Did you know that a single tree provides the same cooling as two household aircons? We are usually not much aware of the impact of forests and trees. After all, do not we need more space to live, build roads, set up industry - in short, to develop? And what do we need forests for anyway, apart from the wood and a pleasant Sunday walk?
But this idea misses something quite essential: the finely-woven interdependence between everything that exists on our planet. Climate change is precisely a consequence of disturbing that fine balance. Countless measures were then planned out to counteract the rise in Earth's temperature, with forests mainly intended to function as carbon storage. After all, carbon is the major villain of climate change.
A large group of scientists from around the world – with the support of the Belgian Development Cooperation via the Flemish Interuniversity Council (VLIR-UOS) – has, however, brought a note of caution to this vision (*). While forests are indeed genuinely important for the carbon cycle, they also play a perhaps even greater role in the housekeeping of water and energy. These scientists are therefore calling on policy-makers to move away from a carbon-focused vision and instead mobilise our forests as powerful tools for cooling and improved water availability.
Through their impact on climate change forests and trees will be even indispensable for guaranteeing our food and water supply in the future. Beyond that, trees and forests provide much more than just wood, including fruit, cattle feed, honey and medicines. They also host 80% of national biodiversity, which in turn holds enormous economic significance.
And yet, human beings continue to chop down en masse. Between 2000 and 2012, 1.5-1.7 million km² of forest – 3.2% of total forest coverage – disappeared due to logging, conversion to agricultural land, city expansion and forest fires. Around 18% of the rise in the Earth's temperature can be attributed to deforestation. So it's high time to make conscious use of our forests in the battle against climate change.
The scientists have discerned five major effects of forests on water and energy housekeeping.
1. Forests promote precipitation
- The leaves of forest trees cause a good deal of water from the soil to evaporate through their leaves. Around 40% of precipitation on land is caused by this transpiration from trees, along with spontaneous evaporation of water.
- In the tropics, air passing over forests causes twice as much rain as air passing over land with sparse vegetation.
- The effects of forests can sometimes be felt thousands of miles away. The Congolese forests, for example, are the cause of rainfall in the Ethiopian highlands or the Amazon Rainforest in the Argentinian Andes.
- Trees release biological particles such as pollen, fungal spores and bacterial cells. These can serve as the nuclei of rain droplets or snowflakes, and are better at doing so than flakes of dust.
- Deforestation leads to reduced evaporation and, thereby, less precipitation.
- At greater heights, cloud forests can draw moisture from clouds and mist.
2. Trees and forests are natural cooling systems
- The temperature beneath the crowns of trees and forests is appreciably lower than in open spaces or urban environments.
- Trees use solar energy to evaporate moisture. The spontaneous evaporation of water, both at ground level and at the forest crown, also uses energy. Just as with evaporating sweat on the human skin, this causes the earth's surface to cool.
- A single tree displays a cooling effect similar to the aircons of two average households. A cheaper and more energy-efficient solution!
3. Forests generate air and moisture flows
- Trees and forests are the root cause of air flow in the atmosphere.
- Large areas of continuous forest, from the coast through to inland regions, carry atmospheric moisture deep into a continent's interior.
- Coastal forests draw in the majority of moist air from across the sea. The moist air is then carried into the inland regions.
4. Trees and forests improve groundwater recharge
- Water is better absorbed into the soil thanks to trees and forests. Soil quality is known to degrade when deprived of the shelter of trees. It loses nutrients and organic carbon, develops a weaker structure and is less able to retain water. As a result, rainwater is more likely to run off, taking fertile soil particles with it (i.e. erosion).
- Trees create macropores into the earth with their roots. The ground-dwelling animals living around tree roots contribute to this as well. Thanks to these tunnels, the water can flow quite freely into the ground.
- The fallen leaves and shade cast beneath the trees also lead to better water absorption, less evaporation and more ground dwellers.
5. Trees regulate river flows
- River discharge varies according to precipitation levels. Trees act as a buffer against these fluctuations because they stimulate the replenishment of groundwater (see 4). Because more groundwater is available, rivers and springs will less likely turn dry during dry periods.
- Deforestation can make it more difficult for water to enter the soil, causing the surface to run off and carry soil particles along with it. This can lead to a build-up of silt in the water flow.
- If the forests are missing, river discharge becomes a lot less predictable and there is a higher risk of floods and droughts.
(*) Trees, forests and water: Cool insights for a hot world. Ellison et al. Global Environmental Challenge 43 (2017), 51-61.