At another event, the International Poplar Symposium held last summer in Nanjing, China, I had the opportunity to organize a session on using poplars and willows to provide ecosystem services such as clean water, soil stabilization, and wildlife habitat, as well as bioenergy. I grouped applications in the following categories based on the benefit provided, with the implicit understanding that in most cases multiple benefits, including energy production, can be provided simultaneously by these versatile tree species.

Phytoremediation and Wastewater Renovation

Poplars are particularly attractive for pollution control applications. They grow quickly and take up large amounts of water and nutrients such as nitrogen and phosphorus as well as other compounds. They remove water from the soil through transpiration, leaving pollutants behind to be held onsite, a process called phytoremediation. Nutrients are retained in the plant and promote growth. Other compounds may be held (sequestered) in the plant, or transformed into nontoxic compounds in plant or soil. Phytoremediation can also refer specifically to treating toxic nonnutrients such as industrial wastes, particularly complex hydrocarbons such as benzene. A related process, wastewater renovation, involves treating nonindustrial wastes such as residential sewage.

Windbreaks, Bank Stabilization, and Water Detention and Spreading

Planting trees around field boundaries as windbreaks and along streams to reduce soil movement by wind or water is an ancient practice which has continued into the present day, especially in drier climates such as the Great Plains of the United States. Blowing soil damages tender young crops; in areas with a lot of wind such as our Midwest or oceanic islands such as New Zealand, planted windbreaks line fields for miles. In northern climates, willows are often planted as living snow fences; by trapping blowing and drifting snow, they maintain visibility for motorists and protect roads.

In wetter conditions unsuitable for poplars, willows are used to stabilize streambanks and slopes; willow roots better tolerate flooding and low oxygen conditions. Sometimes the greatest benefit of the trees is to block or control livestock access to streams. In semiarid and arid environments, where rainfall is infrequent and intense, crusts forming on the soil surface keep water from moving into, or infiltrating, the soil. In Iran, poplars are planted in seasonal water courses to slow water flow, allowing greater infiltration and reducing local flooding. The water spreads over a larger area, in effect irrigating the trees.

Forest Restoration

Forest restoration includes reclaiming degraded land, rehabilitating forest stands, and planting idle agricultural fields to reconstruct native forests. The specific problems addressed by planting short-rotation forests differ by country, site condition, and climate.

In the wheat belt of Western Australia, where rainfall is very low, replacing the deep-rooted native plants with cereal crops eventually led to the salinization (high concentrations of salt) of soil and ground water and the degradation of local lakes. Because the crops extracted water from less of the soil than did the native vegetation, the excess water caused a rise in the saline ground water. Salt-laden water then moved to the soil surface by capillary action or flowed to nearby lakes. Poorly designed irrigation systems can also cause soil salinization. Reversing the trend takes time, but planting deeprooted eucalypts (called oil mallees) in rotation with pasture and cereal grains is being tried in Australia. The oil mallees will be harvested for essential oils and biofuels if the economics work out favorably.

Desertification processes can be reversed at the margins by protecting cropland from blowing and drifting sand. Sometimes the ability of deeprooted trees to tap ground-water sources can be critical to the success of these green fences. In Northern China, drought-resistant poplars, pines, and other species are planted to push back the encroachment of deserts onto arable land.

Highly disturbed surface-mined land is another tough environment for plants to grow. Older mining processes brought unweathered soil and rock material to the surface where chemical reactions produced highly toxic compounds such as sulphuric acid. Newer technologies stockpile surface soils removed early in mining, which are then spread over the mine spoils to create a more hospitable habitat for trees. Many local and exotic species are used in mined land reclamation to establish a tree cover for erosion control, aesthetics, and wildlife habitat. In Estonia, hybrid aspen (aspen is a poplar species) is used to reclaim spoils from calcareous oil shale mined to produce energy.

The most common motivation for forest restoration is to maintain or enhance wildlife habitat and biodiversity. Planting short-rotation woody crops such as cottonwood or willow in with slower growing species such as oaks is one approach to reconstructing native forests on former agricultural land, a process called afforestation. After one or two rotations of the short-rotation crop, the other species is released to form a new forest. If all goes as planned, other desired native species will establish naturally, their seeds dispersed by wind, water, birds, and mammals into the quickly established forest.

Biofuels Complete the Circle

Projects that yield the combined benefits of the ecosystem services described above with biofuels offer several advantages. For private landowners, the potential financial return from harvesting short-rotation crops is attractive. Fuel production from biomass sources benefits society because it is "carbon neutral"; it doesn't convert carbon that has been stored for a long time as coal or petroleum into carbon dioxide in the atmosphere. The carbon in the tree that would have been released by decomposition in a relatively short time (geologically speaking) is used immediately for energy.

The amount of biofuels produced in the multipurpose situations I've described may be less than could be produced in dedicated bioenergy plantations because the trees are not grown as intensively or cannot be harvested as completely without reducing other benefits, but the current interest in biofuels may mark a return to a greater dependence on woody crops for energy. Although most of the world's population still depends on firewood to provide warmth and cooking fuel, the hallmark of an industrialized nation in the last century was the shift to fossil fuels. Perhaps we are returning, in a figurative sense, to our roots in the forest.

For more information:
John Stanturf at 706–559–4316 or
jstanturf@fs.fed.us.

John Stanturf is project leader of the SRS Disturbance Ecology unit in Athens, GA.