Seed dispersal turns an experimental plantation on degraded land into a novel forest in urban northern Puerto Rico

Planting tree species with desirable traits may catalyze forest regeneration in increasingly common degraded lands by restoring soil properties and attracting seed dispersers. We sampled forest regeneration in an experimental plantation of Albizia lebbek, an introduced N-fixing species, on a degraded pasture in northern Puerto Rico, 27 years after its establishment. We hypothesized that the abundance of animal-dispersed tree species within this unmanaged plantation would be (1) greater than in adjacent pastures; (2) higher at distances closer to secondary forest; and (3) increased over the 20 years since floristic surveys were last conducted in the site. We sampled forest structure and tree species composition, and compared the abundance of animal-dispersed tree species on plots located within the adjacent pasture and the A. lebbek plantation, within the plantation in plots distanced at 10, 35, and 60 m from secondary forest, and within the plantation at 7 vs. 27 years after plantation establishment. We found higher abundance of animal-dispersed tree species in the A. lebbek plantation than in the pasture. Distance from secondary forest had no effect on the abundance of animal-dispersed species. The abundance of animal-dispersed species increased by 14% between 7 and 27 years after plantation establishment. In all, these results indicate that A. lebbek trees facilitated the establishment of native animal-dispersed species by shading out tall grasses and acting as perches for animals throughout the plantation. We found that A. lebbek lost its dominance in the site to another introduced N-fixing species, Leucaena leucocephala, possibly due to unfavorable traits such as susceptibility to pathogens. Our findings document a transition from an experimental plantation to a novel urban forest that harbors an array of native tree species, largely due to the dispersal of seeds by animals. A noteworthy example is the presence of Guaiacum officinale, an animal-dispersed and endangered tree species native to the dry forests of southern Puerto Rico. Our results highlight how monocultures of introduced tree species with desirable traits can catalyze the regeneration of native tree species in chronically degraded lands.