Trees save lives

somali_flood.jpgWe often hear that nature provide “ecosystem services” to humanity that would be difficult or impossible to replace if lost, and that support various aspects of human well-being.  The evidence and rationale for this view was recently synthesized in the monumental work of the Millennium Ecosystem Assessment.

But ecosystem services strikes many people as a somewhat vague and squishy term.  What does it really mean?  And with the inevitable trade-offs between the engine of human economic development and nature conservation, how can we estimate the value of these so-called services?

mahattan.jpgPerhaps the most famous example that addresses this question comes from a historic decision made a decade ago by the government of New York City.  The Big Apple was concerned about the declining quality of water in the area that supports its gigantic population and how to ensure a stable and safe water supply into the future.  To do so, the city could either (1) build new water filtration plants with a price tag of four to six billion dollars, and annual operating costs of 250 million, or (2) pay to preserve a large forested area in the Catskill Mountains that supplies most of the state’s water, in effect employing a natural ecosystem as a water purification plant.  After careful analysis, the City opted to invest $250 million to buy and prevent development on the land in the watershed to protect the natural filtration system.  The overhead on this filtration plant involves paying farmers $100 million a year to take measures that keep fertilizers and pesticides out of the waterways. In a nutshell, a cold, economic cost-benefit analysis favored the natural alternative.

But such analyses depend on an openness to alternative ideas (thinking outside the box, to use the current cliche) and, importantly, good data on how the natural system works and its influence on services of value to us.  Now a new study published in Global Change Biology provides this kind of data on a global scale for the flood-protection services provided by natural forests. Deforestation has accelerated tremendously in recent decades, with many costs in terms of lost biodiversity, loss of soil fertility, and so on.  Forests are also widely believed to protect lowlands against flooding, but this idea has been controversial.  The authors of the new study compiled data from 1990 through 2000 from 56 developing countries, and used various sophisticated statistical techniques to show that (1) the frequency of floods is lower in areas with greater natural forest cover, and (2) floods are more frequent in areas that have experienced greater losses of natural forest area.  Importantly, these results remained strong even after controlling statistically for effects of rainfall, slope, and area of degraded landscape. Surprisingly, despite the fact that the study compared a wide range of forest types across a global area, with all kinds of other factors that might potentially obscure these trends, the best models nevertheless accounted for more than 65% of the variation in flood frequency, and roughly 14% was explained by forest cover variables alone.  And here’s the kicker:

“During the decade investigated, nearly 100 000 people were killed and 320 million people were displaced by floods, with total reported economic damages exceeding US$1151 billion . . . Based on an arbitrary decrease in natural forest area of 10%, the model-averaged prediction of flood frequency increased between 4% and 28% among the countries modeled. Using the same hypothetical decline in natural forest area resulted in a 4–8% increase in total flood duration. These correlations suggest that global-scale patterns in mean forest trends across countries are meaningful with respect to flood dynamics.”

rainforest.jpgThe bottom line, therefore, is that rampant forest loss is likely to exacerbate the frequency of flood-related disasters, potentially impacting millions of poor people throughout the world, and causing trillions of dollars (yes, that’s a “t”) in damage in developing economies in the coming decades. This synthesis of global data emphasizes that protection of existing forests and active reforestation of appropriate degraded land may reduce flood-related catastrophes.

So forests are not only important sources of renewable products like wood, or nice places for a stroll or camping trip to regain your mental balance, they are natural sources of flood insurance. And we need all the insurance we can get in this rapidly changing world.

[Source: Bradshaw, C.J.A., N.S. Sodhi, K. S.-H. Peh, and B.W. Brook. 2007. Global evidence that deforestation amplifies flood risk and severity in the developing world.  Global Change Biology (OnlineEarly Articles). doi:10.1111/j.1365-2486.2007.01446.x]

About Emmett Duffy

I am a Natural Patriot and an ecologist with expertise in biodiversity and its importance to human society. My day job is Professor of Marine Science at the College of William and Mary in Virginia.
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4 Responses to Trees save lives

  1. Sarah Duffy says:

    Thanks for the kick in the pants, Emmett. I haven’t volunteered with TreePeople in a while, and you’ve motivated me to sign up for one of their events. It’s an urban planting, so the chances of it affecting flood control are pretty much nonexistent, but the city of LA can use all the trees it can get.

  2. Emmett Duffy says:

    Yes, in modern America, most of us outside the greater New orleans area are mercifully free of the threat of catastrophic floods. But of course, trees have many other benefits, not least their ability to impart inner peace. Om!

  3. Pingback: Riverside Rambles — by Larry Ayers » The 18th Festival of the Trees, or November Arborea

  4. Stacey says:

    The amount of tree loss per day is sickening. It simply has to slow, if not stop. What are people waiting for? Do they need to get hit over their heads with a hurricane to understand what we’re doing to the earth?