[see American Forests Fall 2012], poplar borers are having a population boom due to fewer extended cold snaps in winter and fewer killing frosts in spring and fall — symptoms of climate change. When a late freeze does arrive to mercifully reduce the borers, aspen can still be doomed if an early thaw — another symptom of climate change — has prematurely melted the insulating snowpack, as aspen roots can be damaged by severe cold.
Green aspen contrast with the charred landscape after the Schultz Fire. Credit: Tyler Williams
THE FUTURE OF ASPEN
So what does the future hold for aspen in a warming world? The answers may depend in part on how the aspen reproduce. Aspen have two means of reproduction: Some are seed spreaders, and some are sprouters. In the East, where aspen are seeders — reproducing via cottony seeds floating on the wind — the future looks brighter. Like many trees, they may be able to simply move up in elevation to find suitable habitats in a warmer world. However, aspen is not a dominant species in these forests, nor is it likely to be in the future.
In the West, where aspen play a bigger role in forest composition, they might be slower to make the transition because there, they are sprouters, not seed spreaders. Most aspen in the West reproduce from the roots of existing trees, sprouting new trunks, called ramets, in a steady march across the landscape. Entire groves, then, can be clones of a single original tree.
The largest of these clonal groves ever documented is a 108-acre stand in Utah called “Pando,” Latin for “I spread.” The Pando Grove contains more than 40,000 trees and is estimated to have existed for at least tens of thousands of years. For a single clonal grove to be successful, it must have just enough disturbance to keep conifers from taking over, while escaping any grove-eliminating catastrophic events. Pando, in the rolling snow-covered plateau country of central Utah, has found that perfect place.
he Pando Grove located in Fishlake National Forest spans more than 106 acres. Credit: J Zapell
Groves of clones like Pando add a unique beauty to the landscape: They can be distinguished from neighboring aspen families during spring, when the trees leaf out, because a single clonal grove will usually leaf at the same time. The same visible distinctions are seen in autumn, when aspen turn their brilliant yellow and orange, sometimes directly adjacent to a grove that still retains its summer green.
Fall colors in the relatively drab West might be the single most attractive element of aspen to humans, but food and shelter are primary drivers for a multitude of animal species that call aspen groves home. Mice, voles, picas, rabbits, beaver, porcupine, deer, moose and elk all prefer aspen-dominated habitats. Ruffed grouse depend on them for forage, breeding and nesting. Goshawk, Cooper’s hawk, sharp-shinned hawk and pygmy owl nest and hunt and live there, too.
For these aspen-loving species, the forecast doesn’t look good. On the one hand, more droughts and warmer temperatures — conditions forecasted by most climate scientists for much of the aspen’s prime range — spell disaster for this moisture-loving species. There is little doubt that in marginal habitats — Arizona’s pine-oak woodlands, Colorado’s south-facing slopes, Alberta’s prairie edges — aspen is on its way out. Conversely, more forest fires could open new habitat for aspen, and some studies have shown an increase in atmospheric carbon dioxide aids aspen, producing longer roots and faster growth rates. The prevailing consensus, however, is that while some areas might see more aspen in the years ahead, overall aspen range is retracting. “I think aspen will have a more difficult time adjusting to climate change than many other species,” says Dr. Kathryn Ireland, who researched aspen decline in the Southwest at Northern Arizona University’s School of Forestry.
The northern pygmy owl nests in the cavities of quaking aspen. Credit: David Mitchell
In many ways, aspen is a resilient tree, boasting two means of reproduction, high levels of genetic variability and productive living bark. It’s little wonder that P. tremuloides spans from the Arctic Circle to the Tropic of Cancer. In a changing world, however, its time of glory might be waning. A lauded study by Gerald Rehfeldt of the U.S. Forest Service’s Rocky Mountain Research Station used three future climate models to extrapolate aspen success for the remainder of this century. The verdict? More than half of current aspen stands in the central Rockies will no longer be there by 2060. The huge iconic groves of the Rockies will have to rely on high-altitude fires to clear out spruce forests adjacent to existing aspen groves, where new ramets can crawl uphill. Will they be able to keep up? Only time will tell.
Tyler Williams is a big-tree hunter, adventure seeker and author of Big Tree Hikes of the Redwood Coast: A Guide to the Giants. To learn more, visit his website www.funhogpress.com.