Trends in tree species composition, structure, and aboveground biomass of tropical secondary forests suggest long recovery time to unconverted forest conditions

Background/Question/Methods

Demographic and economic shifts in Latin America and the Caribbean are leading to increases in forest cover in many areas due to forest regrowth on abandoned agricultural lands.  Though often different in species composition and structure, these secondary forests can provide many of the same ecosystem goods and services as primary forests. For this study, we were interested in whether or not secondary forests regain key characteristics of primary forests over time. Specifically, we asked: Does time since establishment affect (1) species composition (determined by changes in species’ importance values), (2) forest structure (basal area and stem density), and (3) aboveground biomass accumulation? We sampled well-replicated primary forest plots and a chronosequence of secondary forest plots (19, 29, 39, 69, 89 years of age) on abandoned pastures in the subtropical wet forest life zone of Puerto Rico.  We identified trees to the species level and recorded all diameters ≥10cm. Trees were divided into five classes according to diameter, and biomass calculations were carried out using established allometric equations based on this forest type for Puerto Rico. Ongoing analyses of data documenting changes in the plots from 2003 to 2012 will provide further insights into successional trends and dynamics.

Results/Conclusions

Species composition varied with forest age or time since pasture abandonment. Of the most important species in the study, the common pioneers Cecropia schreberiana, Schefflera morototoni, and Tabebuia heterophylla all reached maximum importance values in younger (≤ 39 years) forests. Another colonizer species, the nonnative Syzigium jambos, reached peak importance in 89-year-old forests. The trend among mature forest species was not as clear. Stand age explained 86% of the variation in basal area. Basal area generally increased with age, peaking in 89-year-old plots. The effect of stand age on stem density varied by size class. In younger stands, densities were greatest in smaller size classes, whereas this pattern reversed in older stands. Forest age also explained 85% of the variation in aboveground biomass. Biomass increased with age, reaching a maximum at 89 years. Dominance of the monocot Prestoea montana in primary forests likely explains lower basal areas and biomass estimates relative to 89-year-old forests. Contrary to suggestions of previous studies, the abundance of the prevalent nonnative Spathodea campanulata increased, while numbers of S. jambos decreased from 2003 to 2012. While approaching similarity with primary forests over time, secondary forests remained distinct in terms of species composition and structure.