Recurring floods and landslides affecting various regions in Indonesia, including Aceh, North Sumatra, and West Sumatra, show a strong correlation with changes in land cover.
Deforestation has emerged as a primary driver, as biodiverse tropical rainforests have been converted into monoculture plantations, particularly oil palm, as well as mining areas and settlements.
These changes have drastically reduced the ecosystem’s capacity to absorb, store, and regulate water flow.
How Forests Regulate Water Systems
According to Dr. Ir. Hatma Suryatmojo, S.Hut., M.Si., IPU, a lecturer and researcher at the Faculty of Forestry, Universitas Gadjah Mada (UGM), forests play a critical hydrological role.
Under ideal conditions, the forest’s multilayered structure (comprising the upper canopy, understory, and ground cover) functions as an effective system for intercepting and absorbing rainfall. Up to one-third of rainfall can be temporarily retained by the canopy (interception) and subsequently returned to the atmosphere through evaporation.
More than half of the remaining water infiltrates the soil through leaf litter and complex root systems, where it is stored as groundwater or released gradually as base flow into rivers.
When forest cover is lost, these natural mechanisms are disrupted. Rainfall strikes the ground surface directly, reducing infiltration capacity and increasing runoff.
Large volumes of water then move rapidly and simultaneously toward drainage channels, triggering flash floods and sharply increasing peak discharge.
“The water balance inevitably changes, and peak discharge rises dramatically,” Hatma said during a Pojok Bulaksumur discussion published on UGM’s official website.
In addition, the absence of root networks that bind soil particles increases vulnerability to erosion and mass soil movement, ultimately leading to landslides.
Oil Palm Plantations Cannot Replace Forest Functions
This assessment is supported by Dr. Ir. Mahawan Karuniasa, M.M., an environmental expert from the University of Indonesia, who emphasized that monoculture oil palm plantations cannot replace the hydrological functions of natural forests.
Oil palm trees have relatively shallow and uniform root systems, unlike the diverse tree species in natural forests that develop deep buttress roots and layered root structures. Deep root systems in natural forests enhance soil porosity and water infiltration capacity.
By contrast, oil palm plantations allow significantly less infiltration. As a result, under the same rainfall intensity, the risk of flooding and surface runoff is substantially higher in oil palm plantation areas than in forested landscapes.
What Types of Vegetation Help Prevent Floods and Landslides?
Effective land rehabilitation and reforestation of degraded areas and river basins require a well-designed technical approach. Plant selection must take into account ecological functions such as erosion control, water absorption, slope stabilization, and—where possible—the provision of economic benefits for local communities.
Based on various studies, including research by Budiwati of Yogyakarta State University (UNY), ideal plant species for ground cover and land rehabilitation should meet several criteria: they should be easy to propagate, possess strong root systems capable of binding soil, require minimal soil fertility, grow rapidly, produce abundant biomass, and pose no risk of becoming invasive weeds.
Planting recommendations should be implemented in a layered system that mimics the structure of natural forests:
1. Low-Stratum Ground Cover Plants
These plants protect soil from the direct impact of raindrops, reduce surface runoff velocity, and increase soil organic matter.
- Vetiver (Chrysopogon zizanioides): Highly recommended for short- to medium-term slope stabilization. Vetiver roots grow vertically deep into the soil—reaching depths of 3 to 5 meters—acting as “living anchors” that reinforce soil structure. Vetiver is non-competitive, as it does not produce widely dispersing seeds.
- Other species: Centrosema pubescens (leguminous ground cover), Paspalum dilatatum (Australian grass), and Ageratum conyzoides (billygoat weed).
2. Medium-Stratum Plants (Shrubs)
Shrubs help strengthen vegetation structure, trap materials carried downslope, 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 honeybees.
- Gamal (Gliricidia sepium): Commonly used in intercropping systems; its leaves serve as livestock feed and green manure.
- Other species: Turi (Sesbania grandiflora), lamtoro (Leucaena leucocephala), and sunn hemp (Crotalaria juncea).
3. Upper-Stratum Plants (Protective Trees)
These trees function as windbreaks, increase biomass, store carbon, and—through deep root systems—play a crucial role in slope stabilization and water absorption.
- Bamboo: An exceptionally effective option. Bamboo’s dense fibrous roots and spreading rhizomes form a strong underground network that binds soil securely. Bamboo grows rapidly, tolerates a wide range of site conditions, and improves watershed function. Recommended species include Apus bamboo (Gigantochloa apus), Betung bamboo (Dendrocalamus asper), and Wulung bamboo (Bambusa vulgaris). Bamboo products can be utilized for handicrafts, construction materials, or edible shoots.
- Sengon laut (Albizia falcata): Fast-growing, economically valuable timber, suitable for agroforestry systems.
- Jabon (Neolamarckia cadamba): A fast-growing pioneer species well suited for revegetation, with commercially valuable wood.
- High-value species: To increase community participation, fruit trees such as durian (Durio zibethinus) or timber species like meranti (Shorea spp.) may be planted in suitable locations.
Disaster Risk Reduction Through Ecosystem Restoration
Mitigating flood and landslide risks in Sumatra requires strategic ecosystem restoration. Replacing monoculture systems with layered planting patterns that resemble natural forests is a key step.
A combination of ground cover plants (such as vetiver), fodder-producing shrubs (such as calliandra and gamal), and fast-growing, economically valuable trees (such as bamboo and sengon) can restore the hydrological function of degraded land while simultaneously delivering socio-economic benefits.
This approach not only rehabilitates the environment but also strengthens the resilience of communities living in disaster-prone areas.
