Every now and then we encounter reforestation projects with the goal of replacing the old, harvested forests. Although this is a good initiative, there are reforestation projects that bring more harm rather than help. For instance, in a new study, it is revealed that river flow is reduced in areas where forests have been planted and unfortunately, it does not recover over time. The study highlights the need to consider the long-term effects of tree-planting activities especially on the regional water availability and climate.
Botanist Laura Bentley from the Cambridge Conservation Research Institute and first author of the study explains that while reforestation is an important part of negating climate change, organizations and even individuals should consider the right places to do it. If not, there will be dire effects and a perfect example would be the change in the availability of water which will then affect the local cost-benefits of tree-planting.
For the longest time, it has been suggested that the best way of reducing atmospheric carbon dioxide levels is to plant trees in large areas simply because of the fact that trees have the ability to absorb carbon dioxide. However, trees also absorb water and there are species of trees that absorb water in huge amounts. While the knowledge regarding planting trees and the reduction of the amount of water flowing to nearby rivers is not exactly new, there is no understanding of how this effect changes as the forest area starts to age.
Bentley and her team looked at 43 forested sites across the world and used the flow of the river to measure the water availability in the region. The team discovered that within the first five years of planting trees in these areas, the river flow had been reduced by an average of 25%. In 25 years' time, river flow had gone down by 40% and there are already cases of rivers drying up. The biggest of these water reduction was observed in Australia and South Africa. Professor David Coomes, director of the Cambridge Conservation Research Institute and team leader for the study, explains that river flow does not recover after tree-planting once disturbances in the watershed and the effects of the climate are accounted for.
This study's findings are published in Global Change Biology and it reports on which type of land determines the degree of impact on local water availability. Trees planted on natural grasslands where soils are healthy, for example, decrease river flow significantly. On the other hand, when trees are planted in lands previously used for agriculture, it can help repair the soil and can hold more water and decreases the flow of the nearby river is a lesser amount.
It is quite interesting to note that the effect of trees on river flow is quite smaller in a drier season that during the wet season. The explanation behind this is simple: whenever a tree is experiencing some sort of stress brought by drought, it closes the pores on its leaves to conserve water and because of that, they require less water from the soil. However, in rainy seasons trees absorb water from the soil through its roots and catches rainwater using their leaves. Bentley explains that with the changing climate, water availability around the world will surely be affected. "By studying how forests affect water availability, we can work to minimize any local consequence for people and the environment."
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