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Savannas pose a bit of a problem for ecologists. There is an axiom in ecology that ‘complete competitors cannot coexist’:in other words, where two populations of organisms use exactly the same resources, one would be expected to do so slightly more efficiently than the other and therefore come to dominate in the long term. In temperate parts of the world, either trees dominate (in forests) or grasses dominate (in grasslands). Yet, in savannas grasses and trees coexist. The classic explanation proposes that trees have deep roots while grasses have shallow roots. The two plant types are therefore able to coexist because they are not in fact competitors: the trees increase in wetter climates and on sandier soils because more water is able to penetrate to the deep roots. Trees do indeed have a few small roots which penetrate to great depth, but most of their roots are in the top half-metre of the soil, just where the grass roots are.

Question : According to this paragraph, trees have deep roots while grasses have shallow roots, so they are not in fact competitors. But, I don't understand what the bold sentence means.

in the top half-metre of the soil, is there any competition between trees and grasses?

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  • It would be more idiomatic to say "What does it mean?"
    – stangdon
    Commented Jul 21, 2021 at 16:23

2 Answers 2

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It's saying that while trees do have deep roots, most of their roots are not deep and are as shallow as grass roots.

"top half-metre" means 50 centimeters down from the surface, which is around 20 inches.

"soil" means upper layer of the ground

So, breaking it down:

  • Most of their roots = great part of the roots of the trees
  • are in the top half-metre = are in the shallowest 20 inches
  • of the soil = of the ground
  • just where the grass roots are = in the same place where the roots of the grass are

Simplifying we get to "a great part of the roots of the trees are at most 20 inches deep, in the same place the grass roots are"

So yes, there should be competition between trees and grasses if the axiom mentioned in the first line is true. But that paragraph doesn't go deep enough to explain why it isn't true in savannas, and it certainly is implying that it is not true in that savannas "pose a bit of a problem for ecologists."

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The author is trying to point out that the fact that trees and grass do use exactly the same resources does not make them competitors. He says that along with deep roots, trees also have small roots, so according to what he calls the "traditional theory", they should be competitors.

The excerpt stops short though. It doesn't explain what makes this coexistence possible without "competition".

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