Effects of a changing light regime on the invasive plant, Alliaria petiolata (garlic mustard), in eastern deciduous forest

Alliaria petiolata (M. Bieb), Cavara and Grande, (garlic mustard) is a biennial herb native to Europe that has become a widespread, invasive species in North American deciduous forests over the last century .  In the past twenty years, garlic mustard has spread in deciduous forests along the Missouri River valley near Omaha, NE.  Efforts are underway to restore some of these degraded forest areas to the natural bur oak savanna ecosystem that once occupied the region.  Garlic mustard’s response to the cleared understory conditions in oak savanna restorations has not been well-studied, and previous reports suggest that garlic mustard may become less abundant with savanna restoration.  We used a greenhouse and field study approach to evaluate the effect of changing light levels on garlic mustard.  Our greenhouse study indicated that rosettes grown under full sun (simulating savanna conditions) may be physiologically well-suited to high light conditions, at least under well-watered greenhouse conditions.  We set up plots in both savanna and forested regions in Fontenelle Forest (a 567 ha preserve near Omaha, NE) and monitored populations of garlic mustard rosettes in their first growing season and after overwintering.  Savanna areas had higher levels of photosynthetically active radiation and lower levels of soil moisture than did forested areas.  Percent change in rosette number was similar in both savanna and forested sites, although the sites receiving the most light tended to have the highest mortality.   Our results suggest that restoration of oak savanna in these closed-canopy forests changes resources in a way that may ultimately lessen the impact of this invasive understory plant.