The clearing of primary forest for agriculture, urban expansion and industrialisation has increased across the tropics. In addition, marginal agricultural lands are being abandoned as communities seek more lucrative careers in urban centres. As a result, the proportion of secondary forests is increasing, and accounts for the majority of remaining vegetation cover in some regions. For example, ~63% of all remaining forest in Southeast Asia is classified as secondary forest. Secondary forests occur where primary forest (i.e. pristine natural forest) has previously been cleared, and the land subsequently allowed to undergo natural restoration. Secondary forests are, unfortunately, poor in terms of carbon storage and species richness compared to primary forests. Nonetheless, a new study, published this month in Global Change Biology, shows that secondary forests in the Amazon can accumulate carbon at a rate of 2.25 Mg/ha/year and support 85% of the species diversity of primary forest after ~40 years of regeneration.
Figure 1. Mangrove and woodland along the Amazon river. Image credit: Cesar Paes Barreto (2003).
Lead author, Dr Gareth Lennox from Lancaster University, states “We found that the carbon and biodiversity of secondary forests recovered to more than 80 per cent of levels found in undisturbed primary forests. This is undoubtedly good news for climate change mitigation and biodiversity conservation. Nevertheless, the regions we assessed provide very favourable regeneration conditions, with greater than 50 per cent remnant primary forest cover and, consequently, large populations of forest species that can colonise secondary forests.”
He cautions however, that “Even in this situation, secondary forests cannot substitute for undisturbed primary forests, which must remain a priority of conservation efforts.”
This study has particular relevance for projects currently being developed by C4 EcoSolutions. Ecologists at heart, we may wish for full restoration to primary forests, but the reality is that functional, rehabilitated secondary forests may be the only reasonable expectation in most cases. But perhaps this is a more practical approach for adaptation and mitigation. Secondary forests can be managed to allow regulated offtake of ecosystems goods and services by communities and used to buffer remaining primary forest fragments from further degradation. Millions of rural communities are dependent on forests to provide fuel, fodder, food and medicinal plants. These systems are an important supplement to household income and food security. Well-managed secondary forests can accumulate carbon, harbour biodiversity and still fill the needs of communities across the tropics.