Twenty years of change in the Luquillo Forest Dynamics Plot, Puerto Rico

Background/Question/Methods

The Luquillo Forest Dynamics Plot (LFDP) is a 16-ha research grid located in the subtropical wet forest of northeastern Puerto Rico. Major hurricanes struck the LFDP in 1928 and 1932, and subsequently in 1989 and 1998, causing significant damage. Clear patterns in species distribution reflect significant tree felling and farming in the northern two-thirds of the plot, where Casearia arborea dominates, and light disturbance from selective logging in the southern area, where old-growth species such as Dacryodes excelsa are abundant. Disturbance by Hurricane Hugo in 1989 caused massive recruitment of the pioneer tree, Cecropia schreberiana. Since 1990 we have conducted five censuses of all self-supporting, woody stems ≥ 1.0 cm at a height of 130 cm from the ground. In each census, occurring at approximate five-year intervals, individual stems were tagged, identified, and measured for diameter, and their locations mapped. The most recent census was completed in March, 2012 and we report on long-term trends in the structure and composition of the forest community and population dynamics of dominant species. As hurricane disturbance dominated the dynamics of the forest plot during the initial three censuses, we address key changes over the past five years, a period without significant wind disturbance.

Results/Conclusions

We recorded 37,297 individuals representing 123 species in the LFDP, a slight reduction in density and diversity since the last census, but strongly reduced from the 89,267 stems recorded in 1992 (when the first census was completed). A grand total of 128,217 individuals have been recorded in the LFDP, representing 157 species. Since the last census, two species were lost among the 8,186 individuals that died. The latter represented 19% mortality over five years. As in previous censuses, over 90% of mortality was among stems with a diameter <10 cm. Recruitment of new individuals was 4.2%, thus mortality continues to dominate over recruitment in the community. Between the two dominant species representative of the land use history of the LFDP, we found evidence of increased relative recruitment of C. arborea in the least human-impacted portion of the plot where D. excelsa dominates. This result supports previous conclusions that C. arborea will increase relative to D. excelsa as a result of their differential response to canopy disturbance caused by hurricanes. The population dynamics of the pioneer C. schreberiana largely represents that of a single-aged cohort; the massive recruitment of C. schreberiana after Hurricane Hugo was not repeated after Hurricane Georges (1998).