Estimating the changes in tree carbon stocks in Galician forests (NW Spain) between 1972 and 2009

Abstract

Almost 50% of the land in Galicia (NW Spain) is covered by forest (minimum area of land of 1 ha with tree crown cover of more than 10%) and the region is the top timber–producing area in Spain. Regional changes in forest live tree biomass and carbon stocks were estimated from National Forest Inventory (NFI) data covering almost 40 years, i.e. between NFI-1 (1972) and NFI–4 (2009). Carbon stocks were estimated from biomass of different tree species and components, including roots. Biomass equations were applied to the tree stock data for different species and diameter classes. The biomass increased in all species or groups for the period considered. However, although the biomass of softwood species (mainly Pinus spp.) increased, because of the increase in the number of trees belonging to the largest diameter classes (DC), the balance of the number of small trees (DC < 25 cm) was negative. The estimated total tree dry biomass (above- and below-ground) in the region increased from 94.77 Tg (teragrams) to 217.09 Tg between the first (1972) and fourth NFI (2009). This is the equivalent of an increase in carbon stocks from 45.51 Tg to 102.83 Tg. Most of the carbon stocks were estimated by applying regional-specific biomass equations and C-content data for different tree species and components. Assuming a global carbon content of 50% led to overestimation of the total carbon stocks by between 4.1% and 5.6%. Although harvesting intensity in the region is one of the highest in Europe, the live tree C stocks have increased; this was attributed to the increase in forestland area as well as to the increased C density, i.e. from 40 Mg (megagrams) C ha−1 in NFI-1 to 72 Mg C ha−1 in NFI-4.