The study analyzed tree survival and growth data from 176 restoration sites in tropical and sub-tropical
The findings are published today in the Philosophical Transactions of the Royal Society B: Biological Sciences.
Forest restoration is a powerful tool to tackle biodiversity loss and climate change, by locking away carbon and supporting important habitats. Reforestation projects are also used widely for carbon offsetting. While the main measurement used for many projects is the number of trees initially planted, the research shows that many of these trees are not surviving long-term. In some sites, survival rates were high, showing that with the right approach restoration has the potential to be successful.
About 15 per cent of the world's tropical forests are found in Southeast Asia and they are amongst the most carbon-dense and species-rich in the world, providing habitat for tigers, primates and elephants. However, in recent decades the region has also seen major deforestation, with forest cover reducing by an estimated 32 million hectares between 1990 and 2010.
The region has therefore become an important focus for forest restoration projects. The research - by an international team of scientists from 29 universities and research centres - is the first to bring together data to evaluate the long-term outcomes of restoration projects.
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The team found that, when an area had been fully deforested, reforestation efforts were less successful than in areas where some trees remained. Saplings planted in areas with existing mature trees had roughly a 20 per cent higher chance of survival. In more disturbed areas, more intensive measures for protection and maintenance may be needed.
The study also found some evidence that active restoration provides faster results than simply letting nature take its course. Sites which included tree planting activities gained forest cover more quickly than sites which were left to regenerate naturally. But many more studies tracked the fate of planted trees rather than the structural properties of the whole community. The research team believes that collating both types of data in the same study areas will help to determine acceptable levels of mortality that will still deliver a return of forest cover. More experiments are needed to help hone the most appropriate and cost-effective methods of restoration across sites under different conditions.
We need to understand better how to improve the survival chances of saplings on these sites, to ensure restoration has positive outcomes. But the study also provides a warning, to protect our remaining forests as much as possible, both because restoration outcomes are uncertain and to provide the diverse seed sources needed for restoration activities."
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