Increased carbon sequestration of actively restored tropical forests in Sabah, Malaysia : a comparison of natural regeneration and active restoration

Abstract: As restoration with assisted natural regeneration and enrichment planting becomes a more important and popular method to restore ecosystem services and mitigate negative climate effects, these treatments and their effectiveness is heavily debated. There are three main groups of opinions regarding restoration, one is that humans actively need to help nature recover and the second is that nature itself does a better job without our help and the third is somewhere in between. In my study, I compared active and passive restoration in degraded tropical forests within the INIKEA project in Sabah, Malaysia. Along with previously collected data from 2017, I used my own data collected in 2022 to investigate the effects of active restoration with focus on enrichment line-planting followed by continuous maintenance for 10 years. My result showed that some control plots were still almost completely bare of trees, even 40 years after the last severe disturbance. This indicates that heavy degradation sometimes hinders natural regeneration for decades, whereas no such delayed regeneration was found in actively restored forests. In contrast, after 23 years actively restored forests had 41.8 Mg C ha-1 higher carbon storage (p=0.015) compared to naturally regenerated control plots. However, the increase in carbon sequestration cannot be attributed to planted trees since only 30 trees per hectare out of 300 planted had grown into the measurable diameter size (dbh ≥ 10). The average diameter of these planted trees was still low, 17.4 cm, which contributes 4.4 % or 1.84 Mg C ha-1 of the added carbon sequestration as a result from active restoration. I conclude that most of the additional carbon sequestration from active restoration came from the continuous maintenance where lianas, climbers and other competing vegetation is removed which led to an increased growth of the already established and naturally regenerated trees. Consequently, knowledge like this could be useful in decision-making concerning restoration methods to maximize benefits in a cost-effective way.