Black Locust & Drought

Field Study Suggests that Drought Diminishes its Nitrogen-Fixing Ability

Black locust (Robinia pseudoacacia)
Black locust is a native species that grows on disturbed sites and can convert atmospheric nitrogen into plant-available forms. Image courtesy of rawpixel.

With its symbiotic bacteria, black locust (Robinia pseudoacacia) makes its own nitrogen fertilizer – and can share it with other tree species.

“In early successional temperate forests, symbiotic nitrogen fixation is often the main source of new nitrogen,” says USDA Forest Service scientist Chelcy Miniat.

But drought could slow the rate of symbiotic nitrogen fixation, according to a recent study led by Jeffrey Minucci, a Ph.D. student at the University of Georgia at the time of the research. Miniat contributed to the study, and the results were published in the journal Ecology.

Black locust is relatively drought-tolerant, but when water is scarce, sugars are more difficult to make. And sugars are what black locust provides to its symbiotic nitrogen-fixing bacteria. Black locust can allocate more or less sugar to its symbiotic bacteria, but even if it provides less to them during drought, it may be at a competitive disadvantage.

The three-year field study took place on the Nantahala National Forest in western North Carolina.

In 2010, about 75 percent of the trees were harvested. The remaining trees provided seeds for the next generation, and the silviculturists on the Nantahala Ranger District painted the stumps of harvested trees with herbicide so that seeds would come from tulip poplar (Liriodendron tulipifera) and oak species (Quercus spp.). Black locust is an early successional species and was probably rare in the study plots before harvest, but it is such a dependable colonizer of disturbed areas that some people in the South think of it as a weedy tree.

The site had also been logged about a century ago.

In the early 20th century, millions of acres of land in the eastern U.S. were left bare after intensive logging and farming. Without roots to hold it, soil washed away. “Rivers were filled with sediment from mountain slopes and poor water quality was common,” says Miniat.

No one wants to return to practices that hurt water quality. But timber harvests continue – they are a type of active management. Active management can restore wildlife habitat, protect water resources, and keep forests healthy. And as the climate warms, less dense stands may become a healthier new normal.

This photo was taken in 1910 and shows a logged-over hillside. Photo courtesy of USDA.

“Southeastern forests are going to be under more pressure to provide wood and fiber plus clean and stable surface water, all as droughts become more frequent,” says Miniat. “So for this study, we wanted a forest that had been harvested and contracted out for a timber sale. We asked Joan Brown, the forest silviculturist, to show us sites that had been cut in the last year. The site we selected was close to the Coweeta Hydrologic Laboratory and with a range of soil conditions.”

To test the effects of drought in the field, the scientists isolated some of the plots from surrounding soil. Around each plot, they dug trenches that were about two feet deep. They lined the trenches with a thick plastic, which isolated plots from water moving through the soil. In 2013, they installed panels over the plots. The panels diverted 0 percent, 20 percent, or 40 percent of the rainfall, preventing it from falling into the plots.

Plants need water, light, and nutrients. The scientists made extensive measurements of each of these resources. They estimated total carbon and nitrogen stocks in the soil, assessed the amount of plant-available nitrogen in each plot, and estimated competition for light. They tested the amount of nitrogen concentrated in the leaves and leaf water potential, transpiration, and stomatal conductance. Every 15 minutes, soil moisture in the center of the plot was estimated, and every hour the average soil moisture was recorded.

As black locust biomass increased, the amount of plant-available nitrogen in the soil also rose, and non-nitrogen-fixing trees grew faster.

But in drier soils, black locust grew more slowly and did not fix as much nitrogen, as the study also showed.

“Water can be a primary constraint to photosynthesis and a key limiting factor for plant carbon fixation,” says Miniat. “Because black locust eventually enriches soils for other plants, too, our study implies that drought could indirectly diminish forest productivity and slow forest recovery after disturbances.”

A greenhouse study published in 2014 found that during drought, black locust grew bigger root nodules for its symbiotic bacteria.

“Drought triggered an 80 percent increase in biomass of root nodules. That suggested black locust would outcompete non-fixing tree species during drought,” says Miniat. She also contributed to that study, which was led by Nina Wurzburger, an associate professor at the University of Georgia’s Odum School of Ecology and another coauthor of the recent study.

But the greenhouse study was in a highly controlled environment – the exact amount of nutrients in the soil was measured, and temperature and moisture were also controlled.

This image shows how gradients in water, light, and available nitrogen affect black locust and three other native tree species. From left to right, the leaves are black locust, black oak, red maple, and tulip poplar. Black locust is at an advantage when soil moisture is high and soil nitrogen is low. Leaf darkness denotes nitrogen concentration.

In 2017, the team conducted a second greenhouse study. That study showed that the length of the dry period is important – black locust seedlings tolerated long but infrequent droughts. They did not tolerate frequent, short droughts as well.

“Greenhouse studies are valuable, but it’s not always clear how they translate to the field,” says Miniat. “What is clear is that black locust is plastic in how it responds to drought and allocates carbon to its root nodules.”

As the climate changes, droughts are expected to become more frequent and severe. This field study suggests that southern forests may be less resilient because of drought’s effects on black locust.

Currently, black locust thrives after disturbances. “That’s a really good thing for long-term forest recovery,” says Miniat. “Black locust stabilizes soils from erosion and nutrient poor conditions. It helps the entire system recover.”

Read the full text of the article.

For more information, email Chelcy Miniat at chelcy.f.miniat@usda.gov.

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