Article in San Francisco Chronicle re Stew Winchester:
http://www.sfgate.com/cgi-bin/article.cgi?file=/c/a/2002/03/02/HO159531.DTL Plus this piece from Olivia Judson on the influence of science and biology on modern life, sent by Dave Mrus
April 6, 2010, 6:15 pm
Tree-mendous By OLIVIA JUDSON
Olivia Judson on the influence of science and biology on modern life.
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.
Monica Almeida/The New York Times 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, for example, 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.
What forces produce trees, and could any plant, in principle, evolve into one?
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.
http://opinionator.blogs.nytimes.com/2010
