Tree Age Distributions Reveal Large-Scale Disturbance-Recovery Cycles in Three Tropical Forests

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.

Stationary Forestry with Human Interference

“…Consequently, some amount of uncertainty in the present results obviously is related to the reliability of the used models. However, qualitatively, the appearance of slow recruitment, in accordance with Equation (5), appears to agree with quite a few observations [6,23,[39][40][41]. Also, observations indicating a higher rate of recruitment exist [42][43][44].…”

Does Cedrela always form annual rings? Testing ring periodicity across South America using radiocarbon dating

“…Brienen and Zuidema 2006;De Ridder et al 2013;Schöngart 2008), study forest dynamics (e.g. Brienen et al 2010;Vlam et al 2017) and potentially to monitor forest responses to climate change (e.g. van der Sleen et al 2015;Zuidema et al 2012, but also see Brienen et al 2016a).…”

Unfolding long‐term Late Pleistocene–Holocene disturbances of forest communities in the southwestern Amazonian lowlands

“…This variation in plant communities was attributed to the global increase in tree growth in response to the increased anthropogenic CO 2 atmospheric concentration in the last decades (Baker et al 2014, Levis et al 2017). A growing perception, however, has been that variations within present-day tropical forests were imprinted by long-term (i.e., secular to millennial scale) environmental changes (Patton et al 2000, Vlam et al 2017, and references contained therein). For instance, African rainforests showed large-scale changes even with small fluctuations in intensity or duration of the dry season (Malhi et al 2013).…”

“…A recent publication has also related large-scale changes in the Amazonian rainforest to seasonal variations (Zemp et al 2017). A growing perception, however, has been that variations within present-day tropical forests were imprinted by long-term (i.e., secular to millennial scale) environmental changes (Patton et al 2000, Vlam et al 2017, and references contained therein). Thus, identifying the timing and mode of forest establishment and development is critical to examine the origin of the forest's present-day diversity and understand its functionality.…”

“…Large-scale tree mortality and regeneration over a decadal timescale can be detected by forest monitoring, but many of these processes in modern tropical forests are inherited from past disturbances at a centennial (Burslem et al 2000, Baker et al 2005, millennial (Da€ ınou et al 2010, Lebamba et al 2012, or perhaps an even longer timescale. Some studies on tree rings from non-Amazonian areas have provided information on long-term (i.e., centennial) disturbances in tropical forests (Baker et al 2005, Middendorp et al 2013, Nock et al 2016, Vlam et al 2017. Untangling the long-term history of forest disturbances in Amazonia requires floristic and geological data, which can be obtained through the analysis of the sedimentary record.…”