Race Against Time: USU Ecologists Probe Climate Effects on Forests

Across the western United States, a noisy, inquisitive bird called Clark’s nutcracker and piñon pine trees share a symbiotic relationship. A relative of crows and jays, the bird relies on pine seeds for food. By storing a winter cache of seeds in the ground and on steep slopes, Clark’s nutcrackers increase the trees’ chances of sprouting new growth. Without the trees, the birds would lose a valuable food source and without the birds, the trees would struggle to reproduce.

That’s a scenario that could be approaching and it concerns Utah State University ecologists Tom Edwards and graduate student Jacob Gibson MS’11. Weakened by drought, large numbers of pines in the American Southwest’s piñon-juniper woodlands are dying off.

Clark’s nutcrackers are just one species that would feel the impact of reduced piñon-juniper habitat. The woodlands provide critical habitat for dozens of wildlife species, produce forage for livestock, protect watersheds and are a source of fuel and commercial products for humans.

With research partners from the U.S. Forest Service’s Rocky Mountain Research Station, Edwards and Gibson analyzed forest and woodland data collected in 17 western states by the USFS Forest Inventory and Analysis Program to model how piñon-juniper might change under varied climate scenarios.

“Climate change appears to be altering the region’s temperature and precipitation regime,” says Edwards, professor in USU’s Department of Wildland Resources and a member of the U.S. Geological Survey’s Utah Cooperative Fish and Wildlife Research Unit. “This will likely cause piñon-juniper woodlands to shift northward and to higher elevations.”

Already, the team has observed that new, isolated populations of piñon pine, more sensitive to drought and high temperatures than the hardy juniper, are taking root at higher elevations in Utah and other locations. The question is: Can these trees migrate quickly enough to continue to provide critical habitat?

Based on their research so far, the team has found the most sensitive part of piñon-juniper distribution is the “leading edge” at higher elevations where new populations are striving to establish. The conditions for seed germination and establishment, Edwards says, require a high quality, narrow subset of environmental conditions conducive to the growth of saplings.

Behind the newer populations is the “trailing edge,” where established woodlands are shrinking.

“If the leading edge can keep up with the die-off at the trailing edge, the woodlands will successfully migrate with changing climate,” he says. “This means not only maintaining the same amount of trees, but also the ratio of young and old trees.”

The sustainability risk occurs, Edwards says, if the leading edge can’t keep up with the die-off of the trailing edge.

“This could be caused by competition with other tree species blocking northward movement or by the unavailability of soil, moisture and temperature favorable for growth of seedlings and saplings.”

It’s possible, he says, that the trailing edge of woodlands will spring forward toward the leading edge “like a slinky.”

“This situation would cause a sudden and potentially dramatic loss of mature piñon-juniper woodlands,” Edwards says. “The remaining woodlands would consist mostly of young stands, leaving very few seed-producing mature trees.”

Wildlife species with freedom of movement, such as birds, could initially adjust to the changing vegetation. However, changes in the woodland canopy could reduce habitat for other seed-dependent animal species.

With their findings, Edwards, Gibson and colleagues are providing recommendations for site-specific regeneration and management plans to maintain the woodlands.

“Piñon and juniper are slow-growing species and typically require 20 to 25 years to mature to the point of bearing seeds,” Edwards says. “Research of these species must therefore consider not only short-term movement but long-term implications and projections.”

This article is an excerpt from “A Race Against the Clock: Can Piñon-Juniper Woodlands Migrate Fast Enough?” by USU writer Shauna Leavitt, which appeared in the Fall 2011 issue of The Wildlife Society journal, “The Wildlife Professional.” It is reprinted in an edited form with the permission of the publisher. To access the full article, visit the society’s website.

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