Benefits of tropical peatland rewetting for subsidence reduction and forest regrowth: results from a large-scale restoration trial

Abstract

Drainage and deforestation of tropical peat swamp forests (PSF) in Southeast Asia cause carbon emissions and biodiversity loss of global concern. Restoration efforts to mitigate these impacts usually involve peatland rewetting by blocking canals. However, there have been no studies to date of the optimal rewetting approach that will reduce carbon emission whilst also promoting PSF regeneration. Here we present results of a large-scale restoration trial in Sumatra (Indonesia), monitored for 7.5 years. Water levels in a former plantation were raised over an area of 4800 ha by constructing 257 compacted peat dams in canals. We find peat surface subsidence rates in the rewetted restoration area and adjoining PSF to be halved where water tables were raised from ~ - 0.6 m to ~ - 0.3 m, demonstrating the success of rewetting in reducing carbon emission. A total of 57 native PSF tree species were found to spontaneously grow in the most rewetted conditions and in high densities, indicating that forest regrowth is underway. Based on our findings we propose that an effective PSF restoration strategy should follow stepwise rewetting to achieve substantial carbon emission reduction alongside unassisted regrowth of PSF, thereby enabling the peat, forest and canal vegetation to establish a new nature-based ecosystem balance.

Figure 1

Location of the study area within the broader landscape in South Sumatra, Indonesia. The remaining peat swamp forest (PSF) and mangrove is mostly in Sembilang National Park.

Figure 2

Profile along a blocked E-W canal across the restoration area, near the dipwell transect, showing measured water levels in different months with high (‘wet’) and low (‘dry’) water levels. Water levels were raised the most towards the East of the restoration area, as water flow direction was towards the West. Detailed values for individual canal blocks are presented in Supplementary Note 1 (Supplementary Table 1).

Figure 3

Time series of groundwater table depth (GWD) and peat surface subsidence across the restoration area. Area descriptions in Table 1. LiDAR collection dates refer to Fig. 5.

Figure 4

Profiles of groundwater table depth (top) and peat surface subsidence rate (bottom) across the restoration area and 2 km into the forest. Profiles for groundwater table depth and subsidence rate are shown for different periods over the 7.5-year record, to demonstrate the effects of rewetting and drought events.

Figure 5

(top panel) estimated groundwater depth (GWD) and (lower panel) median vegetation height as determined from LiDAR collected at 4 dates. The 2017–2020 maps on the right side presents the mean estimated groundwater depth and the total vegetation height growth over a 3.5-year period.

Figure 6

Visual summary of steps in restoration as identified in the study, highlighting the interactions between rewetting and peat swamp forest restoration, jointly resulting in nature-based carbon emission reduction and ecosystem enhancement.