Willow biomass crops are being developed as sustainable systems that simultaneously pro­duce a suite of ecological and environmental benefits in addition to a renewable feedstock for bioproducts and bioenergy [5,6,10]. The perennial nature of the willow production sys­tem provides a range of beneficial attributes, such an improved energy return in investment, reduced greenhouse gases, and changes to soil conditions and biodiversity.

A recent life cycle analysis of willow biomass crops in North America covered all the inputs and processes from the nursery through seven three-year harvest cycles. Establish­ment, harvest and delivery of the willow to an end user and removal of the willow stools after seven rotations were included. The study explored eight different scenarios based on differences in transportation distances to an end user (71 or 195 km), high or low-yield scenarios (11.8 or 9.2 odt ha-1 yr-1) and, the use of 0 or 100 kgN ha-1 once every three years following harvest. In addition, uncertainty analysis was conducted using data on vari­ations in leaf litter, yield, and belowground biomass. Across the eight scenarios, cumulative energy demand ranged from 446.5 ± 12.3 to 1055.2 ± 42.9 MJ odt-1 [36], which equates to an energy ratio of between 1:19 and 1:45 for willow biomass delivered to an end user. The largest fraction of energy demand across all scenarios was use of diesel fuel, of which 48-77% was used for transportation of willow chips from the field gate to the end user. Harvesting operations had a greater energy demand than other field processes due to the frequency of occurrence over the life of the crop and the size of the equipment that was used. A recent review of 26 studies of short-rotation willow and poplar systems found that net energy ratio across a range of scenarios ranged from 1:13 to 1:79 at the farm gate and 1 : 3 to 1 : 16 after delivery to an end user [37]. Results from the current study and those from a previous study in North America [38] are at the high end of this range because inputs to the willow system, such are fertilizer or fencing, are generally lower or non-existent, when compared to European recommendations.

Patterns similar to energy demand were also found for greenhouse gas emissions for willow biomass crops across all scenarios because fossil fuel use is the largest source of emissions in the system [36]. Among the eight scenarios, greenhouse gas emissions ranged from-138.3 ± 22.5 to -52.7 ± 14.7 kg CO2-eq odt-1 (~-6.9 to -2.7gCO2 eqMJ-1). Carbon sequestration in the belowground portion of the willow system provided a large sink [39] that more than compensated for carbon emissions associated with crop production and management across all eight scenarios. As a result, the willow biomass crop system ended up being a carbon sequestration system, in addition to producing woody biomass that can be used to generate bioenergy, biofuels or bioproducts.

The perennial nature and extensive fine-root system of willow crops reduces soil erosion and non-point source pollution relative to annual crops, promotes stable nutrient cycling and enhances soil carbon storage in roots and the soil [39-42]. In addition, the crop is constantly in its rapid juvenile growth stage, so demand for nutrients is high, resulting in very low leaching rates of nitrogen even when rates of applications exceed what is needed for plant growth [43-45]. The period with greatest potential for soil erosion and non-point source pollution is during the first 1V2 years of establishment, when cover is often limited because weeds need to be controlled and the willow canopy has not closed. The use of a winter rye cover crop has proven to be effective at providing soil cover without impeding establishment of the willow crop [46] and trials with a spring planting of low growing white clover have also been effective [22]. Since herbicides are only used to control weed competition during the establishment phase, the amount of herbicides applied per hectare is about 10% of that used in a typical corn (Zea mays)-alfalfa (Medicago sativa) rotation in upstate New York [47].

Birds are one indicator of the biodiversity supported by willow biomass crops that have been studied in the United States. 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 [48]. Thirty-nine different species made regular use of the willow crops and 21 of these species nested in them (Figure 12.5). The study found that diversity increased as the age of willows and size of plantings increased, and also that birds have preferences for some willow varieties over others [49]. The number of bird species supported in willow biomass crops was similar to natural ecosystems, such as early succession habitats and natural, intact eastern deciduous forest ecosystems. The positive impact of willow on bird diversity was also supported in a recent assessment as part of a multidisciplinary study in Europe [10]. Instead of creating monocultures with a limited diversity across the landscape, willow biomass crops will increase diversity relative to open agricultural land or arable crop fields.