Abstract
Over the past few decades there has been a growing realization that a large share of apparently ‘virgin’ or ‘old-growth’ tropical forests carries a legacy of past natural or anthropogenic disturbances that have a substantial effect on present-day forest composition, structure and dynamics. Yet, direct evidence of such disturbances is scarce and comparisons of disturbance dynamics across regions even more so. Here we present a tree-ring based reconstruction of disturbance histories from three tropical forest sites in Bolivia, Cameroon, and Thailand. We studied temporal patterns in tree regeneration of shade-intolerant tree species, because establishment of these trees is indicative for canopy disturbance. In three large areas (140–300 ha), stem disks and increment cores were collected for a total of 1154 trees (>5 cm diameter) from 12 tree species to estimate the age of every tree. Using these age estimates we produced population age distributions, which were analyzed for evidence of past disturbance. Our approach allowed us to reconstruct patterns of tree establishment over a period of around 250 years. In Bolivia, we found continuous regeneration rates of three species and a peaked age distribution of a long-lived pioneer species. In both Cameroon and Thailand we found irregular age distributions, indicating strongly reduced regeneration rates over a period of 10–60 years. Past fires, windthrow events or anthropogenic disturbances all provide plausible explanations for the reported variation in tree age across the three sites. Our results support the recent idea that the long-term dynamics of tropical forests are impacted by large-scale disturbance-recovery cycles, similar to those driving temperate forest dynamics.
Highlights
Disturbances are increasingly being recognized as important drivers of tropical forest dynamics (e.g., Newbery et al, 2013; Tanner et al, 2014)
We examined whether tree regeneration rates of the 12 shadeintolerant species were continuous or irregular over the past two centuries and use these regeneration patterns to reconstruct disturbance regimes
Sample trees at the site in Bolivia had a comparatively lower height, dbh (Supplementary Figures S2A–D,I–L) and age, whereas the site in Thailand was intermediate between Cameroon and Bolivia
Summary
Disturbances are increasingly being recognized as important drivers of tropical forest dynamics (e.g., Newbery et al, 2013; Tanner et al, 2014). Age Distributions of Tropical Trees disturbances that impact several hectares to square kilometers at the same time, but occur only at decadal or centennial scales (Burslem et al, 2000; Baker et al, 2005). Decades of research in temperate forest ecosystems have demonstrated that infrequent, large-scale disturbances are critical determinants of forest composition, structure, and dynamics (Oliver, 1980; Oliver and Larson, 1996), resulting for example in highly irregular diameter distributions of the dominant trees (Lorimer, 1980). Forest monitoring studies on the other hand are unlikely to capture the dynamics caused by infrequent severe and/or large-scale disturbances because they are generally conducted in small permanent plots for several decades (e.g., Baker et al, 2004; Clark et al, 2010). Treering research may provide information at this intermediate scale, as has been shown for temperate forests (Lorimer, 1980; Fraver et al, 2008), and to a lesser extent tropical forests (Baker et al, 2005; Middendorp et al, 2013; Nock et al, 2016)
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