Tropical Forests Recover Unevenly After Bauxite Mining, Decade-Long Study Finds

A decade of reclamation work at one of Indonesia’s largest bauxite operations has produced a forest that looks recovered from a distance but tells a very different story in the soil. New research from West Kalimantan shows that trees planted on mined land can regain much of their size and density within ten years, while soil nutrients and biodiversity lag far behind — and that acidity, the very trait the miners suppressed with soil treatments, creeps back year after year. The findings, published in Environmental Challenges, offer one of the most detailed pictures yet of how tropical post-mining ecosystems actually recover, and they carry an uncomfortable message for regulators who judge reclamation success by canopy cover alone.

The study was conducted at the Tayan concession of PT ANTAM Tbk’s Bauxite Mining Business Unit in Sanggau Regency, a landscape of rolling hills that receives between 3,000 and 4,000 millimetres of rain each year and sits under a near-constant temperature of about 27 degrees Celsius. The soils there are highly weathered Ultisols and Oxisols, naturally acidic and poor in nutrients, with high levels of exchangeable aluminium that can stunt plant roots. Bauxite mining strips away the topsoil, dismantles nutrient cycles and compacts or reshapes the remaining substrate, leaving a surface that is hostile to natural regeneration. Indonesia ranked fifth among the world’s bauxite producers in 2022, when roughly 380 million dry metric tons were mined globally, and demand for aluminium in electric vehicles and renewable energy infrastructure is only expected to intensify the pressure on such landscapes.

What makes the Tayan site scientifically valuable is its unusually continuous record of restoration. The company established reclamation blocks in eight consecutive planting years, from 2015 to 2022, all under a single reclamation programme involving land contouring, topsoil redistribution and revegetation with a mix of local and fast-growing species such as Acacia mangium, Gliricidia sepium, Paraserianthes falcataria, mahogany and Shorea species. Each block carries an information board documenting the planting year, area, coordinates, tree count and species, allowing researchers to verify the age of every site against company records rather than guess at it. That arrangement permitted a chronosequence design: instead of monitoring a single site for decades, the team sampled sites of different ages simultaneously and read the recovery trajectory across space as a proxy for time.

Between 24 and 26 January 2025, the researchers collected 27 composite soil samples — three observation points per planting year, plus three from an adjacent unmined natural forest block on the same soil type that served as the reference ecosystem. At each point, five sub-samples were taken along a diagonal pattern with roughly 25-metre spacing, composited into a single kilogram of soil drawn from the 0–30 centimetre rooting zone. Vegetation was assessed in twelve nested plots using a four-stage design covering trees, poles, saplings and seedlings, with stem diameters measured at breast height. Soil samples travelled to the Soil Chemistry and Fertility Laboratory at Tanjungpura University within 24 hours, where pH, organic carbon, nitrogen, phosphorus, cation exchange capacity, base saturation and aluminium saturation were measured using standard analytical procedures.

The results revealed a striking asymmetry in recovery. Vegetation structure rebounded quickly: pole and tree density climbed from 175 individuals per hectare in the youngest sites to 1,569 per hectare in the oldest, about 54.6 percent of the reference forest’s 2,875. Mean stem diameter grew from 5.5 to 13.7 centimetres, reaching 80.1 percent of the reference value of 17.1 centimetres. Both measures differed significantly among age classes in the statistical analysis. Species richness, however, told another story, rising only from 5.5 to 7.8 species per plot — just 43.1 percent of the natural forest’s 18 species — a gap that failed to reach statistical significance but remained biologically sobering. Fast-growing planted pioneers had built the skeleton of a forest without rebuilding its diversity.

Soil chemistry followed its own divergent paths. Organic carbon accumulated at an estimated 0.146 percent per year, reaching 1.72 percent in the oldest sites — a meaningful gain, but only 61.1 percent of the reference forest’s 2.82 percent. Available phosphorus showed the strongest temporal trend of any parameter measured, rising 0.802 milligrams per kilogram annually to reach 74.3 percent of the reference value. Total nitrogen and cation exchange capacity showed no significant differences across the chronosequence. Most telling was the chemistry of acidity: soil pH declined steadily from 5.28 in young sites to 4.96 in old ones, at about 0.070 units per year, while aluminium saturation climbed in parallel from 10.4 to 19.9 percent. Rather than exceeding natural levels, the reclaimed soils were simply converging back toward the naturally acidic baseline of the reference forest as the initial liming and amelioration effects faded.

That re-acidification matters because aluminium, abundant in these highly weathered soils, becomes increasingly soluble and toxic as pH drops, constraining root development, nutrient uptake and the recruitment of late-successional native species. The researchers found that available phosphorus was strongly correlated with mean stem diameter (r = 0.90) and with pole and tree density (r = 0.79), linking nutrient availability directly to structural recovery. Yet organic carbon varied enormously among blocks of the same age — from 0.11 to 1.05 percent at three years and from 1.59 to 2.86 percent at four years — suggesting that differences in how the reclamation was executed, particularly the thickness and evenness of redistributed topsoil, mattered as much as elapsed time itself. The chronosequence assumption that all sites started from comparable conditions, the authors acknowledge, can only be partially verified.

The study also exposes a gap in how Indonesia evaluates reclamation. Under Ministerial Regulation No. 7 of 2014, implemented through Regulation No. 26 of 2018 and the assessment matrices of Decree No. 1827 K/30/MEM/2018, reclamation is scored on land management, revegetation and final completion, using indicators such as plant survival, cover crop establishment and canopy closure. No criterion addresses soil chemical stability, and none goes beyond a generic requirement to plant local species. On the strength of such criteria, the Tayan sites would look like a success. The integrated analysis suggests they are only partially one: total nitrogen had reached just 69 percent of reference conditions, and Shannon–Wiener diversity values in the reclamation plots hovered at low to moderate levels across all vegetation layers compared with the reference forest.

The implications stretch well beyond a single concession. Bauxite mining occupies a share of the estimated 57,277 square kilometres of land disturbed by mining worldwide across 102 countries, and tropical bauxite regions with acidic, highly weathered soils may follow recovery trajectories fundamentally different from natural forest development. The authors argue that reclamation monitoring should incorporate soil organic carbon, aluminium saturation and species richness alongside conventional vegetation metrics, and that restoration programmes should move beyond canopy targets toward enrichment planting with native late-successional species, improved habitat connectivity and periodic soil amelioration. Success, they suggest, should be judged not only by similarity to the pre-disturbance forest but by whether the reclaimed ecosystem achieves long-term functionality, stability and resistance to degradation.

The findings come with caveats: the natural forest vegetation reference rested on a single plot, three observation points per planting year limited statistical power, and the study covers only the first decade of recovery. Whether rising aluminium saturation will eventually constrain ecosystem stability, and whether biodiversity continues to accumulate beyond year ten, remain open questions. Future work should extend to microbial communities, soil fauna, hydrological function and ecosystem services. But the central lesson is already clear and transferable to mining regions across the tropics: a young forest can wear the appearance of recovery long before the ground beneath it has healed, and only integrated, long-term monitoring of soil and vegetation together can tell the difference.

Subject of Research: Ecosystem recovery trajectories and reclamation effectiveness following tropical bauxite mining in West Kalimantan, Indonesia

Article Title: Evaluating reclamation effectiveness and ecosystem recovery trajectories following tropical bauxite mining in Indonesia

Article References: Suryadi, U. E., Sulakhudin, & Surachman (2026). Evaluating reclamation effectiveness and ecosystem recovery trajectories following tropical bauxite mining in Indonesia. Environmental Challenges, 25, Article 101647. https://doi.org/10.1016/j.envc.2026.101647

Image Credits: AI Generated

DOI: 10.1016/j.envc.2026.101647

Keywords: bauxite mining, Indonesia, West Kalimantan, ecosystem recovery, reclamation, soil acidity, aluminium saturation, chronosequence, tropical forest restoration, species richness, soil organic carbon, mine closure