After disturbances, healthy ecosystems are usually resilient enough to return to a pre-disturbance state; however, some disturbances are extreme enough to permanently shift an ecosystem, a phenomenon known as a regime shift. In a study of the effects of clear-cut logging, Forest Service scientists found surprising effects on water chemistry, effects that persisted almost 40 years after the experimental clearcut. In the clearcut watershed, dissolved nitrogen was higher, and the seasonal spike in nitrogen concentration had shifted from summer to winter. Nitrogen enters the forest via several pathways and, in the studied watershed, one of the most important sources was black locust. Black locust dominated the clearcut watershed for the first 10 years after the cut. The species, which is native to the southern Appalachians, has nitrogen-fixing bacteria in its roots. The bacteria can convert atmospheric nitrogen to a form that plants can use for growth. The scientists suggest that the long-term elevation of stream nitrogen concentration and export along with the changes in seasonality of nitrogen export and the relationship between concentration and discharge represent a functional regime shift initiated by forest disturbance. This regime shift was not predicted, nor would it have been understood without the long-term and large-scale experimentation possible in FS Experimental Forests.