Thomas' Plant-Related Blog

On plant science. Mostly.

Why trees have to leave home

with 2 comments

Tropical forests are a challenge for ecologists, because there are so many species. It’s not just a practical challenge to identify them, but a theoretical question of why they’re all there. Why don’t the better adapted ones win out, while others go extinct? Does every species really have its own ‘niche’—some set of conditions where it outcompetes everything else? Or are the different species much the same? (I’ve discussed neutral theory before)

A third idea is that trees can’t grow too close to their parents (or others of the same species), perhaps because pests and diseases spread between them. This is called the Janzen-Connell hypothesis, after the scientists who, independently, thought of it. If each species has to space itself out to escape pests, other species can grow in the gaps, without outcompeting each other. Some new evidence backs this up.

One study analysed where thousands of seedlings were growing, on an intensely studied plot on Barro Colorado Island, Panama. After five years, saplings with others of the same species nearby were less likely to have survived. Interestingly, the effect was apparently greater for rare species than for common ones; perhaps that helps to explain why they are rare.

Another experiment looked at seedlings grown in pots, with soil taken from under the same tree species, or another one. Seedlings grown in ‘home’ soil once again fared worse, suggesting that a disease or a pest in the soil could be the key.

Owen T. Lewis (2010) Ecology: Close relatives are bad news. Nature (News & Views) 466, 698–699

Ecology: Close relatives are bad news


Written by Thomas Kluyver

9 August, 2010 at 11:54 pm

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  1. Interesting ideas. There is also contrasting evidence from some tropical forests that trees can improve their chances of survival by engaging in intraspecies root grafting. This allows the transfer of nutrients and carbon between trees in more areas with better soil and light conditions and trees that are less advantaged. In the Carribean, tree networks formed by grafting can provide structural stability that helps them withstand hurricanes.

    If you’re interested, I have a longer post about this phenomenon here


    8 January, 2011 at 5:47 am

    • Thanks, Maddie. Just looking over at your blog, there’s some interesting stuff there that I’ll go and read. 🙂

      That’s an interesting situation, although the resource transfer sounds a bit like group selection–if that was the only benefit, surely a tree which “joined the network” only if it found itself in a shady situation would be an evolutionary success, and the whole thing would break down?

      Thomas Kluyver

      12 January, 2011 at 12:08 am

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