Why was the Highveld treeless? Looking laterally to the Pampas for global edaphic principles beyond biogeographical accidents

•The ultimate cause of treelessness in grasslands has proven elusive.•We compare the South African Highveld ecologically with the Argentinian Pampas.•Drought, frost, waterlogging, fire and herbivory are proximate not ultimate causes.•Supply versus demand of catabolic nutrients may be a crucial edaphic factor.•If supply meets demand for catabolic nutrients, grasses hypothetically exclude trees.

The ultimate reasons for treelessness in the natural grassland of the Highveld of southern Africa have long eluded biologists. This is partly because of the ambivalence of adaptive determinism versus biogeographical/phylogenetic accidents, and partly because of entanglement between cause and effect. For example, waterlogging and grass fires, arguably unfavourable for tree growth, are themselves promoted by an absence of trees. One approach to evaluating previous hypotheses is ecological comparison of the Highveld with the global epitome of treelessness under a mesic climate: the Argentinian Pampas. This comparison detracts from the explanatory power of climate and biotic disturbance (wildfire and herbivory) while pointing to the explanatory power of edaphic factors in accounting for the competitive superiority of the grass growth-form. To address this shortcoming, we offer a new hypothesis. This is that the natural exclusion of trees in the Highveld, the Pampas, and comparable grasslands stems from a particular nutritional regime, in which wood accumulates as a result of rates of photosynthesis exceeding rates of respiration. The ratio of anabolic rates to catabolic rates – at the level of the whole plant community – theoretically determines whether photosynthate, superfluous to catabolism (i.e. respiration), will tend to be deployed as wood. The crucial environmental factor, according to this rationale, would be the supply of those catalytic elements indispensable for catabolic processes relative to the photosynthetic potential of the site (as determined by both the supply of those nutrients indispensable for anabolic processes and the relevant climatic factors, e.g. temperature and rainfall). The root cause we propose – which transcends any simplistic distinction between overall nutrient-richness (eutrophy) and nutrient-poverty (oligotrophy) – has the potential to disentangle the tautologies that have marred previous attempts to explain on a global basis why certain vegetation types were naturally treeless. A suggested line of future investigation is to determine the extent to which deposition of aerosols across the Highveld – over both nutrient-rich (‘sweetveld’) and nutrient-poor (‘sourveld’) parent materials – boosts the supply of Cu within the topsoil, promotes the catabolism of grasses, maximises the competitive strength of the grasses relative to the photosynthetic potential of the site, and prevents the establishment of tree seedlings.