Broad-leaved Savanna Research Articles

Savanna tree-grass interactions: A phenological investigation of green-up in relation to water availability over three seasons

Phenology of African savannas is considered to have high temporal variability, yet few studies have quantified this variation between seasons. This study assessed the weekly green-up phenology of trees, as well as below- and between-canopy grasses in a broad-leaved savanna woodland in the Nylsvley Nature Reserve (NNR), South Africa over three growing seasons (2012–2014). Tree green-up start dates were highly variable in comparison to the grasses, whose green-up showed close ties to the availability of water, particularly rainfall. Early green-up of Burkea africana trees occurred if rainfall onset was after mid-October, thus long-term rainfall records indicate that trees would benefit from early-greening approximately 46% of the time in the NNR. The effects of tree canopies on the growth and biomass accumulation of below-canopy grasses showed that during periods of irrigation when water was not limited, light availability became the limiting factor for grass growth, with grasses below the higher shading of B. africana producing significantly lower biomass than those below the less shaded Terminalia sericea canopies. Access to higher light conditions at the start of the growing season potentially drives the 2–5days faster green-up of below-canopy grasses compared to the between-canopy grasses. A comparison of the above phenological data to the remotely sensed normalized difference vegetation index (NDVI) was made, to determine if it was possible to detect an early-greening signal from the trees, which the sensor was able to effectively distinguish. This study highlights the variability in temporal separation between tree and grass phenology in an African savanna. Trees take advantage of periods of low competition from herbaceous neighbours at the end of the dry season prior to the onset of seasonal rainfall, and potentially at the end of the growing season when seasonal rainfall concludes through the uncoupling of their green-up cue from seasonal water availability.

No two are the same: Assessing variability in broad-leaved savanna tree phenology, with watering, from 2012 to 2014 at Nylsvley, South Africa

Ecologists and modellers are yet to fully understand and design accurate models simulating the unpredictable green-up of savanna ecosystems, possibly due to the lack of long-term, phenological monitoring of this biome. In this study, we observed the percentage of leaf age class (new, fully expanded and mature) phenology of two dominant, broad-leaved savanna trees at a weekly scale over the green-up period (August–November) and a monthly scale during the rest of the growing season (December–May) between August 2012 and May 2015 at the Nylsvley Nature Reserve, South Africa. We used an irrigation experiment, commencing in the dry season, to determine whether the simulation of a 20mm rainfall event for several weeks prior to the start of rainfall, could influence the leaf phenology of Burkea africana and Terminalia sericea. Each observed season experienced different environmental conditions from early onset average rainfall (2012), to late-onset high (2013) or low rainfall (2014). Irrigation of T. sericea in 2012 increased green-up rates and in combination with a late season fire during September 2013, resulted in a delayed green-up of the watered trees. Early-greening of B. africana was only observed during the 2014 season. Terminalia sericea showed faster maturation rates than B. africana which indicates the use of a facultative-greening strategy, while B. africana had faster green-up rates after the fire when competition for available nutrients was low. This study has demonstrated that using fine-scale phenological measurements can provide a clearer picture of how trees are greening up in the early stages of the green-up period and showcases the seasonal variability in savanna tree phenology. Investment into long-term phenological monitoring across southern Africa's savannas is needed if we intend to understand cues and predict phenology accurately in the future.

Online appendix 2:Vegetation of the eastern communal conservancies in Namibia: I. Phytosociological descriptions

The establishment of communal conservancies aims to have the local communities share in the benefits especially of wildlife resources, in this way spearheading the conservation of the environment. The Desert Margins Programme in Namibia aimed to develop vegetation resource data for the Otjituuo, Okamatapati, Ozonahi, African Wild Dog, Otjinene, Epukiro, Otjombinde, Omuramba Ua Mbinda, Eiseb and Ondjou communal conservancies, in order to assist with natural resource planning. For this purpose, a phytosociological survey of this area, with 422 relevés, was conducted during 2004. The data was captured in Turboveg and forms part of the Namibian phytosociological database (GIVD AF-NA-001). The data was split into two, representing two major land forms, the ‘hardeveld’ and the ‘sandveld’, respectively. A classification was undertaken using the Modified two-way indicator species analysis (TWINSPAN) procedure. Further refinements, based on field observations and literature sources, were performed using Cocktail procedures. Thirteen vegetation associations were formally described in this article, of which two were subdivided into subassociations. These associations can broadly be grouped into broad-leaved savanna types typical of the central and northern Kalahari of Namibia and microphyll savannas found on the transitions to the Central Plateau. One association, the Burkeo africanae–Pterocarpetum angolensis, forms the southern fringe of the Zambesian Baikiaea Woodlands ecoregion of the World Wildlife Fund, whilst all the other associations fall within the Kalahari Acacia–BaikiaeaWoodlands ecoregion. The Combreto collini–Terminalietum sericeae is the most widespread association and dominates the landscape. Threats to the vegetation include overutilisation and regular fires, both of which could easily lead to desertification. This threat is aggravated by global climate change.Conservation implications: This article described 13 plant associations of the central Kalahari in eastern Namibia, an area hitherto virtually unknown to science. The information presented in this article forms a baseline description, which can be used for future monitoring of the vegetation under communal land use.

Online appendix 1:Vegetation of the eastern communal conservancies in Namibia: I. Phytosociological descriptions

The establishment of communal conservancies aims to have the local communities share in the benefits especially of wildlife resources, in this way spearheading the conservation of the environment. The Desert Margins Programme in Namibia aimed to develop vegetation resource data for the Otjituuo, Okamatapati, Ozonahi, African Wild Dog, Otjinene, Epukiro, Otjombinde, Omuramba Ua Mbinda, Eiseb and Ondjou communal conservancies, in order to assist with natural resource planning. For this purpose, a phytosociological survey of this area, with 422 relevés, was conducted during 2004. The data was captured in Turboveg and forms part of the Namibian phytosociological database (GIVD AF-NA-001). The data was split into two, representing two major land forms, the ‘hardeveld’ and the ‘sandveld’, respectively. A classification was undertaken using the Modified two-way indicator species analysis (TWINSPAN) procedure. Further refinements, based on field observations and literature sources, were performed using Cocktail procedures. Thirteen vegetation associations were formally described in this article, of which two were subdivided into subassociations. These associations can broadly be grouped into broad-leaved savanna types typical of the central and northern Kalahari of Namibia and microphyll savannas found on the transitions to the Central Plateau. One association, the Burkeo africanae–Pterocarpetum angolensis, forms the southern fringe of the Zambesian Baikiaea Woodlands ecoregion of the World Wildlife Fund, whilst all the other associations fall within the Kalahari Acacia–BaikiaeaWoodlands ecoregion. The Combreto collini–Terminalietum sericeae is the most widespread association and dominates the landscape. Threats to the vegetation include overutilisation and regular fires, both of which could easily lead to desertification. This threat is aggravated by global climate change.Conservation implications: This article described 13 plant associations of the central Kalahari in eastern Namibia, an area hitherto virtually unknown to science. The information presented in this article forms a baseline description, which can be used for future monitoring of the vegetation under communal land use.

Holocene palynology and palaeoenvironments in the Savanna Biome at Tswaing Crater, central South Africa

A radiocarbon dated pollen and microscopic charred particle record from the Holocene section of the Tswaing Crater in the Savanna Biome of South Africa give new evidence for environmental changes during the period c. 9400–1800calyears BP. Pollen grains are scarce or absent in layers dating from before 9400calyears BP but deposits rich in pollen occur in overlying layers. The section dated between 9400calyears BP and c. 7200calyears BP contains grass, Asteraceae and dry savanna pollen types that suggest fluctuating but generally dry moisture conditions. Later between c. 7200 and 1800calyears BP, broad leaved savanna woodland elements and local swamp pollen indicate relatively stable vegetation and wetter mildly fluctuating climatic conditions, which is consistent with previously published biomarker analysis. Between c. 6200 and 5500calyears BP, the numbers of charred particles increased slightly. This indicates burning activity, which can probably be attributed to dry season ignition of denser fuel under relatively moist conditions. A decrease of local swamp pollen between c. 3600 and 3500calyears BP suggests that conditions became briefly drier again as pollen of woody elements declined in favour of open grassland pollen. A comparison between the Tswaing pollen profile and various other sequences within the central interior of South Africa suggests generally similar conditions over the central interior of the sub-continent during the Holocene deviating from sequences further afield along the coastal areas of southern Africa. Between 9400calyears BP and ca. 7200calyears BP, the western and southern coasts were probably controlled by different atmospheric and oceanic circulation regimes under the influence of a strong westerly winter–rain system while the north-eastern area where, Tswaing is situated, experienced weaker precession and less summer rain from the Intertropical Convergence Zone.

Vegetation of the eastern communal conservancies in Namibia: I. Phytosociological descriptions

The establishment of communal conservancies aims to have the local communities share in the benefits especially of wildlife resources, in this way spearheading the conservation of the environment. The Desert Margins Programme in Namibia aimed to develop vegetation resource data for the Otjituuo, Okamatapati, Ozonahi, African Wild Dog, Otjinene, Epukiro, Otjombinde, Omuramba Ua Mbinda, Eiseb and Ondjou communal conservancies, in order to assist with natural resource planning. For this purpose, a phytosociological survey of this area, with 422 relevés, was conducted during 2004. The data was captured in Turboveg and forms part of the Namibian phytosociological database (GIVD AF-NA-001). The data was split into two, representing two major land forms, the ‘hardeveld’ and the ‘sandveld’, respectively. A classification was undertaken using the Modified two-way indicator species analysis (TWINSPAN) procedure. Further refinements, based on field observations and literature sources, were performed using Cocktail procedures. Thirteen vegetation associations were formally described in this article, of which two were subdivided into subassociations. These associations can broadly be grouped into broad-leaved savanna types typical of the central and northern Kalahari of Namibia and microphyll savannas found on the transitions to the Central Plateau. One association, the Burkeo africanae–Pterocarpetum angolensis, forms the southern fringe of the Zambesian Baikiaea Woodlands ecoregion of the World Wildlife Fund, whilst all the other associations fall within the Kalahari Acacia–BaikiaeaWoodlands ecoregion. The Combreto collini–Terminalietum sericeae is the most widespread association and dominates the landscape. Threats to the vegetation include overutilisation and regular fires, both of which could easily lead to desertification. This threat is aggravated by global climate change.Conservation implications: This article described 13 plant associations of the central Kalahari in eastern Namibia, an area hitherto virtually unknown to science. The information presented in this article forms a baseline description, which can be used for future monitoring of the vegetation under communal land use.

Abstract tree crowns in 3D radiative transfer models: Impact on simulated open-canopy reflectances

Three-dimensional (3D) radiatives transfer models of vegetation canopies are increasingly used to study the reflective properties of specific land cover types and to interpret satellite-based remote sensing observations of such environments. In doing so, most 3D canopy reflectance models simplify the structural representation of individual tree crowns, for example, by using a single ellipsoidal envelope or a series of cubic volumes (known as voxels) to approximate the actual crown shape and the 3D distribution of scatterers therein. Often these tree abstractions ignore or simplify the woody architecture as well. Focusing on broad-leaved Savanna trees, this study investigates the impact that architectural simplifications may have on the fidelity of simulated satellite observations at the bottom of the atmosphere for a variety of spatial resolutions, spectral bands, as well as viewing and illumination geometries. The typical uncertainty associated with vicarious calibration efforts, i.e., 5%, is used as the quality objective for the simulated bidirectional reflectance factors (BRFs). Our results indicate that the size of the voxel as well as the spectral, viewing, and illumination conditions drive the BRF bias at a given spatial resolution. The simulation of remote sensing data at medium spatial resolution is not affected by canopy abstractions except in the near-infrared (NIR) for cases where woody structures are omitted. Here, the BRF simulations of the abstract tree crowns exceeded the 5% tolerance limit even at spatial resolutions coarser than 125m. For high-resolution satellite imagery, i.e., for nominal pixel sizes of 1×1m2 or finer, local BRF biases can be 10 times greater than the 5% tolerance criterion. Both positive and negative local biases are possible depending on the relative weights of the single-collided, single-uncollided, and multiple-collided BRF components.

Estimating leaf area distribution in savanna trees from terrestrial LiDAR measurements

Vegetation structure parameters are key elements in the study of ecosystem functioning and global scale ecosystemic interactions. The detailed retrieval of many of these parameters by direct measurements is impractical due to the quantity of plant material in trees. Terrestrial LiDAR Scanners (TLSs) have been shown to hold great potential as an indirect means of estimating plant structure parameters with a high level of detail, while some studies identified a number of challenges inherent to this approach. In this study we investigate the use of a voxel-based approach to retrieve leaf area distribution of individual trees. The approach is based on the contact frequency method applied to co-registered TLS returns from two or more scanning positions. The contact frequency was computed for voxels being 10, 30, and 50 cm in size and subsequently corrected for the influence of occlusion effects, leaf inclination, the presence of non-photosynthetic material, and the laser beam size. The leaf area of voxels for which occlusion effects were too pronounced was estimated using modeled values based on the availability of light. We compared the TLS derived leaf area estimates against direct measurements, obtained by the harvesting of leaves, in a broad-leaved savanna of central Mali. The measured leaf area values of the sampled trees ranged from 30 to 530 m 2, and crown LAI values between 0.8 and 7.2. The leaf area estimates lay on average 14% from the reference measurements (general bias). Our method provides vertical as well as radial distributions of leaf area in individual trees, and lends itself to the estimation of savanna vegetation structural parameters with a high level of detail.

The plant communities of the Andover Game Reserve, South Africa

Floristic characteristics of the Andover Game Reserve (AGR) were surveyed using an area-based survey technique and classified according to the data recorded from 88 relevés, using the PHYTOTAB-PC software package. Three plant communities, of which two each contain two variants, were described and mapped. The plant communities and their causative environmental factors were validated through detrended- and canonical correspondence multivariate analyses. The plant communities of the AGR were found to typify the floristics associated with the catenal sequences located in undulating areas on granite. Broad-leaved savanna is located at the crest and upper mid-slopes while fine-leaved savanna occurs along the footslopes of the AGR. Seeplines, a characteristic occurrence along catenas, are found at the transitional zone between the upper broad- and lower fine-leaved savanna plant communities. This study forms the basis for the compilation of a revised ecological management plan for the Andover Game Reserve.

The dynamics and trophic ecology of grasshoppers (Acridoidea) in a South African savanna : Trophic ecology of grasshoppers in South African savanna.

1. A grasshopper population was monitored for three seasons in broad leaved savanna in South Africa. The biomass at corresponding times in successive seasons varied by up to a factor of four. Climatic factors may have been responsible. 2. The energetics of the grasshopper population are described for one year of intensive study. In this year a mean grasshopper biomass of 0.73 kg ha-1 ingested 94 kg ha-1 of plant material. In addition, 36 kg ha-1 was wasted in the course of feeding. Grasshoppers removed 16% of grass production, but this dropped to about 7% in the following year. 3. Production efficiency and ecological efficiency were lower than those measured for grasshoppers in other systems. Reasons are suggested. 4. Grasshopper diets were examined by epidermal analysis of crop fragments. All grasshopper species examined fed from a wide range of plant species, but diets could be grouped according to the physical characteristics of the food items. There was limited dietary overlap between the whole grasshopper population and large grazing mammals. 5. Vegetation structure is more significant than species composition as a determinant of habitat selection within the savanna type.

Termites of a South African savanna : IV. Subterranean populations, mass determinations and biomass estimations.

Shallow soil cores taken every four weeks for a year in broad-leaved savanna and in Acacia savanna at Nylsvley Reserve, northern Transvaal, supported evidence on termite distribution and abundance reported in earlier papers. Surface activity was markedly affected by soil moisture, and the extent to which the core results can be used to assess subterranean populations is discussed. Live and dry masses for 14 species of termite are reported, and these results plus epigeal nest populations published elsewhere are used to calculate termite density and biomass in the two vegetation types. Tentative figures are 3,00 workers m-1 (=8 g m-1 fresh mass; 2.5 g m-1 dry mass) in broad-leaved savanna, and 725 workers m-1 (=1 g m-1 fresh mass; 0.3 g m-1 dry mass) in Acacia savanna. Excluding the mass of soil in the guts of soil-feeding species, these masses are reduced to 3 g m-1 (fresh) and 1 g m-1 (dry) in broad-leaved savanna, and 0.5 g m-1 (fresh) and 0.15 g m-1 (dry) in Acacia.

Termites of a South African savanna : III. Comparative attack on toilet roll baits in subhabitats.

Cellulose toilet roll baits were exposed in three different subhabitats at Nylsvley Reserve, northern Transvaal, and respective attacks by up to eight species of termites are described and illustrated. In broad-leaved savanna attack was principally by Microcerotermes, a slow feeder that attacked rolls throughout the year, finding about half the rolls exposed each season. In Acacia savanna attack was largely by Microtermes, a fast feeder active only at wetter times of year. In open, grassy areas there was relatively little attack, except sporadically by Macrotermes which consumed whole rolls within a day or two. Bush-fires delayed initial attack on rolls, perhaps because reduced surface litter gave less protection to soil. The amount of each roll eaten during the trials is shown in histograms; it was consistently greater in Acacia than in broad-leaved savanna. The soil-feeding termites Aganotermes and Promirotermes were attracted during the wet season to the undersides of rolls, perhaps as a source of organic-rich soil (or possibly of water).

Termites of a South African savanna : I. List of species and subhabitat preferences.

This study describes the termite fauna of the South African Savanna Ecosystem Project study area at Nylsvley Reserve, Naboomspruit, northern Transvaal. Twenty species of fifteen genera and two families are recorded, and biological notes are given. The faunal composition is broadly similar to that of West African savannas, but a little impoverished. Main deficiencies are in variety of Macrotermitinae and Trinervitermes; Termitinae are broadly comparable.Most of the species occur in the flatter areas of broad-leaved savanna; three are restricted to open land bordering a marsh. Steeply rocky broad-leaved savanna lacks two species but has two others restricted to it; Acacia patches (including sites of former human settlement) have a much reduced variety of species.

A phytosociological classification of the Nylsvley Nature Reserve

The vegetation of the Nylsvley Nature Reserve in the Transvaal Mixed Bushveld is classified hierarchically by the Braun-Blanquet Method of vegetation survey. The vegetation is seasonal grassland and deciduous savanna with four floristically distinct major groups of plant communities: (I) grasslands and broad-leaved savannas on non calcareous sandy soils on elevated sandstone and felsite areas; (2) microphyllous thorn savannas on calcareous, clayey, bottomland alluvial soils and termitaria thickets; (3) grassland and thorn savanna on calcareous self-mulching vertic soils; and (4) secondary communities on long abandoned native settlements and recently ploughed land. Seven primary communities with 12 community variations and 4 subvariations, and three secondary communities are described on the basis of 216 releves. The survey was carried out at two levels of detail, an ecosystem study area in the broad-leaved savanna being surveyed in more detail, floristically and structurally, than the rest of the Reserve.