Anthony J. Mills

A framework for exploring the determinants of savanna and grassland distribution

Abstract An understanding of the factors governing grass–tree coexistence in savannas and exclusion of trees in grasslands remains elusive. We contend that progress in understanding these factors is impeded by a reliance on a falsification approach and an excessive concern over type I errors (false positives), which results in premature rejection of hypotheses, inadequate attention to scale, and a miring rather than galvanizing of ecological discussions. An additional hindrance to progress may be that investigations tend to focus on processes within either savannas or grasslands, while ignoring the boundary between the two. We propose a new scientific framework for identifying determinants of savanna and grassland distribution, which advocates (a) the recognition of ecosystems and biomes as complex adaptive systems, (b) a scientific methodology based on adaptive inference, and (c) explicit consideration of patch boundaries at various scales. Analysis of processes operating at dynamic savanna–grassland boundaries should permit better separation of ultimate from proximate factors controlling grass–tree interactions within the individual biomes. The proposed savanna–grassland framework has potential for application in other areas of ecology facing similar problems.

Unravelling the effects of soil properties on water infiltration : segmented quantile regression on a large data set from arid south-west Africa

Relationships were sought between infiltrability and the properties of hundreds of surface soils (pedoderms) sampled across Namibia and western South Africa. Infiltrability was determined using a laboratory method, calibrated against a rainfall simulator, which measures the passage of a suspension of soil in distilled water through a small column packed with the same soil. Other properties determined were EC, pH, water-soluble cations and anions, ammonium acetate-extractable cations, organic C, total N, a 7-fraction particle size distribution, water-dispersible silt and clay, and clay mineral composition. Our objective was to ascertain whether general principles pertaining to infiltrability can be deduced from an analysis of a wide diversity of soils. To achieve this we compared correlation analysis, generalised linear models (GLMs), and generalised additive models (GAMs) with a segmented quantile regression approach, in which parametric regression lines were fitted to the 0.9 and 0.1 quantile values of equal subpopulations based on the x variable. Quantile regression demarcated relational envelopes enclosing four-fifths of the data points. The envelopes revealed ranges for soil properties over which infiltrability is potentially maximal (spread over a wide range) or predictably minimal (confined to small values). The r2 value of the 0.9 quantile regression line was taken as an index of reliability in being able to predict limiting effects on infiltrability associated with a variety of soil properties. Prediction of infiltration was most certain from textural properties, especially the content of water-dispersible silt (r2 = 0.96, n = 581), water-dispersible clay (0.88, n = 581), very fine sand (0.86, n = 174), and medium sand (0.84, n = 174). Chemical properties such as EC, sodium status, organic C content, and clay mineralogy were less clearly related to infiltrability than was texture. The role of fine-particle dispersion in blocking pores was highlighted by the stronger prediction in all statistical analyses provided by the water-dispersible as opposed to total content of silt and clay. All the statistical analyses revealed a probable skeletal role of medium and fine sand fractions in shaping pores and a plasmic (mobile) role of finer fractions in blocking pores. A noteworthy discovery was an apparent switch in role from skeletal to plasmic at a particle diameter of about 0.1 mm (i.e. between fine and very fine sand).

Transformation of thicket to savanna reduces soil quality in the Eastern Cape, South Africa

Xeric succulent thicket in the Eastern Cape, South Africa has been used for farming goats since the early 1900s. This habitat is characterised by a dense cover of the succulent bush Portulacaria afra and by a warm, semi-arid climate with evenly distributed annual rainfall of 250–400 mm. Heavy browsing by goats results in the loss of P. afra and transforms the thicket to an open savanna dominated by annual grasses. Eight fence-line comparisons between thicket and savanna were used to investigate differences in soil quality associated with the vegetation change. Composite soil samples were taken to a depth of 10 cm from 1 ha plots on either side of the fence-line. Associated with the change from thicket to savanna, a significant decrease (paired t-test, P < 0.05) was found in total C (respective means of 5.6 vs. 3.0%), total N (0.33 vs. 0.24%), labile C (2.8 vs. 1.5%), CO2 flux (1.9 vs. 0.5 µmol m−2 s−1), soil respiration in the laboratory (144 vs. 79 ng C kg−1 s−1), (NH4)OAc-extractable Mg (55 vs. 28 mmolc kg−1), and laboratory infiltration rate (51 vs. 19 mm h−1). In the same direction there was a similarly significant increase in modulus of rupture (16 vs. 34 kPa), water-soluble Ca (2.3 vs. 3.4 mmolc kg−1) and pH (6.7 vs. 7.7). The soil C content of 5.6% in thicket is surprisingly high in this warm, semi-arid climate and suggests that the dense P. afra bush strongly regulates soil organic matter through microclimate, erosion control, litter quantity and perhaps chemistry. Savanna soils had a greater tendency to crust (as evident in a lower rate of laboratory infiltration and greater modulus of rupture) than thicket soils. This was attributed to their lower organic matter content, which probably reduced aggregate stability. Savannas are likely to be more prone to runoff and erosion not only because of sparser vegetation but also because of a decline in soil quality.

Ecosystem carbon storage under different land uses in three semi-arid shrublands and a mesic grassland in South Africa

Carbon (C) storage in biomass and soils is a function of climate, vegetation type, soil type and land management. Carbon storage was examined in intact indigenous vegetation and under different land uses in thicket (250–400 mm mean annual precipitation), xeric shrubland (350 mm), karoo (250 mm), and grassland (900–1200 mm). Carbon storage was as follows: (i) mean soil C (0–50 cm): thicket (T) = grassland (G) > xeric shrubland on Dwyka sediments (XS) > xeric shrubland on dolerite (XSD) > karoo (K) (168, 164, 65, 34 & 26 t ha−1, respectively); (ii) mean root C: T > G > XS = XSD (25.4, 11.4, 7.2 & 7.1 t ha−1); (iii) mean above-ground C including leaf litter: T>XS>G>K> XSD (51.6, 12.9, 2.0, 1.7 & 1.51 ha−1). Carbon stocks in intact indigenous vegetation were related more to woodiness of vegetation and frequency of fire than to climate. Biomass C was greatest in woody thicket and soil C stocks were greatest in thicket and grassland. Total C storage of 245 t ha−9 in thicket is exceptionally high for a semi-arid region and is comparable with mesic forests. Soil C dominated ecosystem C storage in grassland and was influenced more by soil parent material than land use. The semi-arid sites (xeric shrubland and thicket) were more sensitive to effects of land use on C storage than the grassland site. Effects of land use on C stocks were site- and land use-specific and defied prediction in many instances. The results suggest that modelling of national C stocks would benefit from further research on the interactions between C storage, land use, and soil properties.

The Role of Botanic Gardens in the Science and Practice of Ecological Restoration

Many of the skills and resources associated with botanic gardens and arboreta, including plant taxonomy, horticulture, and seed bank management, are fundamental to ecological restoration efforts, yet few of the worlds botanic gardens are involved in the science or practice of restoration. Thus, we examined the potential role of botanic gardens in these emerging fields. We believe a reorientation of certain existing institutional strengths, such as plant-based research and knowledge transfer, would enable many more botanic gardens worldwide to provide effective science-based support to restoration efforts. We recommend botanic gardens widen research to include ecosystems as well as species, increase involvement in practical restoration projects and training practitioners, and serve as information hubs for data archiving and exchange.

Declining soil quality in South Africa: effects of land use on soil organic matter and surface crusting

Soil conservation in South Africa has historically focussed on preventing soil erosion. Effective maintenance of the soil resource requires, in addition to erosion control, an understanding of how land use practices affect more subtle indicators of soil quality. This review outlines how land use in South Africa can rapidly result in a marked reduction in soil organic matter (SOM) content and a greater tendency of soils to crust. Removal of a cover of vegetation whether by ploughing, grazing or burning tends to reduce SOM due to reduced organic matter inputs and enhanced soil microbe activity. Loss of SOM, particularly from the top few centimetres of soil (named here the pedoderm) has a disproportionately large effect on soil infiltrability and nutrient supply. The mineralogy of the clay fraction also has great bearing on the response of soil to land use effects. The unexpected role of quartz in soil dispersion and crusting in South Africa has only recently been unveiled. Apart from SOM effects, land use can lead to subtle changes in soil chemistry. Plantation forestry has resulted in an increase in soil nitrate in many areas, possibly due to greater mineralisation under forests than grasslands. Annual burning in the Kruger National Park bushveld has been shown to increase clay dispersibility and crusting of the pedoderm, which was ascribed to a reduction in electrical conductivity and SOM as well as an increase in the exchangeable sodium percentage. Soil quality is a multifaceted concept. One aspect stands out, however, as critical and that is the conservation and replenishment of nitrogen which is all important for retaining humus and maintaining soil quality.

Soil infiltrability as a driver of plant cover and species richness in the semi-arid Karoo, South Africa

Data from 199 plots in the semi-arid Karoo showed that relationships between soil infiltrability and plant cover/species richness, as depicted by boundary lines, yielded ecological insights not evident if only commonly measured soil properties such as pH, electrical conductivity and the content of clay, silt, sand, nitrogen and carbon were considered. For example, the common grass Stipagrostis obtusa, herb Lepidium africanum and shrub Pentzia incana showed potentially maximal cover at high, low and intermediate infiltrability, respectively (r2 > 0.65 for boundary lines derived by segmented quantile regression), but did not show distinct boundary lines for sand, silt or clay content data (r2 < 0.5). Potentially maximal species richness was revealed under soil conditions of low infiltrability, high nitrogen content and low pH. Distinct boundary lines suggested that the drivers of species richness at any particular point in the Karoo landscape may operate in opposing directions simultaneously.

Ecological effects of fire-breaks in the montane grasslands of the southern Drakensberg, South Africa

Fire-breaks, by legislation, are burnt annually before mid-winter in the southern Drakensberg, affecting 5–10% of the landscape, and resulting in marked selection for the early season flush by both livestock and wildlife. This study investigated whether this severe defoliation regime has had an effect on surface soil properties, botanical composition, plant diversity, phytomass production, and the nutrient content of phytomass by comparing paired samples of grassland and immediately adjacent fire-breaks. Firebreaks differed quantitatively in composition from adjacent grassland although both were characterised as Themeda triandra–Tristachya leucothrix grassland. Only two grass species of lesser abundance, Brachiaria serrata and Eragrostis racemosa, were clearly more abundant on fire-breaks. Differences in plant diversity (species m–2, species 20m–2) were slight. Surface soil of fire-breaks compared with grasslands was slightly more acidic (0.2 units), lower in carbon (1% difference), and lower in total nitrogen (0.03% difference), suggesting deterioration of soil conditions on fire-breaks. Phytomass of fire-breaks and of grasslands burnt in early spring in the season sampled (biennial or greater burning frequency) were equivalent by February, indicating there has been no decline in productivity over time. Concentration of nitrogen, phosphorus, calcium, magnesium, potassium, sodium, zinc and copper in above-ground phytomass did not differ between fire-breaks and grasslands, although concentration of manganese tended to be greater in grassland. Expected seasonal trends, especially for nitrogen and phosphorus, were apparent. Fire-breaks therefore appear to be a sustainable management practice, in part because mid-winter burning allows sufficient recovery of canopy cover to mitigate against erosion by high intensity spring storms and the duration of grazing impact is alleviated by spring burning elsewhere. Grazing impact on fire-breaks may become more severe if grassland burning becomes more infrequent.

SOIL carbon and nitrogen in five contrasting biomes of South Africa exposed to different land uses

Stocks of soil C to a depth of 50 cm in untransformed, indigenous veld ranged from 21 t ha-1 in karoo to 168 t ha-1 in thicket and stocks of N ranged from 3.41 ha-1 in karoo to 12.8 t ha-1 in grassland. Mean soil C in thicket (5.6%, 0–10 cm) was approximately five times greater than expected for a semi-arid region. Removal of vegetation due to cultivation, grazing or burning reduced soil C and N at all sites. Soil C under intact thicket was greater than at sites degraded by goats (71 vs 40 t ha-1, 0–10 cm). Restoration of thicket could potentially sequester -40 t C ha-1. The sale of this sequestered carbon to the international market may make restoration of thousands of hectares of degraded thicket financially feasible. Soil C under plant cover was greater than In exposed soil in renosterveld (28 vs 15 t ha-1) and in karoo (7 vs 5 t ha-1). Parent material was also related to soil C content. In grassland, soil C was greater in dolerite-derived than sandstone-derived soils (54 vs 271 ha-1); and in bushveld it was greater in basalt-derived than granite-derived soils (28 vs 14 t ha-1 in unburnt plots). Annual burning in bushveld reduced soil C, particularly at the surface. Soil C in the 0–1 cm layer of unburnt plots was 2 to 3 times greater than in burnt plots.

Does life consistently maximise energy intensity

We propose that a basic biological imperative of all organisms is to maximise energy (E) intensity, defined as the average rate of energy use per unit area of the Earths surface. The dominant organism in any given environment is predicted to be that exerting the greatest E intensity regardless of evolutionary origin. Our ‘theory of biological E intensity’ thus explains variation in life form in terms of adaptations as opposed to accidents of biological history. It defines the competitive criterion in all metabolic pathways and industrial processes as the average rate of kinetic energy use, excluding heating but including all directed biological kinesis at scales up to the whole organism. A suggested unit for E intensity is joules per square meter per year. Because catalysts are crucial to extremely rapid use of energy (and therefore maximisation of E intensity), catalytic nutrient elements can be viewed as the ultimate means of life. It follows that a common denominator of all dominant organisms would be the acquisition of an optimal catalytic formula as determined by concentrations and ratios of C, H, O, N, S, Na, Mg, P, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Se, Mo, Cd, I, W, and Hg. The likely local shortages of various of these elements can theoretically be alleviated by various changes in the size, shape, and/or behaviour of organisms, depending on the environment. Thus, the availability, and potential for supplementation, of catalytic elements would be the ultimate basis for adaptation, largely determining which life form dominates in any particular location. The theory predicts the following. (1) In nutrient‐rich environments offering the optimal catalytic formula, dominant organisms will be microbes. This is because microbes, and prokaryotes in particular, excel in E intensity through rapid biomolecular turnover, enabling them to usurp resources despite minimising biomass, complexity, and information. (2) Where the environment is catabolically dystrophic (i.e. scarce in certain nutrients required for catabolism), macrobes (e.g. humans and trees) will be superior competitors because they can collect and supplement nutrients and thereby approach the optimal catalytic formula. This enables macrobes, despite having considerably slower metabolism per unit body mass, to enhance E intensity relative to competing microbes constrained by catabolic dystrophy. Finally, (3) where the environment is anabolically dystrophic (i.e. scarce in certain nutrients required for anabolism) microbes will again dominate because biomolecular turnover can be relatively free from constraint given the limited fuel available. We suggest that an important and overlooked way to achieve power is to reuse energy, and that all organisms maximise E intensity by converting chemical potential energy (i.e. in fuel) into circuits of electromagnetic energy comprising electric charge, photons, and excited electrons. Because space and time merge subatomically, these electromagnetic circuits represent a concentration in spacetime of energy that (1) is concurrently kinetic and static, hence available for immediate use yet also conserved with minimal dissipation, and (2) ultimately promotes catalysis, which we assert is the primary biological tactic for maximising E intensity and thus fitness.