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The international research team used satellite data to chart the height of forests around the world for the first time. They zoomed in on forest areas 50 by 50 km in size. ‘We then mapped the 10 percent tallest treetops in those grid cells,’ explains Scheffer.
That revealed a surprising phenomenon. Giant trees, more than 25 metres tall, only grow in places with annual rainfall of more than 1500 mm. What is more, this lower limit applies everywhere, from tropical regions to temperate zones. That is remarkable. ‘If the climate is temperate, it is not so hot, there is less evaporation and so the trees need less water. Yet this phenomenon is universal,’ says Scheffer.
The threshold of 1500 mm of rain is not entirely random. According to Scheffer, it is the same threshold that determines the transition from savannah to tropical rainforest. ‘In the tropics, that 1500 mm rainfall is as much as the evaporation via the leaves. So the net value of rainminus evaporation per surface area is zero. But in temperate zones it’s not zero. That’s the weird part.’ Scheffer speculates that this deviation has to do with the fact that the few giant forests found in temperate regions are exceptional anyway. ‘Those regions have been impacted so much by humans that the remaining forests are the most resilient ones.’ They are super-survivors.
The good news for giant forests is that climate change works to their advantage. The climate will become drier in some areas but a lot wetter in others. ‘As a result, the potential area for giant forests will increase by an amount eight times as big as Spain,’ predicts Scheffer. ‘But we do then need to make room for the forest. And that is no easy matter because there is huge pressure on tropical forests. There are also some uncertain factors. The increase in extreme conditions such as droughts and heatwaves can also cause tropical forests to die.’