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Data capturing multiple axes of tree size and shape, such as a tree's stem diameter, height and crown size, underpin a wide range of ecological research-from developing and testing theory on forest structure and dynamics, to estimating forest carbon stocks and their uncertainties, and integrating remote sensing imagery into forest monitoring programmes. However, these data can be surprisingly hard to come by, particularly for certain regions of the world and for specific taxonomic groups, posing a real barrier to progress in these fields. To overcome this challenge, we developed the Tallo database, a collection of 498,838 georeferenced and taxonomically standardized records of individual trees for which stem diameter, height and/or crown radius have been measured. These data were collected at 61,856 globally distributed sites, spanning all major forested and non-forested biomes. The majority of trees in the database are identified to species (88%), and collectively Tallo includes data for 5163 species distributed across 1453 genera and 187 plant families. The database is publicly archived under a CC-BY 4.0 licence and can be access from: https://doi.org/10.5281/zenodo.6637599. To demonstrate its value, here we present three case studies that highlight how the Tallo database can be used to address a range of theoretical and applied questions in ecology-from testing the predictions of metabolic scaling theory, to exploring the limits of tree allometric plasticity along environmental gradients and modelling global variation in maximum attainable tree height. In doing so, we provide a key resource for field ecologists, remote sensing researchers and the modelling community working together to better understand the role that trees play in regulating the terrestrial carbon cycle.

• Ecological mechanisms underlying species diversity and productivity are not mutually exclusive. • Energy-related climatic factors increased large-diameter trees. • Ecogeographic gradients in large-diameter trees increased aboveground biomass. • Available energy promoted big-sized trees for higher forest functioning. • Sites variation shaped forest diversity – biomass relationship. The macro-ecological mechanisms underlying species diversity and productivity are not mutually exclusive, but our understanding is limited to the climatic variation in species richness and productivity along ecogeographical gradients. Yet, the ecogeographical gradients in a few large-diameter trees are not clearly tested on the linkage amongst forest diversity-structure attributes and aboveground biomass in natural forests. Here, we hypothesized that energy-related climatic factors (i.e. available energy) promote few big-sized trees compared to species or functional diversity or remaining-diameter trees, thereby explains the linkage between forest diversity-structure attributes and aboveground biomass (i.e. the 'big-trees – energy hypothesis'). We used a piecewise structural equation modelling on 238 forest plots from three different forest or vegetation types, i.e., moist temperate, semi-humid, and semi-arid, across 12 sites in Iran, in order to test the hypothesized causal relationships amongst geo-topographic, climatic factors, edaphic factors, top 1% large-diameter trees, forest diversity-structure attributes (stand density, species richness or functional diversity or 99% remaining-diameter trees, and functional dominance) and aboveground biomass. Energy-related climatic factors increased large-diameter trees and functional dominance, but not species richness, functional dispersion, remaining-diameter trees, and stand density. Climatic factors generally increased species richness, functional dispersion, stand density, and remaining-diameter trees via ecogeographic gradients in large-diameter trees. Ecogeographic gradients in large-diameter trees and functional dominance overruled the effects of species richness, functional dispersion and remaining-diameter trees on aboveground biomass, and hence providing support to the big-sized trees effect. Our results suggest that positive effects of energy-related climatic factors (i.e. available energy) promote big-sized trees which in turn enhance aboveground biomass, and hence forest functioning. However, all these underlying ecological mechanisms and drivers were highly dependent on sites variation, and hence context-dependent. We argue that the ecogeographic gradients and sites variation in big-sized trees should be included into the integrative ecological modeling for better understanding the species – energy mechanism as well as the diversity – functioning relationship in large-scale natural forests. [ABSTRACT FROM AUTHOR]