continually move upwards when the stomata are open and if the atmospheric conditions are favourable, the stomata can open and close when needed. The water transpires from the leaves
using heat from the surrounding air to change from liquid to vapour, which in turn cools the air. The more trees, the more transpiration, the cooler the surrounding air, the cooler the local climate. The more forests, the cooler the global climate. Not only does this process have a direct cooling effect, but water vapour above forests creates clouds that reflect sunlight, having an additional cooling effect on the planet. The clouds produce rain, sending water back into the ground for the cycle to continue. In the tropics, this process is supercharged. Rainforests release billions of tonnes of water vapour into the atmosphere every day, which is why the vast belt of clouds that flanks the equator can be seen from space. All life on Earth depends on water
being available in the right places at the right times. Clouds formed in
the tropics can be carried by wind to faraway places where the rain falls, often watering our crops. However, this cycle is increasingly threatened by climate change and deforestation. A rapidly warming atmosphere and more frequent intense droughts are putting pressure on trees’ delicate transpiration systems. This is already affecting the natural world, causing water insecurity and jeopardising harvests. Our Science Engagement Team
is working with researchers from the University of Exeter who are investigating how climate change affects the water cycle of plants in tropical habitats. Ecologist Professor Lucy Rowland and her group are studying the world’s tallest trees in the rainforests of Brazil and Southeast Asia. They’re testing the limits of the trees’ growth and survival to understand how far climate change (and resulting local changes to environmental conditions) could push their amazing but delicate water transport systems before they fail and the trees die. To collect
data, they trek to remote parts of the Tumucumaque National Park in the Amazon and the Sepilok Forest Reserve in Borneo and work with local guides, researchers and tree-climbing teams to reach the tops of these tropical giants – the Dinzia excelsa in the Amazon and the dipterocarps in Borneo. Some are so big that their trunks are over two metres in diameter, and a fallen branch could easily be mistaken for a separate tree. Their findings are alarming –
they suggest that the largest trees, especially in the Amazon, are also the most vulnerable to the effects of climate change. Their ability to move water higher than any other living being comes at a cost: they experience much greater pressure on their internal ‘plumbing’ system. And because they tower over the protective shade of the canopy, they’re more exposed to hotter, drier conditions, which are becoming more frequent. In addition, they’re under increasing threat of logging for timber. These forest giants are often very old, having spent hundreds of years
Photo: Buradaki/Shutterstock
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