Declaration 2: Publications
1.4 Encroachment by Acacia mellifera
The ecophysiological mechanisms and interactions between trees and grasses on which most of the assumptions and models are premised have seldom been tested empirically. The vertical separation of infiltrated soil moisture between upper and lower soil layers (Walter 1939) has not been conclusively established whereas direct competition for moisture in the upper 300 mm soil layer between woody plants and grasses has been shown by Smit and Rethman (2000). The ecophysiological effects of intraspecific woody plant interactions have rarely been quantified in the field. Intraspecific competition among trees is assigned a major role in creating open savannas by the patch-dynamic model of Wiegand et al. (2006). An additional component of the patch dynamic model is that nitrogen fixation by the trees gives them a competitive advantage over grasses under low soil nutrient conditions (Ward 2010). The addition of nitrogen fertiliser in greenhouse experimental trials conferred a significant competitive advantage to grasses over trees in the initial stages of recruitment (Kraaij & Ward 2006). Anecdotal data, from preliminary investigations that failed to detect nodules associated with nitrogen-fixing rhizomes in established Acacia mellifera trees (Ward and Cramer, pers. comm.), may point to an absence of nitrogen-fixing ability among mature trees of this species. The possibility exists of Acacia trees relying on nitrogen-fixing bacteria during initial stages of establishment and discontinuing the association at a mature age when a competitive advantage has been established over grasses. An ability to derive N from the atmosphere could thus help the persistence of woody plants on nutrient-deficient soils of semi-arid regions, possibly conferring a competitive advantage to woody plants over non-N2- fixing herbaceous layer vegetation (Milton 1980, Ward 2005, Wiegand et al. 2006). This may be an important factor in the initiation of the processes of shrub encroachment in savannas. One of the most common encroaching species is Acacia mellifera which occurs as a widespread shrub on rangelands in
18 Botswana, Namibia and northern parts of South Africa (Skarpe 1991, Molele et al. 2002, De Klerk 2004, Kraaij & Ward 2006).
This document synthesises the results of greenhouse and field-based investigations of the underlying ecological mechanisms and ecophysiological interactions between encroaching Acacia mellifera ([M. Vahl] Benth) trees from the semi-arid Northern Cape province of South Africa and grass species as well as intraspecifically among A. mellifera trees. Key questions that this thesis addresses concern the sources of soil moisture available to trees and grasses to ascertain if indeed trees and grasses access water from different depths in the soil and whether such a separation, if established, is distinctly two-tiered in support of Walter‟s (1971) model of niche separation in resource acquisition.
Investigations also covered the role of nutrient supply and limitation in the competitive interplay between grasses and trees as well as the function of N2 fixation by Acacia mellifera seedlings in their interaction with grasses and conspecifics. A. mellifera is known to encroach on rocky and sandy substrates, but does so more frequently on rocky substrates (Britz & Ward 2007). Hence, the effects of these substrate typess on the probability of bush encroachment occurring are also considered. Other than the two-way tree-grass competitive interactions on biomass output and nutrient status under sustained grazing pressure on different substrate types, intraspecific tree competition and its effect on tree vitality, plant water relations, nitrogen content and water use efficiency (WUE) is also covered.
The working hypothesis of this dissertation was that savannas are shaped by a multitude of plant interactions with both tree versus grass and tree versus tree interactions playing major roles in the process. The overarching hypothesis postulated competition from grass and conspecific neighbours for nutrients and soil moisture to negatively influence the growth, biomass, nutrient content and
ecophysiology of A. mellifera so as to limit its proliferation into thickets, with disturbance of the grass component favouring trees. It is also the underpinning assumption of this thesis that both inter- and
19 intra-specific competition is critical during tree seedling establishment stages and a competitive
advantage established by grass over trees, trees over grasses or individual trees over cohorts is perpetuated thereafter until disrupted by disturbance. Given sparse grass cover on rocky terrains that formed part of our field study sites and the results from field experiments in the same area that indicated fire and herbivory to have reduced effects on A. mellifera recruitment (Kraaij & Ward 2006, Ward and Esler 2010), the focus of the investigations described here was on the mechanistic ecophysiology of inter and intraspecific competition. Chapter 2 tested the effect of intraspecific competition among field plants on the growth, mortality, water relations, nutrient concentration and WUE as key factors undepinning the theory of savanna dynamics. The influence of repeated grass herbivory and substrate type (rocky, sandy) in the competitive interactions between A. mellifera and grasses relative to biomass
accumulation, nutrient concentration (Chapter 3) and soil moisture uptake with depth by either growth form (Chapter 4) were also investigated. Naturally limited N availability in savannas and competition for nutrients by grass may enhance a reliance on fixing N2 (Cramer et al. 2007) and confer a competitive advantage to woody legumes over grasses. The nitrogen-fixing capacity of A. mellifera and its
significance in conferring competitive advantage to tree seedlings over grasses was experimentally tested by evaluating the biomass accumulation, 15N isotopic signatures and WUE of A. mellifera plants growing with and without grass competition at different concentration levels of fertilizer N in the greenhouse (Chapter 5).
The work undertaken under this doctoral research is intended to establish the main
ecophysiological factors and feedback mechanisms or processes in the competitive interplay between Acacia mellifera trees and grasses in semi-arid savannas as well as the intra-tree competitive influences on plant ecophysiology. The most important expected outcome of this research was the establishment of a mechanistically appropriate representation of the factors causing a drastic increase in woody plant
20 density that lead to bush encroachment in savanna ecosystems in the Northern Cape and Northwest provinces of South Africa, as well as in Botswana and Namibia. This will ultimately help the development and use of both predictive and management models or tools to control and limit the phenomenon of bush encroachment in order to maintain or increase agricultural productivity and biodiversity of rangelands.
The synthesis has four major thematic areas of focus, all of which seek to contribute to the understanding of bush-encroachment processes/mechanisms in the context of tree-grass coexistence paradigm as well as tree-on-tree interactions. Consequently, four manuscripts were prepared based on the investigations and experiments undertaken under the current study. Different ecophysiological methodologies, protocols and techniques were followed in addressing the different research questions and these are elaborated in each chapter. Chapter 2 covers a field study investigating the effects of tree- on-tree interaction on growth, partial canopy die-back, mortality, water relations, nitrogen and carbon isotopic signatures of mature shrubs from a neighbour removal experiment. Chapter 3 focuses on tree biomass production and nutrient concentration when grown with and without grasses. Niche separation of soil moisture uptake by tree seedling and grasses with depth in a greenhouse experiment is the subject matter of chapter 4 whereas chapter 5 investigates the nitrogen isotopic signature of trees and grasses as a function of intra-specific competition. The ecological significance and implications of the results from this work is synthesised in the concluding chapter 6. Chapters 2-5 were prepared for submission to particular peer-reviewed journals and therefore follow the format of the target journals.
All experimental set-ups, sample collection, direct measurements and data analysis as well as interpretation and write ups were done by the candidate. The field tree removal experiment was set-up by David Ward (see e.g. Meyer et al. 2008). Mass spectrometry and isotopic composition of samples, including nitrogen and carbon concentration, were analysed at the Stable Isotope Laboratory in Cape
21 Town. Nitrogen and phosphorus nutrient composition analyses were carried out by the Animal Science Department of the University of KwaZulu-Natal.