Bringing Back Sierra Aspen
By Jody Holzworth | December 16, 2014
New Study: Aggressive Conifer Removal Benefits Aspen and Birds
It may seem counterintuitive to cut down the iconic pine and fir trees from a Sierra Nevada forest in order to restore another tree species.
That’s just what the U.S. Forest Service did on over 140 sites in Lassen National Forest beginning in 1999. Each site contained a remnant quaking aspen (Populus tremuloides) stand and they used mechanical means to remove all but the largest conifers. Then Point Blue Conservation Science monitored for birds.
“The density was so high. There were so many birds singing that we had trouble writing them down,” says Brent Campos, Point Blue’s northern Sierra project leader. “Birds are good indicators of an aspen stands’ health and we were surprised by the dramatic response so quickly.”
Healthy aspen stands are known for creating their own micro-climate, increasing groundwater, richening soils, and attracting a higher diversity of plants and wildlife, including bird species like the red-breasted sapsucker and mountain bluebird. Yet, most of the aspen stands that dotted the Sierra Nevada less than a century ago are gone or in poor health.
“Aspen thrive in the time between wildfires,” says Ryan Burnett, Sierra Nevada Group director for Point Blue. “Without fire, the conifers take over. Aspen stands now make up less than one percent of the Sierra Nevada landscape. We wanted to see if past wrongs could be mitigated with a fairly aggressive approach to rehabilitating aspen stands.”
This aggressive approach meant using traditional logging equipment to remove the Ponderosa pine, white fir and other conifers blocking sunlight from shade-intolerant aspen.
According to Tom Rickman, U.S. Forest Service wildlife biologist with the Eagle Lake Ranger District in Lassen National Forest, they only left legacy trees that were alive before fire suppression policies resulted in excessive conifer densities. Site size ranged from seven to 99 acres and they removed the majority of the conifer canopy cover.
“We’re harvesting timber in and around these aspen stands in ways we hadn’t done before, and it pushed our comfort level,” Rickman says. “Since this was new for us, we wanted to monitor the effects of such aggressive treatment.”
Point Blue began annual spring bird and vegetation surveys in 2004 on both treated and untreated aspen stands, surveying 180 different sites during the nine-year study. While they documented a large variety of bird species, they selected two groups of six or more focal bird species as the primary indicators of ecological change, one group associated with the aspen habitat and the other with conifer habitat.
Within a few years, the aspen stands teemed with focal bird species during the breeding season, such as mountain bluebirds, tree swallows and hairy woodpeckers. And the trees showed revived health through their messy habits, growing twisted masses of root suckers, raining down honeydew from armies of aphids, and letting fungi rot and disfigure their trunks. This messiness creates nesting cavities for woodpeckers, protection for ground-nesting birds, a buffet of insects for food, and a rich understory of grass and wildflowers.
“The birds tell us a story of whether the habitat was restored,” Burnett says. “This is a cool story of how to bring the ecologically diverse aspen community back. It will be amazing to see these sites ten years from now.”
The bird numbers Campos and Burnett published in a recent Restoration Ecology article do not seem significant at first glance, but the results matter when considering multiple species over the thousands of acres the Forest Service has now restored. For example, the red-breasted sapsucker increased from .84 birds per acre before treatment to 2.85 birds per acres on treated sites. Multiply that by 1,000 acres and that means habitat for nearly 2,000 more red-breasted sapsuckers.
“The woodpecker community really responded,” Rickman says. “And you can’t help but notice the tree swallows and bluebirds.”
He says the results of the Point Blue study validate the district’s aspen rehabilitation program, along with providing an opportunity to see his first warbling vireo nest in a restored Sierra aspen community.
“The aspen and the understory vegetation are really responding on these sites, especially when we fence out the cattle and deer where needed,” Rickman says. “It’s gratifying to see there were no negative impacts.”
However, some of the conifer bird species Point Blue identified did decrease during the study, Burnett says. But those species, such as golden-crowned kinglet and red-breasted nuthatch, are among some of the most common bird species occupying Sierra forest habitat.
“These aspen stands were being swallowed up by the conifer sea around them,” Burnett says. “This work suggests aggressive mechanical treatment should be considered for their restoration. I’d like to see land managers increase the pace and scale of aspen rehabilitation throughout the Sierra Nevada.”
Study citation: Campos, B. R. and Burnett, R. D. 2014. Avian Response to Mechanical Aspen Restoration in Sierra Nevada Coniferous Forest. Restoration Ecology, Vol. 22, No. 5, pp. 616-624.
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