Second-growth forests comprise more than half of all tropical forests, yet changes in species composition as these forests regenerate are poorly understood. Knowledge of disturbance history is critical to understanding tree community composition and successional trajectories. Agricultural disturbance can affect species composition long after the land use has subsided, and natural disturbances such as hurricanes and severe droughts can alter successional trajectories. Using historical aerial photographs (1937, 1954, 1977) and tree census data collected over 7 years from three forests ranging from 41 to 82 years since agricultural abandonment and one unconverted stand in the Luquillo Experimental Forest in northeastern Puerto Rico, we studied changes in species composition over time and in response to a category 4 hurricane. We predicted that (1) past agricultural activities caused differences in species composition that can be detected today, (2) the relative importance of human factors in determining species composition declines and the relative importance of hurricane disturbance increases as forests age, (3) sites with more intense human disturbance will contain a greater prevalence of exotic tree species, and (4) sites with more intense human disturbance will exhibit greater hurricane-related mortality.
Results/Conclusions
NMDS Ordination analyses revealed that species composition was distinct among the four plots, with moderate compositional overlap. The oldest stand is characterized by native, old-growth tree species such as Dacryodes excela, Manilkara bidentata and the endemic Simarouba tulae, while the exotic ornamental Syzygium jambos dominates the youngest site. Mid-successional sites contain primarily native pioneer trees (e.g. Myrcia deflexa, Casearia arborea) and shrubs (e.g. Miconia prasina). Two stands managed for forestry in the past, one early-successional and one mid-successional, contained tree species (e.g. Callophyllum calaba, Tectonia grandis, Swietenia macrophylla) not present in other sites. The youngest site had the greatest prevalence of exotic species and rate of change in species composition, with an 86% decline in abundance of S. jambos in only 6 years due to infection by an exotic rust. The passage of Hurricane María (9/20/17) caused widespread tree damage at all sites. Old-growth stands with large trees suffered the greatest crown damage, while plots with more pioneers and exotic species suffered greater mortality. These results suggest legacies of human land use impact the resistance of a forest to extreme events such as hurricanes.