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Will Trees Save Us From Climate Change? A Doubtful Tale

I have nothing against trees. Love them, in fact -- their bark: rough, smooth, or grooved; their needles or leaves: angular, flat, jagged, or pointed; branches crooked or straight; roots gnarled. I'll forbear quoting Joyce Kilmer about how lovely trees can be (but you hear that poetic ditty in your head, don't you?), and will just confess that I'm a big fan of the arboreal.

If you'd like a more scientific reason to applaud their virtues, then consider Jim Robbins' op-ed in the New York Times late last week. Central to his claim for why trees matter is the positive role they can play in sustaining public health.

He observes not surprisingly that trees are quite efficient water filters. They're "capable of cleaning up the most toxic wastes, including explosives, solvents and organic wastes, largely through a dense community of microbes around the tree's roots that clean water in exchange for nutrients, a process known as phytoremediation."

Because of their remarkable -- their life-essential task -- of converting carbon dioxide into organic compounds such as sugar and then releasing oxygen (their waste, our gain), trees have the unusual ability to cleanse the air we breathe.

By themselves, they cannot compensate for the all toxins that blow out of the tailpipes, smokestacks, or cooling towers in Los Angeles or Houston -- cities with some of the nation's dirtiest air. Yet at a more micro-level, such as a blockface, their presence has been linked to the reduction of asthma.

Like a host of foresters and urban planners, Robbins is convinced too that trees form a kind of green shield. Their canopy as forests, and as cover along streets, can cool soil and concrete, decreasing temperatures and moderating heat-island effects; by releasing an aerosol mist (forest bathing, the Japanese call it) they can make life more bearable for any species inhabiting these particular landscapes.

But can they help humanity adapt to and mitigate the immensity of climate change?

That they can sequester carbon has been much touted in policymaking circles as one tool to help shrink our carbon footprint; and thus trees seem critical to the larger effort to reduce global warming.

Yet it does not necessarily follow, as Robbins assumes, that we must reforest the planet as rapidly as possible; his evidence and motivation for this conclusion comes through his citing of a much-loved aphorism: "'When is the best time to plant a tree?' The answer: 'Twenty years ago. The second-best time? Today.'"

That's where I balk.

Sure: if we had a more complete picture of the variations of potential temperature change across ecosystems and typographies; if we could pinpoint when and where alterations in precipitation will occur; and were we able to calibrate the shifting influence that heat, light, and wet will have on differing soil types, then we might have a clue about what tree species to plant in which biota and at what times.

But we don't. So to plant trees in hopes that they will survive -- and thus increase the odds of us doing so -- seems, at best, random.

Take a local analogy. In the scorched aftermath of the Station Fire the U. S. Forest Service feared that the erosive force of coming rainy seasons would strip the burned-over district of its soil. It thus launched an aggressive restoration project. Beginning in April 2011, contract labor planted one million seedlings of an expected three million over five years. The goal was to re-green approximately 11,000 acres of the 160,000 that burned at a white heat during August and September 2009. The Angeles National Forest, or at least a portion of it, would be reborn.

It has not happened. Only about 25 percent of the seedlings dug into charred slopes, cindered meadows, and blackened canyon floors have survived, a mortality rate that has stunned agency foresters. "When we planted seedlings, conditions were ideal in terms of soil composition and temperature, rainfall and weather trends," one of them told the LA Times. "Then the ground dried out and there just wasn't enough moisture after we planted."

The Forest Service has gone back to the drawing board, shrinking the number of acres to be planted and, where possible, switching to tree species that are indigenous to the San Gabriel Mountains.

Critics are unappeased. One of them is Rick Halsey, of San Diego's California Chaparral Institute. He was skeptical a year ago when he first heard about the project, lambasting the choice of Coulter pine as the keystone tree in the regenerative mix. The news of the past year's die-off came as no surprise: "The reality we live in is a Mediterranean climate, and there is just not enough water to create what they have in mind. I do not believe they will succeed because this is Southern California, not rain-drenched Oregon."

This climatic reality is part of the reason why there has been a very long history of flawed regeneration projects on county and federal lands in the San Gabriels. When he was hired in 1911 as the County's first trained forester (and later its fire warden), Stuart J. Flintham instigated an aggressive firefighting operation throughout the foothills; it included lookout towers, an extensive network of firebreaks, and the massing of tools and men to help suppress wildfire. He also promoted, in the aftermath of the big burns of the 'teens and early 'twenties, reforestation and afforestation initiatives.

Stuart J Flintham | Photo courtesy of the L.A. County Fire Department
Stuart J Flintham | Photo courtesy of the L.A. County Fire Department

Convinced that if they could replace the Mediterranean-zone plants dominating the flammable landscape with pine and fir they might decrease the major fires that cyclically swept across this rugged terrain, and also slow hillside erosion, the county forester's office (which evolved into the L.A. County Fire Department) planted tens of thousands of seedlings.

After 20 years of backbreaking labor, and with little to show for it, the local agency abandoned the project. Flintham's successor, Spencer Turner, "was disillusioned with the high tree mortality," according to a county report, and recognized the "value of chaparral as a precious watershed cover that perhaps is fire dependent and best adapted to the site."

In 1930, three years after Turner and his colleagues paid tribute to the late Flintham's devoted service by planting in his memory a 20-acre Coulter-pine plantation in the Angeles National Forest, (it was incinerated in the 2002 Williams Fire), the new chief issued a restraining order: his foresters henceforth would "plant less, plant better."

The Forest Service has never quite learned L.A. County's hard-won lesson. Despite what federal foresters long have understood about the low fertility of local soils, mercurial weather patterns, and steep canyon walls, they have repeatedly endeavored to re-engineer the San Gabriels' ground cover.

Early in the 20th-century, they collaborated with county efforts to plant seedlings in this rough terrain, with predictable results: "In the 1920s, a million trees including exotic Canary Island pines were planted in the San Gabriel Mountains in a misguided effort to fix something that was not a problem -- a predominance of native chaparral," asserted Rick Halsey. "Most of those trees died because of drought."

Undaunted, as the smoke cleared from a 1960 blaze that roared through upper San Dimas Canyon, agency researchers decided again that chaparral had to go. As part of a revegetation project, scientists at the San Dimas Experimental Forest randomly selected eight watersheds to seed with annual grasses and eight with perennial grasses; four were left untouched as a control group. What they (re)discovered was that native plant material is a fierce competitor.

To restrain it, the research team sprayed the grass-planted watersheds with herbicides, a strategy that comes with two ironies. The first is cultural: Rachel Carson's "Silent Spring" had just been published, a book that decried the very poisoning of America's waterways and life systems in which the agency was complicit. The second is ecological: whatever the downstream consequences of the herbicidal spray, upslope the grasses' growth rate slowed over the project's lifetime, and, despite the chemical assault they endured, indigenous plants took root.

Back in the set of control-group watersheds, chaparral, Manzanita, and buckwheat, ceanothus, deervetch, and morning glory -- which began sprouting within ten days of the fire -- did even better. After four years these and other opportunistic plants had revegetated approximately 50 percent of the burned watersheds, a speed of recovery and density of cover that modern technology could not replicate.

The scientists did not fully accept the evidence their research revealed; instead, they insisted that under ideal conditions -- ample rain, significant labor, and chemical applications -- "land managers appear justified in trying to establish a grass crop." Still, they admitted, "this treatment should not be taken as a cure-all."

Why this institutional memory has not surfaced to check the Forest Service's current aspirations to reforest portions of the Angeles is an open question.

More to the point, the agency's century-long inability to rearrange the San Gabriels' biota to its liking is a powerful rejoinder to those who so confidently believe that planting trees, indiscriminately and in large number, will help resolve some of the challenges that a climate-changed world is bringing.

After all, when some of the world's most-accomplished land managers have a hard time getting it right, it's difficult to believe that modern-day Johnny Appleseeds will do better.

If they persist, however, their success will depend on how they approach their self-appointed task. Only the most humble will have a chance, for at least they will recognize that they'll be mucking with complex ecosystems whose mysteries we have yet to penetrate.

What they'll need most of all is the kind of knee-bending humility evoked in the final line of the poem earlier I promised not to quote: "...only God can make a tree."

Char Miller is the Director and W.M. Keck Professor of Environmental Analysis at Pomona College, author of "Public Lands, Public Debates: A Century of Controversy" (Oregon State University Press), and editor of "Cities and Nature in the American West." He comments every week on environmental issues. Read more of his columns here

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