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

Abstract

a b s t r a c t a r t i c l e i n f o Available online xxxx Edited by A Potts Keywords: Highveld Pampas Treelessness Edaphic EcologyThe 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.

The Role of Environmental Factors in Promoting and Limiting Biological Invasions in South Africa

“…The theory has three main premises: first the availability of catabolic versus anabolic nutrients has marked effects on vegetation structure; second that short plants (e.g. shrubs and grasses) are more competitive than tree seedlings where demand for catabolic nutrients is met by supply; and third that demand for catabolic nutrients is dependent on the rate of photosynthate production (Milewski and Mills 2015;Mills et al 2013aMills et al , b, 2016.…”

Influences on plant nutritional variation and their potential effects on hominin diet selection

“…Some have questioned whether the grassland component of these habitats would have been as prevalent as they are today, given that human activity including fire management, grazing, and tree-clearing may contribute to the relatively treeless aspect of the current-day landscape (Mucina and Rutherford, 2006). A variety of data suggests that these grassland biomes are in fact ancient, likely due to naturally occurring fires (Bond et al, 2003), climate regimes (particularly rainfall under b1000 mm) (Staver et al, 2011), and/or soil nutrient availability (Milewski and Mills, 2015). Even if wildfires were not as prevalent in the past, the cold climate and low rainfall that characterizes this region is likely sufficient for the presence of open grassy biomes, as suggested by the fossil grassland-adapted fauna.…”

Effects of anabolic and catabolic nutrients on woody plant encroachment after long-term experimental fertilization in a South African savanna

“…Such an understanding of vegetation structure has remained elusive. Although the literature on the treelessness of grasslands has emphasised the adverse effects of drought, fire and frost on woody plants [ 4 , 5 , 6 , 7 ], these factors are likely to be mechanisms rather than causes underlying the natural competitive power of grasses and the consequent suppression or exclusion of even those taxa of trees adapted to such adversities [ 8 ].…”

“…This is accordingly one approach which would offer a ‘fresh look at how nutrients influence vegetation structure’. Recently, Milewski and Mills [ 8 ] provided a theoretical framework (hereafter referred to as the Anabolic/Catabolic Theory) for why ratios of anabolic nutrients (e.g. Mn and Mg) to catabolic nutrients (e.g.…”