The heavy rainfall that powers rainforests is caused by the complex interaction of multiple geographic and climatic factors, including mountain ranges, ocean and wind currents, and the intensity of solar exposure. The forests themselves contribute to rainfall, with evaporation from plant biomass contributing significantly to the water cycle.
Disruptions to any of these factors can lead to drought – altering the delicate balance that sustains them. Periodic droughts are normal in rainforests but they regenerate in short order once the rains return. However, they are increasingly stressed by decreased rainfall resulting from climate change and deforestation.
When the rain doesn’t come, a cascade of harmful effects ensues. Trees become stressed and close their stomata – pores in their leaves that facilitate gas exchange – thus reducing intake of carbon dioxide and release of water vapour and oxygen. Eventually, they may shed their leaves. Hot, dry conditions can result in the rupture of the xylem cells that conduct water and ultimately kill the tree. Understory plants that rely on shade from the taller trees perish, too.
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When trees become stressed and die, the forests become more susceptible to wildfires. Fires may occur naturally due to lightning strikes but some are set intentionally by humans in order to clear space for agriculture. After the fires subside, invasive plant species can take hold, reducing the chances that the forest will rebound.
Decreased leaf cover and fruit production affect the animals reliant on these resources. They may be forced to migrate to more abundant regions, creating competition with other animals, or simply starve. Smaller animals that need regular access to water will die because they are unable to move over long distances.
The bodies of water that thread through rainforests reduce in volume or disappear entirely. This can lead to large-scale death of fish and other aquatic organisms – the Amazon drought has killed hundreds of river dolphins due to overheating.
Indigenous people who make their livelihoods from the rainforest may face extreme privation as well. In some cases, they cannot even leave due to the reduction of the waterways they rely on for transport.
Rainforest droughts also have worldwide implications: these forests are some of the world’s largest carbon sinks, holding millions of metric tonnes of carbon. These forests are crucial to the global ecological balance.
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Top image: Madidi National Park in the Amazon rainforest, Bolivia. Credit: Getty