4 Free-air CO 2 enrichment (FACE) site in Duke Forest near Research Triangle Park, NC. FACE sites across the United States are used <strong>to</strong> study how <strong>forests</strong> will s<strong>to</strong>re and cycle carbon under <strong>climate</strong> <strong>change</strong>. (Pho<strong>to</strong> by Chris Maier, U.S. Forest Service) compass—february 2008
Forests & Global Climate Change by Allen Solomon Forest Service scientists <strong>have</strong> been studying <strong>global</strong> <strong>change</strong> and its effects on <strong>forests</strong> and ranges informally for many decades, and formally since passage of the U.S. Global Change Research Act of 1990. More recently, the Forest Service as a whole has come <strong>to</strong> recognize the threats and opportunities involved <strong>with</strong> maintaining ecosystem services and products under rapidly changing <strong>climate</strong> and <strong>climate</strong> variability. Forest Service Chief Dale Bosworth, now retired, identified <strong>global</strong> <strong>change</strong> as “the greatest threat <strong>to</strong> our natural resources in the 21st century . . . .” Our present Chief, Gail Kimbell, has named <strong>climate</strong> <strong>change</strong> and related water issues as two of the three greatest challenges facing the Forest Service. So, if we know in general what the threats of a changing <strong>climate</strong> are, and we know how plants and ecosystems respond <strong>to</strong> changing <strong>climate</strong> and atmospheric chemistry, why <strong>do</strong> we not simply implement the management actions we <strong>have</strong> available <strong>to</strong> reduce the risks and take advantage of the opportunities <strong>global</strong> <strong>change</strong> presents? The answer, of course, is that we <strong>do</strong> not yet know the threats well enough, particularly at the local level where actions can be taken—and surprising <strong>to</strong> some—we <strong>do</strong>n’t really know how plants and ecosystems respond <strong>to</strong> changing <strong>climate</strong> and atmospheric chemistry that well. I first became involved in <strong>global</strong> <strong>change</strong> research a little over 30 years ago at Oak Ridge National Labora<strong>to</strong>ry. Then, the fundamental question <strong>to</strong> be solved was whether the Earth would become a source or a sink for carbon as <strong>climate</strong> and atmospheric chemistry continued <strong>to</strong> <strong>change</strong>. The answer <strong>to</strong> this question is critical for defining the nature of the risks we face in the future. If <strong>global</strong> warming and increases in atmospheric CO 2 result in the Earth sequestering and s<strong>to</strong>ring more carbon (say, because trees cover more area and CO 2, a plant nutrient, makes them grow at greater densities than <strong>to</strong>day), the impacts “...things we can and must <strong>do</strong> in response...our options include protecting the existing carbon sink through forest conservation and increasing carbon sequestration through reforesting degraded land, improving forest health, and supporting sustainable forest management...forest biofuels for energy and the substitution of wood for manufactured products are the other opportunities for managing carbon.” —Gale Kimbell, Chief, U.S. Forest Service from <strong>climate</strong> <strong>change</strong>s forced by excess atmospheric CO 2 will be considerably less than our calculations suggest. On the other hand, if warming forces the s<strong>to</strong>red carbon from <strong>forests</strong> and rangelands (say, because <strong>to</strong>day’s trees become climatically “obsolete” and undergo widespread dieback), then the problem becomes even more challenging than we thought, <strong>with</strong> more CO 2 begetting more warming, more warming begetting still more CO 2, and so on. In the intervening 30 years since 1977, when we began obtaining research grants <strong>to</strong> study this issue, much has been learned about the <strong>global</strong> carbon cycle. There is even a “current” answer <strong>to</strong> the question: The vegetation of the Earth is now a net carbon sink and should continue sequestering carbon at least until about mid-21st century, when it is expected <strong>to</strong> become “carbon saturated.” Yet, this outcome rests on <strong>global</strong> vegetation models that assume, rather than know, the answer <strong>to</strong> the question we were asking in 1977: They assume that warming will permit trees <strong>to</strong> cover more area, and that CO 2 will enhance vegetation density. At the same time, the models <strong>do</strong> not simulate such things as the consequences if trees undergo significant dieback when the <strong>climate</strong> they require (their “<strong>climate</strong> envelope”) moves away <strong>to</strong> higher latitudes and altitudes. Yet how we manage the land <strong>to</strong> reduce <strong>climate</strong> impacts depends entirely on that unanswered question. One reason the question has not been answered is that we still <strong>do</strong> not yet know if, or how, increasing atmospheric CO 2 will <strong>change</strong> the <strong>climate</strong> envelope <strong>to</strong> which each species is thought <strong>to</strong> be limited. For example, Forest Service research has shown that tree seedlings grown under higher concentrations of CO 2 more efficiently use water <strong>to</strong> pho<strong>to</strong>synthesize; that is, they can sequester more carbon for every gallon of water they use. If this process works the same way in wildland vegetation as in controlled experiments, the moisture limits of the <strong>climate</strong> envelope for say, loblolly pine, www.srs.fs.usda.gov 1
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