Plant diversity and foliar elemental profiles in relation to soil chemistry and altitude on ultramafic edaphic islands in Kinabalu Park (Malaysia)

Abstract

This research project was multifaceted and aimed to develop an understanding of how plant diversity and foliar elemental chemistry interact with soil chemistry and altitude on ultramafic outcrops in Kinabalu Park (Sabah, Malaysia, Borneo Island). The research site, Kinabalu Park, is the worldrs most species-rich hotspot (g5000 plant species in l1200 km2), and the presence of widely-scattered ultramafic outcrops is thought to have stimulated plant evolution in the area. Ultramafic outcrops were conceptualised as edaphic islands of contrasting soil chemistry. The results show that ultramafic soils host radically different species assemblages even at the same altitude. The vegetation analysis supports the distinction of six main vegetation classes, with associated soil types: (1) Sub-alpine scrub; (2) Graminioid shrub; both associated with skeletal hypermagnesian soils; (3) Montane cloud-forest, associated with montane soils with mor humus; (4) Mixed-dipterocarp lowland forest, associated with deep laterite soils (oxisols); (5) Pioneer Casuarina shrub; and (6) Mature mixed Casuarina forest, both associated with serpentinite soils. The sub-type of ultramafic soils with the most extreme chemical properties are the serpentinite soils and the hypermagnesian soils, the former characterised by very high Mg:Ca molar quotients (5n25), and the latter also having extremely high phytoavailable Ni and Mn (50n180 mg/g and 250n500 mg/g DTPA-extractable). Although generally adverse to plants, the hypermagnesian soils on Mount Tambuyukon host exceptionally species-rich plant communities, but also contribute to the extreme stunting of this vegetation. The vegetation of serpentinite soils, however, are characterised by trees in the family Casuarinaceae and is species-poor. Although vastly different chemically, both hypermagnesian and serpentinite soils host high levels of ultramafic obligate species. In contrast, obligate ultramafic species are virtually absent in the lowland forest, but the combination of a number of dipterocarps is a characteristic of these forests. The foliar elemental profiles of plants growing on ultramafic soils were characterised by higher concentrations of Fe, Mg, Co, Cr, and Ni, but lower concentrations of Ca, K and P. This research identified hyperaccumulator behaviour in a range of plant species: Al g1000 mg/g (38 spp.), Mn g10 mg/g (7 spp.), Cu g300 mg/g (1 sp.), Cr g300 mg/g (3 spp.), Co g300 (3 spp.), Ni g1000 mg/g (24 spp.) and Zn g3000 mg/g (2 spp.) The foliar frequency distribution of Ni follows a distinct bimodal distribution, with the modes (lnon-accumulatorsr and lhyperaccumulatorsr) separated at approximately 850 mg/g Ni (the lower limit of the lhyperaccumulator moder). This indicates a distinct ecophysiological trait, as opposed to just the upper tail of a normal frequency distribution. In Sabah, Ni hyperaccumulation occurs most frequently in the Order Malpighiales (families: Dichapetalaceae, Phyllanthaceae, Salicaceae, Violaceae), and is particularly common in the Phyllanthaceae (genera Phyllanthus, Glochidion). Ni concentrations belong to the highest recorded globally attaining up to 2.4% in foliage (Psychotria sarmentosa complex) and up to 16.9% in the phloem exudate (Phyllanthus balgooyi). The phloem tissue of many of these species appears to act as a lNi-sinkr, which could function to re-distribute Ni to emerging shoots, hence potentially aiding insect herbivory-protection, although specialist Ni-tolerant insects were also found. Ni hyperaccumulators were localised in successional habitats on serpentinite soils with active mineral weathering. These soils showed a lthreshold responser with g20 mg/g carboxylic extractable Ni or g 630 mg/g pseudo-total Ni, and pH g6.3. Ni hyperaccumulators have the potential to be used in future phytomining operations, and knowledge of multi-element concentrations in their tissues could assist in selecting species with high Ni, but low concentrations of other unwanted elements (e.g. Ca, Mg, K, P, S) that could act as contaminants in the lbio-orer. The results of this study showed that ultramafic outcrops of Kinabalu Park support exceptionally high plant diversity with 2854 plant species in 742 genera and 188 families recorded (14 662 unique collections) during the fieldwork. Vegetation altitudinal zonation is compressed on tropical ultramafic mountains, as shown for Mount Tambuyukon (2579 m asl) in comparison with the non-ultramafic Mount Trus Madi (2643 m asl). The results demonstrated that plant diversity and endemicity were higher on Mount Tambuyukon in the summit zone, but not on the lower slopes. The soil chemistry in this zone is extreme, and the most probable cause for the stunted vegetation. Carnivorous Nepenthes provide examples of ultramafic obligate and endemic species, and N. rajah, N. villosa, N. burbidgeae and N. edwardsiana were studied. The results showed that these species are restricted to a very unusual combination of factors: extremely high insolation and permanently wet soils, brought about by the combined effects of an edaphic filter (ultramafic soils) and a climatic filter (altitude).