Plant invasions are often implicated in declines of native plant species. However, common experimental designs have received criticism questioning the assumption that invasive plants are the primary cause for ecosystem deterioration. We used a combination of field observations and a transplant experiment to investigate the influence of an exotic invasive shrub, Ligustrum sinense (Chinese Privet) on native plant species in Piedmont floodplain forests of South Carolina, USA. We conducted vegetation surveys of 12 floodplain forests documenting abundance and cover of all herbaceous and woody plant species. Additionally, we established an experimental garden to compare survival and growth of L. sinense and four common native species transplanted into a mature L. sinense stand and an adjacent uninvaded area over two growing seasons. The vegetation survey demonstrated a strong negative relationship between L. sinense presence and herbaceous vegetation. As L. sinense cover increased, herbaceous cover and height, plant abundance, and native species richness decreased. In our transplant experiment we found drastic effects of L. sinense on native plant seedling survival and growth. Survival for all native species was lower under the L. sinense canopy and native seedling growth was substantially reduced.Results from both the vegetation survey and transplant experiment show that invasion of L. sinense suppresses herbaceous understory and prevents regeneration of native species by reducing seedling survival and growth. With an approach that combines multiple field sites and local site-specific investigations our research provides strong evidence that L. sinense is an agent of change in floodplain forests.

1. Trophic linkages between terrestrial and aquatic ecosystems are common and sensitive to disruption. However, there is little information on what causes variation in the strength and spatial scale of these linkages. 2. In the highly aquatic adults of the headwater salamander Gyrinophilus porphyriticus (family Plethodontidae), use of terrestrial prey decreases along a gradient from early-to late-successional riparian forests. To understand the cause of this relationship, we tested the predictions that (i) terrestrial prey abundance is lower in late-successional forests, and (ii) G. porphyriticus adults cannot move as far from the stream to forage in late-successional forests, thus limiting access to terrestrial prey. 3. We established 100-m long study reaches on six headwater streams in the Hubbard Brook Experimental Forest, New Hampshire. Three reaches were in early-successional forests and three were in late-successional forests. We conducted pitfall trapping for invertebrate prey in June and July of 2005, with three traps at 0, 2, 5 and 10 m from the stream at each reach. In June, July and August of 2004 and 2005, nighttime salamander surveys were conducted at each reach along ten, 10-m long by 2.5-m wide transects perpendicular to the stream. 4. Abundance of terrestrial prey was consistently lower in late-successional forests, suggesting that consumption of terrestrial prey by G. porphyriticus is affected by prey abundance. Contrary to our prediction, G. porphyriticus adults moved farther from the stream in late-successional forests, suggesting that habitat conditions in late-successional forests do not limit movement away from the stream, and that lower abundances of terrestrial prey in these forests may cause salamanders to move farther from streams. 5. Our results provide novel insight on the extent of terrestrial habitat use by G. porphyriticus. More broadly, these results indicate that major habitat gradients, such as forest succession, can affect the strength and scale of terrestrial-aquatic linkages. Application of this insight to the design of vegetation buffers along headwater streams would have widespread benefits to freshwater ecosystems.

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