There is growing recognition of the relationship that trees and forests play regarding climate change. The interactions between forests, water, and energy provide not only the foundations for carbon storage but for cooling the Earth’s surfaces and distributing our global water resources.
Trees and forests:
- are linked to rainfall;
- provide systems that naturally cool the earth;
- transport water both locally and globally; and
- regulate water supplies.
There is no doubt of the intrinsic link between forest cover and water. In a country like Australia, where water is a coveted resource, increasing green cover is something that we need to do urgently. We also need to protect our existing canopy cover and old-growth forests as a priority.
Evaporation and plant transpiration from the Earth’s surface to the atmosphere (known as evapotranspiration) is responsible for around 40% of all rainfall over land. The more cleared land a region has, the less rainfall it may receive because there is less water in the atmosphere. Biological particles emitted by forests during transpiration allow cloud droplets to form at much lower temperatures than they do in clean air, which can also increase rainfall at lower altitudes.
Forests remain much cooler during the daytime due not only to shade but also because evapotranspiration helps to reduce sensible heat. Individual trees can transpire hundreds of litres of water each day – a cooling power equivalent to two average household central air-conditioning units. The organic particles emitted into the atmosphere within forest areas create low-level cloud systems, increasing solar reflectivity and contributing to regional and global cooling. Through transpiration and condensation processes, forests actively create low-pressure regions that draw in moist air from the oceans, generating prevailing winds capable of carrying moisture and sustaining rainfall far inland. Broad and continuous areas of forest that continue from coastline to inland regions promote atmospheric circulation that delivers rainfall to continental interiors. Deforestation leads to a reduction in the predictability, extent, and quantity of rainfall.
Trees and forests improve the infiltration of water into soils, while the soil of cleared land becomes degraded, suffering reductions in organic carbon, nutrient content, soil structure, and water-holding capacity. As a result, cleared land develops surface runoff and increased erosion. Though deforestation may increase the amount of water in the rivers, this water availability is less reliable, and cleared land areas are at risk of increased floods and droughts.
Integrating forests into the landscape, especially in heavily cleared areas, is a viable and cost-effective solution for flood mitigation. In water-rich regions, fast-growing and high water-consuming species can reduce flood risk. In contrast, low water areas slow growing and low water consuming species can increase soil infiltration and help moderate flooding. When restoring cleared land, mixed species forests outperform monoculture forests in both drought resistance and tree growth, and can lead to healthier, more productive forests, more resilient ecosystems, and more reliable water services. Old Growth Forests provide even more resilient ecosystems.
There is a popular Chinese proverb that notes “The best time to plant a tree was 20 years ago. The second best time is now.”
This autumn, we will begin planting at our project sites and, given time, the reforestation of this cleared land may help to reverse many of these impacts described above and we will be doing our bit to restore our climate.
You can assist us by donating to our native tree planting projects across Australia. $4 plants one tree.