Our research found that islands within the Balbina Hydroelectric Reservoir harboured significantly lower densities of adult and sapling trees, compared to mainland continuous forest. In other words, tree biomass is being lost on those islands.
The new study assessed the long-term impact of dam-induced fragmentation on tropical tree communities isolated on islands following dam construction, with the aim of generating robust recommendations for dam developers and decision-makers to mitigate some of the detrimental impacts associated with dam development.
The research was carried out across 89 permanent forest survey plots on 36 islands across Balbina, as well as three nearby mainland continuous forest sites. The network of forest survey plots is the most extensive established within a hydroelectric dam system. Large trees were surveyed in 2012 and saplings in 2014.
Dr Jones explained: “Our research found that islands within the Balbina Hydroelectric Reservoir harboured significantly lower densities of adult and sapling trees, compared to mainland continuous forest. In other words, tree biomass is being lost on those islands. This is important to know considering the carbon emissions associated with tropical forest loss and degradation.
“Furthermore, the species composition of saplings was significantly different to those of the relict adult trees on islands, indicating that future tree communities on islands will be very different to those originally there when the forest was continuous.
“We also showed that the overall composition of tree communities on islands was different to those in mainland continuous forest, which means that island tree communities are changing due to being isolated within the reservoir. Loss of tree species and biomass within these remnant tree communities is largely ignored in Environmental Impact Assessments. We show that the degradation of remnant tree communities on islands is an additional impact of dams, and must be explicitly considered in the planning and licensing of future dam construction. ”
The researchers also found that fires – which can spread through reservoir island systems in periods of drought when water levels are not maintained – can cause even more degradation to tree communities and that the damage is particularly severe for hard-wooded species, those which store the most carbon.
The research was developed with Professor Carlos Peres, University of East Anglia; Dr Maria Benchimol, Universidade Estadual de Santa Cruz in Brazil; as well as Dr Daisy Dent and Dr Lynsey Bunnefeld, of the University of Stirling. It was part-funded by grants from the Natural Environment Research Council and a Carnegie Trust for the Universities of Scotland.
The paper, Instability of insular tree communities in an Amazonian mega-dam is driven by impaired recruitment and altered species composition, is published in the Journal of Applied Ecology.