17 June 2019

Watch where you plant your trees

Planting large amounts of trees may add a lot to improving an ecosystem, and in mitigating atmospheric CO2-levels. But planting trees in arid areas can sometimes worsen the environmental circumstances, especially the water availability for people living downstream. Environmental researchers from Utrecht University studied over 400.000 watersheds worldwide. Their results have been published in Nature.

Deforestation and afforestation have been studied many times worldwide, mostly in terms of climate impact. The effects of forest management on streamflow however is less well-known. For over 100 years researchers have been studying the prediction of streamflow response to forest planting and removal. But until now the results were exceptionally unclear and largely unpredictable.

In de wetenschappelijke literatuur wordt ongeveer evenveel aandacht besteedt aan bosbeheer vs. klimaat en bosbeheer vs. watermanagement. Maar in populaire media zijn de verhoudingen volledig anders. Beeld: Jaivime Evaristo
Only a little less amount of attention is spent on 'forest and climate' and 'forest and water' in scientific publictions, whereas popular media is overloaded with 'forest and climate' compared to 'forest and water'. Image: Jaivime Evaristo

A comprehensive database

First author and environmental scientist dr. Jaivime Evaristo from Utrecht University teamed up with Dr Jeffrey J. McDonnell from the University of Saskatchewan. Together they compiled the most comprehensive database of forest cover studies worldwide ever made. The database contains details of over 400,000 watersheds located in 137 different countries. A watershed is an area of landscape of a particular size where precipitation collects and drains off into a common outlet, such as into a river, bay, or other body of water. The researchers created a model that uses seven key landscape factors in order to predict how the streamflow responds to forest planting and removal.

Ontbossing in beheerde bossen in Washington (VS). Foto via iStock/halbergman
Deforestation in managed forests in Washington, USA. Image via iStock/halbergman.

Calculating the non-linear decrease and increase

Using their model the researchers have calculated for the first time the non-linear decrease or increase in streamflow when trees are planted or removed. The most important factor for predicting how the streamflow responds to the removal of trees is the amount of subsurface water stored in the watershed. An increasing amount of water will be available downstream when trees are cut down in a watershed that has a large amount of subsurface storage. To predict the streamflow when trees are planted the amount of evaporation and transpiration is the principal factor, the researchers conclude.

People downstream are significantly affected

By and large, planting trees is still beneficial for numerous forms of life. But be careful where you plant them. In areas already affected by droughts planting schemes can decrease the flow of water in streams to such an extent that people downstream are significantly affected. For this reason, for example, a law has been passed in South Africa which prohibits the planting of forests in certain areas. But in general, successful afforestation can only happen when other water resources are sufficiently available, the researchers conclude.

Publication

Jaivime Evaristo and Jeffrey J McDonnell. “Global Analysis of Streamflow Response to Forest Management.” Nature, 2019.