Scale-dependence of aboveground carbon accumulation in secondary forests of Panama: A test of the intermediate peak hypothesis

Accumulation of aboveground carbon is one of the most important services provided by tropical secondary forests-a land-cover type that is increasing in importance globally. Carbon accumulates rapidly for the first 20. years of succession, but few studies have considered forests older than 20. years, and the available data do not yield a consistent pattern. Two alternative hypotheses have been proposed: (1) an asymptotic increase, with the highest carbon stocks occurring in the oldest stands, and (2) an intermediate peak, caused by roughly synchronous tree maturity (and thus high carbon stocks) after which time treefall gaps cause carbon stocks to regress. Here we revisited a well-studied tropical moist forest chronosequence in Barro Colorado Nature Monument, Central Panama, consisting of 35, 55, 85, and 115-year-old stands, as well as old-growth stands, to determine whether past evidence for the intermediate peak hypothesis was influenced by the spatial limitations of the field plots used to assess forest structure. We used airborne LiDAR (light detection and ranging) to measure carbon stocks at the scale of the original transects (0.16. ha), in surrounding forest of the same age (up to 20. ha), and at a landscape scale incorporating thousands of hectares not previously measured. We also compared forest structure as measured in three dimensions by LiDAR, considering vertical and horizontal variation in canopy organization, as well as the abundance of treefall gaps. Our results suggested a strong scale-dependence of aboveground carbon accumulation, supporting the intermediate peak hypothesis at the fine scale of the 0.16-ha transects, but an asymptotic model at the landscape scale incorporating thousands of hectares. Further analyses of forest structure suggest that both the limitations of small plots and intrinsic scaling of forest structure and carbon dynamics account for the scale-dependence of aboveground carbon accumulation in this secondary forest matrix.