Replanting Logged Forests With Diverse Mixtures of Seedlings Accelerates Restoration
Replanting logged forests with a mixture of seedlings significantly accelerates recovery, reports a new study.
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Satellite observations of one of the world’s biggest ecological experiments on the island of Borneo have revealed that replanting logged forests with diverse mixtures of seedlings can significantly accelerate their recovery. The results have been published today in the journal Science Advances.
The experiment was set up by the University of Oxford’s Professor Andy Hector and colleagues over twenty years ago as part of the SE Asia Rainforest Research Partnership (SEARRP). This assessed the recovery of 125 different plots in an area of logged tropical forest that were sown with different combinations of tree species. The results revealed that plots replanted with a mixture of 16 native tree species showed faster recovery of canopy area and total tree biomass, compared to plots replanted with 4 or just 1 species. However, even plots that had been replanted with 1 tree species were recovering more quickly than those left to restore naturally.
Lead Scientist of the study, Professor Andy Hector (Department of Biology, University of Oxford) said: ‘Our new study demonstrates that replanting logged tropical forests with diverse mixtures of native tree species achieves multiple wins, accelerating the restoration of tree cover, biodiversity, and important ecosystem services such as carbon sequestration.’
Greater diversity gives greater resilience
According to the researchers, a likely reason behind the result is that different tree species occupy different positions, or ‘niches’, within an ecosystem. This includes both the physical and environmental conditions that the species is adapted to, and how it interacts with other organisms. As a result, diverse mixtures complement each other to increase overall functioning and stability of the ecosystem. For instance, some tropical tree species are more tolerant of drought because they produce a greater amount of protective chemicals, giving the forest resilience to periodic times of low rainfall.
Professor Hector added: ‘Having diversity in a tropical forest can be likened to an insurance effect, similar to having a financial strategy of diverse investment portfolios.’
In turn, a diverse mix of trees can support a much wider range of animal life. For instance, hornbills specifically require large mature trees with holes where the females can nest.
One of the world’s biggest ecological experiments
Tropical forests cover just 6% of the planet’s land surface but are home to around 80% of the world’s documented species (WWF), and act as major carbon sinks. However, these critical habitats are disappearing at an alarming rate, chiefly due to logging for timber and conversion to palm oil plantations. Between 2004 and 2017, 43 million hectares of tropical forest were lost - an area roughly the size of Morocco (WWF).
Restoring logged tropical forests is a crucial component of efforts to tackle both the nature and climate crises. Up to now, however, it has been unclear whether this is best achieved through allowing forests to restore themselves naturally (using dormant seeds in the soil) or through active replanting.
The recovery of the plots was assessed by applying statistical models to aerial images captured by satellites. Within a few years, it became apparent that those with 1 species did worse than those planted with a mixture of 4 species, and those enriched with 16 species did best of all.
Lead author Ryan Veryard (who analysed the data as part of his PhD at the University of Oxford), said: ‘Importantly, our results show that logged forest can recover so long as it is not converted to agricultural uses like oil palm plantation. They also emphasise the need to conserve biodiversity within undisturbed forests, so that we can restore it in areas that have already been logged.’
The Sabah Biodiversity Experiment team are now starting a new three-year project funded by the UK Natural Environmental Research Council to take a census of all the surviving trees in the experiment. This will be combined with a wider range of remote sensing methods (including lidar sensors carried by a helicopter and smaller sensors carried by drones) to give a more comprehensive analysis of forest health.
Reference: Veryard R, Wu J, O’Brien MJ, et al. Positive effects of tree diversity on tropical forest restoration in a field-scale experiment. Sci Adv. 2023;9(37):eadf0938. doi: 10.1126/sciadv.adf0938
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