Recent findings by U.S. Forest Service researcher Chelcy Ford Miniat and University of Georgia cooperator Nina Wurzburger suggest that trees such as black locust that fix nitrogen symbiotically may fare better than others under moderate drought conditions, and may benefit drought-stressed forests by supplying them with new nitrogen. The findings were recently published in the journal Oecologia.
General circulation models predict more intense and frequent droughts over the next century as a result of climate change. The temperate forests of the Southeast already experience more dry periods during growing seasons, with extended droughts causing widespread death of some tree species. A chronic reduction in the amount of precipitation could lead to a substantial shift in forest composition and ecosystem function.
“The combinations of temperature and precipitation regimes predicted by the models are like nothing we’ve experienced in modern history,” says Miniat, project leader of the Forest Service Southern Research Station Watershed Science Research units, “How southeastern forest ecosystems will respond to these conditions is unknown, and could have consequences not only for forests but for the people who depend on the clean water and other benefits they provide.”
Because of continuing and emerging demands for timber, fiber, and other wood products, a significant portion of southeastern forests will be in various stages of early succession in the future. To look at effects of drought on these regenerating forests, the researchers grew four early successional tree species in a greenhouse experiment. They measured physiology, growth, and competition under the reduced soil moisture conditions present during moderate drought. A major factor they considered was growth among trees that acquire nitrogen by different mechanisms.
Nitrogen is essential for the growth of plants. Although nitrogen is abundant in the atmosphere, plants cannot use it until it’s been transformed, either by free-living bacteria or by symbiotic bacteria that live in the root nodules of leguminous plants. Reduced soil moisture affects nitrogen availability by limiting mineralization and reducing the mobility of nitrogen compounds in soil.
Of the four tree species in the experiment—northern red oak, red maple, tulip poplar, and black locust—only black locust, a leguminous species, can acquire nitrogen from the atmosphere, and “fix” it into a plant-usable form with the help of symbiotic bacteria that reside in root nodules. In normal conditions, black locust and other leguminous species introduce a substantial amount of new nitrogen into the ecosystems where they grow.
“We expected to see black locust with its high water demands to decline with drought, resulting in a marked reduction in new nitrogen entering the ecosystem, which would in turn impair recovery,” says Miniat. “Instead we found something remarkable. Drought triggered an 80 percent increase in the root nodules where nitrogen fixation takes place and in the amount of nitrogen fixed by black locust.”
This is good news for regenerating forests in the Southeast, where locusts are some of the first trees to grow after harvests and other disturbances that open up the canopy. “Under scenarios of moderate drought, increased nitrogen fixation by species such as black locust could make early successional forests more resilient to drought by supplying more new nitrogen to the ecosystem.”
For more information, email Chelcy Miniat at email@example.com .