Forest ecosystems are vital for clean, healthy and abundant water. They provide a wealth of ecosystem services; they capture rainwater and feed the underground aquifers many of us rely on, and they slow down and filter water run-off, preventing floods, soil erosion and pollution of water sources.

Tree roots keep soil in place. When trees are cut down, soil washes into watercourses, polluting the water with sediment that can be detrimental to ecosystem health, rendering the water too polluted for many species to nest, feed and mate. Nutrient pollution – mainly nitrogen and phosphorus – also enters the watercourse from natural and artificial sources, including synthetic agricultural chemicals and animal waste.

These pollutants stimulate the excessive growth of algae, which results in so-called dead zones: The overgrowths of algae – also known as algal blooms – lower the oxygen levels in the water, suffocating fish. They also block sunlight, preventing aquatic plants from photosynthesising. Entire ecosystems from watershed to ocean have been devastated by sediment and nutrient pollution. But when there are trees, these pollutants are prevented from entering watercourses.

A healthy forest composition has layers, including the emergent layer, canopy, understorey and forest floor, which slow water’s journey after falling as rain – it’s worth noting that tree plantations rarely have these layers. Water stumbles through the layers slowly. Eventually, it meets the forest floor, where the detritus – the dead organic matter, including fallen leaves – holds onto the water, enabling it to slowly soak into the ground. The decay of old roots creates pores and small spaces in the ground, allowing water to seep deeply into the soil and recharge the groundwater table.

Trees and forest vegetation also drink up lots of water, which historically was considered to reduce water availability for humans. However, research shows that in some water-scarce areas, the reduction and removal of vegetation correlates with a decrease in groundwater. Additionally, the water consumed by the trees is subject to evapotranspiration, in which water reenters the atmosphere as vapour, accumulates and then falls again as rain. This cycling of freshwater over land is important to keep ecosystems healthy and our water supplies replenished.

At a planetary level, networks of trees and forests are important for regulating climate and rainfall. Planetary systems like the water cycle connect us all. As a global community, we are suffering the collective impact of the disruption to such systems. Protecting and restoring forests is one way to get us back on course.

Sources upon request.