As we face the consequences of climate change there is renewed impetus to restore forests so that they can once again provide ecosystem services, such as protecting species, sequestering carbon and securing livelihoods. But planting trees in areas that have been disturbed or denuded is costly, as is the maintenance of the new saplings after planting. This makes it especially important to understand the best conditions for successful reforestation.
A new study by a large international team of scientists has reviewed the success of reforesting programs at 176 sites in tropical and sub-tropical Asia. The researchers scoured the literature for published records of tree survival and growth data in areas where forest restoration has been attempted. They synthesized the evidence of forest recovery and the efficacy of tree planting as an intervention for ecosystem restoration.
In particular, the researchers used data on planted tree survival, planted tree size and/or growth, and area-based metrics of forest structure and diversity in planted and unplanted adjacent plots. They also collected data on other variables, such as location, duration of monitoring, disturbance history, site condition prior to planting, restoration methods, diversity of trees planted, planting density, and height of trees at planting. This was to identify the factors most closely correlated with the success of the replanting initiatives.
Overall, the researchers compiled data from 221,199 planted trees at sites across South and Southeast Asia. Of these trees, 207 224 were monitored in terms of survival, while data on growth was collected for 102 412 trees. In total, 625 tree species (252 genera; 81 families) were planted but species richness of plantings at individual sites was typically low (median of only three species planted per site). Although the maximum study duration was 33 years, only 49 (28 percent) sites conducted surveys at 5 years or more after planting, and the median study length was only two years.
The results of their analysis, published in the Philosophical Transactions of the Royal Society B: Biological Sciences, make for sobering reading. They found that, on average, 18 percent of planted saplings died within the first year, and that this mortality rose to 44 percent after five years. However, survival rates varied greatly between sites and species, with some sites seeing over 80 percent of trees still alive after five years, while at others a similar percentage had died.
Policy makers are aware that forest restoration is a powerful tool to tackle biodiversity loss and climate change, as growing trees lock away carbon and support important habitats. Reforestation projects are also used widely for carbon offsetting. While the main measurement used to gauge success is often the number of trees initially planted, this research shows that many of these trees are not surviving long-term. In addition, the ultimate goal of restoration is the structural and compositional recovery of the whole plant community and broader ecosystem, and monitoring often ceases before this can be assessed fully.
The analysis was not able to identify exactly what was responsible for different success rates at the various sites. Tree survival was not significantly correlated with climatic variables, soil type, species planted, or planting densities, but was highly dependent on the particular site of the reforestation project. This indicates that there are environmental, methodological and social factors that operate and affect whole sites, making restoration outcomes context specific.
“The large variability in survival we found across sites could be for a number of reasons, including planting densities, the choice of species, the site conditions, extreme weather events or differences in management and maintenance. Local socio-economic factors may also be important,” said Dr Lindsay Banin, co-lead author based at the U.K. Centre for Ecology and Hydrology.
“What’s clear is that success is very site-dependent – we need to understand what works and why, and share that information, so we can bring all sites up to the level of the most successful and harness the full potential for restoration. There’s likely no one-size-fits-all approach and restoration action should be tailored to local conditions. This will help ensure the scarce resources and land available to restoration are used to best effect,” she added.
The researchers found that planting taller saplings did give better survival rates, and that reforestation efforts were more successful in areas where some established trees remained than in areas that had been fully deforested. Saplings planted in areas with existing mature trees had roughly a 20 percent higher chance of survival. Furthermore, there was evidence that active restoration provides faster results than simply letting nature take its course. Sites that included tree planting activities gained forest cover more quickly than sites that were left to regenerate naturally.
“The sites where active restoration is most needed – those that have already been cleared of trees – are also those where restoration is most risky and prone to higher numbers of trees dying,” said study co-author Professor David Burslem. “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.”
“Replanting is only going to be an answer to excess carbon dioxide in the atmosphere if we can guarantee that carbon is being successfully drawn out of the atmosphere and locked away – and be able to quantify the amounts and timescales involved,” said study co-author Professor Robin Chazdon. “This is why assessing restoration outcomes over the long-term, and gathering information that helps to maximize success rates, are so important. We need the focus to shift away from simply planting trees toward growing them and helping our forests thrive.”
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