If the investment is in a carbon credits company, which plants new trees to absorb carbon as they grow, a new study says they may be doing more harm than good. Planting large numbers of trees in new regions depletes existing rivers within a decade, and those rivers never recover. It's not a surprise. Trees, like humans, breathe as part of energy production, and that requires a great deal of water.
The study looked at 43 sites across the world where forests have been established, and used river flow as a measure of water availability in the region. It found that within five years of planting trees, river flow had reduced by an average of 25%. By 25 years, rivers had gone down by an average of 40% and in a few cases had dried up entirely.
River bed in Buderim Forest Park, Queensland, Australia. Credit: Laura Bentley
The biggest reductions in water availability were in regions in Australia and South Africa
The type of land where trees are planted determines the degree of impact they have on local water availability. Trees planted on natural grassland where the soil is healthy decrease river flow significantly. On land previously degraded by agriculture, establishing forest helps to repair the soil so it can hold more water and decreases nearby river flow by a lesser amount.
Counterintuitively, the effect of trees on river flow is smaller in drier years than wetter ones. When trees are drought-stressed they close the pores on their leaves to conserve water, and as a result draw up less water from the soil. In wet weather the trees use more water from the soil, and also catch the rainwater in their leaves.