Incomplete recovery of tree community composition and rare species after 120 years of tropical forest succession in Panama

Determining how fully tropical forests regenerating on abandoned land recover characteristics of old-growth forests is increasingly important for understanding their role in conserving rare species and maintaining ecosystem services. Despite this, our understanding of forest structure and community composition recovery throughout succession is incomplete, as many tropical chronosequences do not extend beyond the first 50 years of succession. Here, we examined trajectories of forest recovery across eight 1-hectare plots in middle and later stages of forest succession (40 – 120 years) and five 1-hectare old-growth plots, in the Barro Colorado Nature Monument (BCNM), Panama. We first verified that forest age had a greater effect than edaphic or topographic variation on forest structure, diversity and composition and then corroborated results from smaller plots censused 20 years previously. Tree species diversity (but not species richness) and forest structure had fully recovered to old-growth levels by 40 and 90 years, respectively. However, rare species were missing, and old-growth specialists were in low abundance, in the mid- and late secondary forest plots, leading to incomplete recovery of species composition even by 120 years into succession. We also found evidence that dominance early in succession by a long-lived pioneer led to altered forest structure and delayed recovery of species diversity and composition well past a century after land abandonment. Our results illustrate the critical importance of old-growth and old secondary forests for biodiversity conservation, given that recovery of community composition may take several centuries, particularly when a long-lived pioneer dominates in early succession.

Methods
The dataset was collected over a five-year period (2011 – 2016) in the Barro Colorado Nature Monument, Central Panama. Eight permanent forest plots (1-ha) were established around existing transects (0.32-ha) that comprise a secondary forest chronosequence in stands of known ages (see Denslow and Guzman, 2000). The chronosequence has previously been used to study forest structure, diversity, and community compositional recovery through succession (Denslow and Guzman, 2000; Dent et al., 2013; DeWalt et al., 2003; Jones et al., 2019; Mascaro et al., 2012). The 1-ha plots (50 x 200m) were located in the same secondary forest stands as close as possible to the original transects, but in some cases were reorientated due to the presence of forest edges. All tree, palm, and shrub stems ≥ 50 mm DBH (diameter at breast height) were censused within the plots by a team of field assistants, following ForestGeo protocols (Condit, 1998). Trees, palms, and shrubs were identified to a species level where possible (98.3% of individuals identified to species). A total of 9,478 stems were censused. The dataset has been cleaned following data entry and is ready for analysis.

References:

Condit, R., 1998. Tropical forest census plots: Methods and results from Barro Colorado Island, Panama and a comparison with other plots. Springer-Verlag, Berlin. https://doi.org/10.1007/978-3-662-03664-8

Denslow, J.S., Guzman, S.G., 2000. Variation in stand structure, light and seedling abundance across a tropical moist forest chronosequence, Panama. J. Veg. Sci. 11, 201–212. https://doi.org/10.2307/3236800

Dent, D.H., DeWalt, S.J., Denslow, J.S., 2013. Secondary forests of central Panama increase in similarity to old-growth forest over time in shade tolerance but not species composition. J. Veg. Sci. 24, 530–542. https://doi.org/10.1111/j.1654-1103.2012.01482.x

DeWalt, S.J., Maliakal, S.K., Denslow, J.S., 2003. Changes in vegetation structure and composition along a tropical forest chronosequence: implications for wildlife. For. Ecol. Manag. 182, 139–151. https://doi.org/10.1016/S0378-1127(03)00029-X
Jones, I.L., DeWalt, S.J., Lopez, O.R., Bunnefeld, L., Pattison, Z., Dent, D.H., 2019. Above- and belowground carbon stocks are decoupled in secondary tropical forests and are positively related to forest age and soil nutrients respectively. Sci. Total Environ. 697, 133987. https://doi.org/10.1016/j.scitotenv.2019.133987

Mascaro, J., Asner, G.P., Dent, D.H., DeWalt, S.J., Denslow, J.S., 2012. Scale-dependence of aboveground carbon accumulation in secondary forests of Panama: A test of the intermediate peak hypothesis. For. Ecol. Manag. 276, 62–70. https://doi.org/10.1016/j.foreco.2012.03.032