List of tables
Chapter 3 Study Area
4.2 Experimental design .1 Nkuhlu experimental site
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Chapter 4
Experimental- and sampling design
Chapter 4: Experimental- and sampling design
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Figure 13: Aerial view of the Nkuhlu exclosures indicating the three herbivore treatments and location of sampled sites (1-5) within the sodic zone.
LMH exclosure, all LMH excluded; Elephant exclosure, elephants (and giraffes) excluded; Sodic zone, sodium-rich, deep, duplex soil associated with the footslopes of undulating granitic landscapes.
4.2.1.1 Fire treatments
Each of the three treatments are further divided into a fire and no-fire area, resulting in a total of six herbivory-fire combinations (Figure 14) (O’Keefe & Alard, 2002). Located within the larger KNP burn blocks (i.e. fire management units), occurrence of fire in these treatments are dependent on fire management strategies set out by the KNP (Van Coller et al., 2018).
Prescribed burn events occurred approximately once every five years between June and
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October, with four burn events taking place between the establishment of the exclosures and sampling i.e. 2002, 2007, 2012, 2017 (Vermeire et al., 2021). Although fire is a major driver of savanna ecosystem dynamics (Van Coller et al., 2013; Pausas & Bond, 2020; Vermeire et al., 2021), it was not considered for the purpose of this study. The primary reason for the exclusion of fire as a variable pertains to the design constraints of the Nkuhlu exclosures. The main focal point of the study, the sodic zone, does not extend into the sixth herbivore treatment i.e. the Control site that is exposed to fire. As such, fire was not included as it would result in an unbalanced experimental design. A detailed graphical representation of the Nkuhlu exclosure layout and the associated constraints of the six herbivore-fire treatments are provided as supplementary material in Appendix A, Figure A1. Furthermore, evidence has been found that the fuel load within intensively utilised sodic patches, characterised by palatable forage, are kept relatively low as a direct result of higher grazing pressure (Van Wilgen et al., 2003;
Govender et al., 2006). As such, it is often the case that fire, within the sodic zone, does not represent a continuous burn as a result of high structural patchiness of the system. Moreover, initial statistical analyses revealed no significant variation between fire and no fire treatments within the Elephant- and LMH exclosure, further justifying the pooling of these sites. Statistical results are provided as supplementary material in Table B1 of Appendix B.
4.2.1.2 Experimental plots
Surveys were conducted within five sites (Figure 13), each comprising five plots (2.75 m x 1.5 m; Figure 15) located across the three herbivore treatments (Elephant exclosure, Control
site with all LMH present, LMH exclosure; Figure 14), resulting in a total of 25 plots.
Figure 14: Experimental layout of the Nkuhlu long-term research exclosures indicating the five sampled sites and associated plots located across the three herbivore treatments within the sodic zone.
Chapter 4: Experimental- and sampling design
46 4.3 Field surveys and sampling
This study was initiated in January 2019 and concluded in January 2020 with sampling taking place along four intervals of three months, each representing a different season. At the time of data collection, herbivores have been excluded from the LMH exclosure for approximately 18-19 years. Sampling encompassed the collection of aboveground productivity (i.e.
herbaceous biomass), plant species composition and functional traits data, decomposition data as well as soil samples for the purpose of soil microbial genomic analyses. Soil physicochemical properties of the studied site have also been sampled for the purpose of another research project in which data were pooled into the larger herbivore treatments (i.e.
herbivore presence or absence) and analysed accordingly (Van Coller et al., in prep). Results obtained from soil analyses will be used to facilitate the interpretation of observed patterns of change in potential soil-based decomposition and stabilisation as well as belowground microbial community composition. Observed patterns of changes in vegetation structure and edaphic properties are contextualised in terms of soil function as key regulatory constituents of belowground processes and structure.
Chapter 5, 6 & 7
Decomposition-based sampling conformed to the extended, site-specific TBI approach as described by Erasmus et al. (2021) -Appendix D. Initiated in January 2019 with the burial of 1 000 tea bags across three herbivore treatments (Figure 14) within 25 experimental plots (Figure 15), this study ran for a period of one year with sampling taking place along four intervals of three months. The development and validation of this extended approach is comprehensively described in Chapter 5. Application of the extended TBI approach from a global comparative perspective (Erasmus et al., 2022; Appendix E) is described in Chapter 6, while the application thereof along a temporal gradient is described in Chapter 7.
Chapter 8
Soil samples were collected during the dry (October 2019) and wet (January 2020) season of this particular savanna ecosystem. Soil samples (25 per season, i.e. one composite sample per plot; Figure 15) were taken at a depth of up to 8 cm and stored at -80°C to maintain DNA integrity. Extracted DNA was analysed through the application of NGS techniques. A detailed account on the methodology used to identify microbial taxa across the three herbivore treatments is presented in Chapter 8.
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47 Chapter 9
Primary productivity was estimated for each of the three herbivore treatments (Figure 14) by using a Lowveld Savanna-calibrated disk pasture metre (DPM). DPM readings were recorded for both the dry (October) and wet (January) season and converted to kg/ha. Additional productivity data in the form of aboveground herbaceous biomass clippings were collected in January 2020 for a more accurate approximation of dry weight of aboveground phytomass during the wet season. Clippings were dried and separated into forbs and grasses where after they were weighed and converted to kg/ha. Plant species composition data (sampled in January 2020) were collected from five 50 m transects that were positioned in close proximity to the decomposition and soil microbial sites. Local scale floristic data were collected to better understand the observed patterns in decomposition, stabilisation and microbial community composition. The floristic analyses are therefore not aimed at describing vegetation patterns across the herbivore treatments. A detailed account of the herbaceous floristic patterns in the sodic zone has already addressed floristic responses to herbivory in the studied site (Van Coller et al., 2013).
Figure 15: Positioning of (a) temporary experimental plots within the sodic zone of each herbivore treatment of the Nkuhlu exclosures, and (b) layout of plots for the purpose of decomposition-based sampling. A detailed explanation of the experimental plot layout is provided in Chapter 5. (Adapted from Erasmus et al., 2021).
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48 4.4 Data preparation
Decomposition data (TBI data) were recorded in the TBI 2.0 NW data sheet for non-woven tea bags provided by Keuskamp et al. (2013). Floristic data were recorded and consolidated in a Microsoft Excel (2013) database. Vegetation-based data consisted of quantitative floristics (i.e. species counts) whereas biomass data consisted of aboveground primary productivity estimates per hectare. Data were extracted from the separate databases and formatted to be imported into various statistical software –Microsoft Excel (2013), Paleontological Statistics Software (PAST) (Hammer et al., 2001), STATISTICA version 13.3 (TIBCO Software Inc., 2017) and R version 4.1.0 (R Core Team, 2021). Existing soil physicochemical data was already consolidated in Microsoft Excel and analysed. Plant species that could not be identified in the field were collected for accurate identifications from specimens housed at the AP Goossens herbarium of the North-West University, Potchefstroom Campus.