A predicament the United States faces is the simultaneous threat of climate change and the rising energy demands of its growing population. This, in combination with the economics and politics surrounding crude oil production, has created a national interest in diversifying and growing the domestic energy portfolio.
In 2007, the U.S. Congress enacted Renewable Fuel Standard Provision (RFS2), which aims to decrease the carbon footprint related to transportation by increasing domestic production of renewable fuels. The RFS set a goal for the United States to produce 36 billion gallons of liquid biofuels per year by 2022, more than tripling biofuel production from 2007 levels. Related policy in the United States and the European Union fosters the development of bioenergy (e.g., using biomass for electricity) more broadly.
Currently, most U.S. biofuels are made by converting starches in corn into ethanol, though modern technologies are making it possible to create ethanol more efficiently using cellulose – the substance that gives structure to plants and is abundant in wood, grass and other fibrous plant materials. This technique will primarily use tree plantations and grasses as a feedstock for its fuel source. In addition, bioenergy from tree biomass grown in the U.S. is expected to play an important role in meeting sustainability guidelines for the European Union.
All energy sources have environmental impacts, and it is important to understand and minimize these impacts to conserve the atmosphere, landscapes, and wildlife. Bioenergy production requires land for row crops and plantation forests, and land-use change for bioenergy may have impacts on local wildlife communities. Because bioenergy can come from several different crops and types of land management, comparisons between these alternatives are necessary for understanding the environmental costs and benefits for wildlife.
Isabel Gottlieb and I led a team of scientists from the University of Florida, Emory University, and the Joseph W. Jones Ecological Research Center measured impacts of bioenergy alternatives for wildlife in Florida, Georgia, and Alabama, three states projected to become major producers of bioenergy. They studied two major bioenergy crops, corn, and pine.
In pine plantations, land can be managed to harvest biomass in a variety of ways. The team studied the effects of thinning plantations, removing woody residue (debris left on fields after plantations have been cleared), and the use of short-rotation plantations that would focus on biomass specifically, rather than traditional uses (e.g., paper, timber). These land-uses were compared with native longleaf pine forests, which were historically the common habitat in the region.
Using Corn For Bioenergy Is Bad For The Birds
The most profound finding was the difference in the abundance and diversity of birds between cornfields and pine forests. Pine plantations supported far greater biodiversity of birds than corn fields. Over the course of the study, they observed 42 bird species in the natural pine forest, and only 10 species in the corn fields. In the pine plantations, they found 41-53 species in each class of plantation stand, with a total of 78 species found using plantation forests in various stages of its life cycle. Most species seen in cornfields were observed only once or twice.
This suggests first that corn is not a suitable habitat for most bird species in the region, and that plantation forests can provide habitat for many species of birds including species of conservation concern such as the Bachman’s Sparrow (Peucaea aestivalis), Hooded Warbler (Setophaga citrina), and others. The major effect of corn land-use on wildlife has been observed throughout the country, but this was one of the first examples illustrating how in a bioenergy context, this crop is more detrimental to other alternatives.
The Good And The Bad: Alternative Options For Pine Biomass
Pine plantations clearly harbor a greater diversity of birds, yet the way in which pine biomass is generated could have a wide array of effects on wildlife. The team found that thinning plantations had a generally positive effect on bird biodiversity, with the strongest benefit to birds that forage in tree bark or in the canopy, and birds that nest in trees or tree cavities. Thinning is the only biomass harvest method that was found to positively impact the bird community.
Residue harvest from cleared pine plantations had a negative effect on bird activity. Sites, where residues were unharvested, contained a well-developed understory and often standing dead pine trees that were not removed which can be used by a surprising diversity of forest birds including Summer Tanager (Piranga rubra), Brown-headed Nuthatch (Sitta pusilla), and several species of woodpeckers. Very few birds remained in clear-cut stands where the residues were harvested. Groups of birds most affected were birds that nest in trees or forage in the lower canopy.
The team estimated that short-rotation plantations would have the largest negative effect on the bird community because these stands are cut after 8-10 years and never reach older age classes of conventional timber plantations. Birds that were most strongly affected by short-rotation plantations included birds that nest in tree cavities and species that use large trees to forage in the bark or on the wing, including several species of conservation concern. These results suggest that short-rotation energy plantations may provide habitat for some common species, but are not valuable for protecting rare species that rely on a mature forest.
Context For The Landscape And Conservation
Short-rotation plantations had an overall negative effect on birds, but their biomass yield per acre is several times greater than the other options considered. Consequently, much less land would be required to meet bioenergy production goals. Across a region, minimizing the impact of bioenergy may mean choosing the most intensive management regime that requires a much smaller land footprint.
The research team recommends that thinning should be used as a fuel source whenever possible. Short-rotation energy plantations may help reach energy goals, but to minimize impacts to wildlife these stands should be established preferentially on former cornfields or degraded agricultural land rather than replacing conventional timber stands or natural forest. These recommendations could help guide the growing bioenergy market toward environmental sustainability over the next decade.
The study, Alternative biomass strategies for bioenergy: implications for bird communities across the southeastern United States was recently published in the journal GCB Bioenergy.