Friday dinosaur blogging: tall stories

Did tall trees evolve to escape munching sauropods?

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 © National Geographic, 2007. Brachiosaur added by me

Author: James Wimberley

James Wimberley (b. 1946, an Englishman raised in the Channel Islands. three adult children) is a former career international bureaucrat with the Council of Europe in Strasbourg. His main achievements there were the Lisbon Convention on recognition of qualifications and the Kosovo law on school education. He retired in 2006 to a little white house in Andalucia, His first wife Patricia Morris died in 2009 after a long illness. He remarried in 2011. to the former Brazilian TV actress Lu Mendonça. The cat overlords are now three. I suppose I've been invited to join real scholars on the list because my skills, acquired in a decade of technical assistance work in eastern Europe, include being able to ask faux-naïf questions like the exotic Persians and Chinese of eighteenth-century philosophical fiction. So I'm quite comfortable in the role of country-cousin blogger with a European perspective. The other specialised skill I learnt was making toasts with a moral in the course of drunken Caucasian banquets. I'm open to expenses-paid offers to retell Noah the great Armenian and Columbus, the orange, and university reform in Georgia. James Wimberley's occasional publications on the web

8 thoughts on “Friday dinosaur blogging: tall stories”

  1. You might want to watch the documentary – “National Geographic: Climbing Redwood Giants”. It’s on Netflix in HD, and it is great.

    Incredible fact from the show; The coastal redwood forest has 5 – 10x the biomass per acre compared to a tropical forest. These are the most productive ecosystems on the planet. The bigger the trees get, the faster they grow.

    And I’m with CM, access to sunlight is the most important factor driving height.

  2. I should have pointed out that the picture of the entire redwood used in this post is from the show I mention above.

  3. Also, the show suggests that fog is a big factor in how tall redwoods get. They absorb huge amounts of water from fog in the summer, and they don’t have to do all the work of lifting the water to the top.

    Notably, the Sequoias over in the Sierra Nevada get as big or bigger, but never as tall. They don’t have the fog.

  4. We learned in forest ecology in CA that it was likely a combination of factors, most importantly fog and dispersal. The hydrology of their internal structure and gravity limits their height.

  5. Sunlight could be a differential as compared with tropical trees.

    In the temperate zones, there is a more pronounced seasonal difference between winter & summer.
    Also, the light comes in more obliquely. Could that be a factor in an arms race for solar cross section?

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