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Helping our too-thirsty forests

Posted: May 9, 2012 - 11:00pm

Ronald Reagan once justified logging with “a tree is a tree; how many more do you need to look at?” Besides, he warned, “trees cause more pollution than automobiles.” We cringed at his biases. Yet due to forces none foresaw, Reagan’s gaffes may now ring true.

Today, the hottest and thirstiest parts of the United States are best described as over-forested. Vigorous federal protection has stocked semiarid regions of public land with several billion trees too many. And day after day these excess trees deplete a natural resource that has become far more precious than toilet paper or 2-by-4’s: water.

Scientists and water managers report that 39 states face water scarcity. Much of the nation’s freshwater shortfall comes from our population growth, waste, hunger and contaminants. But we must also now implicate the escalating thirst of unnatural forests.

Water depletion from afforestation — the establishment of trees or tree stands where none previously were — is the unintended consequence of a wildly popular federal policy. For millennia, fires set by lightning or Native Americans limited forest stocks to roughly a few dozen trees per acre. All that changed after the nationally terrifying Big Blowup wildfires of 1910, which led the United States to in effect declare war on wildfire. Decades of heroic victories against fire led to gradual defeat in the larger war. Fuel builds up, and when it ignites, the fires burn hotter, faster and more destructively. More new trees compete for less sunlight, thinner soil nutrients and scarcer water resources. Native wildlife suffers. Insects and diseases spread faster. Public subsidies protect private properties at the wildland-urban interface.

Call it the water-fire nexus. To be sure, the dynamics are complex. Impacts fluctuate locally depending on forest slope, aspect, age, altitude, density, latitude, species composition and natural history. But adjusting for these variables reveals the nexus’ overarching pattern.

First, the past century of fire suppression has resulted in roughly 112 to 172 more trees per acre in high-elevation forests of the West. That’s a fivefold increase from the pre-settlement era.

Second, denser growth means that the thicker canopy of needles will intercept more rain and snow, returning to the sky as vapor 20 percent to 30 percent of the moisture that had formerly soaked into the forest floor and fed tributaries as liquid.

Third, that pattern adds up. Applying low-end estimates to the more than 7.5 million acres of Sierra Nevada conifer forests suggests the water-fire nexus causes excess daily net water loss of 58 billion liters. So each year, post-fire afforestation means 17 million acre-feet of water can no longer seep in or trickle down from the Sierra to thirsty families, firms, farms or endangered fisheries.

So how do we unlock the nexus to replenish the Earth? A century’s accumulation of dry fuel in public lands makes it too expensive and risky — for people, property, habitats or carbon emissions — to unleash prescribed fires throughout our 16-million-acre ponderosa tinderbox. Mechanical thinning generates popular distrust as long as timber industry chain saws try to cut “high grade” valuable mature growth to compensate for less profitable small-diameter “trash trees.”

Happily, a lumber mill’s trash has now become a water user’s treasure. Thirsty downstream interests could organize to restrict thinning to scrawny excess trees simply for the purpose of releasing the liquid assets they consume. Western water rights markets value an acre-foot at $450 to $650 and rising. So rather than compete with forests for rain and snow, private and public institutions could invest $1,000 per acre (average U.S. Forest Service price) to cut down fire-prone trash trees, yielding at least $1,100 to $1,500 worth of vital water. To reduce fuel loads and increase runoff, the water-fire nexus pays for itself.

This pragmatic approach has regional precedents. The U.S. Endowment for Forestry and Communities cites forest-to-faucet agreements emerging from Denver to Raleigh, N.C. The only obstacle is our century-old cultural mind-set that if a dozen trees are good, 100 trees are better. But as temperatures rise, too much forest strangles too many watersheds. To replenish streams before they dry up, we lifelong tree-huggers must learn when and where to let go.

• Poulos is a fire ecologist and postdoctoral teaching fellow at Wesleyan University’s College of the Environment.

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