Risk and reward

Natural History, July-August, 2004 by Stephan Reebs

As plants have evolved, they have had to pick a lifestyle and marshal their resources accordingly: to grow woody or not, to live as annuals or perennials, to adapt to wet climates or dry.

Another forced choice has been even more fundamental: Do you live fast (and die young), or take it slow (and survive to old age)? Do you accept the risks of investing for quick returns, or do you aim for slow but steady augmentation of capital? Ian Wright, a biologist at Macquarie University in Sydney, Australia, and thirty-two colleagues throughout the world recently completed the most comprehensive global plant survey ever made. And one of their most surprising findings is that both "day traders" and the most risk-averse, long-term "bondholders" coexist in every kind of climate.

Focusing only on leaves, the team found that some plants invest a lot in photosynthesis but little in defense. Their leaves have a large surface area compared with their dry mass, and they require a generous supply of enzymes and nutrients. Such leaves are good at responding rapidly to growth opportunities, and they produce a lot of energy. Their downside is that they dry out more readily and are vulnerable to disease, herbivores, pollution, or weather. The leaves of other species invest less in photosynthesis and more in defense: they're tougher, and make various nasty chemicals that deter consumers. Those are the leaves that produce energy at a slower rate, but they do live longer.

The biologists' key discovery is that all species of plants, regardless of habitat or climate type, take their primary cue from one central trade-off--maximizing photosynthesis versus ensuring survival. Once that choice is made, the remaining spectrum of choices is really rather narrow. ("The worldwide leaf economics spectrum," Nature 428:821-27, April 22, 2004) --S.R.