A little girl walks under the shade of trees along Lytle Creek stream flowing…
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 millenniums, 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. The government's wartime-like tactics included security watchtowers, propaganda, aerial bombing and color-coded threat alerts. Uncle Sam trained elite Hotshot and Smokejumper crews to snuff out enemy flames. Congress annually funded the war effort with an emergency blank check, now $2.5 billion.
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.
Ironically, Congress enacted the anti-fire 1911 Weeks Act and 1924 Clarke-McNary Act to prevent erosion and thus secure downstream navigable rivers. That logic made sense in damp Eastern states, but it had the opposite effect in the semiarid West. There, fire exclusion degraded the integrity and runoff of high-elevation watershed recharge zones.
Naturally, forest managers focus on forest health. Yet combing through their extensive upland research, our analysis also found the larger scope of downstream casualties: suppression of fire causes suppression of flows.
Indeed, in some landscapes, you literally can't see the river for the trees.
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% to 30% of the moisture that had formerly soaked into the forest floor and fed tributaries as liquid. But let's conservatively ignore potential vapor losses. Instead, assume that the lowest average daily sap flow rate is 70 liters per tree for an open forest acre of 112 new young trees. Even then, this over-forested acre transpires an additional 2.3 acre-feet of water per year, enough to meet the needs of four families.
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."