Wood density is a crucial variable in carbon accounting programs of both secondary and old-growth tropical forests. It also is the best single descriptor of wood: it correlates with numerous morphological, mechanical, physiological, and ecological properties. To explore the extent to which wood density could be estimated for rare or poorly censused taxa, and possible sources of variation in this trait, we analyzed regional, taxonomic, and phylogenetic variation in wood density among 2456 tree species from Central and South America. Wood density varied over more than one order of magnitude across species, with an overall mean of 0.645 g/cm3. Our geographical analysis showed significant decreases in wood density with increasing altitude and significant differences among low-altitude geographical regions: wet forests of Central America and western Amazonia have significantly lower mean wood density than dry forests of Central and South America, eastern and central Amazonian forests, and the Atlantic forests of Brazil; and eastern Amazonian forests have lower wood densities than the dry forests and the Atlantic forest. A nested analysis of variance showed that 74% of the species-level wood density variation was explained at the genus level, 34% at the Angiosperm Phylogeny Group (APG) family level, and 19% at the APG order level. This indicates that genus-level means give reliable approximations of values of species, except in a few hypervariable genera. We also studied which evolutionary shifts in wood density occurred in the phylogeny of seed plants using a composite phylogenetic tree. Major changes were observed at deep nodes (Eurosid 1), and also in more recent divergences (for instance in the Rhamnoids, Simaroubaceae, and Anacardiaceae). Our unprecedented wood density data set yields consistent guidelines for estimating wood densities when species-level information is lacking and should significantly reduce error in Central and South American carbon accounting programs.

Summary 1.To investigate life-history differentiation and an objective functional classification of tree species we analysed the demography of 29 species in subtropical montane forests in north-western Argentina. 2. We computed 13 growth, demographic, abundance and distribution variables based on: (i) two 5-year re-measurements of stems ≥ 10 cm diameter at breast height (d.b.h.) in 8 ha of old growth forest and 4 ha of secondary forest; (ii) assessments of tree crown illumination; and (iii) sapling counts under shade and on landslides. 3. We assessed the potential confounding effects of stem size and crown illumination on absolute stem diameter growth rate for the 24 most abundant species. As diameter increased, one species showed significant increases in growth rate and five showed significant reductions. Seventeen species grew significantly faster with increased exposure to light and we controlled for this confounding effect in the computation of diameter growth rates for subsequent analyses. 4. A principal component analysis resulted in three meaningful and interpretable axes of demographic variation across species. The first axis (interpreted as shade tolerance) indicates that trees of species with inherently high growth rates tend to have wellexposed crowns at 10-30 cm d.b.h., have high density of trees in secondary forest and are less tolerant of shade. 5. The second axis (turnover) shows that in old-growth forest short-lived species, with high mortality rates, size-class distributions with a steep negative slope and low dominance, persist due to high rates of recruitment (to ≥ 10 cm d.b.h.). 6. The third axis indicates that species that colonize landslides have lower tree recruitment rates and greater growth variability in secondary forest, reflecting spatio/ temporal differences in species' recruitment linked to differences in their substrate requirements for regeneration. 7. Maximum height and diameter are correlated with the first and second axes, indicating that higher rates of both growth and survival permit some species to attain large size. 8. All three demographic axes depict separate trade-offs that confer competitive advantage to each 'demographic type' under contrasting ecological conditions (of light availability, disturbance frequency and disturbance intensity), thus underpinning species' coexistence in dynamic forest landscapes.

Uncertainty about the mechanisms driving biomass change at broad spatial scales limits our ability to predict the response of forest biomass storage to global change. Here we use a spatially representative network of 874 forest plots in New Zealand to examine whether commonly hypothesised drivers of forest biomass and biomass change (diversity, disturbance, nutrients and climate) differ between old-growth and secondary forests at a national scale. We calculate biomass stocks and net biomass change for live above-ground biomass, belowground biomass, deadwood and litter pools. We combine these data with plot-level information on forest type, tree diversity, plant functional traits,

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