Trees, forests and water: Cool insights for a hot world, Ellison et al. 2017

This article (also highlighted in Compendium v2n1) reviews research on the benefits of tree cover in relation to water and energy cycles.

Forests help produce rain. Vegetation releases water vapor through transpiration, increasing atmospheric moisture that is then transported by wind. In fact, “over most of the tropics, air that passes over forests for ten days typically produces at least twice as much rain as air that passes over sparse vegetation” [Ellison 2017: 53]. Forests also release biological particles, such as spores, bacteria and pollen into the atmosphere. Water condenses around these particles, forming raindrops.

In addition to the atmospheric moisture produced by forests that is transported by prevailing winds to generate downwind rain, forests also help transport moisture from the coasts to the interior of continents. According to the biotic pump theory [Makarieva & Gorshkov 2007], evapotranspiration over coastal forests creates low pressure zones that draw in atmospheric moisture from the ocean. This oceanic moisture eventually comes down as rain over land. Deforestation of coastal forests thus reduces this influx of moisture to land, while deforestation anywhere can decrease the reliability of rainfall downwind.

Through shading and evapotranspiration, forests cool the Earth’s surface in tropical and temperate climates. Due to a lower albedo compared to other land cover types at high latitudes, boreal forests potentially contribute to local warming. However, forests also increase cloud cover and thus albedo, higher in the atmosphere. In the absence of vegetation, such as in cities, solar energy remains in the environment in the form of heat, rather than driving evapotranspiration.

Using the sun’s energy, individual trees can transpire hundreds of liters of water per day. This represents a cooling power equivalent to 70 kWh for every 100 L of water transpired (enough to power two average household central air-conditioning units per day) [Ellison 2017: 54].

High-elevation forests have a unique potential to intercept fog and cloud droplets, which boosts tree growth, evapotranspiration, groundwater infiltration, and ultimately contributes up to 75% of catchment runoff. Tree cover can improve water infiltration due to increased organic matter to hold water and the presence of tree roots, which loosen and shade the soil and channel water into the ground. In areas where infiltration rates are greater than transpiration rates, the presence of trees increases groundwater recharge.

All of the aforementioned mechanisms distribute water naturally, hence reducing floods.