One of the main reasons for growing bio-energy plantations is a reduction of the carbon emissions whilst obtaining energy benefits. Most of the intensive silvicultural treatments needed to ensure high levels of productivity in plantations (e. g. site preparation, weeding and fertilization — see Sect. 5.5) require some energy inputs. For example, fertilizer application uses fossil fuel based energy (and is responsible for some carbon dioxide emissions) during manufacture and/or mining, transport and application. In this section we will contrast the carbon costs of fertilization and other silvicultural treatments with the potential carbon gains from increased growth in short-rotation plantations.

Energy and carbon budgets for specific scenario’s in bio-energy crop systems are usually calculated through a life cycle analysis (LCA) approach, which takes account of energy inputs and outputs, as well as carbon gains and losses associated with every step of the production process. The major steps to construct carbon and energy budgets have been proposed by Schlamadinger et al. (1997) and subsequently implemented by inter alia Matthews (2001) Heller et al. (2003) and include:

• Definition of the system boundaries to do all calculations. A reference system is normally chosen, against which different alternative management scenario’s are contrasted.

• Estimating the total energy benefits from a specific scenario (including waste products).

• Estimation of total energy inputs in the production system (including energy investments in infrastructure and energy losses along the fuel chain).

• Estimating the carbon sequestration that takes place.

• Estimating of total carbon emissions from each specific scenario.

• Estimating the net emission of other greenhouse gasses, e. g. N2O, which can be presented as CO2 equivalents for the purposes of the budget.

Most authors stress the importance of detailed carbon and energy budgets for every step in the LCA because changes in management regime of biomass production systems can lead to large variations in the carbon and energy budget. Mead and Pimentel (2006) make a good case to show that individual silvicultural operations should be evaluated to decide on the optimum energy production system, as their efficiencies may differ widely. In order to obtain meaningful results, these calculations will have to be done on a site-specific basis because recommendations on the type and intensity of soil preparation (Smith et al. 2000, 2001; Zwolinski et al. 2002), fertilization (du Toit et al. 2010; Kotze and du Toit 2012) and weed control (Little and Rolando 2008) differ strongly across site types.

The figures most commonly used to evaluate the suitability of bio-energy systems are:

• The energy ratio (energy produced per unit of energy input)

• Net energy yield (energy output minus energy input per hectare)

• The carbon emission coefficient (inclusive of the greenhouse gas emissions expressed as CO2 equivalents)

Despite the strong dependence of the carbon and energy balance on individual cultural practices when applied to specific site types, a few useful generalisations can be made: Energy ratio’s of forestry crops (in temperate climates) typically vary between 10 and 25, compared with annual crops that vary between 1 and 5 (Mead and Pimentel 2006). Energy ratio’s as high as 55 have been reported for temperate climate forest crop systems under intensive management and fertilization (Heller et al. 2003). Energy ratio’s of 42 have been calculated for warm climate eucalypt crops in semi-arid areas (Wu et al. 2007) and ratio’s in excess of 60 have been estimated for a hypothetical Pinus taeda and Eucalyptus grandis systems that included site preparation and fertilization inputs (Mead and Pimentel 2006). The three warm-climate case studies have all shown considerable room for improvement of the energy ratio if specific vegetation management, harvesting and/or transport regimes are adopted. New research should focus on the ability of warm climate tree crops across different site qualities to produce energy efficient biomass through appropriate silvicultural management strategies.