The BBC ran a nice part-documentary last week (How to Grow a Planet) on the evolution of plants. The presenter, geologist – not botanist – Iain Stewart, climbed a coast redwood in California to impress viewers with the height of these monsters. His theory was arresting too: tall woody trees evolved as a response to browsing by the huge herbivorous sauropods of the Jurassic. The programme featured animated herds of sauropods advancing on forests of helpless lycopods and cycads like giant combine harvesters. Here’s a reconstruction of the tallest sauropod species, the brachiosaur. Its head could munch branches at 18m, three times the height of a giraffe. Cut to intrepid presenter roping his way up giant redwood.
Nice, but how credible? There are two obvious problems with this fun theory. (Big and tall page below the jump.)
Wood evolved long before the sauropods – in fact in the Devonian, before there were any land animals at all.
Here’s an amazingly clear photo of the fossil stem of a small herb dated to 400m BP. Credit: New Scientist.
However, woody plants were niche players for the next 200m years, and the cycads and horsetails had things very much their own way. At most Stewart can claim that sauropod browsing was bad news for these rivals. Woody trees allowed greater height, an inedible trunk, and fast growth – all key advantages over their rivals. It’s reasonable to suggest that in the long run sauropods promoted the triumph of woody trees.
But the second problem is insoluble. The brachiosaur’s 18m probably represents something of a technical limit on the height of a workable animal. No mammal ever reached half its height. So a modest 25m tree would be pretty safe. Why go on to 90m? See the photo at right of a fine Californian coast redwood with superimposed brachiosaur silhouette to rough scale. TANSTAFL: there must be costs involved with such overkill, and pushing the envelope quite so far. The hydraulics must be very tricky, and you are at greater risk from lightning and storms. We need something else to explain adaptations to such great height. [Update: the arms race for light explains some of it – see comments – but not I think all.]
Tropical rainforests are made up made up almost entirely of flowering trees. The canopy consists of a dense layer of trees around 30-45m tall, with a few emergents at 55m or so, rarely going up to 80m.The super-tall >90m species are temperate and almost all conifers: sequoias, douglas firs, spruce. What’s the big difference?
Undramatically, I suggest pollination and seed dispersal. Advanced flowering plants like deciduous trees depend on a variety of insect and animal pollinators: bees, wasps, flies, butterflies, moths, bats, and hummingbirds. For seed dispersal, some rely on the wind, like maples and willows; but many more on fruit-eating birds, squirrels and monkeys. For these ecological partners, some height in trees is an advantage, as it keeps them clear of ground-level predators; but too much exposes them to increasing risks of predation from birds, being blown away by high winds, and (if you are an animal) of a fatal fall.
Their older cousins the conifers on the other hand largely pollinate and disperse their seeds by the wind. To a first approximation, the area covered by a given quantity of pollen or seeds will be proportional to the wind speed. But this increases with height above ground by a power law. Getting a 10m height advantage over your neighbours increases your reproductive chances dramatically. I suggest that the wind factor economically explains the fantastic arms race for height among conifers.
The exception among macroscopically flowering trees that proves (in the proper sense) the rule is the great eucalyptus regnans of South Australia and Tasmania, which also regularly achieves 90m. It’s largely self-pollinated, and is not dependent on specific insects. The dispersal of its seeds is eccentric: they are locked up in gummy pods that stick for years to the parent tree. In normal times the pods eventually fall off and are consumed by ants without germinating. Come a forest fire, the pods explode, and the tiny seeds, in vast quantities, are spread by the wind. The non-giant Monterey pine, an undemanding forestry workhorse, has a similar pattern.
[Update 2: the best test of my theory would be systematic differences in pollination and seed dispersal between emergent and ordinary canopy trees in tropical rainforests. There are too many species involved for this to be a feasible amateur project, but one common emergent tree is the kapok. It’s pollinated by bats, which doesn’t help my theory much – though bats are strong fliers and nocturnal, so the risks of height to them are the lowest of all the possible pollinators. However, seed dispersal is by wind, the fluffy fibres acting as sails. Points to me.]
It’s a real shame about the clear-munching sauropods though.
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Photo: A 300-foot (90m) Coast redwood, composite photo Â© National Geographic, 2007. Brachiosaur added by me