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Published online 7 November 2009 | Nature | doi:10.1038/news.2009.1057
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Planting trees can shift water flow
Creating forests where none existed may affect long-term hydrology.
Planting trees, which can significantly help to reduce atmospheric carbon dioxide, nevertheless comes with potentially damaging side effects. According to two new studies, planting forests in areas that currently don't have trees — a process called afforestation — can reduce the local availability of water.
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Baseflow is just one component of streamflow. There is much less storm runoff from forests than open fields. So while baseflow may be reduced in forest regions, high flows, land erosion, and downstream flooding are also greatly reduced.
George Perkins Marsh said it best in his 1864 book 'Man and Nature':
"With the disappearance of the forest, all is changed. The face of the earth no longer a sponge, but a dust heap, and the floods which the waters of the sky pour out hurry swiftly along its slopes, carrying in suspension vast quantities of earthy particles which increase the abrading power and mechanical force of the current, and augmented by the sand and gravel of falling banks, fill the beds of the streams, divert them into new channels and obstruct their outlets. The rivulets, wanting their former regularity of supply and deprived of the protecting shade of the woods, are heated, evaporated, and thus reduced in their summer currents, but swollen to raging torments in autumn and in spring.
"The surface of a forest, in its natural condition, can never pour forth such deluges of water as flow from cultivated soil."
It's clear that tree cover moderates the flow of water from the headwaters of any sub-watershed, whether it's in a tropical, temperate, highland or lowland situation. Naturally diverse forests not only eliminate rill erosion, reduce flooding and loss of topsoil to the rivers below, but they also allow moisture to slowly seep into the small creeks and rivulets that feed the larger rivers.
The problem I have with this article, is that it shows us once again the biasness that our single-discipline approach to science brings. Foresters want to see trees grown that grow into lumber. Hydrologists see the increased evapotranspiration and reduced baseflow as a water quantity issue, while the ecologist wouldn't want to see a single species pine tree plantation as it limits the ability of the forest to withstand diseases, pest outbreaks or fire. Sociologists may want that forest to provide medicinal drugs and economic enefits to local populations. All are doing what they believe to be the right thing – and they all are good benefits, but we can do better by understanding the others' perspective intuitively.
Better integration of these disciplines needs to be taught at all levels in our post-secondary educational instututions. This lack of integration will continue to hamper our abilities to address the long term cliamte change problems we face as a group. As Lester Brown in Plan B 3.0 – Mobilizing to Save Civilization points out, "The Billion Tree Program, inspired by Nobel Laureate Wangari Maathai, would cost the first world countries $20B per year for a decade yet would sequester 3.5 billion tons of CO2 per year, to give climate stabilization a decisive boost". Where are we doing this in the West? China is building a green wall 10 km thick and 1500 km long along the edge of the Gobi desert. We should be planting native trees in every denuded watershed, on every street corner, vacant lot and piece of marginal farmland throughout the world. Water conservation is undoubtedly important, but tackling climate change needs bold new solutions such as Brown discusses, and these will only be found through integrating each discipline, working across the socio-economical scale, and setting aside our scientific biases for the greater good.
Shawn Taylor
River Doctor
Many caveats deserve to be addressed regarding forestation(i.e., reforestation and afforestation) policies that may cause alarming effects on local water resources: 1) timing. One would not expect large reduction of streamflow right after tree planning since it takes years or decades for forest stands to research their maximum evapotranspiration potentials; evidence from the longterm studies in Southern African by David Scott suggests that as trees get mature and the evapotranspiration rates actually decline and thus the impacts on water yield will decrease. 2) Physiographic conditions. The impacts on streamflow are most pounced and of worth of concerns in regions with precipitation around 400-1000 mm range where precipitation barely meet evapotranspiration demand of plantations. Planting trees on wet environments such as wetlands is not likely to decrease streamflow significantly. 3. Land use legacy . Hanqin Tian’s modeling group at Auburn University suggests that planting trees on former croplands in southern China that were previously subject to irrigation such as paddy fields is not likely to decrease streamflow, but will increase baseflow and ground water recharge, since irrigated lands may use more groundwater and surface water than forests. 4. Planting area. It is true that planting trees will reduce streamflow on a per unit area base, such as small watershed that most experiments are conducted and described in the article. However, in most situation, the area subject to complete reforestation is only a small percentage of the landbase, and thus the impacts on water quantity are not likely great for large water supply basins.
Ge Sun