Soil geochemistry and constraint of tree seedlings immediately after germination on Macrotermes termite mounds in the Kruger National Park, South Africa.

Macrotermes termite mounds in the Kruger National Park occupy a significant part of the savanna landscapes, occurring at densities of up to 70 km-2 and often exceeding 10 m in width and 4 m in height. The mounds are usually devoid of trees, but have dense grass cover in wet years. As a result, these mounds form large patches of grassland amongst the wooded savanna. To our knowledge, it is not known why trees are largely excluded from the mounds. We analysed soil surface nutrient concentrations on and off mounds (0-2 cm deep, n = 80) to ascertain whether the availability of nutrients could be influencing competition between grasses and tree seedlings. The results showed that potential deficiencies in P, Ca, Cu, Zn and B in soils off the mounds are likely to be constraining plant growth. Notably, only B, with an average concentration of 0.19 mg kg-1, was likely to be limiting plant growth on the mounds. Notwithstanding likely interactions with herbivory and fire, we hypothesise that because grasses are far less susceptible to deficiencies of B than dicotyledonous trees, it is likely that grass competition with tree seedlings is considerably greater on mounds than off mounds.

A biome-wide experiment to assess the effects of propagule size and treatment on the survival of Portulacaria afra (spekboom) truncheons planted to restore degraded subtropical thicket of South Africa.

Insights from biome-wide experiments can improve efficacy of landscape-scale ecological restoration projects. Such insights enable implementers to set temporal and geographical benchmarks and to identify key drivers of success during the often decades-long restoration trajectory. Here we report on a biome-wide experiment aimed at informing the ecological restoration of thousands of hectares of degraded subtropical thicket dominated by the succulent shrub, Portulacaria afra (spekboom). Restoration using spekboom truncheons has the potential to sequester, for a semi-arid region, large amounts of ecosystem carbon, while regenerating a host of associated ecosystem services. This study evaluates, after about three years post-propagation, the effects of spekboom truncheon size and treatment on survivorship in 40 fence-enclosed (0.25 ha) plots located in target habitat across the entire spekboom thicket biome. In each plot, locally harvested spekboom truncheons, comprising eight size/treatment combinations, were planted in replicated rows of between 24 and 49 individuals, depending on treatment. The experiment assessed the role of truncheon size, spacing, application of rooting hormone and watering at planting on survivorship percentage as an indicator of restoration success. All eight combinations recorded extreme minimum survivorship values of zero, while the range of extreme maximum values was 70-100%. Larger truncheons (>22.5 mm diameter) had almost double the survivorship (ca. 45%) than smaller truncheons (< 15 mm) (ca. 25%). Planting large, untreated truncheons at 1 m intervals-as opposed to 2 m intervals recommended in the current restoration protocol-resulted in no significant change in survivorship. The application of rooting hormone and water at planting had no significant effect on restoration success for both large and small truncheons. While our results do not provide an evidence base for changing the current spekboom planting protocol, we recommend research on the financial and economic costs and benefits of different propagation strategies in real-world contexts.

Herbivory and misidentification of target habitat constrain region-wide restoration success of spekboom (Portulacaria afra) in South African subtropical succulent thicket.

Restoration of degraded subtropical succulent thicket, via the planting of Portulacaria afra (spekboom) truncheons, is the focus of a public works programme funded by the South African government. The goals of the programme, which started in 2004, are to create jobs, sequester carbon, restore biodiversity, reduce erosion, improve soil water holding capacity and catalyse private sector investment for upscaling of restoration. Here we report on a region-wide experiment to identify factors that can improve project success. Measures of success were survivorship and annual aboveground biomass carbon sequestration (ABCsr) of spekboom truncheons some 33-57 months after planting-starting in March 2008-into 173 fenced plots (0.25 ha) located throughout the global extent of spekboom thicket vegetation. We also collected data for 18 explanatory variables under the control of managers, and an additional 39 variables reflecting soil physical and chemical characteristics and rainfall patterns post restoration, all beyond the influence of managers. Since the latter covariates were available for only 83 plots, we analysed the two data sets separately. We used a prediction rule ensemble to determine the most important predictors of restoration success. There was great variation in percentage survivorship (median = 24, range = 0-100%) and ABCsr (median = 0.009, range = 0-0.38 t C ha-1 yr-1). The model using management variables explained less variance (53%) in survivorship than the model incorporating additional soil and rainfall covariates (62%). ABCsr models were better fits (78 and 88% variance explained, respectively). All model configurations identified browse intensity as a highly influential predictor of restoration success. Predicted success was highest for plots located in target habitat; however, only 45% were thus located, suggesting the need for expert input and habitat modelling for improving target habitat identification. Frost exposure was another important predictor influencing all models but was likely a consequence of locating sites off target habitat. Sites planted on equatorward slopes during the warm season showed reduced carbon sequestration, possibly due to elevated soil moisture stress associated with high radiation loads. Physiographic factors associated with improved restoration success were location on sloping ground (reduced frost exposure), increasing longitude (more warm-season rainfall) and increasing latitude (less frost coastwards). Few trends were evident among post-restoration climatic factors beyond the control of managers. Higher rainfall during the year post restoration had a negative impact on carbon sequestration while higher rain during the early months post restoration had a positive effect on both carbon sequestration and survivorship. Soil factors showed little importance for the survivorship model, whereas silt content, % K and Mg CEC emerged as predictors of carbon sequestration. Our results have direct relevance for improving the success of landscape-scale restoration projects envisioned for the ca. 8,930 km2 of degraded spekboom thicket.

Mapping soil organic carbon stocks and trends with satellite-driven high resolution maps over South Africa.

Estimation and monitoring of soil organic carbon (SOC) stocks is important for maintaining soil productivity and meeting climate change mitigation targets. Current global SOC maps do not provide enough detail for landscape-scale decision making, and do not allow for tracking carbon sequestration or loss over time. Using an optical satellite-driven machine learning workflow, we mapped SOC stocks (topsoil; 0 to 30 cm) under natural vegetation (86% of land area) over South Africa at 30 m spatial resolution between 1984 and 2019. We estimate a total topsoil SOC stock of 5.6 Pg C with a median SOC density of 6 kg C m-2 (IQR: interquartile range 2.9 kg C m-2). Over 35 years, predicted SOC underwent a net increase of 0.3% (relative to long-term mean) with the greatest net increases (1.7%) and decreases (-0.6%) occurring in the Grassland and Nama Karoo biomes, respectively. At the landscape scale, SOC changes of up to 25% were evident in some locations, as evidenced from fence-line contrasts, and were likely due to local management effects (e.g. woody encroachment associated with increased SOC and overgrazing associated with decreased SOC). Our SOC mapping approach exhibited lower uncertainty (R2 = 0.64; RMSE = 2.5 kg C m-2) and less bias compared to previous low-resolution (250-1000 m) national SOC mapping efforts (average R2 = 0.24; RMSE = 3.7 kg C m-2). Our trend map remains an estimate, pending repeated measures of soil samples in the same location (time-series); a global priority for tracking SOC changes. While high resolution SOC maps can inform land management decisions aimed at climate mitigation (natural climate solutions), potential increases in SOC are likely limited by local climate and soils. It is also important that climate mitigation efforts such as planting trees balance trade-offs between carbon, biodiversity and overall ecosystem function.

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

The causes of the worldwide problem of encroachment of woody plants into grassy vegetation are elusive. The effects of soil nutrients on competition between herbaceous and woody plants in various landscapes are particularly poorly understood. A long-term experiment of 60 plots in a South African savanna, comprising annual applications of ammonium sulphate (146-1166 kg ha-1 yr-1) and superphosphate (233-466 kg ha-1 yr-1) over three decades, and subsequent passive protection over another three decades, during which indigenous trees encroached on different plots to extremely variable degrees, provided an opportunity to investigate relationships between soil properties and woody encroachment. All topsoils were analysed for pH, acidity, EC, water-dispersible clay, Na, Mg, K, Ca, P, S, C, N, NH4, NO3, B, Mn, Cu and Zn. Applications of ammonium sulphate (AS), but not superphosphate (SP), greatly constrained tree abundance relative to control plots. Differences between control plots and plots that had received maximal AS application were particularly marked (16.3 ± 5.7 versus 1.2 ± 0.8 trees per plot). Soil properties most affected by AS applications included pH (H2O) (control to maximal AS application: 6.4 ± 0.1 to 5.1 ± 0.2), pH (KCl) (5.5 ± 0.2 to 4.0 ± 0.1), acidity (0.7 ± 0.1 to 2.6 ± 0.3 cmol kg-1), acid saturation (8 ± 2 to 40 ± 5%), Mg (386 ± 25 to 143 ± 15 mg kg-1), Ca (1022 ± 180 to 322 ± 14 mg kg-1), Mn (314 ± 11 to 118 ± 9 mg kg-1), Cu (3.6 ± 0.3 to 2.3 ± 0.2 mg kg-1) and Zn (6.6 ± 0.4 to 3.7 ± 0.4 mg kg-1). Magnesium, B, Mn and Cu were identified using principal component analysis, boundary line analysis and Kruskal-Wallis rank sum tests as the nutrients most likely to be affecting tree abundance. The ratio Mn/Cu was most related to tree abundance across the experiment, supporting the hypothesis that competition between herbaceous and woody plants depends on the availability of anabolic relative to catabolic nutrients. These findings, based on more than six decades of experimentation, may have global significance for the theoretical understanding of changes in vegetation structure and thus the practical control of invasive woody plants.

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 Earth's 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.