies. These are habitats in which each buckthorn species is abundant and colonizes rapidly. The objective of this project was to compare light use efficiency (LUE), water use efficiency (WUE), chlorophyll content and stomatal characteristics of both buckthorn species with two locally common co-habiting native species, hawthorn (Crataegus spp.) and gray dogwood (Cornus racemosa Lam.), acclimated to forest edge and understory environments on the Calvin campus in 2016 and 2017. Between 380 and 480 light use efficiency curves were measured with a LI-6400XT Portable Photosynthesis System using common and glossy buckthorn, hawthorn and gray dogwood leaves during two growing seasons from plants acclimated to edge and understory habitats. Common buckthorn assimilated carbon faster than glossy buckthorn when leaves were exposed to moderate or high light intensities. Water use efficiency was advantageous to glossy buckthorn at low to moderate light intensities, potentially enabled by the high density of stomates on lower leaf surfaces. Shade-acclimated common buckthorn displayed only minor decreases in photosynthesis rate relative to leaves in sun-acclimated plants after exposure to a wide range of light intensities possibly because of its higher leaf chlorophyll content. Sun-acclimated hawthorn grew faster than both buckthorn species when exposed to higher light intensities and produced comparable growth rates to buckthorn species in the shaded understory. Gray dogwood produced the slowest growth rate of the four species in all environments. Restoration of buckthorn infested areas is problematic in both sunny edge and shaded understory habitats and will require continuous mitigation of common and glossy buckthorn along with the concomitant establishment of more competitive native species like hawthorn.