Afforestation & reforestation: rooting for trees

Before we begin, there is an important land-cover distinction to be made between afforestation and reforestation:

• Afforestation is the establishment of forest cover on previously non-forested land
• Reforestation is the re-establishment of forest cover on previously forested land (UNFCCC, 2013)

Afforestation is valuably regarded for its carbon sequestration potential, i.e. capturing excess atmospheric carbon and storing it long-term in tree biomass (Caldeira et al, 2013). As a result, afforestation is classed under the Carbon Dioxide Removal (CDR) subcategory of geoengineering strategies; which aims to decrease excess atmospheric CO₂ and reverse anthropogenic CO₂ emissions. Although carbon is sequestered in above-ground and below-ground biomass, soil forms the primary carbon sink (Fonseca et al, 2012).

Figure 1: Subcategories of the CDR geoengineering technique

Carbon capture potential

Forest ecosystems are significant components in the global carbon cycle, climate & hydrological system. This is due to their albedo, evapotranspiration, surface roughness and carbon capture influences (Caldeira et al, 2013). Their global carbon sink absorption potential has been estimated at ~3 Pg C year^-1, or ~30% of total CO₂ emissions (Fonseca et al, 2012).  It has been estimated that by 2100, forests can sequester ~80-140 PgC globally (Lenton, 2010). However, rapid deforestation in the tropics releases ~1-2 Pg C yr^-1 (Houghton et al, 2015).

Yet, various forest biomes influence the carbon cycle differently. There is also uncertainty in regards to forest age, species richness and density. Research suggests tropical secondary rainforests have a carbon capture potential of 11 times more than primary forests (Poorter et al, 2015). Similarly, indigenous, mixed-tree plantations have been found to sequester significantly more carbon in ecosystems and soil than monoculture forest-plantations (He et al, 2013).

Socio-economic benefits

 Recently, the concept of agroforestry has incorporated afforestation with sustainable development for local farmers (Alao and Shuaibu, 2013). Cocoa afforestation of savannah grassland in Cameroon, for example, has proven beneficial; with an increase in soil carbon concentration, from 1.7-2.25% and the Shannon-Weaver (species abundance) index from 1.96-2.26 that has resulted in economic security for farmers and ecological resilience (Jagoret et al, 2012). Reforestation schemes in the Peruvian Amazon by Nestlé have provided a multitude of ecological and socio-economic benefits, as shown in the video below. 

Reforestation in Peruvian Amazon (AFP News)

From an ethical governance perspective, the empowerment of indigenous communities may mean afforestation CDR is preferable than supranational, top-down SRM techniques (Scheer and Renn, 2014).

Boreal/temperate forests vs tropical forests

Despite their carbon storage potential, some studies have suggested boreal and temperate forests can reduce surface albedo; resulting in a net warming effect through enhanced heat absorption (Alkama and Cescatti, 2016). On the other hand, tropical forests, such as the Amazon rainforest, produce a cooling effect due to their equatorial location. Intense heating at the equator causes an increase in evapotranspiration, thus releasing latent heat energy (Bathiany et al, 2010). This intense heating also causes the formation of the low-pressure Intertropical Convergence Zone (ITCZ) which brings heavy precipitation. Furthermore, the white clouds associated with the ITCZ belt enhance equatorial albedo by reflecting solar insolation (Met Office, 2010).  

Figure 2: ITCZ clouds around the Equator (NASA)

Yet, recent global climate models propose that the cooling and soil carbon stock potential of boreal and temperate biomes has been underestimated (Tupek et al, 2016). Historical land-use decisions in boreal and temperate regions for agricultural purposes involved the clearing of productive land: i.e. land with long snow-free intervals and high carbon stock potential (Pongratz et al, 2011). The global warming mitigation potential of forests, therefore, should be assessed through temporal albedo change and spatial heterogeneity (Mattsson et al, 2016).

Dr Julia Pongratz discusses her research on boreal & temperate reforestation cooling potential in-depth (Carnegie Institution for Science)

The exact implications for biogeochemistry, i.e. water and carbon systems, is uncertain. Mass-afforestation modelling indicates a strengthening of the carbon cycle. Greater interception and transpiration may lead to significant changes in global water dynamics. Similarly, enhanced surface roughness may alter local, regional and via teleconnections, even global climate. Climate modelling in West Africa suggests the potential for extreme rainfall events in coastal areas, with droughts in arid regions (Abiodun et al, 2013). Therefore, afforestation schemes may just be a Pre-Neolithic fantasy: as they have the potential to alter Earth system's more than historical land use (Heck et al, 2016).

Limitations

1) Rapid population growth and changing economies have resulted in the mass deforestation of  tropical forests for agricultural land, with ~0.84 million ha of Indonesian rainforest lost in 2012 (Drescher al, 2016). This may significantly limit reforestation techniques in the tropics.

2    2) The carbon storage potential, and thus cooling effect, of boreal and temperate forests is uncertain.

      3) There is a danger of monoculture forestry for maximising carbon stock at the expense of biodiversity (Bayrak and Marafa, 2016). 

      4) Whilst SRM techniques are seen as a 'quick fix', afforestation CDR mechanisms would require decadal-centennial timescales to be effective (Caldeira et al, 2013).

      5) Uncertainty surrounding impacts on biogeochemical cycles: therefore not as 'green' as once assumed (Heck et al, 2016).

      Conclusion 

     I believe that afforestation/reforestation techniques should be holistic in their approach by considering carbon, cooling, biodiversity, socio-economic and ecosystem service potential. It may be a unique opportunity to incorporate local decision-making in geoengineering governance. Although more modelling is needed into the cooling potential of different vegetation types at various latitudes, I feel combining it with another ‘quick fix’ may mitigate climate change and get to the root of the problem.