Study shows binary switches between forest, savanna, desert: macro-scale climate-biodiversity tipping points

Researchers of Wageningen University find in South America, Africa and Australia under climatic change forest, savanna and treeless systems don’t gradually phase across, but rather tip over.

climate change biodiversity Amazon rainforest
[Picture: Amazon rainforest, close to 100 percent tree cover optimum, under condition of daily oscillating rainy cycle. Wageningen University handout.]

That is because intermediate states, with around 5% and around 60% tree cover in nature do not seem to exist, as somehow in the tropics and subtropics the ecology-climate interaction works towards one of three optimum states.

This means species have far less time to migrate – and a much smaller chance to survive. It also means there could be truth in the feared transition of the Amazon, where drought leads to vegetation damage, which in turn may lead to further disturbance of the rainy cycle. Where now is tropical rainforest (maximum biomass density), in the future savanna could remain – with an average tree cover of only around 20 percent. [Speaking of tipping points: apart from biodiversity loss, this transition would also result in the release of many gigatonnes worth of carbon to the atmosphere.]

Climate-biodiversity threshold mechanisms at every ecosystem level

Earlier today we´ve looked at the biodiversity impact of climate change on the smallest ecosystem level: a dried-out aquatic desert ecosystem. It is a classical case study to show ecological damage most likely occurs after a disturbance (like a drought) passes a threshold. A normal intermediate drought may have no effect, because the stream will only temporarily decrease in size, or there may at least remain puddles for aquatic species to find shelter – until the rains come back. With the introduction of more extreme drought the whole ecosystem may dry out – and suddenly local extinctions occur, as has recently happened in the Arizona desert.

Now the new research, which has been published in Science, shows such thresholds exist in macro tropical ecosystems as well, possibly across the entire biomes of tropical forest, savanna and ecosystems of the ‘treeless state’ (treeless scrubland, treeless grassland and desert).

Satellite observations show current tipping points

The researchers hope their findings will also serve a practical use. The same satellite data that revealed ecosystem transitions to them also shows where precipitation changes come may lead current biome boundary to the brink. There, local conservation measures could be put in place to for instance try and improve vegetation resilience.

Considering a recent Nature publication which has quantified what is actually at stake under the [conservative] IPCC scenario range, we are slowly starting to think perhaps the wisest thing would be to consider something of a top-down approach. Let’s not forget in the end it is all about emissions.

© Rolf Schuttenhelm |

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