tropical rainforests harbor exceptionally high biodiversity and store large amounts of carbon in vegetation biomass. However, regional variation in plant species richness and vegetation carbon stock can be substantial, and may be related to the heterogeneity of topoedaphic properties. therefore, aboveground vegetation carbon storage typically differs between geographic forest regions inassociation with the locally dominant plant functional group. A better understanding of the underlying factors controlling tropical forest diversity and vegetation carbon storage could be critical for predicting tropical carbon sink strength in response to projected climate change. Based on regionally replicated 1-ha forest inventory plots established in a region of high geomorphological heterogeneity we investigated how climatic and edaphic factors affect tropical forest diversity and vegetation carbon storage. plant species richness (of all living stems >10 cm in diameter) ranged from 69 to 127 ha −1 and vegetation carbon storage ranged from 114 to 200 t ha −1 . While plant species richness was controlled by climate and soil water availability, vegetation carbon storage was strongly related to wood density and soil phosphorus availability. Results suggest that local heterogeneity in resource availability and plant functional composition should be considered to improve projections of tropical forest ecosystem functioning under future scenarios.Tropical forests host two thirds of terrestrial biota 1 and comprise one fourth of the planet's terrestrial carbon (C) stored in aboveground vegetation biomass (AGB) 2 . It has been proposed that biodiversity positively affects carbon storage in hyper-diverse tropical forests 3 , but this finding has been repeatedly challenged by studies showing that relationships between species diversity and ecosystem functioning are dependent on the scale of observation 4,5 , and usually saturate at high levels of species richness, such as in tropical forests 6,7 . As a consequence, relationships between biodiversity and C storage remain poorly resolved for tropical forests [6][7][8] . It is inherently difficult to disentangle factors determining tropical ecosystem functioning and isolating possible effects of species diversity,

Recent studies have reported a consistent pattern of strong dominance of a small subset of tree species in neotropical forests. These species have been called "hyperdominant" at large geographical scales and "oligarchs" at regional-landscape scales when being abundant and frequent. Forest community assembly is shaped by environmental factors and stochastic processes, but so far the contribution of oligarchic species to the variation of community composition (i.e., beta diversity) remains poorly known. To that end, we established 20.1-ha plots, that is, five sites with four forest types (ridge, slope and ravine primary forest, and secondary forest) per site, in humid lowland tropical forests of southwestern Costa Rica to (a) investigate how community composition responds to differences in topography, successional stage, and distance among plots for different groups of species (all, oligarch, common and rare/ very rare species) and (b) identify oligarch species characterizing changes in community composition among forest types. From a total of 485 species of trees, lianas and palms recorded in this study only 27 species (i.e., 6%) were nominated as oligarch species. Oligarch species accounted for 37% of all recorded individuals and were present in at least half of the plots. Plant community composition significantly differed among forest types, thus contributing to beta diversity at the landscape scale.Oligarch species was the component best explained by geographical and topographic variables, allowing a confident characterization of the beta diversity among tropical lowland forest stands.Abstract in Spanish is available with online material. K E Y W O R D Sbeta diversity, community composition, neotropical forests, oligarch species, topographic habitats

& Key message Quercus secondary forests show a gradual transition toward mixed forests, with sweet chestnut (Castanea sativa) becoming increasingly abundant in the western Spanish Central System. Additionally, in chestnut-dominated stands, it shows a certain resistance to competitive displacement by Quercus pyrenaica. Our results partially refute the traditional view that C. sativa is unable to recruit in the absence of cultural inputs. & Context Sweet chestnut, Castanea sativa, is a component of European broadleaf forests and is one of the most managed trees. Due to a reduction in cultural inputs, chestnut-dominated stands tend to be invaded by other species, and it is unclear how chestnut is able to persist in natural mixed forests. & Aims Our work aimed to identity the main factors that limit the establishment of C. sativa and to analyze the recruitment and mortality processes of C. sativa trees. & Methods The age, growth ring patterns, regeneration density, and the spatial structure of trees and saplings in 11 plots in the Spanish Central System were analyzed. & Results Chestnut seedling density increased with C. sativa basal area, but transition toward the sapling stage appeared limited owing to light availability. In Quercus pyrenaica secondary forests, sparse canopies did not constrain chestnut regeneration, and in old chestnut stands, C. sativa showed a certain resistance to competitive displacement. By contrast, mixed young coppices showed a high mortality, most likely due to competition with other vigorous resprouters. & Conclusion Quercus secondary forests showed a gradual transition toward mixed forests with sweet chestnut becoming increasingly more abundant. In old stands, C. sativa is likely to persist under a gap-phase mode of regeneration. Our results partially refute the traditional view that C. sativa is unable to recruit in the absence of cultural inputs.

Aims Carbon accumulation in terrestrial ecosystems is inherent to the vegetation development and ageing process. Primary productivity synthetize biomass which is constantly incorporated to soil. Vegetation community composition, and other ecological drivers, are known to mediate biomass production. However, links between forest developmental stage and ecological drivers of carbon stocks are unexplored. We address this topic under the prediction that species-rich and uneven-sized forests can improve carbon storage potential in biomass and topsoil fraction across its development. Methods The study was carried in forest stands growing under Mediterranean conditions in Central Spain. Carbon content in both above- and below-ground tree biomass and in topsoil organic matter (0–40 cm) was measured in 30 sampling plots of variable size (900–3000 m2). We also assessed Shannon species diversity index, Gini tree-size inequality index and forest developmental stage using dendrochronological procedures to derive the mean age of the oldest trees. First-order interaction terms between diversity factors and forest age were regressed against carbon density in compartment-independent regressions. Results Forest-age and tree-size heterogeneity coupling was the main factor driving carbon density of both compartments. The interaction showed that woodlands maximize density in aged forests composed by uneven-sized trees. Models gave not support to consider species diversity as a mediator of carbon stocks. Conclusion Our results shed light on how tree-size heterogeneity can regulate the temporal dimension of forest ageing to rise the carbon storage potential. Mature forests in semi-arid environments cannot store carbon due to their intrinsic ontogeny, they need to grow structurally diverse.