Recurrent floods and landslides affecting various regions of Indonesia, including Aceh, North Sumatra and West Sumatra, show a robust correlation with land cover changes.
Deforestation has emerged as a significant cause as biologically diverse tropical rainforests have been converted to monoculture plantations, especially oil palm, in addition to mining areas and settlements.
These changes have drastically reduced the ecosystem’s ability to soak up, store and regulate water flow.
How forests regulate water systems
According to Dr. Ir. Hatma Suryatmojo, S.Hut., M.Si., IPU, lecturer and researcher on the Faculty of Forestry, Universitas Gadjah Mada (UGM), forests play a key hydrological role.
Under ideal conditions, the multi-layered forest structure (consisting of the upper cover, understory and undergrowth) functions as an efficient system for capturing and absorbing atmospheric precipitation. Up to one-third of rainfall could also be temporarily retained by the cover (interception) after which returned to the atmosphere through evaporation.
More than half of the remaining water infiltrates the soil through leaf litter and complicated root systems, where it’s stored as groundwater or released steadily as the bottom flows into rivers.
When forest cover is lost, these natural mechanisms are disrupted. Rainfall hits the bottom surface directly, reducing infiltration capability and increasing runoff.
Large volumes of water then move rapidly and concurrently towards drainage channels, causing flash floods and rapidly increasing peak discharge.
“The water balance is inevitably changing and the peak discharge is increasing rapidly,” Hatma said during Bulaksmur corner discussion published on the official UGM website.
Moreover, the dearth of a root network to bind soil particles increases susceptibility to erosion and large soil movements, which ultimately results in landslides.
Oil palm plantations cannot replace the functions of forests
This assessment is confirmed by Dr. Ir. Mahawan Karuniasa, MM, an environmental expert from the University of Indonesia, who emphasized that monoculture oil palm plantations cannot replace the hydrological functions of natural forests.
Oil palms have relatively shallow and uniform root systems, unlike quite a lot of tree species in natural forests that develop deep support roots and layered root structures. Deep root systems in natural forests increase soil porosity and water infiltration capability.
In contrast, oil palm plantations allow much less infiltration. As a result, for a similar rainfall intensity, the danger of flooding and surface runoff is way higher in oil palm plantation areas than in forested areas.
What varieties of vegetation help prevent floods and landslides?
Effective land reclamation and reforestation of degraded areas and river basins require a well-designed technical approach. Plant selection must consider ecological functions resembling erosion control, water absorption, slope stabilization and, where possible, providing economic advantages to local communities.
Various studies, including research by Budiwati of Yogyakarta State University (UNY), conclude that ideal plant species for land cover and land reclamation should meet several criteria: they needs to be easy to propagate, have a robust root system able to binding soil, require minimal soil fertility, grow quickly, produce abundant biomass, and pose no risk of becoming invasive weeds.
Planting recommendations needs to be implemented in a layered system imitating the structure of natural forests:
1. Low-layer cover crops
These plants protect the soil from the direct motion of raindrops, reduce the speed of surface runoff and increase the content of organic matter within the soil.
- Vetiver (Chrysopogon zizanioides): Particularly beneficial for brief and medium-term slope stabilization. Vetiver roots grow vertically deep into the soil, reaching a depth of three to five meters, acting as “living anchors” that strengthen the soil structure. Vetiver is uncompetitive since it doesn’t produce seeds that spread widely.
- Other species: Centrosema pubescens (ground cover plant), Paspalus dilated (Australian grass) i Conizoids of Ageratum (goat weed).
2. Medium-layer plants (shrubs)
Shrubs strengthen the structure of vegetation, retain materials carried downhill, and in some cases fix nitrogen, thereby improving soil fertility.
- Calliandra (Calliandra calothyrsus): Fast growing, with strong roots; its flowers are also a source of nectar for honey bees.
- Gamal (Glyricide hedges): Commonly utilized in cover cropping systems; its leaves function livestock feed and green manure.
- Other species: It has (sesbania granddiflore), lamtoro (Leucaena leucocephala) and sunhemp (Crotalaria juncea).
3. Plants of the upper layers (protective trees)
These trees act as windbreaks, increase biomass, store carbon and – through their deep root systems – play a key role in stabilizing slopes and absorbing water.
- Bamboo: An extremely effective option. Bamboo’s dense, fibrous roots and spreading rhizomes create a robust underground network that securely binds the soil. Bamboo grows rapidly, tolerates a big selection of habitat conditions, and improves watershed function. Recommended species are Apus bamboo (Gigantochloa apus), Bime Bamboo (Dendrocalamus asper) and Wiluung bamboo (Common bamboo). Bamboo products may be used for handicrafts, constructing materials or edible shoots.
- Sengon Sea (Albizia falcata): Fast-growing wood, economically invaluable, suitable for agroforestry systems.
- Jabon (Neolamarckia cadamba): A quick growing pioneer species, well suited to revegetation and containing commercially invaluable timber.
- High value species: To increase community participation, fruit trees resembling durian (Durio zibethinus) or wood species resembling meranti (Shorea spp.) may be planted in suitable places.
Reducing the danger of natural disasters through ecosystem restoration
Reducing the danger of floods and landslides in Sumatra requires strategic ecosystem restoration. A key step is to switch monoculture systems with layered planting patterns that resemble natural forests.
A mix of canopy crops (resembling vetiver), forage shrubs (resembling calliandra and gamal) and fast-growing, economically invaluable trees (resembling bamboo and sengon) can restore hydrological functions to degraded land while providing socio-economic advantages.
This approach not only regenerates the environment, but additionally strengthens the resilience of communities living in disaster-prone areas.
