Spatial heterogeneity is the unequal distribution of landscape features and consists of diversity in vegetation structure, number and size of woody plants, patchiness in grass cover, sub-canopy habitats, etc. A granite catena (hillslope) comprises of a gradient of soils, hydrology patterns and vegetation composition, creating a spatially heterogeneous area with variety in animal habitats. Objectives were to determine small-scale spatial heterogeneity along a catena near Skukuza, such as vegetation structure, patchiness, size and cover of woody and grass components, to describe certain catenal processes. Tree sizes and canopy cover were measured and the point method used on seven 100 m transects representing different catenal zones. Grasses were categorised according to grazing value, ecological status and percentage shade tolerant grasses. A total of 155 tree canopies were present. Large trees (> 5 m) occurred in riparian zone and upper midslope, but were low in number (< 4 per transect). Woody plants ranged in number from 8 to 32, canopy cover 4.5% – 33.6%, and grass cover from 47% to 69% between zones. A strong correlation was found between canopy cover and shade-tolerant grasses. Size of sub-canopy habitats are mostly determined by size of woody plants and both are important to animals. Various factors related to vegetation contributed to heterogeneity and spatial stratification patterns of the catena ecosystem.
Concerns about the decline in tree numbers inside Kruger National Park are addressed. Mammal habitats and plant communities are impacted by the decline. The research can be linked to the long-term exclosure studies on granites at Nkuhlu.
Spatial patterns in plant communities, including vegetation structure, patchiness and density, as well as the factors that generate them, have been receiving increased attention from ecologists (Adjorlolo & Mutanga
A landscape with spatial heterogeneity [
Several spatially stratified heterogeneous phenomena can be described, such as vertical layering of plants in an environment; differences in population densities between areas, ecological zones or climates; and distribution of soil types, land cover and land use (Pickett, Cadenasso & Benning
Local spatial patterning forms part of the description of spatial heterogeneity and is associated with alternation between vegetated and bare areas that commonly occur in savanna systems. This variability and patchiness of vegetation cover creates different habitats for wildlife species (Augustine
Vegetation structure and plant species composition play an important role in the suitability of a habitat to animals. This is because different animal species are sensitive to the size of vegetated and bare patches in the landscape, and this makes the state of the spatial heterogeneity (i.e. patchiness) a valuable indicator of the suitability of a habitat to animals (Turner
Many savanna systems across the world have undergone some dramatic changes in terms of tree abundance (Bond
Adjorlolo and Mutanga (
The granite landscape in the south of KNP is finely dissected with a high density of streams and hillslopes or catenas (Smit et al.
Soil, hydrology and vegetation form the foundation of the catena ecosystem; therefore, it is important to give an accurate description of these. This study forms part of a bigger study presented in this Special Issue where multidisciplinary research fields are focussed on the same local study area (small area) with the ultimate goal of finding links and processes that can drive the catena functioning. The multidisciplinary research has been conducted on a small scale to provide a basis for further studies in the same area, or to be expanded to the larger Southern Granite Supersite, or to be used as comparison for similar studies.
This study is an extension of the vegetation studies that formed part of the multidisciplinary research and expands on the description of plant communities along the environmental gradient (from top to bottom of the hillslope) done in the study area (Theron et al.
Supersites were established in 2013 to give researchers an opportunity to focus research geographically in specific areas of KNP and to allow long-term monitoring and integration of data across different research themes on specific large study areas with similar climate and geology (Smit et al.
The study focussed on one catena (or hillslope) from the crest down to the third order drainage line (or watercourse) in the Sabie River catchment. A Global Positioning System (GPS) was used to indicate the highest point (370 m above sea level [m.a.s.l.]) that was taken as the crest. The lowest point was inside the drainage line at 354 m m.a.s.l. It is not a steep slope (1%), with a gradual drop of 16 m over a 200 m length. A seepline was present in the transitional area between the upper midslope and the sodic patch on the lower midslope. This formed a green grass belt that could clearly be differentiated from the rest of the sodic patch during the time of study. The riparian zone contained a small floodplain and the banks of the drainage line. Jacobs and Naiman (
The vegetation on the hillslopes is generally described as moderately dense bush or shrub savanna, with a riverine forest at the valley bottom in areas closer to the river (Smit et al.
The study followed on the vegetation classification surveys (description of plant communities) that were conducted as part of a bigger multidisciplinary research project in the same study area during 2015. Line transects of this study were placed in the length of the catenal zones, in other words, perpendicular to the single long belt transect that ran from the crest down to the drainage line, that was used for the description of the vegetation gradient from top to bottom of the catena by Theron et al. (
Two measurements were taken to describe the canopy of trees and shrubs, where crown refers to an individual plant and canopy refers to more crowns together in the same area. The first measurement was taken for the widest part of the crown (if the largest part of that crown fell inside the transect). This was done to describe maximum crown width of individual plants. An adapted Biomass Estimates of Canopy Volume (BECVOL) method (Smit
The point method was used to determine grass cover and species composition (Evans & Love
Woody plants were grouped into different height classes based on about 1 m increments, that is, 0 m – 0.9 m, 1 m – 1.9 m, 2 m – 2.9 m, etc., up to 5 m and then the trees higher than 5 m were grouped together for each transect. The number of species per transect, as well as the number of individual woody plants for each height class, was counted and tabulated. A difference was made between rooted woody plants where the trunk was within the transect (3.5 m × 100 m) and woody plants that were not rooted in the transect but their canopies covered part of the transect. Similar size classes were used for categorising the maximum crown width of woody plants (where the broadest part was measured). This differs from the more commonly used average crown spread where the longest and shortest extent of the crown is measured and divided by two (Blozan
The number of points where a grass tuft (crown or shoot) touched the metal rod with the point method (hits) was totalled to indicate the grass cover of living grasses, while points with ‘no hit’ were divided into nearest grass (30 cm radius) and bare soil (with no grass present in a 30 cm radius of the rod). Grass species were grouped into different categories based on their grazing value, plant succession stage and ecological status, following Van Oudshoorn (
Ethical approval for the multidisciplinary project as a whole was obtained from the Interfaculty Animal Ethics Committee at the University of the Free State (UFS-AED2019/0121).
A total number of 155 woody plant canopies partially covered the seven transects (
Numbers of individual woody plants and woody species present in each transect (3.5 m × 100 m). The total number of woody plants indicates all individuals where the crown only, or crown and trunk (rooted), was within the transect. Canopy cover and the number of plants directly on the line were indicated for all woody species where the crown covered the 100 m measuring tape. The number of grass species and percentage of grasses that grow in the shade, determined by hits of the point method on the 100 m line, are also indicated.
Catenal zone | Total nr woody plants in transect | Nr woody species in transect | Nr woody plants on 100 m line | Canopy cover (%) on 100 m line | Nr of grass species on 100 m line | Shade grasses (%) on 100 m line |
---|---|---|---|---|---|---|
Crest | 26 | 9 | 14 | 17.9 | 15 | 28 |
Upper midslope | 23 | 8 | 10 | 10.5 | 16 | 26 |
Above sodic site | 25 | 12 | 8 | 21.5 | 12 | 45 |
Upper sodic site | 10 | 6 | 5 | 4.5 | 7 | 18 |
Lower sodic site | 8 | 6 | 3 | 2.7 | 9 | 20 |
Shrub veld | 31 | 12 | 13 | 17.5 | 8 | 39 |
Riparian zone | 32 | 13 | 11 | 33.6 | 11 | 55 |
All height classes of the woody plants (from < 1 m to > 5 m) were represented on the crest and upper midslope of the catena (
The number of woody plants (different species) in each height class for a transect that represents the following catenal zones: (a) crest, (b) upper midslope, (c) above sodic site, (d) sodic patch, (e) shrub veld and (f) riparian zone in the granite study area. The sodic patch was divided into an upper and a lower site, but both are indicated separately on the same graph.
The maximum crown width of individual woody plants that were grouped into the mentioned size classes is indicated in
The numbers of woody plants with maximum crown width in a specific size range are indicated for each transect that represents a catenal zone.
Grass cover ranged from 47% to 69% during 2015 in different catenal zones of the study area, with the lowest cover in the riparian zone on the banks of the dry drainage line and on the lower midslope sodic site (
Comparison between two years of grass cover in the exact same locations in each catenal zone. It is based on point method data on a 100 m line transect. If the crown (base) or shoot of a grass tuft was touched by a metal rod, it was indicated as grass cover. If no tuft was touched by the rod, the nearest grass (30 cm radius) was noted, and if no living grass was within the radius, it was noted as bare soil or dead grass, respectively. No grass was noted as bare soil or dead grass, respectively. Forbs were not included.
Percentage of grass tufts of a certain size touched by the metal rod (hit) of the point method (100 points per line) at the end of the growing season of 2016, during the drought. The percentage of dead grass and bare soil (no hit) was also included.
Catenal zone | Description | Percentage cover (%) |
||||||
---|---|---|---|---|---|---|---|---|
Small tuft < 3 cm across, 2 cm high | Short < 10 cm across, grazed down to 1–3 cm | Medium, grazed to 5–15 cm height | Tall, grazed to 30 cm or higher | Total living grasses | Dead grass | Bare soil - no nearest plant in 30 cm | ||
Upper midslope | Grass tuft | 2 | 12 | 1 | 1 | 15 | - | |
Nearest grass | 4 | 14 | 5 | 1 | 9 | - | ||
No grass | - | - | - | - | - | - | 36 | |
Upper and Lower midslope transition | Grass tuft | 3 | 12 | 6 | 2 | 13 | - | |
Nearest grass | 5 | 12 | 6 | 2 | 14 | - | ||
No grass | - | - | - | - | - | - | 25 | |
Sodic patch on lower midslope | Grass tuft | 5 | 11 | 7 | 1 | 13 | - | |
Nearest grass | 7 | 10 | 5 | 0 | 18 | - | ||
No grass | - | - | - | - | - | - | 23 | |
Shrub veld on footslope | Grass tuft | 0 | 13 | 16 | 3 | 39 | - | |
Nearest grass | 0 | 3 | 1 | 0 | 18 | - | ||
No grass | - | - | - | - | - | - | 7 | |
Riparian area at drainage line | Grass tuft | 6 | 12 | 8 | 1 | 14 | - | |
Nearest grass | 5 | 10 | 2 | 1 | 13 | - | ||
No grass | - | - | - | - | - | - | 28 |
About 58% – 74% of the grass species present on the upper midslope transects during 2015 were of high grazing value (
Contribution (%) of grass species present on 100 m point method line to different categories for each catenal zone.
Catenal zone | Grazing value Low (%) | Grazing value Average | Grazing value High | Decreaser | Increaser I | Increaser II | Increaser III | Pioneer | Subclimax | Climax |
---|---|---|---|---|---|---|---|---|---|---|
Crest | 26 | 7 | 67 | 66 | 0 | 34 | 0 | 20 | 14 | 66 |
Upper midslope | 39 | 3 | 58 | 57 | 1 | 40 | 2 | 12 | 27 | 61 |
Above sodic site | 26 | 0 | 74 | 70 | 0 | 26 | 4 | 8 | 18 | 74 |
Upper sodic site | 17 | 45 | 38 | 1 | 0 | 99 | 0 | 74 | 25 | 1 |
Lower sodic site | 21 | 42 | 37 | 2 | 0 | 98 | 0 | 57 | 24 | 19 |
Shrub veld | 16 | 8 | 76 | 72 | 13 | 15 | 0 | 0 | 15 | 85 |
Riparian zone | 19 | 21 | 60 | 48 | 0 | 52 | 0 | 32 | 20 | 48 |
A catena is a dynamic arrangement of vegetation and localised soils in a gradient pattern resulting from geological features, hydrology and topography that cause apparent irregularities in vegetation distribution (Emmet & Pattrick
A total of 137 individual woody plants were rooted in the seven transects covering the four catenal zones during the vegetation survey. Several trees were uprooted by elephants (
There have been some concerns from conservationists and researchers about the general decline in the number of trees inside KNP over the past 50 – 75 years, especially in the large tree component (> 5 m tall) (Druce et al.
Large trees might be low in numbers in the study area, but woody cover in general seems to be increasing on the granite catenas over time and that may change the vegetation structure normally associated with these areas. Eckhardt et al. (
Herbivores have a definite effect on spatial patterns involving vegetation structure, diversity and cover (Augustine
Canopy cover creates patchiness in the environment in the form of sub-habitats, each with its own microclimate underneath the canopy, and areas that are not covered (Jennings et al.
Canopy cover varied from 11% to 18% on the upper midslope transects, while 18% and 34% was calculated for the shrub veld and riparian zone transects, respectively (
The composition of grass species usually differs significantly between canopied and open areas, thus affecting the vegetation structure and patterning (Riginos et al.
Environmental conditions such as different soil properties in various areas, namely, wetness, leaching, clay or sand content, available nutrients, acidity (pH), etc. (Bouwer et al.
The presence or absence of grasses contributes to vegetation cover and thus to the patchiness (bare and vegetated areas) associated with heterogeneity of the catena. Grass tufts provided cover to the soil during 2015, but in 2016 many tufts were dead, possibly as a result of extreme heat, low rainfall and animal grazing during the intensifying drought conditions (
A decline in grass cover was observed in the study area during the drought, mostly because of the climate and the impact of grazing on top of that, and this lowered the heterogeneity in the patchiness of the area by creating large bare areas and reducing vegetated areas. The effect of the drought (Van Aardt et al.
Vegetation patchiness is common in the savannas and can be observed at specific scales, from landscape scale to individual tree or grass scale. A key driver of savanna ecosystem structure and functioning is the impact of large herbivores. Large herbivores alter standing biomass, woody and herbaceous diversity, and soil characteristics that form part of the heterogeneity in the structure of plant communities. Herbivory can also act as a disturbance by reducing the biomass and canopy cover of specific species and by increasing the spatial heterogeneity in the process (Jacobs & Naiman
This study focussed on reporting the spatial heterogeneity of the catena vegetation structure and its patchy vegetation cover. Different zones are created on the catena because of soil types and characteristics that differ from the crest to the drainage line, and this causes alterations in plant species composition, vegetation structure and patterns in the zones. Vegetation structure ranged between the zones, from a high density of woody plants (including different height classes and crown widths) and a high grass cover to a low density of woody plants (where they were almost absent) and low grass cover. The number of woody plants in different height classes also differed between the upper midslope, sodic site, shrub veld and riparian zone, creating a variation in vertical layering on the catena. Patchiness was observed in alternating areas of different vegetation cover and bare areas that also led to certain patterns in the vegetation. Canopy cover increased the patterning and patchiness of vegetation by creating sub-habitats under the canopies that differed from the inter-canopy open areas. The drought caused changes in the distribution of patterns in the vegetation on the catena by increasing the bare areas to most of the catena, reducing the vegetated areas, and the spatial heterogeneity in the process. Herbivores (including elephants) can impact the observed spatial heterogeneity of the area by changing the vegetation structure and vertical layering (felling, removing and breaking woody plants), species composition and density (feeding more on palatable plants and opening up the vegetation), plant cover and patchiness (intensive grazing in higher nutritional areas and by trampling) and thus also impacting the patterns observed in the vegetation on the catena.
Spatial heterogeneity was found between the catenal zones as differences in the number of woody plants present, various sizes of trees and shrubs, grass cover (bare areas and vegetated areas), patchiness, sub-canopy habitats, etc. Large trees were scarce in the study area and mainly medium-to-small woody plants were present in the vegetation structure. A positive correlation was found between canopy cover and the percentage of shade-tolerant grasses that impact sub-canopy habitats and vegetation patchiness. Significant differences in grass cover were found between the two survey years, with larger bare areas noted in the drought year. Various factors from the literature that could have contributed to the heterogeneity and spatial stratification patterns of the catena ecosystem were mentioned. Some of these factors also play a role in the processes and functioning of the catena ecosystem, such as the nutrient cycling and the impact of animal presence on the habitat and sub-habitats created by the different vegetation structures. This small-scale vegetation structure study was used successfully to describe specific aspects of spatial heterogeneity on the catena and to provide information that can be used as a potential warning system for changes in vegetation structure with regard to specifically the low numbers of large trees. This data can be used by management as an indicator to identify when the study area will reach a Threshold of Potential Concern (TPC) for woody vegetation structure and cover.
The author wants to thank the late Fred Kruger from the Organisation for Tropical Studies and the Centre for Environmental Management at the University of the Free State for valuable input into this project and especially for his assistance with fieldwork for this article. The author would also like to thank the following persons and institutions: The University of the Free State Strategic Research Fund for largely funding the multi-disciplinary project; the National Research Foundation for partially funding the research in this article; the staff of SANParks Scientific Services for their friendliness and all the administrative arrangements; Martin Tinneveld (student) for field assistance; and Dr Tascha Vos for all her time and effort with table and figure formatting.
The author declares that she has no financial or personal relationships that may have inappropriately influenced her in writing this article.
B.B.J. is the sole author of this research article.
The University of the Free State Strategic Research Fund largely funded the multi-disciplinary project as a whole, including this part of the study, and the National Research Foundation Thuthuka Grant also partially funded this research.
Data from all research done within Kruger National Park is placed within the SANParks repository (not for free, open access).
The views and opinions expressed in this article are the author’s own and do not necessarily reflect the official policy or position of the institution or funder.