OpenUCT :: Browsing by Author "February, Edmund C"

Browsing by Author "February, Edmund C"

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Open Access
An alternative hypothesis for explaining anomalies in the fine scale distribution patterns of Colophospermum mopane : Are shrub and tree forms genetically distinct ?
(2004) Hempson, Gareth; Verboom, George Anthony; February, Edmund C
Abiotic and top down control hypotheses do not adequately explain the fine scale distribution patterns of shrub and tree Colophospermum mopane (Caesalpinioideae). Genetic distinctiveness between growth forms is investigated as an alternative hypothesis. Tree and shrub C. mopane from the riparian and inland savanna zones were sampled at four sites in the northern Kruger National Park. Molecular DNA sequences were obtained for four plastid and one nuclear region, and the inter simple sequence repeat (ISSR) technique used to fingerprint individuals. Very low levels of sequence divergence were observed. The ISSR technique revealed no genetic structure between plants when grouped by growth form or by habitat in an analysis of molecular variance (AMOV A). Soil profile and xylem pressure potential data also did not explain the distribution of growth forms. A principle component analysis and a discriminant analysis of five leaf and branching characters identified a significant difference in the shape of shrub and tree C. mopane leaves. It is concluded that the C. mopane growth forms are not genetically distinct and that their fine scale distributions may be due to top down controls such as large mammals and fire.
Open Access
Analysis of long-term changes in populations of the Clanwiliam Cedar (Widdringtonia cedarbergensis) using repeat photography
(2013) White, Joseph Douglas Mandla; Hoffman, Timm; Jack , Sam; February, Edmund C
Repeat photography photosets were used to analyse 20th century changes in populations of Widdringtonia cedarbergensis from four sites in the Cederberg Mountains, South Africa. Systematic evaluation of 115 photosets was combined with field observation and environmental data to determine the age class and degree of foliage cover, as well as fire frequency, rockiness of habitat, altitude, aspect of slope, annual mean temperature and annual precipitation for each tree viewed in photosets. Mortality and recruitment events were documented in each of the photosets. A total of 1315 trees were recorded in historical photographs (1931- 1982), with 968 of these shown to have died in the repeat photographs (2007-2013), indicating 74% mortality. With only 45 (3.4%) recruits being recorded, the total current living population of W. cedarbergensis in the repeat photographs was 392. There was no significant difference found in mortality or recruitment across the sampled sites. From the historical to repeat photosets there was an 8% increase in the proportion of mature adults and a 13% increase in the proportion of W. cedarbergensis individuals with sparse foliage cover. A generalized linear model was used to determine the effects of environmental factors on W. cedarbergensis mortality in natural populations.
Open Access
Another look at the savannah conundrum : evidence of niche segregation and competition avoidance with grasses in establishing trees
(2014) Lewis, Joel Robin; February, Edmund C
The savannah vegetation, although iconic, is a curiosity due to the coexistence of the two dominant, but remarkably different life-forms: trees and grasses. However, despite an apparent competitive advantage grasses do not dominate over trees in these systems and this presents a conundrum: how are savannah trees are able to establish in the face of such strong competition from grasses? As this is when trees are most susceptible to competition from grasses. This makes the knowledge of establishment phase critical for understanding the tree-grass co-existence. Despite this, there is a paucity of knowledge of this phase and the long-term aspects of the competitive interactions between trees and grasses, where the focus should be on seedling establishment and survival rather than biomass accumulation This study investigated the natural levels of tree establishment amongst differing amounts of grass as we sought to identify the limitation, both aboveground for light and belowground for resources, imposed by grasses on tree establishment. To uncover this limitation we searched for naturally established Acacia nigrescens (the dominant savannah tree species in our research area) seedlings at 12 sites in the Limpopo Province, South Africa. We also collected data on seedling health to determine the effect of proximate grass canopy on their performance. Finally, we compared the root lengths of established seedlings and co-occurring grasses to determine if seedlings were rooted in the same soil horizons as grasses. We hypothesised that tree establishment would be facilitated by low levels of grass biomass, as the lower the grass biomass, the more gaps would be present in the grass layer for seedlings to establish. We found that establishing seedlings were more resilient to grass competition than had previously been appreciated, as we found established seedlings (at a density of 9 seedlings per 100m2) with a grass canopy cover of 65%. We also found no correlation between our measure of grass competition and seedling performance. Finally, we found that even very young tree seedlings consistently root deeper than co-occurring grasses, despite establishing on different geologies, soils and across a rainfall gradient (p < 0.0001) - demonstrating that this resilience to grass competition is due to a specialised rooting strategy present in seedlings that allows them avoid belowground competition with grasses. In light of this, we propose a dynamic two-niche hypothesis of savannah tree-grass co-existence.
Open Access
Anthropogenic modification of the natural fire landscape and its consequences for vegetation patterns on the Cape Peninsula
(2017) Rogers, Annabelle J; February, Edmund C; Moncrieff, Glenn R; Slingsby, Jasper A
Understanding the spatial probability of fire and how urban development may alter natural patterns is particularly important in areas where alternate ecosystem states occur at fine spatial scales. The Cape Peninsula, South Africa, is a one such region where fire-sensitive forest patches occur interspersed in a sea of fire-dependent fynbos. Fire is believed to be an important determinate of forest distribution, with absence or occurrence of fires potentially allowing patch contraction and expansion. In this thesis I use a series of computer models to determine the extent to which anthropogenic development and land transformation have altered the spatial variation in fire likelihood, or the 'burn probability', and its consequence for the distribution of forest on the Cape Peninsula. The two multi-model, fire behaviour simulation systems I use are FlamMap and FARSITE. FARSITE is a deterministic simulation package used globally for discrete event simulation. In an effort to assess the viability of using the FARSITE model for fire prediction in fynbos and the determinants of model accuracy, I predicted fire area for a historical fire on the Cape Peninsula using a variety of fuel models and wind conditions. Following this validation, FlamMap was used to simulate the burn probability of the Cape Peninsula under natural conditions – no urban development present – and transformed conditions – where urban areas mapped as non-burnable fuel models. I then determined changes in forest distribution documented over the last 50 years relative to changes in burn probability as a result of urbanisation. My results show that an increase in urbanisation on the Cape Flats has produced a significant urban shadow effect due to the interruption of natural fire catchments. This urban shadow effect has resulted in an overall increase in area of fire refuges on the Peninsula and expansion of forest, particularly on the more mesic eastern slopes at Kirstenbosch and Newlands. The results strongly support that urban-mediated changes to fire patterns are drivers of forest expansion in this region, and adds further evidence to support the significance of fire in determining biome boundaries in the fynbos.
Open Access
Branch junction constriction and hydraulic limitation in two species in the Cape Proteaceae : a mechanism explaining the trade-off between longevity and degree of ramification in the Cape Proteaceae
(2004) Slingsby, Jasper; February, Edmund C
The purpose of this study was to establish if hydraulic limitation and branch junction constriction explain the trade-off between longevity and increased ramification in species of the Cape Proteaceae. This was done by establishing if branch nodes are regions of greater resistance to xylem sap flow in the study species; attempting to quantify nodal resistance for a plant as a whole; and by determining if there is any evidence for hydraulic limitation or compensation in Leucadendron laureolum (Lam.) Fourc. and Leucospermum oleifolium (Berg.) R. Br. individuals. The extent of branch junction constriction was established by comparing the ratio of the xylem sap flow rates of adjacent branch segments of equal length. The ratio of a proximal intemodal to a distal intemodal segment was compared against the ratio of a proximal intemodal segment to a distal node. Branch nodes were found to create significant resistance to xylem sap flow rates in both species (p < .005 for L. laureolum, and p < .05 for L. oleifolium). Total nodal resistance in L. laureolum was modelled as equivalent extra branch length, and as loss of sap flow rate. Equivalent extra branch length increased exponentially with increased branching order, and increased much faster in more ramified plants. Loss of flow rate increased linearly with increased branching order and was unaffected by differences in ramification. This was because the nodes in more ramified plants had smaller basal sapwood areas, and the resistance imposed by nodes decreased with decreased basal sapwood area. As more ramified plants tend to branch earlier, and/or with greater temporal frequency, they will still accumulate greater nodal hydraulic resistance faster than less ramified plants, and thus may be limited to smaller size and younger maximum age. Branch specific conductivities (kₛ), leaf specific conductivities (LSC) and transpiration rates were investigated in higher (11 nodes) and lower (10 nodes) order branches of L. laureolum. All were found to be significantly lower in higher order branches (11 nodes), which can be related to greater nodal resistance to sap flow limiting the amount of water available to the area of developing xylem, causing the vessels to have narrower diameters. There was no significant difference in leaf to sapwood area ratios (Aₗ/As) between the two branching orders. Previous studies have suggested that changes in Aₗ/Aₛratios occur to compensate for hydraulic limitation. Reductions in Aₗ/Aₛin plants with greater hydraulic restriction allow LSC values, and thus transpiration rates, to stay as high as plants with less hydraulic restriction. As there was no such compensation, LSC values, and thus transpiration rates, were much lower in branches of higher order. Transpiration rate is a good indication of rates of photosynthesis. Lower transpiration rate in higher order branches is thus a good indication of hydraulic limitation on rates of photosynthesis with increased branching order. Hydraulic resistance in nodes thus imposes an important limitation on size and/or age of L. laureolum individuals, and explains the apparent trade-off between longevity and degree of ramification in the Cape Proteaceae.
Open Access
Can the pathway of abalone aquaculture effluent be traced using carbon and nitrogen stable isotope analysis of kelp Ecklonia maxima and mussels Mytilus galloprovincialis?
(2007) Thomas, Alicia; Bolton, John J; Anderson, Robert J; February, Edmund C
Abalone farms are becoming more common along the coast of South Africa. The effects these farms are having on the coastal habitats they occupy are becoming of increasing concern due to the possible negative effects of the effluent discharged from the farm. This study was conducted along the coast of Jacobsbaai (32°58'22.09" Sand 17°53'10.56" E) and Mauritzbaai (32°58'50.75"S and 17°52'59.44"E) near Saldanha on the South African west coast, situated approximately 120 Km from Cape Town. The aim of this study is to determine if the outflow of effluent from the Jacobsbaai Sea Products (pty) Ltd, abalone farm in Jacobsbaai can be traced using stable carbon and nitrogen isotopes. Kelp (Ecklonia maxima) and blue mussels (Mytilus galloprovincialis) along the coast near the farm (in Jacobsbaai) were collected from 8 sites and used as tracers of the farms effluent. A ninth site was sample in another bay (Mauritzbaai) which functioned as our control. We hypothesised that if the kelp and mussels are taking up the nitrogen and carbon from the farm, the amount of nitrogen and carbon taken up would decrease with decreasing distance from the effluent outfall. Using this we could map the geographic distribution of the abalone effluent alone the coast to measure the extent of eutrophication due to abalone effluent. Our results suggest that the effluent from the farm is having a localized effect on both Ecklonia maxima and Mytilus galloprovincialis.
Open Access
Changes in abundance and distribution of Protea caffra in the central and northern Drakensberg as a consequence of climate and land use change
(2014) Poultney, Daniel Mika-Nsimbi; Hoffman, Timm; February, Edmund C; Rebelo, Anthony G; Puttick, James; Jack,Sam
The study investigated changes in populations of Protea caffra at five sites in the central and northern regions of the uKhlahlamba Drakensberg Park, a UNESCO world heritage site. Based on an archaeological study in 1994 that used charcoal remains to compare the size of Protea species populations in the Drakensberg to a much earlier era (between 1260 to 1880 BP), it was hypothesized that there would be a further decline in the number of individuals. A total of 76 historical photographs (dated from 1950-1995) were re-photographed in 2014 and the number of individual P. caffra plants in the two sets of photographs counted to assess the extent and rate of population change. The landform units were demarcated on each photograph according to their catenal position, aspect, elevation and the extent of rocky cover. Geological parameters for each landform unit were obtained from GoogleEarth, climatic data from the South African Atlas of Climatology and Agrohydrology and fire data from MODIS. In 76% of the landform units there was an increase in the number of individuals over time, in 17% a decrease and in 7% there was no change over time. Several hypotheses were generated from a conceptual model based on ecological insight into the study area to explain change in the abundance and distribution of P. caffra. Using an Akaike test, the input models were ranked according to how representative the explanatory variables were of the observed change. The model with aspect and elevation as explanatory variables was ranked the best predictor of change. A greater proportion of increase in the size of the population was found on lower slopes. With increasing atmospheric CO2, P. caffra on the warmer lower slopes are likely to have increased photosynthetic rates and increased productivity. A greater proportion of landform units showing a decrease in P. caffra was shown at higher elevations. The reason could be higher fire intensity at higher elevations. The variation in change influenced by fire suggests it is an important regulator of P. caffra numbers. The fact that there is no relationship between fire frequency and landform units showing an increase in P. caffra implies that the majority increase in the species is more likely to be driven by climatic changes, i.e. increases in atmospheric CO2 rather than a change in the fire regime. Increasing atmospheric CO2 levels lead to enhanced growth rates, postburn recovery and resprouting of tree species in grasslands and savannas, which could account for the majority increase in P. caffra populations in the grassland of the Drakensberg. This has important conservation management implications for the reserve, in that the current fire regimes do not appear to be having a negative impact on the abundance in P. caffra. Further, fire will continue to be an important management tool in maintaining the vegetation structure and grasstree codominance in the reserve.
Open Access
A dendrochronological investigation of Pinus radiata from Silvermine Nature Reserve
(2002) Midgley, John Claude; February, Edmund C
Whole trunk cross sections of Pinus radiata were collected from Silvermine Nature Reserve, Cape Town. The site was located on a hill in the nature reserve where precipitation was assumed to be a limiting factor in tree growth. Ring widths along 82 radii on 41 samples were measured to the closest 0.1 mm. Samples displaying the highest correlation between ring width values were selected to create a growth index chronology for the site. The resulting chronology was compared to various aspects of precipitation and temperature. Correlation coefficients between tree growth and annual, winter and growth season precipitation levels, minimum and maximum growth season temperatures, mean summer and mean winter temperatures were calculated with the use of a regression analysis. None of the correlation coefficients were above 0.1 , leading to the conclusion that the growth of trees was possibly disturbed by humans while the stand was under Cape Nature Conservation management. An alternative explanation could be that the height at which trees were sampled is likely to cause lowered correlation between samples as proposed by Fletcher (1974b). Although it is not statistically significant, trees displayed similar patterns to precipitation levels since 1955. The conclusion of this study is that Pinus radiata could provide a useful proxy record of climate in South Africa, but an undisturbed site where trees are limited by precipitation needs to be located before attempting the development of another ring index chronology.
Open Access
Disturbance ecology and size class structure of the Mulanje cedar of Malawi, Widdringtonia whytei and associated broadleaved forest
(2010) Burger, Niel; February, Edmund C
The disturbance ecology and regeneration patterns of the emergent conifer, Widdringtonia whytei, and five broadleaved species, Aphloia theiformis, Rapanea melanophloeos, Maesa lanceolata, Maytenus accuminata and Psychotria mahonii, were inferred from population size structures in the mixed conifer-broadleaved forests of Mt. Mulanje, Malawi. The size-class structures of the emergent cedar populations were characterised by even-sized, disjunct frequency distributions. Seedling recruitment was found in recently burnt sites and not in middle-aged or oldgrowth forest. This indicates reliance on the catastrophic mode of regeneration for W. whytei, which takes advantage of the well-lit, competition free environment after large-scale disturbances provided by fire. The sub-canopy angiosperms primarily had all-sized frequency distributions, with at least some individuals found in each class within their size range, indicating continuous regeneration under a closed forest canopy. These results emphasize the dependence of long-lived conifers such as the Mulanje cedar on large-scale disturbance for regeneration and long-term persistence of the species.
Open Access
Exploration of climate mediated decline in a critically endangered southern hemisphere conifer over the last 40 years / Annabelle J. Rogers
(2013) Rogers, Annabelle J; February, Edmund C; Verboom, Tony
Widdringtonia cedarbergensis is a critically endangered conifer restricted entirely to the Cederberg Mountain range in the Western Cape of South Africa. This study aimed to assess whether contemporary climate change could be a driver in the recently documented tenfold decline of W. cedarbergensis in the last 40 years. By building distribution models using Maxent bioclimatic modelling for pre- and post- 1970â€™s point occurrence data, the distribution of the species pre- and post-1970â€™s were compared and assessed for significant change. Models predicted a clear range contraction in W. cedarbergensis from pre- to post- 1970. The present model predictions of suitability occur in a tight altitudinal band on the mid to upper slopes of the middle Cederberg (minimum of 1048m to a maximum of 1530m). However, the climatic predictions associate the present model distribution with relatively warmer and drier areas compared to that of the pre- 1970 model, suggesting a shift down slope rather than upslope. This is possibly due to the exaggeration of the upper limit of distribution for the pre-1970â€™s model that predicts W. cedarbergensis to occur as high as 1850m. This is unlikely due to the high prevalence of frost at this elevation. Though any climatic signal was complicated due to historical data error, high confidence in the present model prediction adds a valuable contribution to a body of literature that documents the decline of W. cedarbergensis and has important implications for conservation management of the species.
Open Access
Flammability traits of fynbos species with different post-fire regeneration strategies
(2010) Burger, Niel; February, Edmund C
Wild fire is an important disturbance regime that shape global biome distributions and maintain the structure, function and biological diversity of plant communities in fire-prone environments. Physical, chemical and architectural properties are known to affect plant flammability, but few studies explore how these traits contribute to fire properties at the individual level and how flammability is influence by regeneration strategy. The flammability and traits that contribute to it is investigated in 15 co-occurring fynbos and forest species with burning experiments performed at Bain's Kloof Pass, Western Cape of South Africa. Eight species are characterised by fire-stimulated (mostly seeders) and seven by non-fire-stimulated recruitment (mostly resprouters). Results across all species are consistent with other studies which indicate that the proportion of dead and fine fuel is significant predictors of flammability. Significantly higher flame temperatures and values for % fuel burned were recorded for FSR's relative to NFSR's and these differences are attributed to significant differences in % fuel dead, fuel bed porosity and packing ratio. This is consistent with the hypothesis that flammability is most like to be selected for in fire-stimulated recruiters (first colonisers after fire) where a combination of increased flammability and recurrent success enhance their fitness in the fire-prone environment. This study indicates that cultivation of the highly flammable species E. rhinocerotis, Stoebe sp., M. muricata, Erica sp. and P. amplexicaulus on urban fringes should avoided to reduce fire-risk and thus increase human safety.
Open Access
Functional divergence between Vachellia and Senegalia could underpin differences in invasiveness and Eltonian niche partitioning in African savannas
(2019) Lewis, Joel Robin; February, Edmund C; Verboom,Tony G
For several decades Vachellia and Senegalia have been assumed to be ecological equivalents in African savannas. Their supposedly close evolutionary relationship has, however, been totally revised in light of recent molecular phylogenetic data. These data highlight the deep divergence (± 30 Ma) between these genera and also their independent transitions into African savannas. Distant divergence and independent transitions into savannas between Vachellia and Senegalia suggest their possible ecological niche divergence, which could explain the observed differences in invasiveness between them. This thesis provides the first ecophylogenetic study to assess the level of ecological niche divergence between these economically, socioeconomically and ecologically important tree genera in Africa. I address this question by first determining the scale(s) of coexistence between Vachellia and Senegalia and determine their phylogenetic community structure at both the landscape-regional scale and the plot-scale. I then conduct a large glasshouse experiment using seven species from each genus to determine conserved functional differences between them during seedling establishment in the presence, and also the absence, of grass competition. While I find evidence of some Grinnellian niche differentiation between Vachellia and Senegalia at the landscape-regional scale, their greater-than-expected co-occurrence in plots suggests they generally partition an Eltonian niche in savannas. Glasshouse trait data also reveal that Vachellia has a conserved strategy for rapid vertical growth when free from grass competition and Senegalia has a conserved root tissue density response to grass competition. I argue that the rapid growth (bolting) strategy of Vachellia in the absence of grass competition provides a functional explanation for why this genus is observed to be relatively more invasive in grassy ecosystems. I also argue that the root tissue density response of Senegalia enables stable coexistence at the plot scale with Vachellia as it increases competitive ability among grasses leaving establishment less tied to times of low grass biomass. I conclude that this study provides overwhelming support for ecological niche divergence between Vachellia and Senegalia and that this divergence may underpin their observed differences in invasiveness. That the coexistence of these genera is largely attributable to Eltonian niche partitioning also provides fresh support for the contention that tree-tree competition structures tree communities in African savannas. Finally, niche divergence suggests that, when promoting diversity and managing invasion, Vachellia and Senegalia species will require different management practices.
Open Access
Genetic and morphological means of differentiating within and between populations of Widdringtonia whytei and Widdringtonia nodiflora on Mount Mulanje, Malawi
(2010) Hall, Stuart; Hedderson, Terry A; February, Edmund C
There has been confusion in the past as to the taxonomy of Widdringtonia on Mount Mulanje. At present it is accepted that there are two species present. The aim of this study was to assess the level of genetic variation present across six populations of Widdringtonia whytei and Widdringtonia nodijlora on the mountain, and determine whether there are two genetically separate species present. Analysis of genetic variation included PsbA (chloroplast DNA), ITS (nuclear DNA), microsatellite and ISSR gene region markers. Out of the markers that amplified successfully, it was found that there is little genetic variation present with no differentiation between or within taxonomic groups for the gene regions analyzed. Morphometric analysis of cones and leaves grouped W. whytei and W. nodijlora separately, with a midgroup more closely associated with definite W. whytei. This compared favourably with observations of individuals from the different groups in the field. It is suggested that based on the low level of genetic variation across the mountain, W. whytei and W. nodijlora have diverged very recently from a W. nodijlora ancestor.
Open Access
Geohydrological characteristics of Table Mountain Group aquifer-fed seeps and the plant ecophysiological consequences
(2007) Aston, Tim; Bond, William J; February, Edmund C; Midgley, Jeremy J
Fynbos, the native vegetation of the Western Cape of Southern Africa experiences a mild, Mediterranean type climate with hot dry summers and cool wet winters. In terms of climate, fynbos is comparable with other Mediterranean systems found around the Mediterranean in Europe, in parts of Chile, south-western Australia and in the Chaparral in California (Aschmann, 1973). The Cape Floristic Region, of which fynbos is part, is one of the world's most botanically diverse regions, home to an estimated 9030 vascular species (Goldblatt, 1978; Goldblatt and Manning, 2002). The region has exceptionally high levels of endemism. Almost 69% of its 8920 species of flowering plants are endemic (Goldblatt and Manning, 2002), and, despite its small area, it is regarded as one of the six global plant kingdoms (Takhtajan, 1986). Ericaceae, Iridaceae, Proteaceae and the Restionaceae are well represented and there are a number of families that are endemic or nearly so (Goldblatt and Manning, 2002). The largest is the Penaeaceae, followed by Grubbiaceae, Roridulaceae and Geissolomataceae, which together contain 15 endemic genera (Goldblatt and Manning, 2002). These families are almost without exception evergreen sclerophyllous shrubs and are thought to be palaeoendemic remnants from an ancient temperate flora, when conditions were cooler and wetter (February et al., 2004). As a result, many of these species are restricted to wetter areas such as wetlands and mountain seeps (February et al., 2004). Many of these seeps, as well as other groundwater-fed ecosystems, are likely to be connected to the Table Mountain Group (TMG) aquifer from which the city of Cape Town may begin to abstract water.
Open Access
Habitat use and diet preference of extralimital giraffes in the Kgalagadi Transfrontier Park
(2013) Viljoen, Storme; February, Edmund C
Extralimital giraffe introductions in the south-western region of Kgalagadi Transfrontier Park, South Africa, may result in changes to the vegetation structure for the region through overutilization of two keystone tree species in the area. In the areas where giraffe density was highest, I determine the relative abundance of giraffe; their diet preference and the potential reasons for this preference. Using both spoor and dung counts as indicators of giraffe presence, transects were carried out over approximately 20 km of the Auob River. Percentage occurrence was plotted onto a map of the river, along with the location of boreholes that supply the only surface water. Little trend could be seen, although occurrence appeared slightly higher near boreholes. The two trees that most commonly occur in the Auob duneveld, Acacia erioloba and Acacia haematoxylon, were examined for nutritional content, and both chemical and physical defences - all of which are thought to affect herbivore selection. Stable isotope analysis and linear mixing model were used to determine the proportion of each species in the diet of giraffe using the leaves of both species and giraffe dung. The majority of the giraffe's diet consists of A. haematoxylon (mean = 79 %, s.d. = 20.5 %) despite having less crude protein and similar condensed tannin content. Relative lack of physical defences appears to encourage giraffes to exploit A. haematoxylon, which has shorter, weaker thorns. Without management intervention, A. haematoxylon could suffer selective mortality through the impacts of giraffe browsing, resulting in a loss of species and structural diversity of the landscape.
Open Access
Hydraulic trait variation in Protea repens with change in climate in space and time
(2016) Kellermann, Jacobus; February, Edmund C; Slingsby, Jasper
Global climate change and atmospheric CO₂ concentrations are affecting all levels of biodiversity in a number of ways. For example, the unique vegetation of the Cape Floristic Region (CFR) is expected to experience increased temperatures while rainfall becomes more seasonal, resulting in stronger summer drought with greater hydraulic stress in plants. Increased CO₂ concentrations, on the other hand, are expected to relieve hydraulic stress in plants that utilize the C3 pathway for photosynthesis (most fynbos species), by reducing the amount of time they have to keep their stomata open to take up the CO₂ they require. Observed weather data suggest that rainfall has remained relatively stable over the last 21 years, while temperatures for the region have increased marginally. Here I explore variation in the hydraulic traits (leaf and xylem anatomy) of Protea repens (L.) across a spatial climatic gradient in the CFR relative to a common garden experiment. I then compare the contemporary trait-climate relationships with a 21 year old xylem anatomy dataset. In the common garden experiment I explore xylem and leaf trait variation in P. repens from thirteen populations representing a gradient in temperature and mean annual precipitation. Because trait-gradient relationships can be confounded by genetic differences between populations along the gradient, I used a common garden experiment to test the degree to which trait variation was genetically constrained among populations. My results show that xylem vessel diameters and an estimate of hydraulic conductance increased with increases in maximum temperature and soil moisture days across the spatial gradient. My results for the common garden experiment does however show genetically constrained intra-specific differences in xylem vessel morphology between populations. Despite this, differences in xylem vessel and leaf morphology between plants in the common garden and their source locality demonstrate that P. repens has some ability to respond to changes in the environment through phenotypic plasticity. To determine the response of P. repens to changes in climate over the past 21 years, I compared contemporary xylem anatomy to an existing dataset collected from the same sites in 1994. My results show no significant change in vessel diameters since 1994 even though temperatures and atmospheric CO₂ have increased, with no change in rainfall amount. These results suggest that either P. repens is not experiencing increased drought stress under current climate conditions, or that xylem vessel anatomy is not a good proxy for small changes in drought stress in this species. The effect of increased drought stress due to higher temperatures and associated evaporative demand may be alleviated by increased atmospheric CO₂ reducing the amount of time the plants have to keep their stomata open to take up the CO₂ they require. Similarly, it is possible that drought stress has not changed substantially over this period, because a reduction in wind run across the CFR may have balanced the increase in evaporative demand created by higher temperatures. At a plant level, P. repens may not respond to small increases in drought stress by utilising deep water. In addition, P. repens is potentially able to reduce stomatal conductance thereby alleviating xylem anatomical responses to the small change in temperature since 1994.
Open Access
The influence of rainfall variability on savanna tree seedling establishment
(2006) Botha, Susan; Bond, William J; February, Edmund C
My thesis investigates the rainfall requirements necessary for tree seedling survival of different savanna species in the first year of growth. Results from a field drought experiment showed that dry season survival for seedlings of Acacia nigrescens and Philenaptera vialacea was not related to wet season length. Instead mortality was highest immediately after germination despite irrigation to minimize dry spells. Furthermore there was no relationship between seedling size and the length of the dry spell seedlings of Acacia nigrescens and Philenaptera vialacea grown in basalt clay in the greenhouse, could survive. This was in contrast to seedlings of Scleracarya birrea and Terminalia sericea grown in granite sand, where number of dry days survived increased with number of growth days received. There was no significant differences in rainfall variability (t-test: tL III = 1.56, P = 0.121, F-ratio = 1.13) between a semi-arid (Satara) and semi-mesic (Pretoriuskop) site within the Kruger National Park despite significant differences in rainfall totals (t-test: tl, III = 4.40, P < 0.000, F-ratio 3.41). Results from a soil-water model show that seedling recruitment can occur at least twice a decade in the Satara area, which should be frequent enough to allow a transition to a woodland state over time. These results suggest that a mechanism other than the effect of rainfall variability on seedling establishment is required to explain the co-existence of trees and grasses at Satara. The lack of field germination, the absence of a seed bank and the germination of added seed at the field experiment site after rainfall emphasizes seed limitation as an important bottleneck at Satara. Higher herbivore biomass at Satara compared to Pretoriuskop may limit seedling establishment opportunities by destroying seed and seedlings of tree species.
Open Access
Is the grass really greener on the other side?
(2013) Lane, Wade Howard; February, Edmund C; Woolley, Leigh-Ann
While many studies focus on identifying pollutants within an ecosystem or how they affect primary producers, few look at how pollutants move through trophic levels or their influence on animal demography. The aim of this study was to determine whether additional nitrogen, phosphorus and water, to a savanna would alter the vegetation quality enough to influence the feeding behaviour and diet of the ungulate populations both of which would alter the vegetation quantity. The study was conducted in the Kruger National Park. One site was supplied with additional nutrients and compared to three control sites. The nitrogen and phosphorus content of grass and tree leaves collected at the enriched site were higher than the leaves collected at the control site, indicating the additional nutrients are improving the vegetation quality. Feeding rates (determined from photos captured by camera traps) indicated a higher degree of herbivory at the enriched site. However, there was no difference in the δ¹³C value, nitrogen and phosphorus content in the ungulate dung collected amongst the study sites suggesting no change in the diet. The dominant grass was significantly shorter at the enriched site suggesting that increased grazing was diminishing grass biomass. Basic assessment of the trees indicated that the additional water at the enriched site seemed to be triggering an earlier start to the growing season for the trees. It was concluded that the additional nutrients have altered the vegetation structure enough to potentially influence animal demographics.
Open Access
An isotopic assessment of the water sourced by Ischyrolepis Sieberi (Restionaceae) growing at high altitudes in the Cedarberg : does fog play an important role?
(2004) Wigley, Benjamin; February, Edmund C
The main aim of this study was to determine if Ischyrolepis sieberi, a common restiod species found at high altitudes in the Cederberg captures fog to supplement its water needs. This was done by comparing the isotopic ratios of δD and δ¹⁸O in captured fog, rainwater, and stream water and comparing these to the δD and δ¹⁸O values found in the xylem water of Ischyrolepis sieberi plants growing at the study site. The δD values of the collected fog samples were enriched relative to rainwater, stream water, and xylem water from I. sieberi, however these differences were not significant. The δ¹⁸O values of the I. sieberi xylem water were significantly (p < 0.01) more enriched than rain, stream, and fog water. The xylem water of I. sieberi was depleted in δD and enriched in δ¹⁸O relative to all other water sources. The plants therefore did not appear to be utilizing fog during the three months of this study. The δ¹⁸O values of I. sieberi were consistently 4-6‰ enriched compared to rainwater, suggesting that they are utilizing rainwater, which has undergone fractionation due to evaporation in the soil before being taken up by the plants. This study was run during the wet winter months. The δD and δ¹⁸O values from previous data suggest that the I. sieberi plants do utilize fog during the dry summer months.
Open Access
The lazy root : Will a change in the rainfall regime or increased anthropogenic nitrogen result in an alteration in the competitive balance between trees and grassed in a semi-arid South Africa savanna?
(2012) Bell, Wesley Drummond; February, Edmund C
In an attempt to determine why trees and grasses are able to coexist in savannas, Walter (1971) first proposed the root niche separation model for savannas which suggests that trees and grasses have differential access to water due to spatial differences in their rooting profiles. Subsequent studies in various savannas around the world have found that this is often not the case. In the central region of the Kruger National Park February & Higgins (2010) found that trees and grasses root at the same depths within the soil profile. In this study I determine whether the existence of fine roots in the soil profile necessarily indicates root activity. I also aim to determine the effect of increased water and nitrogen availability on root activity of trees and grasses. Pits of 20 x 20 em were dug to a depth of 40 em in plots that were either irrigated with the equivalent of 30 ml of rainfall per month or left without irrigation. This study was performed at the end of the rainfall season. I found that trees and grasses root at the same depths under moist and dry edaphic conditions as well as in plots with increased nitrogen. Tree roots are however, significantly more active than grass roots in non-irrigated plots. Temporal separation in root activity therefore seems to be a factor that allows for the coexistence of trees and grasses in savanna ecosystems.