Nitrogen-Fixing Tree Paves the Way for Other Invaders

Invasion-resistant ecosystems can be breached by introducing a single nonnative plant that makes it easier for other nonnatives to grow. New research at the Cape Cod National Seashore shows that black locust trees that enrich poor soils by fixing nitrogen have dense undergrowths of invasive species. In contrast, the native trees do not fix nitrogen and have hardly any nonnative understory plants.

“The introduction of a novel functional type—leguminous nitrogen-fixing trees—into this upland forested ecosystem facilitates invasion by other nonnative species into the naturally invasion-resistant system,” say Betsy Von Holle of Harvard and three coauthors in Biodiversity and Conservation.

The researchers investigated how black locust trees (Robinia pseudoacacia) affect soil chemistry and understory plants in upland forests along the Cape Cod National Seashore, which are naturally low in nutrients. They compared plots in 20 black locust stands with those in 20 adjacent native stands. The paired stands were only 20 meters apart, and the native stands were dominated by pines and oaks.

The results suggest that black locust trees transformed the ecosystem by adding nutrients. The soil analysis revealed that locust trees increased soil organic matter and nutrients, nearly doubling local nitrogen levels. The understory plant survey showed that native stands had very little nonnative undergrowth but that locust stands averaged 10 invasive species, accounting for nearly half the understory cover. The nonnatives flourishing under the locust trees included roses (Rosa multiflora), morning glories (Convulvus arvensis), and honeysuckles (Lonicera morrowii).

Once established, these lush nonnative shrubs and vines may further accelerate their invasion by attracting more animals, which could then spread these understory plants to other black locust stands. “Native bird species appear to occur in greater abundances within the black locust communities,” note the researchers, suggesting that managers should focus on controlling the invasive plants that are dispersed by birds.

By Robin Meadows

Von Holle, B. et al. 2006. Facilitations between the introduced nitrogen-fixing tree, Robinia pseudoacacia, and nonnative plant species in the glacial outwash upland ecosystem of Cape Cod, MA. Biodiversity and Conservation 15:2197-2215.

Photo: iStockphoto.com/Roger Whiteway

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