The notion of storing CO2 by planting trees was developed as early as the
1970s (Dyson, 1977) and proposed again when the climate change discussion became more public following the 1992 Earth Summit in Rio de Janeiro (Marland and Marland, 1992). The development of a global afforestation programme to sequester carbon in plantations was proposed (Nilsson and Schopfhauser, 1995). At that time, it was estimated that this could result in 345Mha of new forest plantations and agroforestry plantings, which would sequester up to 1.5Gt C per year, equivalent to about 30 per cent of the anthropogenic carbon emissions. A decade later, although afforestation for
CO2 sequestration is still of interest, its expected contribution of carbon is
significantly smaller. The IPCC Third Assessment Report estimated that 12–15 per cent of fossil fuel emissions up to 2050 could be offset by improved management of terrestrial ecosystems globally (Sathaye and Bouille, 2001). A more recent regional study in the Midwestern United States reported that the afforestation of marginal agricultural land, comprising 24 per cent of
agricultural land area, could offset 6–8 per cent of current CO2emissions from
the regional fossil fuel combustion (Niu and Duiker, 2006).
To assess the net carbon mitigation impact of an afforestation programme, consideration must be given to the balance of deforestation and afforestation. For example, Woodbury et al (2006) calculated that, in the period 1990–2004, deforestation in the south-central and south-eastern US resulted in the release of 49Tg C from the soil; more than 50 per cent of the 88Tg C in soils sequestered by afforestation in the region during this time period. The net carbon sink in the tree biomass was about 240Tg C, resulting from a much larger afforested area compared to the deforested area. However, globally, the latest Forest Resource Assessment by the United Nations Food and Agriculture Organization (FAO, 2005) estimates the current rate of afforestation, landscape restoration and natural expansion of forests at 5.7Mha per year, in contrast to a loss of 13Mha per year through deforestation.
The area of forest plantations increased by 2.8Mha per year between 2000 and 2005, mostly in Asia (FAO, 2005). According to the Millennium
Ecosystem Assessment scenarios (MEA, 2005), the forest area in industrialized regions will further increase between 2000 and 2050 by about 60–230Mha. These areas of new forests are characterized by a very juvenile age structure (e.g. as for the old coppices or post-war plantations of European forests) that show high increment rates and increasing growing stock. The sustained accumulation of carbon in these forests also results from harvest rates that are lower than the increment because harvesting has not adapted to the increase in productivity (Ciais et al, 2008).
Afforestation typically leads to increases in biomass and dead organic matter carbon pools and, to a lesser extent, soil carbon pools (Paul et al, 2003). However, biomass removal and site preparation prior to afforestation also may lead to carbon losses. The carbon sequestration potential of afforestation depends on many different factors such as previous land use, soil type or tree species (Guo and Gifford, 2002; Jandl et al, 2007). Carbon sequestration is higher on sites deplete in soil carbon, e.g. due to unsustainable agricultural practices (De Koning et al, 2005; Jandl et al, 2007). Conversely, on sites with high initial soil carbon stocks, (e.g. grassland ecosystems), soil carbon stocks may decline following afforestation (Davis and Condron, 2002; Thuille and Schulze, 2006). However, depending on the ecosystem type and productivity, more often the accumulation of carbon in the forest biomass compensates for this loss after about 10–15 years (Thuille and Schulze, 2006). The degraded soils on land available for afforestation in the tropics primarily constitute low-carbon systems where a significant net C sink after afforestation is expected. Grasslands with high initial carbon stocks that more likely lead to C release after afforestation are more often associated with dry or cold/wet climates.
Thuille and Schulze (2006) described the development typical of different ecosystem C pools following an afforestation of former grasslands (Figure 3.3). After tree planting, carbon is released from the mineral soil for at least several years, whereas carbon is sequestered mainly in the above ground biomass. With increasing tree biomass in regions where decomposition is restricted by dry and/or cool periods, litter production increases, building up an organic layer and, in the longer term, the carbon pool, at least in the top soil, is partly replenished (Zerva and Mencuccini, 2005).
In their comprehensive review of the change in soil carbon after afforestation, Paul et al (2002) found a clear age effect, indicating carbon losses in most young afforestations, variable trends in 5–30-year-old plantations and prevailing soil carbon gains 30 years and more after afforestation. Furthermore they documented species differences: in surface soil (<10cm or <30cm), C amounts increased under hardwoods (poplar, mahogany, etc.) and softwoods (mixed pines, spruce, etc.), yet changed little under eucalypts and decreased under radiata pine. In deeper soil layers (>10cm depth), C increased under eucalypts and other hardwoods, and decreased under radiata pine and other softwoods. It should be noted that the different species were planted in different climatic regions of the world. However,
species-specific responses are also documented from targeted experiments by planting several species along a site fertility gradient (e.g. Vesterdal and Raulund-Rasmussen, 1998).
We can conclude that creating new forests results, in most cases, in significant carbon sinks. However, the influence of former land use and selected species stresses the importance of management choices for optimal carbon management of these areas.
Figure 3.3 Changes in carbon stocks in stem biomass, organic layers and
mineral soil to 50cm depth, averaged over several chronosequences
Note: Previous land use was pasture. The bars on the right hand side show the carbon stocks in continuously
forested control plots