Introduction, concepts and indicators

1 Introduction, concepts and indicators

Fixation and accumulation of carbon in forest ecosystems has to be based on data of the forest stand inventory including annual and mean increment but also on data obtained by studies carried out in long-term field research stations. Particularly carbon fluxes between atmosphere, forest stand biomass and soil are affected by forest practices including methods of the stand establishment, soil preparation and fertilization, species composition, logging methods and using the wood material but also by factors such as fires, windbreaks and insect pests under concrete climatic conditions. We considered in this review the forest ecosystem including the soil and vegetation under influence of forestry practices. The carbon element is the major constituent of anthropogenic greenhouse gases molecules

such as the carbon dioxide, CO 2 , methane, CH 4 , and halocarbon (CFC). The uptake, release and storage of carbon in soil, biomass and wood products are the main process by which forests and forest-wood chains affect the biosphere-atmosphere interactions and the atmospheric greenhouse effect. In addition, forests and forest-wood chains may potentially interfere with climate through other mechanisms such as the surface energy balance, a biophysical effect including the albedo and the Bowen ratio, and additional greenhouse gases

such as N 2 O and ozone. This biophysical effect must be taken into account as far as a complete assessment of the environmental impacts of forest is the objective (Betts, 2000; Betts et al., 2007; Gibbard et al., 2005). The potential for enhancing carbon sequestration by forestry, including land use changes, is estimated to 11-15% of the actual fossil fuel emissions at the global level and 5-11% in the EU15 (Brown et al., 1996; Cannell, 2003). In the case of EU15, the potential including fossil fuel substitution by energy would be raised to 25% of the fossil fuel emissions. The analysis of the management impact on the forest carbon cycle must assess the entire life cycle from tree regeneration to final harvest and wood products use. The life cycle assessment has become a standardised protocol to examine the environmental impacts of product or process (Sonne, 2006). Unfortunately, most of the available studies available so far do consider only a part of the forest life cycle or a fraction of the ecosystem which renders their interpretation difficult in the context of the global carbon cycle. For instance, the fossil fuel consumption associated with management operations, the energy use for seedling production and transportation or the energy use for fertilisers production and pllication are rarely considered.

The terrestrial carbon cycle includes five major pools, the atmosphere, soil, biomass, harvested products and fossil fuel (Figure 1). Their turnover-rate is decreasing from the atmosphere to the biomass, soil, fossil carbohydrates and sediments. The residence time of carbon, is typically one to two orders of magnitude lesser in the atmosphere (5-7 years) than in biomass (1-250 years) and soil (5-10 000 years). For example, carbon stock in the wood biomass of forests of EU countries is dependent both on the area of forests and the production level of forest stands reaching following values: > 1000 mil. t (Sweden) 500 – 1000 mil. t (Germany, France, Finland, Austria, Poland) 200 - 500 mil. t (Romania, Italy, CR) 100 – 200 mil. t (Spain, Slovakia, Lithuania, UK, Hungary, Latvia, Estonia, Bulgaria)

50 – 100 mil. t (Netherlands, Denmark, Luxemburg, Cyprus, Malta) (according to Paschalis- Jakubowicz 2004). The highest value of carbon accumulation in wood biomass is mentioned by Paschalis- Jakubowicz (2004) on the example of Austria (more than 70 mil. t carbon per 1 mil. ha forest) and the lowest value on the example of Cyprus and Spain (less than 20 mil. t per 1 mil. ha forests).

The soil C pool is characterised by a wide range of turnover rates, from 1 to 10 -1 yr which are determined by the chemical quality of the organic matter and the biophysical environment

which makes it available for decomposition rates (temperature, oxygen, water, pH etc.). Carbon is withdrawn by vegetation from the atmosphere through the photosynthesis (gross primary production) which fixes carbon atoms in carbohydrate molecules such as sugars. When the energy encapsulated in this process as covalent bonds is being used, carbon is

oxydised as CO 2 which returns into the atmosphere. This mineralisation occurs in plants as ”autotrophic” respiration, and animals and decomposers as ”heterotrophic” respiration and during other mineralisation processes (combustion in industrial processes, fires). The balance

of CO 2 exchanged by a given forest ecosystem with the atmosphere is called the net ecosystem exchange (NEE). The net carbon balance of the only vegetation compartment is the net primary production (NPP). Other molecules such as methane, CH 4 , volatile organic compounds (isoprene etc.) contribute marginally to the exchange between the ecosystem and the atmosphere. Methane is produced under anerobic conditions, during soil organic matter decomposition and this emission may be taken into account for managing wetlands and peatlands. Methane has been demonstrated to be also produced by green leaves under normal conditions but the cause and the relevance of this emission in the carbon cycle is still under debate (Keppler et al., 2006). The relative importance of the dissolved carbon flow in the forest carbon cycle is not crucial in the context of this chapter even if recent studies show that it may account for several percents of the total carbon exchange between ecosystem and the environment.

ATMOSPHERE

Net ecosystem exchange

Gross Primary Production

Net Primary Production Respiration

Wood

Species composition

Vegetation management

life cycle

Pest management Fertilisation

Fertilisation

Fossile C

Drainage

SOIL

Soil preparation

Dissolved Carbon

Figure 1. Carbon cycle in a managed forest ecosystem and main effects of management practices.

Soil appears to be the highly important sink of carbon. The ratio of soil: vegetation C density increases with latitud. Land use change particularly conversion to agricultural ecosystems, depletes the soil C stock. Consequently, afforestation of agricultural soils and management of forest plantations can enhance SOC stock through C sequestration. The rate of SOC sequestration, and the magnitude and quality of soil C stock depend on the complex interaction between climate, soils, tree species and management, and chemical composition of the litter as determined by the dominant tree species. Soil C sequestration in boreal and temperate forests may by an important strategy to ameliorate changes in atmospheric chemistry (K. Lal 2005).

Tab. 1 Carbon stock in selected biomass of the world (Lal 2005). Biom

Area

C density Mg/ha

C stock (Pg)

vegetation soil Tundra

vegetation

soil

8 97 Boreal/Taiga 1372

88 471 Temperate

57 96 59 100 Tropical

216 Wetlands

6 202 Total

Mean 54

Mean 189

The global carbon cycle includes five pools, the atmosphere, biomass, harvested products, soil and fossil carbon pools which must be included in the assessment of the greenhouse gas balances of forestry scenarios. Two methods can be used for calculating the greenhouse gases balance and evaluate the impacts of forest and land management, namely the “flux” and “stock” methods. For convenience, practicability and consistency with IPCC guidelines (Naaburs et al., 2004), we recommend to adopt the stock change approach for instance for the TOSIA model. The stock change approach quantifies the net change in stock of each of the five pools between two dates. Not all the greenhouse gases considered have the same global warming potential (Houghton et al., 1990). Releasing in the atmosphere 1 kg of carbon as

CO 2 or CH 4 affects differentially the climate (Table 2). It is therefore recommended to distinguish each greenhouse gases as far as possible. Table 3 lists the main operational

Trace gas

Estimated lifetime in Global Warming potential the atmosphere (years)

integration time (years)

Table 3. Indicators list proposed for assessing the forestry scenarios impact of the greenhouse gas balance.

Carbon sequestration in the forest ecosystem

carbon in the above ground living tree MCPFE (C.1.4)

tons C pr ha

compartment biomass Eforwood WCI (16) carbon in the root biomass

tons C pr ha

MCPFE (C.1.4) Eforwood WCI (16)

carbon in above ground living herb and MCPFE (C.1.4)