Robinia pseudoacacia, a nitrogen-fixing tree, facilitates the future growth of neighboring trees in Black Rock Forest

Trees that form symbioses with nitrogen (N)-fixing bacteria (N-fixers) have long been thought to facilitate neighbors’ growth by providing new N, but some data do not show facilitation. Two hypotheses to explain the lack of facilitation are that (1) N-fixers may decrease the amount of N they fix over time, reducing the N supply to their neighbors and (2) the effect of facilitation might take longer than the 1-5 y interval of most studies. In this study, we tested these hypotheses with the N-fixing tree Robinia pseudoacacia L. and its non-fixing neighbors, using tree ring N isotopes to estimate past N fixation and tree ring width to estimate past size and growth. Where the isotopes allowed a constrained estimate of N fixation, we did not find support for the first hypothesis: 51% of Robinia’s N came from fixation throughout its 60-y lifespan, with little variation across age. In support of the second hypothesis, we found that Robinia enhanced growth of neighboring trees in the future (up to 14 y) more than in the present. This delayed effect matches the expected timing of facilitation by N-fixers and indicates that single-census studies might underestimate the degree to which N-fixers facilitate forest growth.