## 4 Life-Cycle Assessment
### Life-Cycle Assessment and Forest Sustainability  
Sustainable forestry certification systems have been developed in over 150 countries, often by intergovernmental, national, or local organizations. Methods and tools have been developed to evaluate these systems: criteria and indicators, cost-benefit analysis, knowledge-based systems, environmental impact assessment, and life cycle assessment [50]. While criteria and indictors are the popular method to assess forest sustainability, LCA is also a tool for this purpose, but it is product-focused and evaluates the whole product system. One of the major strengths of LCA is that it is transparent. Many consumers are demanding a means to compare products in terms of impact on the environment and LCA allows for development of quantitative indexes that allow for this comparison [51]. Another strength is that the process is input/output based. LCA evaluates a closed system and measures inflow and outflow, allowing for measurements of changes in key elements. Forest sustainability measures tend to concentrate on traditional environmental values like timber, soil and water. LCA can measure use of those variables, but it is ideal to measure more complicated variables (such as greenhouse gases) in terms of input and output. Greenhouse gases (GHG) are a good example and one reason LCA is currently a very popular tool for evaluating forest-based products and services. Forests are an important means to control GHG because carbon is stored in forest products and remains out of the atmosphere during this storage. Incrementally, about 100 million t of carbon is sequestered annually U.S. forests [52]. The net impact of forest products on carbon in the atmosphere is complex. Forest products tend to have superior thermal properties, require less energy to produce than other building products, and produce fewer carbon dioxide emissions. One reason for this is that much of the energy used to produce forest products come from biomass, and biomass is somewhat carbon-neutral (fossil fuels introduce \"new\" carbon to the atmosphere, while biomass recycles carbon in the atmosphere) [53]. Time frame impacts carbon neutrality of biomass. As long as there is no net temporal change in forest land or productivity, a specific timber harvest management regime should equilibrate to some level of carbon stock on the forest landscape, plus a carbon yield from harvest. However, any change in a harvest management regime (defined at the landscape level) will cause a change in overall levels of carbon stocks on the landscape (positive or negative depending on whether harvest rates increase or decrease), and this will produce a transient net flux to or from the atmosphere. Biomass use is carbon-neutral in the long run as long as vegetation is allowed to grow back and the ecosystem retains as much carbon as before. For example, cutting an old-growth forest on the American West Coast that is 1,000 years old and contains a huge amount of biomass will not be carbon-neutral because the same amount of biomass may never be allowed to grow back. Also, in some cases, a forest is harvested and the soil is disturbed to increase productivity, which releases carbon that was locked in the soil. So, although the forest might regrow quickly, it is important to determine how much carbon stored in the soil was released and how much is going back. Carbon neutrality is a tough question, but it is certainly better, for example, to burn biomass from the forest that does not require as much fossil fuel energy to produce as agricultural biomass. Also, harvesting forest biomass requires some external energy and this means the entire system is not balanced in terms of carbon neutrality. Plus, when viewed from a global level, GHG emissions from forest industry are mostly offset by carbon sequestration in forests and forest products [54].