In 1983, Borneo was ablaze. Millions of hectares burned during the El Niño drought, and the worst affected were forests that had undergone intensive logging. In the Malaysian state of Sabah, in northeastern Borneo, one million hectares of forest were damaged. Since 1998, 18,500 hectares of these have been restored to regain a structure and biodiversity similar to an untouched rainforest, with a diversity of species, particularly from the dipterocarp family, which is typical for these forests.
The restoration has been scientifically monitored, and in a unique study, researchers can now show its effects over time. This knowledge has long been lacking, as tree planting done to offset carbon emissions, restore ecological functions, and secure livelihoods in tropical rainforest areas often lacks evaluations of how effective these measures actually are.
“Restoration projects often measure the number of planted trees but do not follow up on how well the trees survive over a longer period,” says Petter Axelsson, a researcher at the Swedish University of Agricultural Sciences (SLU).
92 native tree species used
In this restoration project, 92 native tree species have been planted, a significant higher number compared to the average of three species in replanting projects in the region. Thirty of the species used are red-listed, and several of them are endangered. The trees planted are mostly dipterocarps and fruit trees, all native species.
“We see that the highest risk of mortality is during the first three years when the young plants are sensitive and exposed to trampling, rooting, and grazing animals before they grow beyond grazing height. The initial mortality rate aligns with that in natural systems and what has been reported in other projects,” says Petter Axelsson.
However, after the first three years, the risk of mortality decreases significantly. Of the five million trees planted, 15–25 percent survive after 20 years, which is enough to form a new canopy. Some of these trees have also set seeds and are contributing to new seedlings.
There are also differences between species in terms of both survival and growth. In the study, researchers compared these factors among 32 tree species used in several restoration efforts on Borneo.
“Our results show significant differences in both growth and survival between species. Knowledge of this makes it possible to buffer for this in restoration by planting more of the more sensitive species,” says Petter Axelsson.
To plant or not to plant – that is the question
To ensure both general and species-rich survival, many trees may be needed. But is planting really necessary? In the study, researchers have tackled this key question in forest restoration: the usefulness of planting. Is it needed, or can nature just as well recover on its own if left undisturbed?
“One of our most important discoveries is that the need for planting depends on the level of damage,” says Petter Axelsson. “In the most damaged areas, very little natural regeneration of dipterocarp trees has occurred 40 years after the fires. To achieve a more varied forest, it is good to plant a diversity of tree species and also clear vegetation to help the dipterocarps establish.”
The study also show that in more moderately disturbed forests, where some trees survived the fires, it is sufficient to manage the regrowth without planting.
Additionally, the study highlights key lessons from practical restoration efforts and the associated costs. The hope is that these findings will guide future restoration projects, particularly in tropical rainforests. You can read the full article here.
