The prickly pear cactus in the Galápagos Islands.
The garden outside my window is home to an enormous and beautiful tree. I gave it a hug the other day, but the trunk is so huge I could barely get my arms round a quarter of its girth. For now, the branches are bare of leaves, so you can see its form in all its majesty, a triumph of natural architecture. And if you half-close your eyes and dream a little, you can also see its roots, stretching deep beneath the grass, much as its branches and twigs stretch outwards towards the buildings and upwards towards the sky.
Trees figure in our mythologies and metaphors — the tree of life, the tree of knowledge — and we often imagine them to harbor spirits and sprites. They also figure in a big way in our reality: forests (still) cover about 30 percent of the planet’s land, and may make up as much as 80 percent of Earth’s biomass. That is, if you were to put all the organisms on the planet on a giant set of scales, trees would account for 80 percent of the total.
Better yet, trees harbor plenty of non-imaginary beings. Birds like starlings or blue tits nest in tree holes; others, like magpies and crows, build their nests high in the branches. Chimpanzees sleep in trees. A number of fungi — truffles, anyone? — associate with tree roots. Insects like wasps make houses (galls) in the leaves. And so on.
A giant sequoia tree in California.
Some trees — sequoias and eucalypts, for instance — can be prodigiously tall, reaching heights of 90 meters (295 feet) or more. And some are prodigiously old. Plenty of species can live for four, five or six centuries, and some can keep going for several thousand years. The oldest living tree — which is also one of the oldest living beings — is thought to be a bristlecone pine, Pinus longaeva. It is certainly more than 4,600 years old, and by some reckonings, it celebrates its 4,842nd birthday this year. But however you count, when it was a sapling, the great pyramids of Giza had not yet been built.
Yet although trees are familiar to all of us, many aspects of their biology remain enigmatic: because they grow slowly and live for so long, they’ve been hard for us to study in the laboratory. Which is why they are my nomination for Life-form of the Month: April.
Unlike the Life-forms of the Month I’ve nominated so far (dinoflagellates, ciliates and grasses), trees aren’t a natural group. That is, the term “tree” refers to their lifestyle, not their ancestry. To put it another way, beings that we call trees have evolved several times from different ancestors, whereas beings like ciliates, grasses or (for that matter) primates have evolved only once. Palms evolved into trees independently from species like oaks, for example. Moreover, plant species that exist as herbs or shrubs on a continent often evolve into trees when they find themselves on islands. On some of the Galápagos islands, the prickly pear cactus — which is usually low to the ground — has evolved a tree-form. It is tall, with a woody trunk and its leaves high in the air. The island of Socotra, off the coast of Yemen, is home to a species of cucumber that has become a tree.
What, then, is a tree? Precise definitions vary, but most of them mention the words “tall” and “woody,” and add that a tree has a single self-supporting stem (i.e., a trunk) that branches well above the ground.
The first trees appeared more than 375 million years ago, in several different plant lineages, in a burst of evolution that some authors have termed “the scramble for the sky.” If you’d been walking through the Earth’s early forests, you might have seen club mosses that were 40 meters (131 feet) tall, as well as giant horsetails. Both types of tree are now extinct. But what’s interesting about them is that they made wood differently from, say, pine trees. Pine trees grow outwards, forming a solid woody cylinder. In contrast, the trunks of tree-horsetails were hollow tubes, like bamboo. Tree-club mosses produced trunks with a hard outer casing, and a softer interior. Meanwhile, tree-ferns evolved a fourth type of woody structure: they grow several stems that are bound together by other tissues.
(Trees, incidentally, have an excellent fossil record: just think of the vast petrified forests of Arizona, or Patagonia, each of which covers more than 37,000 hectares — more than 90,000 acres. The study of tree growth patterns give us insights into past climates. Early scientists, however, were not sure whether petrified trees were living trees that had become stone, or stones that were becoming trees.)
So what forces produce trees, and could any plant, in principle, evolve into one? The answer to the second part of the question is, maybe. Genetic experiments on the botanists’ lab rat — a weedy little plant called Arabidopsis — have shown that you can make it grow wood by turning off a few key genes. If this is true for other plants too, then growing into a tree may be a matter of a few mutations and the right circumstances.
So what circumstances are those? The evolutionary advantages of being a tree include an ability to get light — especially in dense forests, plants compete for light, and the tallest individuals have the most access to direct sunlight. In addition, their longer lifespan gives them many more chances of reproducing. In places like islands where there are few trees, plants that were previously living as shrubs may find that the tree habit gives them an edge.
But being a tree has challenges, too. Trees are more vulnerable to wind and lightning than shrubs and herbs. And longevity itself creates difficulties. In the course of centuries, situations change: droughts and fires may come and go, soil may erode, water tables may rise and fall. Worse, other organisms — especially enemies — can evolve far faster, because they can go through hundreds of generations during the tree’s life. How can trees avoid succumbing to diseases? Especially as they don’t have an immune system like ours: you can graft tissue from one tree to that of another (think apples and olives) without the kind of rejection that a mammal would experience. Part of the answer may be that many trees have evolved associations with other, fast-evolving organisms, like fungi and ants, that can protect them to some extent.
With all this in mind, I think I’ll go and hug another tree.
Notes:
The estimate that forests cover 30 percent of the planet’s landmasses comes from the Food and Agriculture Organization of the United Nations, 2005 Forestry Report. The Earth’s total biomass, and that proportion made up by trees, is a number that is hard to pin down. I took my estimate from Wikipedia. A higher estimate (90 percent) is given by Petit, R. J. and Hampe, A. 2006. “Some evolutionary consequences of being a tree.” Annual Review of Ecology, Evolution, and Systematics 37: 187-214. However, I was unable to verify the source of their claim. This paper presents an interesting analysis of the pros and cons of the tree lifestyle, as well as several (rather similar) definitions of the word “tree.”
For the heights of the tallest trees, see the Wikipedia entry on trees. For some of the traits that help trees to live for a long time, see Lanner, R. M. 2002. “Why do trees live so long?” Ageing Research Reviews 1: 653-671. Tracking down the age of bristlecone pines is a difficult business. The authority that is usually cited is Schulman, E. 1958. “Bristlecone pine, oldest known living thing.” National Geographic 113: 355-372. However, this paper simply says that the oldest tree so far found is more than 4,600 years old, and that a more precise estimate is not possible. A far more precise age, of 4,842 years, is given on the Gymnosperm Database entry for the species. However, the author’s source for this more precise age is not entirely clear.
For an excellent and clear account of different forms of wood, as well as the importance of petrified forests in reconstructing the plant fossil record, see chapter three of Kenrick, P. and Davis, P. 2004. “Fossil Plants.” Natural History Museum, London. This book also gives the account of early scientists wondering whether petrified trees were once trees, or were stones coming to life (see page 58). These authors also describe the “scramble for the sky” (page 68).
For a wide-ranging account of the evolution of trees in different plant lineages, see Groover, A. T. 2005. “What genes make a tree a tree?” Trends in Plant Sciences 10: 210-214. For trees in lineages extinct and extant, as well as a more detailed discussion of the evolution of wood, see Donoghue, M. J. 2005. “Key innovations, convergence, and success: macroevolutionary lessons from plant phylogeny.” Paleobiology 31 (supplement to issue 2): 77-93.
For the genetics of Arabidopsis and wood, see Melzer, S. et al. 2008. “Flowering-time genes modulate meristem determinacy and growth form in Arabidopsis thaliana.” Nature Genetics 40: 1489-1492.
For wind as a problem for trees, see Ennos, A. R. 1997. “Wind as an ecological factor.” Trends in Ecology and Evolution 12: 108-111. For trees being protected from enemies by other organisms see, for example, Arnold, A. E. et al. 2003. “Fungal endophytes limit pathogen damage in a tropical tree.” Proceedings of the National Academy of Sciences USA 100: 15649-15654 and Heil, M. and McKey, D. 2003. “Protective ant-plant interactions as model systems in ecological and evolutionary research.” Annual Review of Ecology, Evolution, and Systematics 34: 425-453.
Many thanks to Nicholas Mott and Jamie Shreeve for help in tracking down estimates of bristlecone pine ages, to Martin Espindola for suggesting the root-dream, and to Jonathan Swire for insights, comments and suggestions.
Olivia Judson, New York Times
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Full article and photo: http://opinionator.blogs.nytimes.com/2010/04/06/tree-mendous/
