by Colin Tudge
Another approach is to look in the world’s herbaria. For obvious reasons, common species tend to appear in many different herbaria, while rarer ones turn up only in a few. The very rare species are represented in no herbaria at all—meaning that they are as yet unknown. Again, it’s reasonable to suppose that a great many are unknown—precisely because they are rare (and/or extremely inaccessible). In fact, there is good reason to suppose that most of the species in any one genus are rare. Since the rarest are the least likely to be found, this in turn implies that within any one genus, only a minority of species (the commonest ones) have so far been identified (even after three hundred years of exploration, which in some areas has been fairly intense).
Dr. Mike Hopkins and his colleagues at the Brazilian EMBRAPA research center applied comparable chains of reasoning to the known distribution of various species of trees from several families and genera. As with all plants, a few species of Amazonian trees are fairly common over quite a wide area; some are common in one place but rare elsewhere; some are rare all over but widely scattered; and some are rare and seem to occur in only one or a few places. Any one area is liable to contain various species of any one genus, some common and some rare, some belonging to widespread species and some very local. From the limited data available—good studies of a few places, such as the Ducke Reserve, and scattered observations from elsewhere—and by making a few assumptions that are largely commonsensical, the botanists are able at least roughly to work out what the distribution of each species is liable to be. Thus if specimens of one particular species are known only from areas six hundred miles apart, it is obvious that there must be others in between, even though the others are not known. Either that, or the two specimens that are known are the last of their species and are simply waiting to go extinct. By such cogitations (reinforced by computer models), botanists are able at least roughly to estimate the distribution of the trees they have identified, as well as how densely they are liable to occur even in places that no botanist has yet visited.
By putting all such data and estimates together, and applying a series of mathematical projections whose details I won’t try to convey, the botanists can then work out, at least to a first approximation, how many species of plants (or trees) there are likely to be in any one place, and how greatly the list of species is liable to differ between any two places. They are able to do this (approximately) even though they have not visited most of the places they are making guesses about, let alone studied them. They are also able to estimate how many species in any one place are still unknown. Finally, and important, they are able to guess where the “hot spots” are liable to be: the places where there are liable to be most species. All this may sound too rarefied for words—guesswork running miles ahead of data, pulling itself along by its own bootlaces. Indeed, such ways of thinking are known generically as “bootstrapping.” In fact, the method is more robust than it may seem from this necessarily rough description. More important, the estimate arrived at is not just a guess, left hovering in space. It offers testable hypotheses, which are the stuff of real science. That is, if today’s botanists guess that such-and-such an area ought to contain a high number of species, with a high proportion of a particular genus, then, in the fullness of time, when more grant money is available, future botanists will be able to go out and see if the projections are true. The more the predictions do prove to be true, the more the calculations on which they are based are vindicated; and if they prove untrue, that is instructive too. In the short term, such estimates could have great significance for conservation—not least because it’s the hot spots, so far identified only on theoretical grounds, that seem most worthy of protection. In short: even if the estimates are wrong, they are definitely better than nothing.
2. KEEPING TRACK
Jose Eduardo L. da S. Ribeiro, ed. Flora da Reserva Ducke. Manaus: INPADFID, 1999.
1. At least a dozen different species of trees are marketed as “angelim.” All are from the family Fabaceae—but they do come from two different subfamilies. Thus from the subfamily Papilionoideae come at least half a dozen different species of Hymenolobium; at least another three from the genus Vatairea; plus a Vataireopsis and an Andira. From the subfamily Mimosoideae come Zygia racemosa, Dinizia excelsa, and the magnificent Parkia pendula. Similarly, the valued taurai tree commonly includes at least five species (and probably many more) from the Brazil nut family, Lecythidaceae. At least two of the alleged taurais are from the genus Cariniana, and another three (probably more) from Couratari.
3. HOW TREES BECAME
1. Martin Ingrouille. Diversity and Evolution of Land Plants (London: Chapman & Hall, 1992). An excellent general outline of plant evolution.
4. WOOD
Aiden Walker, ed. The Encyclopedia of Wood. London: Quantum Publishing, 2001.
5. TREES WITHOUT FLOWERS
Aljos Farjon. World Checklist and Bibliography of Conifers. 2nd ed. Kew: Royal Botanic Gardens, 2001.
6. TREES WITH FLOWERS
1. In particular I have in mind Heywood’s Flowering Plants.
7. FROM PALMS AND SCREW PINES TO YUCCAS AND BAMBOOS
E. J. H. Corner. The Natural History of Palms. London: Weidenfeld & Nicolson, 1966. A classic by one of the twentieth century’s most original botanic thinkers. In the text I call it “Corner.”
9. FROM OAKS TO MANGOES
Thomas Pakenham. The Remarkable Baobab. London: Weidenfeld & Nicolson, 2004.
1. Thus while in the 1970s Heywood placed both the currants and the hydrangeas within the Saxifragaceae family, Judd (writing in 2002) separates the currants into the Grossulariaceae family and gives the hydrangeas their own family, the Hydrangeaceae—which, for good measure, he transfers to a quite different rosid order, the Cornales. Then again, while Heywood groups the American sweet gum (Liquidambar) in with the witch hazels (Hamamelis) within the Hamameliaceae family, Judd puts the sweet gums together with the ramara tree (Altingia), within the Altingiaceae family. At this stage of taxonomic history, with molecular studies and computer-assisted cladistics rapidly coming on board, life can be very confusing.
2. Cercidiphyllum is often traditionally classified alongside Trochodendron in the Trochodendraceae.
3. Rosaceae is such a big and various family that it has often been divided into tribes: Judd recognizes the Rosoideae (the group with roses) and Maloideae (the group with apples) and several smaller groupings that don’t seem to fit in either. The Maloideae in particular have a remarkable tendency to hybridize not only within species but even between genera. Crataegus demonstrates this in spades: Judd speaks of 265 species worldwide, but others estimate nearer 400; in any case, it is extremely hard to see which is a true species and which a hybrid, and which true species have arisen as hybrids of others. The same is true to a lesser extent of Amelanchier (with about thirty-three recognized species); for good measure, Crataegus and Amelanchier seem at times to have hybridized with each other.
4. Botanists have struggled ever since the time of Linnaeus to bring some order to the prodigious variety of oaks. There are more than twenty different classifications in the literature. But there are big problems. Some individual species of Quercus are enormously variable. Quercus, too, is among those many genera of trees that are prone to hybridize, so it can be hard to see where one species ends and the next begins. As we saw in Chapter 1, the concept of “species” seems to be far more flexible among trees (and, indeed, plants in general) than among animals—although even animals hybridize far more than was traditionally supposed. To cap it all, taxonomists cannot agree on which features reveal true evolutionary relationships and which are incidental. As things stand, many taxonomists at present split the genus into three “series”: the red oaks, restricted to North America; the white oaks; and a mixed bag of intermediates. Some kinds, however, including the holm oak, Q. ilex, do not seem to fit comfortably into any of these groupings. It seems best to treat this genus splitting as work in progress, and hope that in time th
e new molecular studies will throw more light.
5. From their origins in Southeast Asia oaks spread in all directions, and by the Eocene, around 55 million years ago, the fossils show they were common in China, Europe, and North America. This was a warm period: there were cycads, primarily trees of the tropics, as far north as Alaska. The world has been cooling fairly steadily ever since (although there have been a few warm spells). The pending greenhouse effect is returning us (roughly) to the climate of forty to fifty million years ago. Oaks diversified rapidly between thirty-five and five million years ago, a cooler and drier period that gave rise to the vast expanse of modern grasslands. Most of the several hundred modern species of oak were probably extant by fourteen million years ago.
11. HOW TREES LIVE
M. R. Macnair. “The Hyperaccumulation of Metals by Plants.” In Advances in Botanical Research, vol. 40, pp. vi–105. Amsterdam: Elsevier, 2003. On plants that tolerate nickel and other metals.
Peter H. Raven, Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 5th ed. New York: Worth Publishers, 1992. A good general introduction to plant physiology.
1. The evolution of photosynthesis, somewhere around two billion years ago, changed the course of life on earth. The evolution of all creatures changed direction. The cyanobacteria (or their ancestors) that first evolved photosynthesis clearly lived at first in the absence of free oxygen—since before they developed photosynthesis, there wasn’t any. As those first photosynthesizers put more and more oxygen into the atmosphere, so they and all other creatures had to adapt to it (or stay out of its way, as some microbes still do, living in the depths of airless marshes). The adaptation of ancient creatures to the constant presence of oxygen gas was a huge physiological leap, of enormous evolutionary importance. Oxygen is very lively stuff, very reactive, and for creatures that can make use of it, it is extremely useful. In particular, creatures like us use it to break down sugars to provide energy by the method known as “aerobic respiration”; and aerobic respiration is fast and efficient. But for creatures that are not adapted to it, oxygen is highly toxic, one of the quickest and surest killers there is. (Creatures like us, who do make use of oxygen, still need to pack our bodies with “antioxidants,” such as vitamin C, to protect our flesh against its corrosiveness.)
As described in Chapter 3, the chloroplasts, which contain the chlorophyll within green leaves, have evolved from cyanobacteria that, in the deep past, lodged in the host cell.
2. The two forms are known as Pr and Pfr. Pr absorbs red light and Pfr absorbs “far-red” (which effectively means infrared) light. Pfr is biologically active, and causes things to happen. Pr is inactive, its presence leaving the plant unmoved. Light flips the pigment between its two forms, and thus provides an on-off switch. Red light shone on (inactive) Pr converts it to (active) Pfr; and far-red light shone on (active) Pfr converts it to (inactive) Pr. Sunlight contains both red and far-red, so Pr and Pfr are normally in equilibrium. At noon, with the sun at its brightest, about 60 percent of the phytochrome is in the form of Pfr. But in the dark of night, as the hours pass, Pfr steadily spontaneously degrades to become Pr: the active form decays into inactivity. Yet one brief flash will reconvert the accumulating Pr back into the active Pfr. In a long-day (short-night) plant, the burst of Pfr induces flowering. In a short-day (long-night) plant, it suppresses flowering.
12. WHICH TREES LIVE WHERE, AND WHY
Robin L. Chazdon and T. C. Whitmore, eds. Foundations of Tropical Forest Biology. Chicago: University of Chicago Press, 2002.
Verna R. Johnston. California Forests and Woodlands. Berkeley: University of California Press, 1994.
“Plant Phylogeny and the Origin of Major Biomes.” Discussion meeting at the Royal Society, London, March 2004, organized by Toby Pennington, Quentin Cronk, and James Richardson. Proceedings published by the Royal Society in October 2004.
1. Recorded in J. David Henry, Canada’s Boreal Forest. Washington, D.C.: Smithsonian Institution Press, 2002.
13. THE SOCIAL LIFE OF TREES
Egbert Giles Leigh, Jr. Tropical Forest Ecology: A View from Barro Colorado Island. Oxford: Oxford University Press, 1999.
Ghillean T. Prance. “The Pollination of Amazonian Plants.” In Key Environments, Amazonia, ed. G. T. Prance and T. E. Lovejoy. Oxford: Pergamon Press, 1985.
1. S. Patino, E. A. Herre, and M. T. Tyree, “Physiological Determinants of Ficus Fruit Temperature and Implications for Survival of Pollinator Wasp Species,” Oecologia 100 (1994): 13–20. Other key sources for their researches on figs and fig wasps are:
E. A. Herre, N. Knowlton, U. G. Mueller, and S. A. Rehner. “The Evolution of Mutualisms: Exploring the Paths Between Conflict and Cooperation.” Trends in Ecology and Evolution 14 (1999): 49–53.
Edward Allen Herre and Stuart A. West. “Conflict of Interest in a Mutualism: Documenting the Elusive Fig-Wasp–Seed Trade-off.” Proceedings of the Royal Society, Series B, 267 (1997): 1, 501–07.
Edward Allen Herre. “Population Structure and the Evolution of Virulence in Nematode Parasites of Fig Wasps.” Science 259 (1993): 1, 442–45.
Carlos A. Machado, Emmanuelle Jousselin, Finn Kjellberg, Stephen G. Compton, and Edward Allen Herre. “Phylogenetic Relationships, Historical Biogeography and Character Evolution of Fig Pollinating Wasps.” Proceedings of the Royal Society, Series B, 268 (2001): 685–94.
2. Stanley A. Temple, “Plant-Animal Mutualism: Coevolution with Dodo Leads to Near Extinction of Plant,” Science 197 (1977): 885–86. See, too, the protest in Science 203, (1997): p. 1364, from A. W. Owadally of the Forestry Service, Mauritius, and Dr. Temple’s reply in the same issue.
14. THE FUTURE WITH TREES
Global Environment Outlook 3. London: Earthscan/UNEP, 2002.
M. Ibrajim and J. Beer, eds. Agroforestry Prototypes for Belize. Turrialba, Costa Rica: CATIE, 1998.
Wangari Maathai’s Nobel Acceptance Speech and other articles about her work can be found on the Web site of the Green Belt Movement of North America: www.gbmna.org.
Andrew W. Mitchell, Katherine Secoy, and Tobias Jackson, eds. The Global Canopy Handbook. Oxford: Global Canopy Programme, 2002.
L. Szott, M. Ibrajim, and J. Beer. The Hamburger Connection Hangover. Turrialba, Costa Rica: CATIE, 2000.
GLOSSARY
A
ALLELE Many genes are “polymorphic,” meaning they may take more than one form. An allele is any one of the possible variants.
ALTERNATION OF GENERATIONS All land plants practice alternation of generations, in which a diploid generation (the sporophyte) gives rise to a haploid generation (the gametophyte), which in turn gives rise to another sporophyte generation, and so on. (Some animals also exhibit alternation of generations, including the cnidarians, which include jellyfish and anemones. But the basis of this is quite different.)
ANALOGOUS Applied to structures that are similar in function but originate in different ways. Thus the wings of a fly are merely analogous to the wings of a bird. Many plants, including some acacias and conifers, have phyllodes instead of leaves: they do the same job but originated differently.
ANGIOSPERM Technically, the term refers to plants whose seeds are completely enclosed within an ovary. More casually (but accurately) “angiosperm” simply means “flowering plant.”
ARIL A covering around the seed that is often formed by outgrowth from the base of the ovule. Arils are often brightly colored and lure animals that disperse the seeds. Yew “berries” are arils, and so is mace, the aromatic lacy covering of the nutmeg seed.
B
BROADLEAF The term colloquially applied to a dicot tree.
BRYOPHYTE A primitive land plant that lacks specialized conducting tissue (“tracheary elements”) for internal transport of water and nutrients. The most conspicuous generation is the gametophyte. The living examples are hornworts, liverworts, and mosses. In earlier classifications these three were grouped together in the formal taxon “Bryophyta,” spelled with a capital “B.” But the
three do not necessarily share a specific common ancestor, and so do not form a true clade, and so should not be presented as a formal group. But they are all of the same “grade,” which can be denoted by the informal “bryophyte,” spelled with a small “b.”
C
CAMBIUM A meristem that gives rise to parallel rows of cells. The cambium of coniferous or angiospermous trees is responsible for “secondary thickening,” producing xylem tissue on the inside and phloem tissue on the outside.
CARBON FIXATION The process by which hydrogen is combined with carbon dioxide from the atmosphere to produce organic molecules.
CATKIN An inflorescence of single-sexed flowers arranged as on a spike. Catkins are found primarily in woody plants, including trees such as willows and oaks.
CHLOROPHYLL The green pigment that mediates photosynthesis.
CHLOROPLAST The organelle that contains the chlorophyll.
CHROMOSOME Chromosomes are long, thin structures that carry the genes. During most of the life of the cell, each chromosome is spread throughout the nucleus, and in this relaxed form they cannot be seen under the light microscope. But during cell division (mitosis and meiosis) the chromosomes contract to form short rods, which, when suitably stained, can be seen clearly. In this visible, contracted form, each chromosome has its own characteristic size and shape; and each organism has its own characteristic number of chromosomes, each with its own characteristic size and shape. In truth, each chromosome consists of one enormously long molecule (or “macromolecule”) of DNA.
