The Shonai Coastal Forest is located in Jurizuka Town, between Sakata and Tsuruoka in Yamagata Prefecture (38˚ 52' 38" N; 139˚ 47' 53" E). This coastal forest, as it is typical in Japan, is made of black pine (Pinus thunbergii). However, in recent years this forest has experimented an invasion of black locust trees (Robinia pseudoacacia) as a result of opening areas in the forest caused by fallen trees affected by nematode infection. Since the main function of this forest is as a windbreak barrier for extremely strong winds coming from continental Asia, through the Japan Sea, the presence of deciduous trees diminishes its function.  In this forest, we have used UAVs to map the distribution of black locust trees within the forest.  However, the main objective of our research in this forest has been the interaction of black pine trees with the leguminous black locust trees, since these latter ones are able to fix N from the atmosphere and thus fertilize the soil for the benefit of the surrounding trees.  Using stable N isotopes, we demonstrated that after a decade of black locust invasion, that all the N demand of black pine trees was supplied by black locust fertilization.