Plants invest dry matter and mineral nutrients in building green leaves. During photosynthesis, carbon dioxide from the air is converted to carbohydrates using energy harnessed from the sun. These carbohydrates are re-invested in the construction of leaves, in stems that support the leaves, and in other plant parts. They are also used to support metabolism and in the acquisition of mineral nutrients from the soil. Across the world there are more than 250,000 vascular plant species, all engaging in these processes of investment and reinvestment of carbon and mineral nutrients. Plants inhabit a sharply competitive world, and all the species we see around us today are successful "businesses" that have been operating for thousands to tens of millions of years with proven profit strategies. The "invisible hand" of natural selection is very strong—much stronger than what operates in human economics. Any plant business strategy defined by a given mix of DNA has to actually turn a "profit" in carbon terms every generation, or its DNA will become extinct.

A global plant trait network (GLOPNET) was formed by researchers from 15 countries to quantify leaf "economics" across the world’s plant species. The GLOPNET dataset covers more than 2,500 species gathered from 175 sites around the world, a substantially larger dataset than has ever been drawn together before. The dataset extends to all vegetated continents, spanning from arctic tundra to tropical rainforest, from hot to cold deserts, and from boreal forest to grasslands. We suspect that our article has become rapidly cited because in it we establish powerful and general global patterns that are useful to researchers across a wide range of disciplines, from plant physiology to ecology to biogeochemistry. Reliable quantification of global leaf economics and how it varies with climate will prove valuable in modeling how carbon and nitrogen are tied up and used in plant biomass, and how nutrient fluxes and vegetation boundaries will shift with land-use and climate change in coming decades.

The GLOPNET project can be seen as part of a new phase in ecology, where the importance of looking for consistent, worldwide patterns is becoming more recognized. In this paper we described a consistent pattern of relationships among key leaf traits, the "Leaf Economics Spectrum" (described below). By illustrating that plant evolution leads to powerful global patterns we are able to understand the world in a simplified way, while at the same time recognizing that every species is unique and to be valued in its own right.

We identified six key leaf traits that summarize many of the important features of each plant’s business strategy: nitrogen concentration, phosphorus concentration, structure (a leaf’s mass per area), photosynthetic capacity, respiration rate, and leaf lifespan. In economic terms, these traits together tell us about the major costs of constructing a leaf and about its potential to get a carbon profit from that investment, as well as the potential duration of the revenue streams. These are the same things you would want to know if you were investing in a business: what the potential rate of return would be and how long the revenue stream would be sustained before the machinery had to be replaced.

Via GLOPNET we created a global database of these leaf traits. When all six traits were analyzed, around 75% of total variation in the traits could be understood as lying along a single spectrum of leaf economics. At the "quick-return" end of the Leaf Economics Spectrum are species with flimsy leaves that return high yields for short lifespans. At the "slow-return" end are species with robust leaves, low yields, low nutrient concentrations and respiration costs, but generating long-lasting revenue streams. Across all biomes worldwide, species have converged onto this leaf economics spectrum, and also diverged along it. In other words, whether you are looking at plants growing in deserts, rainforests, mountain tops, or grassy prairies, the combination of leaf traits that have evolved in each species allow us to place them along this spectrum. Analyzing the effects of climate on leaf economics led to the conclusion that the return on investments of dry mass and nutrients in leaves varies considerably depending on where plants grow, with differences shaped by millions of years of evolution.

I undertook my Ph.D. as a member of Mark Westoby’s Comparative Ecology Group at Macquarie University. In 1998, Peter Reich from the University of Minnesota spent his sabbatical with us, and we began collaborating on some projects. At that time my research focused on quantifying how relationships between leaf economic traits varied with site factors such as climate or soil nutrients, among shrubs and trees in eastern Australia. As our work progressed, we began to notice similar patterns in our work from Australia and that from the Americas. Clearly, the next step was to go global; indeed, Reich had already begun forming an informal data-sharing network of ecologists. In 2001, and by then a post-doc (funded jointly by Reich and Westoby), I began the task of expanding the network, compiling the data and beginning the analysis of global leaf trait data. "The worldwide leaf economics spectrum" was the first paper stemming from this project. Several others are now published, in press, or in preparation.