Rinorea bengalensis has been on this site’s nickel roster for years as a facultative hyperaccumulator — it hyperaccumulates on ultramafic soils and stays ordinary elsewhere. What it has never had is a whole-stand mass balance: not how much Ni can one leaf hold, but how much of the forest’s total element budget does this one species control, and in which tissues is the metal sitting. Nkrumah, Corzo-Remigio, Sumail, Echevarria, Erskine and van der Ent have now done the destructive sampling required to answer that, in a R. cf. bengalensis-dominated secondary forest just outside Kinabalu Park near the Serinsim substation, Sabah (Ecological Research 41:e70061, CC BY). The answer is more lopsided than the hyperaccumulation literature implies.
Eighteen square metres, one accounting
Two 3 × 3 m plots (18 m² total, with 1 × 1 m subplots) were harvested to the ground, everything weighed and digested, vegetation split into Rinorea and non-Rinorea, plus six intact soil cores to 30 cm mapped by laboratory μXRF. The stand’s total dry biomass was 17.4 kg — a mean of 966 g m⁻² — and R. cf. bengalensis made up 83% of it (82.9% by the paper’s Figure 2). Its share of the stand’s elemental budgets is larger still: more than 75% of every major and trace element, reaching 94% of calcium and 99% of nickel. For context, the Rinorea fraction held roughly ten times the total element mass of all other species combined, and its Ni pool (~2.1 g m⁻², about twenty-fold its Mn and Zn pools) dwarfs the trace-element budgets of the non-hyperaccumulators, where Fe dominates and Ni is a rounding error.
One correction a careful reader must know: the paper’s abstract prints the Ni budget as 0.21 g m⁻², but the Results text, the figure values (~2,000–2,100 mg m⁻²) and the Discussion’s own agromining arithmetic (below) all converge on ~2.1 g m⁻² — the abstract figure appears to be a misprint. This matters, because the per-hectare yield scales directly with it.
The wood is the vault
The tissue partitioning is the mechanistic heart of the paper. In Rinorea, about 70% of the Ni sits aboveground (69% in the Discussion; ~70% in Results) — the reverse of the non-hyperaccumulators, where 76% of Ni is locked in roots. Stems are the main reservoir for most elements in both groups; the exceptions are Fe (root-stored in both) and, in the non-Rinorea plants, Mn and Ni as well. For an agromining operator this is the favourable case: the metal is not in ephemeral leaf tissue that falls off before harvest, but in the standing wood. Yet the leaves still matter for the cycle — mature leaves carry roughly 20% of the plant’s Ni, and they are precisely the fraction that senesces. Prior work on this species already showed the trafficking behind that pattern: Ni isotopes reveal uptake of the lighter isotopes, transport to young leaves, complexation with organic acids (the paper prints “carbolic acids”, an apparent slip for carboxylic), and phloem loading that progressively enriches the woody tissues (Zelano et al. 2020, Plant and Soil 454:225–243). The extreme trunk concentrations this can reach — up to 7.9 wt% Ni in the green phloem (van der Ent & Mulligan 2015), against ~1.2 wt% as the aboveground-tissue record cited here — are why the stem pool, not the leaf pool, anchors any harvest arithmetic.
A self-fertilising nickel pump — hypothesis, not measurement
The non-obvious implication runs the other direction: the same traits that make the species an agromining candidate also make the forest run on Ni. The authors argue, citing the elemental-allelopathy framework of Boyd and Martens (1998), that a hyperaccumulator which dominates biomass and pumps metal into surficial litter creates soil conditions its Ni-sensitive competitors cannot tolerate — reinforcing its own dominance and recycling the Ni stock through the humus layer. Their qualitative μXRF maps do show Ca and Ni concentrated at the soil surface under the stand. But the quantitative anchor for the mechanism is an analogy, not a measurement: the 3.1 vs 0.6 wt% Ni contrast in surface humus under versus away from a hyperaccumulator canopy comes from P. acuminata in New Caledonia (Boyd & Jaffré 2001, South African Journal of Science 97:535–538), not from this site — no humus Ni concentration was measured in this study, and no soil Ni concentration either. Treat the pump as a well-supported hypothesis that now deserves direct testing, in this stand, rather than as established fact.
What it means for agromining — and its honest price
The Discussion converts the stand budget into the number agrominers actually argue about: ~14 kg Ni ha⁻¹ standing in the biomass, worth US$238 ha⁻¹ at the US$17 kg⁻¹ Ni price the authors take from USGS (2025) — before production and processing costs, which for a dispersed, low-yield tropical operation are not trivial. Two readings fall out. Opportunity: a stem-stored, 99%-concentrated pool in a species that can reach 25 m tall is exactly the profile a woody agroforestry “metal crop” would want, and pot trials in Sabah show the yield is amendable. Risk: because ~30% of the Ni cycles through litterfall and the topsoil, a shoot-harvesting operation strips the nutrient/metal recycling loop the stand depends on — sustained yield would need the litter returned, adding handling cost to an already thin margin — and the field-demonstration record belongs to Phyllanthus rufuschaneyi, not to this species, which the authors state plainly has never been field-tested for agromining.
The bounds deserve equal weight: one stand, 18 m², no replication, a species whose very name carries a “cf.” (taxonomy unconfirmed), and a facultative hyperaccumulator whose performance off ultramafic soil is a different question entirely. This paper is a precise accounting of a single, functioning hypernickel ecosystem — and a reminder that in these forests, conservation and agromining are arguments about the same pool of metal.
Source: Nkrumah, Corzo-Remigio, Sumail, Echevarria, Erskine & van der Ent (2026), Ecological Research 41:e70061, DOI 10.1111/1440-1703.70061 (CC BY; received 2025-10-10, accepted 2026-02-18). Full text read from the open-access PDF (Wageningen repository copy of the publisher file). Every load-bearing number was verified verbatim by three independent reviewers via two independent retrieval paths (repository PDF and Crossref abstract); the only genuine discrepancy found — the abstract’s 0.21 g m⁻² Ni budget versus ~2.1 g m⁻² everywhere else in the paper — is flagged in the body, as are the authors’ “carbolic acids” misprint and the New-Caledonia provenance of the humus-Ni analogy. See /methodology/ for how we source and check analysis pieces.