Silva Balcanica, 14(1)/2013 CARBON STORAGE IN SELECTED EUROPEAN CHESTNUT (CASTANEA SATIVA MILL.) ECOSYSTEMS IN BELASITSA MOUNTAIN, SW BULGARIA Miglena Zhiyanski, Maria Glushkova Forest Research Institute – Sofia Bulgarian Academy of Sciences Abstract This work is focused on carbon storage of various components (above-ground biomass, forest floor and soil) of selected European chestnut (Castanea sativa Mill.) forest ecosystems developed on Chromic Luvisols in Belasitsa Mountain, SW Bul- garia. In 2010 two mixed chestnut forests and one pure stand were sampled. Within each experimental site sampling plot was defined and the characteristics of stands were measured. Estimated biomass was calculated per hectare then the values ob- tained were converted to carbon stock. The carbon content of forest floor and differ- ent soil depths (0–10 cm, 10–30 cm, and 30-50 cm) was estimated in 6 replicates per plot. All soil properties were determined in accordance with the standardized meth- ods. Variations were obtained for soil carbon stock in studied chestnut ecosystems. More carbon is sequestered in chestnut biomass of older forest CF1 (31.1 t C ha-1) compared with the other two stands CF2 and CF3 (14.4 – 19.6 t C ha-1). Concerning carbon stored in soil system the picture is different. The mean value of carbon stored in forest floor layers of all studied ecosystems was 19.8 t C ha-1 while the highest stock was determined for the pure chestnut stand CF3 – 23.8±0.8 t C ha-1. In soils carbon is mainly accumulated in the superficial 0 - 10 cm and decreased toward deeper layers. In the mixed forests CF1 and CF2 the carbon stock in this soil layer was estimated at 20.1 t C ha-1 and 21.2 t C ha-1, respectively. The values obtained were lower in comparison with the same layer under the pure chestnut stand CF3 (34.1 t C ha-1). The carbon stock in both forest floor and 0-50 cm soil was higher in CF3 (84.04 t C ha-1) compared with CF1 (59.77 t C ha-1) and CF2 (50.50 t C ha-1). Following the estimations of carbon stock including above ground biomass, the total stock in the studied chestnut forests could be ordered as follows: CF3 (105.8 t C ha-1) > CF1 (102.1 t C ha-1) > CF2 (76.3 t C ha-1). The pure chestnut forest CF3 characterized with the highest total carbon stock per hectare and only 20.6 % from it is accumulated in the aboveground tree biomass. This confirms the high po- tential of carbon sequestration in soil system under pure and mature chestnut forests. At the same time the carbon in older mixed chestnut ecosystems dominated by beech in tree composition (CF1) was also high but 33.8 % of carbon is accumulated in the aboveground chestnut biomass, while 21.0% of carbon is sequestered by chestnut 60 trees in the other mixed stand CF2, dominated by chestnut. In CF3 the accumulation is mainly in forest floor and in mineral soil (especially in the superficial soil layer), which shows favourable conditions for the incorporation of organic substances in soil system under pure chestnut stands in mature growing phase. Key words: Carbon storage, soil, forest floor, tree biomass, Castanea sativa, forest ecosystems, Bulgaria INTRODUCTION Carbon balance is one of the most important biogeochemical parameters in natural systems, since it determines the flow of organic matter and controls the con- tent of CO2 in the atmosphere. The increased concentration of CO2 in the atmosphere due to anthropogenic activities such as emissions from fossil fuels (about 30 Gt CO2 worldwide in 2007) (IEA, 2009) and from land use change, mainly deforesta- tion in tropical regions (~ 5.8 Gt CO2 per year worldwide in the 1990’s) (Denman et al., 2007) emphasises the role of forest for carbon sequestration and buffering. During the last century, the carbon dioxide (CO2) concentration in the atmosphere has increased from 280 to 367 parts per million (IPCC SRES, 2000; IPCC, 2001). About half of the emissions of fossil fuels are released in the atmosphere and the rest are absorbed by the oceans and land surface. Most analyses to date of options for mitigating the risk of global climate change have focused on reducing emissions of carbon dioxide and other greenhouse gases. Much less attention has been given to the potential for storing (or ‘sequestering’) significant amounts of carbon in forests and other ecosystems as an alternative means of offsetting the effect of future emis- sions on GHG concentrations in the atmosphere. Castanea sativa Mill. is a multipurpose species because of various useful properties and a wide range of valuable products (chestnut fruits, honey, timber, etc.). Previous studies have revealed that chestnuts grow faster and larger than other hardwood species allowing them to retain more carbon over a shorter period of time (Jacobs et al., 2009). Chestnut trees can sequester as much as five times the CO2 of any other hardwood tree, and they grow quickly, inhaling massive amounts of carbon dioxide in the early phase of their life, making them a viable near-term carbon stor- age solution. And since the tree is most often harvested for high-quality hardwood products such as furniture, the sequestered carbon would remain trapped in the ob- jects for longer periods of time than in wood used for paper manufacture (Jacobs et al., 2009). Nevertheless despite the importance of its multiple uses, chestnut is insufficiently studied concerning carbon sequestration and is not effectively used as a managed forest species in Bulgaria, being categorised as a ‘negligible’ genetic resource. Peev (2008) and Velichkov et al. (2010) reported that in Bulgaria, the for- est habitat of C. sativa (defined under Directive 92/43/EEC) comprises stands with a total area of 3315 ha. In Belasitsa Mountain the protected zone within the Natural 61 Park covers 2085 ha out of 2560 ha totally protected forests, thus enhancing the habitat favourable conservation status in the site, which is of primary importance for its conservation at a country scale (Velichkov et al., 2010). According to Velichkov et al. (2011) in Belastitsa Natural Park the mixed chestnut forests increased their area from 604 ha in 1964 to 1030 ha in 2009, while the pure chestnut stands decreased from 1319 ha to 648 ha for the same period. The authors underlined that in the mixed forests European chestnut dominates at diameter of breast height and age (Velichkov et al., 2011). Forests and other terrestrial ecosystems have great importance in carbon se- questration, absorbing about 3.3 Gt CO2 per year for the period between 1993 and 2003 (Nabuurs et al., 2007). The site conditions and tree species are key factors in determining the forest potential to sequester carbon. In addition, the forestry and forest management activities, through the management decisions made for different forestry systems (harvesting, rotation systems, etc.) play an important role (Lucas et al., 2010). A longer rotation period is proposed as a measure to promote carbon se- questration in forests (Adams et al., 1999; Sohngen, Mendelsohn 2003). The longer rotation period, with a higher proportion of old trees, leads to higher carbon accu- mulation in the ‘reservoirs’ in comparison with short-rotation systems. The soils in mountainous broadleaved forests are characterized by higher soil carbon content as a result of increased quantity of aboveground and belowground biomass and the more intensive mineralization rates compared with coniferous forests (Reicosky, Forcella 1998). Mature and over-matured stands characterized with higher carbon density in the ‘reservoirs’, while young stands have higher capacity as ‘sinks’. Short rotation where harvesting is carried out at the age of maximum growth are linked to maxi- mum biomass production, but not to carbon accumulation. Growth tables (Nedyalk- ov, Shikov, 1983) show that the biomass productivity of forests strongly decreases after the mature phase. However, research in old forests show that especially in these ecosystems, much more carbon is transferred to the soil and increased stocks of carbon in underground biomass and soil are reported (Harmon et al., 1990; Schulze et al., 2000). Old forests can not maintain high stand densities and thus stimulate the decomposition of organic matter in soil, which is associated with an increase of soil organic carbon stocks (Cannell, 1999; Liski et al., 2001; Harmon, Marks, 2002). Gallardo, González-Hernández (2008) reported the maximum soil C content of 530 t C ha-1 in chestnut forests managed as coppices for timber production and concluded that this value is considered to be the maximum C-storage potential of the chestnut soils. At the same time in chestnut orchard soils in Spain reported values were as low as 40 t C ha-1, indicating that the limited soil organic matter allows for a potential C storage capacity of at least 150 t C ha-1 which could be reached under an appropriate management regime (Gallardo, González 2008). In another study of carbon seques- tration in 25 years old coppice of C. sativa the accumulation of C in the tree biomass was 58 mg C ha-1 y-1; the calculated litter decomposition-constants 0.39 y-1; and the aboveground annual-production 5.3 mg C ha-1 y-1. On calculating an annual overall 62 balance, inputs of C into this forest ecosystem are always higher than the C outputs and C sequestration reported was 4.6 mg C ha-1 y-1 (Gallardo, González 2005). The carbon accumulation rate and duration along the full life cycle of stands are determined by a combination of tree species, site characteristics, management practices and climatic conditions (Sakin et al., 2011).