Functional megadiversity.

Western Amazonian and Andean forests have Earth’s highest biodiversity and are globally important for ecosystem services and climate regulation. Straddling the planet’s longest ecological gradients, they have captured the imagination of scientists since von Humboldt (1). However, ecological studies and theory have incorporated that spectacular biological variation in only the most rudimentary ways, or not at all. It is widely understood that there are many traits that are important to plant performance, consequential not only for things like carbon gain and loss, but also timing of life histories, evolutionary patterns, and interactions with symbionts. In PNAS, Asner et al. (2) focus on one of the best known suites of traits, those comprising the biochemical functions of canopy leaves, on a breathtaking number of species drawn from nearly all major angiosperm clades. The study crosses ecological gradients spanning lowland Amazonian soil fertilities from poor white sands to rich clays, and an altitudinal gradient stretching 3.5 km from the hot lowlands to the frost and ice near the limit of tree growth in the Andes. The authors’ findings show that there is immense variability in functional traits found in the canopies of tropical forests: indeed, as much variability in the forests of western South America as was previously known for all tropical forests on the planet. Understanding how tropical tree communities are put together—what determines their diversities and the relative abundances of their members—is a longstanding and contentious issue in ecology. Given the importance of tropical forests in terms of carbon and hydrological cycles, climate regulation, and a vast array of ecosystem services, the question becomes much more acute as we proceed through the Anthropocene. Human-caused changes to tropical forests can alter not only their biodiversity, but also the basic provisioning, regulating, supporting, and cultural services they provide to humanity. However, for most of the history of their study, tropical forest community members were treated as monolithic; a tree was a tree, even though they are organisms with radically different evolutionary histories, being drawn from every major clade of flowering plants. A rain forest tree may be much more closely related to a temperate zone herb than to the other trees growing near it. Ecologists were forced to ever more esoteric theories of coexistence, diversity, and ecosystem function, relying on communities being assembled randomly from source pools of different sizes, or with functional traits being reduced to average values of demographic performance. That is, until now. Big Science and the -omics revolution is changing the study of tropical forests. Measurements that once required heroic effort to make for a few species can now be done for entire assemblages. The revolution is giving a detailed understanding of evolutionary relationships among organisms and the ways that they make their living. Trees have gone from being anonymous stems in a vast woods characterized only by size, growth, and the rate at which stems appear or disappear, to organisms where we characterize their traits and try to understand how they relate to environmental gradients and the myriad organisms with which they interact. Big Science is vastly increasing the data available, and promises to change and give mechanism to the patterns we see in tropical forests.