By John Bradley
We all know that trees do great things for our environment. Trees and all their plant neighbors use the sun's energy for photosynthesis and provide many ecological benefits. Trees use CO2 from the atmosphere to grow, they sequester carbon in the soil, transpire water vapor into the atmosphere, cool the local air, and add organic matter to the soil. Removing CO2 is an important contribution to climate mitigation and resilience.
Trees also play an important role in the water cycle, transpiring moisture back into the atmosphere. This is called precipitation recycling and is a continuous process as airflows move over distant landscapes. This plays out as water evaporates from leaf surfaces and the ground and the trees transpire moisture into the atmosphere. The process is called evapotranspiration. Over 40% of rainfall on land is from atmospheric moisture from evapotranspiration. This recycled moisture contributes additional rainfall in the local catchment as well as in distant catchments. Satellite observation can actually distinguish the chemical signature of water vapor, whether from ocean evaporation or forest evapotranspiration.
Precipitation recycling is integral to the water cycle on all continents, and when it is disrupted, such as through deforestation and urbanization, rainfall patterns change. On all continents, deforestation reduces the frequency of rainwater being recycled, and can impact rainfall locally and in distant locations. There is extensive research on the impact of forest degradation on weather patterns on all continents; and there is a growing body of practical experience on regenerating ecosystems to restore the water cycle, mostly in arid landscapes. There is much to learn on using this knowledge to build resilience to climate change in varied climates.
The next time you walk in the forest and notice its coolness, this is why.
Notes and References:
Explanation of the Graphic: Effects of forests on water and climate at local, regional and continental scales through change in water and energy cycles.
(1) Precipitation is recycled by forests and other forms of vegetation and transported across terrestrial surfaces to the other end of continents.
(2) Upward fluxes of moisture, volatile organic compounds and microbes from plant surfaces (yellow dots) create precipitation triggers.
(3) Forest-driven air pressure patterns may transport atmospheric moisture toward continental interiors.
(4) Water fluxes cool temperatures and produce clouds that deflect additional radiation from terrestrial surfaces.
(5) Fog and cloud interception by trees draws additional moisture out of the atmosphere.
(6) Infiltration and groundwater recharge can be facilitated by trees.
(7) All of the above processes naturally disperse water, thereby moderating floods.
Graphical Abstract is from: Trees, Forests, and Water: Cool Insights for a Hot World
References:
Trees, Forests, and Water: Cool Insights for a Hot World, D.Elison, et al, Elsevier, Global Environmental Change, VOl 43, March 2017; this article reviews 93 scientific articles on forest, water and energy cycle interactions ; https://www.sciencedirect.com/science/article/pii/S095937801730013
Climate Water Project:
What causes the rainy season to start in the Amazon, Congo, and Great Plains, by Alpha Lo, Feb 21, 2024 https://climatewaterproject.substack.com/p/what-causes-the-rainy-season-to-start
Millan Millan: He helped us to understand rain: A pioneer in precipitation recycling and the small water cycle, by Alpha Lo, Jan 27, 2024 https://climatewaterproject.substack.com/p/millan-millan-he-helped-us-understand
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