Physiological constraints on the spread of Alliaria petiolata populations in Massachusetts

The expansion of plant species into new sites is limited by a combination of environmental conditions and the capacity for adaptive variability in trait expression. Here, we investigated whether and how adaptation to forest edge conditions might be limiting the spread of the invasive plant Alliaria petiolata (garlic mustard) into the forest interior in eastern Massachusetts. We conducted a common garden experiment to test whether plants from forest edge vs. forest interior microhabitats differ in their plasticity and physiological responses to experimental shading. All plants in the experiment responded to shading with reductions in growth, photosynthetic activity, and reproduction regardless of the source environment, indicating a high degree of plasticity and a strong likelihood that most seeds in our study populations are produced at the forest edge. Effects of the source habitat on physiological function were detectable, but small compared to the magnitude in growth and reproductive responses to light. Plants from the forest edge physiologically outperformed those from the interior when grown in the shade, but had equally low reproductive success. Plants originating in the forest interior, in contrast, demonstrated greater allocation to growth in relation to photosynthesis and reproduction compared to plants originating at the forest edge. We compare our findings to earlier work on the importance of plasticity for invasive spread in this species, and conclude that failure by garlic mustard to invade some forest interior sites is due in part to overwhelming reproductive and physiological disadvantages in low light. We further suggest that in some cases shade tolerance in this species is constrained in favor of plastic responses that optimize fitness in high light conditions. The implications for geographic variation in the spread and management of this species are discussed.