Diversity, distribution and dynamics of large trees across an old-growth lowland tropical rain forest landscape

Abstract

Large trees, here defined as �60 cm trunk diameter, are the most massive organisms in tropical rain forest, and are important in forest structure, dynamics and carbon cycling. The status of large trees in tropical forest is unclear, with both increasing and decreasing trends reported. We sampled across an old-growth tropical rain forest landscape at the La Selva Biological Station in Costa Rica to study the distribution and performance of large trees and their contribution to forest structure and dynamics. We censused all large trees in 238 0.50 ha plots, and also identified and measured all stems �10 cm diameter in 18 0.50 ha plots annually for 20 years (1997-2017). We assessed abundance, species diversity, and crown conditions of large trees in relation to soil type and topography, measured the contribution of large trees to stand structure, productivity, and dynamics, and analyzed the decadal population trends of large trees. Large trees accounted for 2.5% of stems and 25% of mean basal area and Estimated Above-Ground Biomass, and produced~10% of the estimated wood production. Crown exposure increased with stem diameter but predictability was low. Large tree density was about twice as high on more-fertile flat sites compared to less fertile sites on slopes and plateaus. Density of large trees increased 27% over the study interval, but the increase was restricted to the flat more-fertile sites. Mortality and recruitment differed between large trees and smaller stems, and strongly suggested that large tree density was affected by past climatic disturbances such as large El Niño events. Our results generally do not support the hypothesis of increasing biomass and turnover rates in tropical forest. We suggest that additional landscape-scale studies of large trees are needed to determine the generality of disturbance legacies in tropical forest study sites.

Large trees in tropical rain forests require big plots

“…We found one recent, directly comparable data set in the published literature which reported trees over 80 cm dbh from 119 ha of plots in Costa Rica (Clark et al., 2019). At La Selva, they recorded 39 species of big trees in 119 ha.…”

“…We note that for La Selva in Costa Rica and Korup in Cameroon, like in our study, there is a clear winner amongst the big tree species. Clark et al., (2019) reported that Pentaclethra macroloba was by far the commonest big tree at La Selva. Similarly, Thomas et al., (2003) identified Lecomtedoxa klaineana as the commonest tree above 60 cm dbh in their 50‐ha plot and reported that it was three times more common than the next most common big tree.…”

“…It would be possible, of course, to make big tree plots smaller than 10 ha, and depending on the questions being asked, it might be better to have smaller plots. Clark et al., (2019), for example used 0.5‐ha plots. However, it will be much easier to make statistical comparisons if plots are the same size.…”

“…Clark et al., (2019) shy away from defining big trees. We agree with their point when they state that the “useful course is to acknowledge the biological diversity contained within any large tree classification.” However, to rapidly inventory the large number of hectares (at least 100 ha) that our study suggests is necessary to sample big trees, we need a lower limit.…”

“…The tallest recorded tropical tree is just over 100 m tall (Shenkin et al., 2019). Such big trees occur at much lower densities, but play important roles in the ecosystem and face increased threats compared to the other trees (Clark et al., 2019; Lindenmayer & Laurence, 2017; Lutz et al., 2018; Pinho et al., 2020). However, in many tropical forests, little is known about the largest trees, including their species composition and distribution.…”