Sustainability of SRWC for Energy
What is Sustainability?
Sustainability is meeting the needs of the present without compromising the ability of future generations to meet their own needs (The Brundtland Commission, WCED, 1987). "Triple-bottom line" of sustainability: economics, environmental, and social.
Environmental sustainability in short rotation woody crop (SRWC) systems must also provide the same benefits as the present generations. Sustainable management maintains the ecological capacity of the site for tree growth over time by avoiding degradation of soil nutrients and water supplies. The intensity of silvicultural practices can have a positive effect by increasing yield, which makes SRWC biomass bioenergy cost competitive and also reduces the amount of land required to produce the necessary tonnage. Negative environmental effects of SRWC systems also arise from the intensity of silvicultural practices, resulting in a trade-off between yield increases and environmental impacts. Following are some factors that negatively impact the environmental sustainability of SRWC systems:
Soil erosion represents possibly the greatest threat to long-term soil productivity due to loss of organic matter and nutrient-rich soil surface. However, since tillage is only required at establishment, erosion is likely limited to the first few years and should be less in coppice regenerated systems compared to annual-tilled systems. Once SRWC crops are established they are perennial crops with low soil erosion potential. As a result of erosion during the establishment phase of shrub willow SRWC systems, the use of cover crops must be explored to minimize such impacts.
During harvest, SRWC systems require the use of machinery with the potential to compact the soil. Compaction may reduce water infiltration causing soil erosion. However, the use of machinery in SRWC plantations is usually less than that needed for traditional forestry as fewer passes are needed, making the SRWC systems more environment-friendly (Hamza and Anderson, 2005). Established root systems are extensive and diffuse enough to support harvesting equipment when soils are dry (Mitchell et al., 1999).
Herbicide application is a practice in SRWC systems to control weeds during establishment and has the potential to enter ground water thus affecting water quality. However, herbicides are only used during the first year or two of establishment; thus over time these inputs are a fraction of what occurs in agricultural rotations (Volk and Luzadis, 2009). SRWCs can be used in the phytoremediation process that can improve water quality compared to traditional agriculture.
Fertilization has the potential to negatively impact the sustainability of SWRC systems when added nutrients are not captured by the crop but instead leached to the ground water or run off to surface waters. It appears that fertilization will likely be much less than in traditional agricultural crops than in SRWC systems; hence lesser nitrogen leaching.
Pest and Diseases
The perennial nature of the SRWC systems and the relatively narrow genetic base in Salix, Populus or Eucalyptus SRWC may promote susceptibility to pest and diseases. Damage of SRWC systems by pests and pathogens has been found to be problematic. Some willow clones are susceptible to potato leafhoppers, while leaf rust (Melampsora medusa) is common disease for Populus species affecting performance. Planting multiple cultivars with different genetic backgrounds when large-scale plantings occur is an effective way to manage this concern.
The perennial nature of SRWC and their changing structure suggests that across the landscape these systems can enhance biodiversity. A study of bird diversity in willow biomass crops over several years found that these systems provide good foraging and nesting habitat for a diverse array of bird species (Dhondt et al., 2007). For example, an increase in abundance and diversity of ground flora and avian species among commercial SRWC systems was observed in Germany (Schulz et al., 2009).
Economic and Social Sustainability
Achieving environmental sustainability is critical in SRWC systems, so as with economic and social sustainability. The current yield potential of SRWC systems with typical operational costs and procedures across the inherent site fertility gradient appears to range from 9-15 ton (oven dry) ha-1 yr-1 (Fig 1).
Selection of the best genetics and intensive silvicultural practices such as optimal fertilization, irrigation, and weed control show the potential to double or triple the yield of SRWC. Yield increases can proportionally reduce the area of land required to meet production goals.
SRWCs have the potential to provide rural development benefits by diversifying farm crops, creating an alternative source of income for landowners, and circulating energy dollars through the local economy. Statistics shows over 80% of the jobs are created when biofuels are produced from locally produced biomass associated with the biomass production, harvesting and transportation portion of the system.
As biomass systems develop and are implemented, the feedstock production and delivery portion of the system will create the majority of jobs with this new industry. Willow biomass crop production, harvesting and transportation can create 45 direct and indirect jobs for every 10,000 acres of willow biomass crops established and managed (Volk et al., 2005).
Overall, extending the concept of sustainability to bioenergy, sustainable bioenergy production systems "would be environmentally, economically, and socially viable now and for future generations and would move the world away from an unsustainable reliance on fossil fuels (Friedman et al., 2010).
Modified from Volk et al., 2011, Nissim et al., 2013, Zamora et al., 2013
Fig 1. First rotation biomass yield [Mg (oven dry) ha-1 yr-1] of top 5 clones with biomass crop yield trials established with new willow genotypes between 1999-2008. Site labels include the year of planting and location