A Physiological Advantage of the Invasive Shrub Buckthorn.
David L. Dornbos and Erica Vandergaast. Calvin College, 1726 Knollcrest Circle SE, Grand Rapids, MI 49546
Restoration of disturbed land areas or maintenance of existing quality land areas often requires effective control of invasive non-native weed species. Buckthorn (Rhamnus cathartica or R. frangula) is a shrub which aggressively competes with native shrubs and ground story plants on abandoned farmland or along woodland edges in most Midwestern, northeastern states and Ontario. Research was conducted at Calvin College (Grand Rapids, Michigan) during 2005 and 2006 to characterize the physiological basis for buckthorn’s competitiveness in comparison with eight native shrub species. Carbon dioxide uptake and transpiration rate was measured on numerous plants growing in situ at a range of quantum flux levels ranging from full sun exposure to deep shade under a forest canopy with a TPS-1 (PP Systems) gas exchange sensor and quantum line sensor. Both buckthorn species exhibited extremely rapid CO2 uptake rates in high light environments, with only two of eight native species (buttonbush [Cephanlanthus occidentalis] and high-bush cranberry [Viburnum trilobum]) exhibiting comparable rates. In shade sufficient to reduce the net photosynthesis rate of buckthorn by eight-fold, from eight to one mg CO2 m-2 s-1, the net photosynthesis rates of all eight native species was comparable to buckthorn (0.5 to 1 mg CO2 m-2 s-1) with the one exception of high-bush cranberry, which exhibited a slightly higher gas exchange rate. These data support field observation of buckthorn's apparent aggressive invasiveness in abandoned fields, wooded edges, or in relatively thin plant canopies where greater quantities of light are available. Restoration efforts in areas where buckthorn is a potential problem should include either planting of sufficient over story plants to quench much of the available light or maintenance of an active maintenance program where buckthorn is removed every couple of years from the area under restoration.