Water managers barking up the wrong tree

It has been a bad week for trees, with two independent reports calling into question their true value as natural guardians of our air and water quality.

According to the UK’s tropical Forestry Research Programme (FRP) huge amounts of money have been squandered on misguided water resource projects because managers have accepted the ‘myth that trees always improve water availability’.

The FRP report From the Mountain to the Tap: How Land Use and Water Management Can Work for the Rural Poor draws its conclusions from extensive case studies in India, Costa Rica, South Africa and Tanzania and challenges the wisdom of believing trees are automatically good for the environment.

The research in India, backed up by findings elsewhere, indicates that efforts to convert agricultural lands to forests in the area could cause a 16% to 26% reduction in water yields.

The FPR believes findings may warrant a review of water management projects that include aggressive tree planting initiatives, as there is strong evidence that rainfall in forests evaporates up to twice as fast as it does in treeless areas.

If increased forestation leads to reductions in streamflows, then trees could be contributing to a drop in groundwater tables and reservoir levels.

“The point of our work is to show that the science surrounding water uses and land use change is constantly evolving and policies and practices need to reflect this,” said John Palmer, manager of FRP.

“Given the large sums being invested in good faith in water management projects that will be critical to the welfare of entire nations, it is critical that these projects be based on scientific evidence of their benefits.”

Trees suffered a second blow from the axe of bad PR when scientists announced that many of Britain’s native broadleaves can actually harm, rather than improve, air quality.

It is no secret that emissions from certain trees can actually increase the level of ozone and certain other pollutants in urban air, while absorbing others and cooling air.

But until now, only well-funded Government scientists were likely to have access to the kind of sophisticated computers capable of modeling the pros and cons and assessing which species were likely to benefit the urban environment the most.

known for nearly two decades that the biogenic emissions from certain trees can increase the levels of some pollutants, particularly ozone, in urban air. Trees also take up pollutants and cool urban air, but until now, only government scientists using sophisticated computers have had access to models for determining when the pros of planting different tree species outweigh the cons.

But now a study by academics at Lancaster University, University of Birmingham and the Environment Agency has shown how new software that can run on a home PC can make the same kind of assessment.

The study, Development and Application of an Urban Tree Air Quality Score for Photochemical Pollution Episodes may be a bit of a mouthful.

But put simply, the team painstakingly analysed emissions from all the trees in huge swathes of central Birmingham and discovered that many deciduous species such as willows, oaks and poplars could actually have a detrimental effect on the environment overall, whereas pines, larches and silver birch had the greatest potential to clean the air.

Despite their findings both groups of scientists were anxious not to drag the good name of trees through the dirt.

“We’re not saying forests never produce water benefits or that they don’t have an important role in the ecosystem,” said Ian Calder, one of the authors of the FRP report and director of the Centre for Land Use and Water Resources Research at the University of Newcastle.

“But if we are trying to manage our water resources effectively, the
overenthusiastic adoption of the simple view that ‘more trees are always better’ is a prime example of how a failure to root decisions in scientific evidence leads to bad water policy.”

Nick Hewitt, from the Lancaster University team was also hedging his bets.
“We’re definitely not saying chop down the oak trees,” he said.

“Trees do benefit air quality if you plant the right kind.”

By Sam Bond

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