Analysis of aboveground biomass and carbon stocks (as equivalent CO2) was performed in the Castilla y Leon region, Spain. Data from the second and third Spanish Forest Inventories (1996 and 2006) were used. Total aboveground biomass was calculated using allometric biomass equations and biomass expansion factors (BEF), the first method giving higher values. Forests of Castilla y Leon stored 77,051,308 Mg of biomass, with a mean of 8.18 Mg ha(-1), in 1996 and 135,531,737 Mg of biomass, with a mean of 14.4 Mg ha(-1), in 2006. The total equivalent CO2 in this region's forests increased 9,608,824 Mg year(-1) between 1996 and 2006. In relation to the Kyoto Protocol, the Castilla y Leon forests have sequestered 3 million tons of CO2 per year, which represents 6.4% of the total regional emission of CO2. A Geographic Information System (GIS) based method was also used to assess the geographic distribution of residual forest biomass for bio-energy in the region. The forest statistics data on area of each species were used. The fraction of vegetation cover, land slope and protected areas were also considered. The residual forest biomass in Castilla y Leon was 1,464,991 Mg year(-1), or 1.90% of the total aboveground biomass in 1996. The residual forest biomass was concentrated in specific zones of the Castilla y Leon region, suitable for the location of industries that utilize biomass as energy source. The energy potential of the residual forest biomass in the Castilla y Leon region is 7350 million MJ per year. (C) 2010 Elsevier Ltd. All rights reserved.