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.
“Ecosystem regime shifts have been well documented in lakes, streams, and oceans,” says Forest Service Southern Research Station (SRS) scientist Jennifer Knoepp. “We identified a functional regime shift in a forested watershed that had been clearcut in the 1970s.” The study was led by Jackson Webster, a professor emeritus at Virginia Tech, and published in the journal Ecosystems.
The scientists paired the clearcut watershed with an adjacent watershed that was similar, except for the fact that it had not been clearcut. Both watersheds are located at the SRS Coweeta Hydrologic Laboratory, where Forest Service research has been conducted since 1934. Knoepp and her colleagues used a 36-year dataset to compare the two watersheds.
“We found surprising effects on water chemistry, effects that persist almost 40 years later,” says Knoepp, a research soil scientist. In the clearcut watershed, dissolved nitrogen was higher, and the seasonal spike in nitrogen concentration had shifted from summer to winter.
“In many ways, the recovery of the clearcut watershed was rapid,” says SRS project leader and study coauthor Chelcy Miniat. “Within 10 years, tree cover had returned and within 30 years the total amount of aboveground biomass was about the same as before logging.” The amount and speed of water flowing through the streambeds increased immediately after the clearcut, but within 7 years, returned to predicted levels.
The scientists expected that the stream nitrogen concentration would also decline. However, about 10 years after the clearcut, it actually increased. Almost 40 years after the clearcut, it remains elevated.
Nitrogen is an important plant nutrient, but when it reaches the water – especially in the form of nitrate and ammonia – it can affect water quality. Nitrogen enters the forest via several pathways, and in the studied watershed, one of the most important sources was black locust.
Black locust trees 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.
Black locust has declined on the study site, which is now dominated by tulip poplar and oak. However, the amount of nitrogen in streams remains elevated. Studies on black locust have found that as it decomposes, it contributes large amounts of relatively inert nitrogen to the soil and may have accumulated in the deeper levels of the soil, as well as areas next to the streams.
“It is very difficult to distinguish a regime shift from a very slow recovery,” says Knoepp. “If we had stopped measuring the amount of dissolved nitrogen in streams after 10 years, we probably would have thought the ecosystem was recovering to its original state.”
Whether stream nitrogen levels in the watershed will eventually return to pre-disturbance levels is unknown, and whether the change represents a regime shift or simply a very slow recovery is unclear. Although the nitrogen concentration is much higher than in other nearby watersheds, it is low in comparison to streams that flow through agricultural lands or cities, and is not a water quality issue.
“However, the long-lasting effects suggest the need for forward-looking management,” says Knoepp. “Management should aim to increase the resilience of forests and create or maintain desired nutrient cycling regimes.”