OpenUCT :: Browsing by Author "Cramer, Michael D"

Browsing by Author "Cramer, Michael D"

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Open Access
A comparison of the influence of different inorganic nitrogen forms on the partitioning of Carbon in C₃ (Triticum Aestivum) and C₄ (Zea Mays) plants
(1992) Cramer, Michael D
Open Access
Are forests restricted by nutrient poor soils? : an analysis of soil nutrient stocks and associated vegetation in the fynbos biome, South Africa
(2011) Donaldson, Jason; Cramer, Michael D
The realized woody biomass of the Fynbos Biome in the Western Cape, South Africa falls below the climatic potential to support woody biomass. The lack of substantial tracts of woody forests has long puzzled ecologists, although patchy nutrient distribution and fire disturbance are thought to play a role. The issue has been confounded in the recent past by the invasion of non-indigenous woody plants into formerly low stature fynbos dominated areas. Despite low wood nutrient concentrations, a substantial proportion of nutrients are locked up in the wood of forests due to the large volume of wood. Nevertheless, nutrient stock analysis indicated that plant available nutrients in the poorest global soils (including fynbos soils) are sufficient to support forests (indigenous and alien). I hypothesized that soil nutrient stocks of the Fynbos Biome are sufficient in quantity to support closed canopy indigenous forests with a woody biomass greater than 225 000 kg ha⁻¹ and that alien Pinus spp. and Eucalyptus spp. have lower nutrient stocks than indigenous fynbos species. The study was conducted in the Orange Kloof Forest Reserve and Jonkershoek Nature Reserve within the Western Cape (South Africa). Soil, wood and leaf samples of representative species were collected from indigenous forest, fynbos, E. globulus and P. halepensis and assessed for nutrient contents. Estimates of potential woody biomass on four different soils indicated that indigenous forests would be limited by fynbos soil stocks to below the 225 000 kg ha⁻¹. Pine forest had lower wood nutrient concentrations (mg kg⁻¹, n=6) for N (2466), K (2433), Ca (383), and Fe (34) than indigenous forest species (n=11) N (3427), K (4254), Ca (1636) and Fe (140). The low nutrient stocks in pine wood may allow them to grow tall and be competitive in the shrubby fynbos biome where indigenous forests are limited by expensive wood costs.
Open Access
Are Namibian "fairy circles" the consequence of self-organizing spatial vegetation patterning
(Public Library of Science, 2013) Cramer, Michael D; Barger, Nichole N
Causes of over-dispersed barren "fairy circles" that are often surrounded by ca. 0.5 m tall peripheral grasses in a matrix of shorter ( ca. 0.2 m tall) grasses in Namibian grasslands remain mysterious. It was hypothesized that the fairy circles are the consequence of self-organizing spatial vegetation patterning arising from resource competition and facilitation. We examined the edaphic properties of fairy circles and variation in fairy circle size, density and landscape occupancy (% land surface) with edaphic properties and water availability at a local scale (<50 km) and with climate and vegetation characteristics at a regional scale. Soil moisture in the barren fairy circles declines from the center towards the periphery and is inversely correlated with soil organic carbon, possibly indicating that the peripheral grass roots access soil moisture that persists into the dry season within fairy circles. Fairy circle landscape occupancy is negatively correlated with precipitation and soil [N], consistent with fairy circles being the product of resource-competition. Regional fairy circle presence/absence is highly predictable using an empirical model that includes narrow ranges of vegetation biomass, precipitation and temperature seasonality as predictor variables, indicating that fairy circles are likely a climate-dependent emergent phenomenon. This dependence of fairy circle occurrence on climate explains why fairy circles in some locations may appear and disappear over time. Fairy circles are only over-dispersed at high landscape occupancies, indicating that inter-circle competition may determine their spacing. We conclude that fairy circles are likely to be an emergent arid-grassland phenomenon that forms as a consequence of peripheral grass resource-competition and that the consequent barren circle may provide a resource-reservoir essential for the survival of the larger peripheral grasses and provides a habitat for fossicking fauna.
Open Access
Biogeography of Isolepis subgenus Fluitantes (C.B. Clarke), Muasya (Cyperaceae): niche conservatism, long distance dispersal and hybridization
(2010) Viljoen, Jan-Adriaan; Verboom, George Anthony; Cramer, Michael D
Numerous lineages of the Western Cape of South Africa show affinities with the flora of tropical Africa and with Australasia. Recent work suggests that most migrations between the Western Cape and tropical Africa occur in a northward direction, and that connections between the flora of regions in the Southern Hemisphere are maintained by wind-assisted long-distance dispersal. The Fluitantes clade of lsolepis (Cyperaceae: Cypereae) is distributed throughout these areas and provides a useful study group to assess the general validity of published biogeographical trends. Furthermore, the cooccurrence of several closely related species in the Cape floristic region allows geographical and ecological patterns to be used for inference of speciation processes in the clade. Sequence data of the ITS and atpi-H gene regions were collected for 82 specimens; these were used to construct haplotype networks and phylogenies. By using the Tristan da Cunha endemics in the genus, as well as results from higher-level studies, a dated phylogeny for the Fluitantes clade was constructed and allowed for ancestral character state optimization of distributions by maximum likelihood. Ecological data were extracted from geographic information systems map to test for environmental differentiation in the Cape taxa. The Fluitantes were found to have originated in the Cape 7 million years ago. From there, they spread east and northwards onto the mountains of East Africa and to the islands of the Indian Ocean. Multiple dispersal events to Australia were recorded. Incongruence between the plastid and nuclear gene trees indicate hybridization to have taken place in Australasia, with possible subsequent speciation. Although the multivariate analysis found some ecological differentiation between the three Cape species, there was substantial overlap in all variables, and interpretation of habitat differences was difficult. It is suggested that, instead, differentiation may have taken place at the microhabitat scale, as I. rubicunda occupies low-lying sandy depressions, I. striata occurs at higher altitudes floating in water, and I. Judwigii inhabits the edges of streams and wetlands.
Open Access
The consequences of precipitation seasonality for Mediterranean-ecosystem vegetation of South Africa
(Public Library of Science, 2015) Cramer, Michael D; Hoffman, M Timm
Globally, mediterranean-climate ecosystem vegetation has converged on an evergreen, sclerophyllous and shrubby growth form. The particular aspects of mediterranean-climate regions that contribute to this convergence include summer droughts and relatively nutrient-poor soils. We hypothesised that winter-precipitation implies stressful summer droughts and leaches soils due to greater water availability (i.e. balance between precipitation and potential evapotranspiration; P-PET) during cold periods. We conducted a comparative analysis of normalised difference vegetation indices (NDVI) and edaphic and climate properties across the biomes of South Africa. NDVI was strongly correlated with both precipitation and P-PET (r 2 = 0.8). There was no evidence, however, that winter-precipitation reduces NDVI in comparison to similar amounts of summer-precipitation. Base saturation (BS), a measure of soil leaching was, however, negatively related to P-PET (r 2 = 0.64). This led to an interaction between P-PET and BS in determining NDVI, indicating the existence of a trade-off between water availability and soil nutrients that enables NDVI to increase with precipitation, despite negative consequences for soil nutrient availability. The mechanism of this trade-off is suggested to be that water increases nutrient accessibility. This implies that along with nutrient-depauperate geologies and long periods of time since glaciation, the winter-precipitation may have contributed to the highly leached status of the soils. Since many of the ecophysiological characteristics of mediterranean-ecosystem flora are associated with low nutrient availabilities (e.g. evergreen foliage, sclerophylly, cluster roots), we conclude that mediterranean-climates promote convergence of growth-forms in these regions through high leaching capacity.
Open Access
Contesting the termite hypothesis for the origins of heuweltjies in Clanwilliam, South Africa
(2010) Innes, Sheona; Midgley, Jeremy J; Cramer, Michael D
The age and origins of large earth mounds (locally termed "heuweltjies") is under debate, with ages being proposed for between 4500 and 30 000 years old, and hypotheses including zoogenic and geological origins of the mounds. The widely accepted hypothesis for the origins of the heuweltjies found in the South Western Cape of South Africa is that mounds are ancient termitaria, belonging to the southern harvester termite Microhodotermes viator. Our idea was to use rocks and stone layer profiles to determine approximate ages of the heuweltjies as well as to gather evidence to either support or refute the termite hypothesis. We excavated ten mounds in Clanwilliam, South Africa, to gather stone and rock profiles throughout the mounds, as well as digging a trench through a heuweltjie with bedrock as its matrix in order to determine the extent of the bedrock into the mound. No stone layers as such were found. What we did find was that the mounds are a whole order of magnitude greater in volume than large termite mounds made by the northern harvester termite Macrotermes in tropical regions, and many of the mounds contain large rocks both throughout the mound as well as on the surface. The trench cut through the mound on bedrock revealed the bedrock extending all the way to the centre of the mound, on the same level as the surface of the surrounding matrix. This evidence can be used to refute the termite hypothesis for the heuweltjies of Clanwilliam as it is virtually impossible for termites to move rocks up to 25 kg to the top of a mound, and mining soil from beneath a mound with bedrock at its base is an impossibility.
Open Access
The contribution of fog to the moisture and nutritional supply of Arthraerua leubnitziae in the central Namib Desert, Namibia
(2018) Gottlieb, Tunehafo Ruusa; Eckardt, Frank; Cramer, Michael D
Fog is a key source of moisture to the diverse coastal Namib Desert biota, delivering five times more moisture than rain. Apart from the importance of fog as a source of water for plants, it is also associated with particulates that may contain essential nutrients for plants. Furthermore, dry deposition can be an important input of nutrients to many ecosystems, but without water, dust deposited on leaves or on soil is inaccessible for plant uptake. In other studies of coastal ecosystems (e.g. Strandveld), it has been found that this combined deposition of nutrients represents a major source of nutrients to terrestrial ecosystems. In the case of the Namib Desert, the range of Arthraerua leubnitziae is limited to those areas where fog occurs. This study was carried out at five meteorological stations in the gravel plains of the Namib Desert, along an east-west transect increasing in elevation inland. I hypothesised that marine-derived deposition contributes to moisture and nutrient supply of Arthraerua leubnitziae, an endemic shrub restricted to the fog zone of the central Namib Desert, and consequently determines its distribution. To test this hypothesis, two sub-hypotheses were developed and tested independently. The first sub-hypothesis was that fog contributed to the distribution range of A. leubnitziae in the central Namib Desert and the second was that fog deposition has a significant potential to supply moisture and nutrients to A. leubnitziae in the central Namib Desert. To test the first sub-hypothesis, I measured fog and rain volume, and plant morphological characteristics at the five sites. In addition, a fog map was derived using climatic variables from the five sites and used with climate and edaphic variables in MaxEnt model of the probability of occurrence of A. leubnitziae. The occurrence of A. leubnitziae was found to coincide with areas with high fog occurrence with fog contributing 36% to the modelled distribution of A. leubnitziae alongside precipitation, elevation and isothermality. In order to test the second sub-hypothesis, I measured nutrients deposited in fog water derived from wet and dry deposition (Ca, K, Mg, Na, Cl, Br, NO₃, PO₄ and SO₄) and plant essential nutrients in plant and soil samples (N, P, K, Ca, Mg, Mn, Fe, Cu and Zn). I also determined the nutrient uptake by stems of A. leubnitziae. During the period of Sep 2015 to Aug 2016, fog provided 92% of the yearly water input across the study sites. Over the course of the sampling cycle, the total annual nutrient content of fog and dust was dominated by Ca and Na. Most of the nutrients (K, Mg, Na, and S) were of marine origin. However, Ca enrichment factors (relative to seawater) were higher than 1, suggesting an input from dust. A. leubnitziae was found to be able to directly intercept and absorb some of the nutrients in the dust and fog via their stems. Even though plant and soil nutrients did not match each other, a contribution to plant nutrition from dust and fog was evident. The low nutrient concentrations in the soils of the Namib Desert and significant inputs from dust and fog suggests deposition is an important source of nutrients for A. leubnitziae. Although work is still required to better understand the importance of fog water uptake for nutrient provision, I have identified that water, nutrients and environmental stress alleviation should not be considered separately in studying the role of fog as a determinant of plant distributions.
Open Access
The Contribution of Occult Precipitation to Nutrient Deposition on the West Coast of South Africa
(PLOS, 2015-05) Nyaga, Justine M; Neff, Jason C; Cramer, Michael D
The Strandveld mediterranean-ecosystem of the west coast of South Africa supports floristically diverse vegetation growing on mostly nutrient-poor aeolian sands and extending from the Atlantic Ocean tens of kilometers inland. The cold Benguela current upwelling interacts with warm onshore southerly winds in summer causing coastal fogs in this region. We hypothesized that fog and other forms of occult precipitation contribute moisture and nutrients to the vegetation. We measured occult precipitation over one year along a transect running inland in the direction of the prevailing wind and compared the nutrient concentrations with those in rainwater. Occult deposition rates of P, N, K, Mg, Ca, Na, Al and Fe all decreased with distance from the ocean. Furthermore, ratios of cations to Na were similar to those of seawater, suggesting a marine origin for these. In contrast, N and P ratios in occult precipitation were higher than in seawater. We speculate that this is due to marine foam contributing to occult precipitation. Nutrient loss in leaf litter from dominant shrub species was measured to indicate nutrient demand. We estimated that occult precipitation could meet the demand of the dominant shrubby species for annual N, P, K and Ca. Of these species, those with small leaves intercepted more moisture and nutrients than those with larger leaves and could take up foliar deposits of glycine, NO3-, NH4 + and Li (as tracer for K) through leaf surfaces. We conclude that occult deposition together with rainfall deposition are potentially important nutrient and moisture sources for the Strandveld vegetation that contribute to this vegetation being floristically distinct from neighbouring nutrient-poor Fynbos vegetation.
Open Access
Determinants of blackwaters in the South Western Cape
(2006) Power, Simon C; Cramer, Michael D; Midgley, Jeremy J
Blackwater rivers and lakelets are a common feature of the landscape in the South Western Cape. Contrastingly, white rivers can also be found in the region. Key to the colour of blackwaters is the increased presence of dissolved organic carbon. The vegetation of the regions is known to posses' large amounts of polyphenols and potentially low microbial decomposition. Therefore, are the plant-soil dynamics a possible answer to variation in river colour? To answer this, the chemical correlates of rivers in the region, along with the in vitro leaching of the fynbos vegetation compared to other types and the effects of nutrient fertilisation on microbial activity in soil were determined. The results revealed that organic carbon, Fe and pH are significantly correlated with blackwaters. The Fe in the water appears to be bound to humic compounds. Fynbos vegetation is able to produce greater concentrations of polyphenol leachates (315 mg/l humic acid) compared to savanna species (246 mg/ humic acid) over a four-day period. P fertilisations increased the concentrations of humic acids from 30 mg/1 to 200 mg/l in some soils through their affinity to bind with humic compounds. The N fertilisations moderately increased the humic acid concentrations and in some cases lowered the concentration by 10 mg/l, indicating that it provided a nutrient source to the microbes for carbon breakdown. The interactions between humic compounds, nutrients and Fe are key to the formation of blackwaters in the region. Attached to this is the limited decomposition that takes place due to nutrient limitations. Therefore, the interactions between the carbon leached from the vegetation and below ground activities are determinants of water colour in the South Western Cape.
Open Access
Distribution of leaf dissection and sclerophylly along microhabitat gradients in pteridophytes of the Cape peninsula
(2010) Viljoen, Jan-Adriaan; Verboom, George Anthony; Cramer, Michael D
Plant species with small leaves and sclerophylls have been reported to occur mainly on dry, low-nutrient soils in situations of high insolation. However; a number of physiological functions have been proposed for the two sets of traits. Ferns are well-suited to the study of leaf structure and its impact in different environments, as they show remarkable variation in leaf dissectedness and sclerophylly, and are able to inhabit a variety of marginal habitats. In this study, ecological and leaf trait data were collected for 17 fern species occurring in Skeleton Gorge, Table Mountain, in the Western Cape. Correlations between the traits and regressions of leaf traits on potential environmental determinants was carried out using both species averages and phylogenetically independent contrasts. The habitat and leaf traits were also subjected to a test of evolutionary trait conservatism. Sclerophyllous plants were found to be have thicker leaves, containing less chlorophyll, but sclerophylly was poorly correlated with leaf dissection. Plants occurring in high-light environments tended to be more sclerophyllous and have more dissected leaves, although these environments also were nutrient-poor. Leaf dissection appears to be primarily a means of dissipating heat by convection, rather than evaporative cooling, but it may also improve nutrient acquisition in low-nutrient soils. Sclerophylly in these ferns was not directly associated with nutrients; instead low sclerophylly seems to be favoured in low-light environments, perhaps because of lower metabolic costs or to reduce self-shading.
Open Access
Does pubescence increase uptake of nutrient deposition on leaves?
(2009) Mendel, Lilyane I J; Cramer, Michael D
1. Leaf hairs have been associated with reducing environmental stress by increasing reflectance and the thickness of the boundary layer, however, such effects are more obvious in species with dense trichome layers. The adaptive functions for species with sparse leaf hair cover, such as in the Cape Proteaceae species of the Cape Floristic Region (CFR), remain unknown. 2. We postulated that leaf hairs of the Proteaceae in the CFR facilitate both nutrient absorption and water retention on the leaf surface. Leaf hair distribution is expected to show a pattern relative to the higher levels of atmospheric nutrients with proximity to the coast. 3. We followed a three pronged approach, this involved (i) evaluating the relationship of leaf pubescence with leaf surface water holding capacity, water uptake and nitrogen absorption from wet deposition for 12 Proteaceae species from the CFR, under controlled conditions; (ii) evaluating the relationship between leaf pubescence and plant height of 39 Proteaceae species from the CFR and (iii) comparing visual scores of pubescence with the relative proximity of the plants to the coast (oceanic index/continentality) of 205 Proteaceae species. 4. There was a positive correlation between leaf pubescence (0 to 8 x 10⁶ m⁻²) and water holding capacity for the 12 experimental Proteaceae species, however, water uptake did not vary with hairiness. ¹⁵NO₃⁻, ¹⁵NH₄⁺ and ¹⁵N-glycine solutions were each independently applied as wet deposition to the 12 experimental Proteaceae species. The uptake of glycine showed a positive correlation with hairiness overall and for Protea, whereas nitrate and ammonium uptake were positively correlated only with hairiness of Leucospermum. 5. Average plant height was positively correlated with leaf hair density of the 39 Proteaceae species. The visual score of pubescence was significantly different for each genus and there was no correlation between leaf pubescence and distance from the coast for the 205 field Proteaceae species. 6. Leaf hairs probably play a role in reducing environmental stresses, however, it is also possible that they play a role in water retention and nutrient absorption. Hairs on leaves vary in arrangement and morphological characteristics, thus they accordingly, probably serve different functions.
Open Access
Does the foliage of Acacia spp. determine their distribution? : a study to determine how two different leaf forms may alter the distribution of Acacia spp. in relation to phosphorus concentration, mean annual precipitation and temperature within Australia and South Africa
(2007) Cyrus, Mark; Cramer, Michael D
The genus Acacia consists of over 1000 species, of which most are native to Australia. An intriguing aspect of this genus is their divergence into two major groups that poses entirely different leaf structures. The ﬁrst of these groups consist of Acacias have fern like bi-pinnate leaves, that are subdivided into small leaﬂets. The other group posses' what are known as phyllodes which are also called 'simple leaves'. The evolution of phyllodes within the genus Acacia seems to have been localized, occurring largely within Australia. Many hypotheses have been put forward in the past to try and explain what advantages phyllodes incur on the species that bear them. Many studies have indicated drought tolerance and resistance as a main evolutionary driver of phyllodes. However due to the very low concentrations of nutrients particularly P within Australia and the generally longer life span exhibited by phyllodes compared to normal compound leaves, we hypothesized that phyllodes were in fact an adaptation to nutrient limitation, and provide a way in which to limit nutrient loss back to the environment. In order to test our hypothesis, we analysed the distributions of 6 Acacia spp., three of which were phyllodinous and three of which bore compound leaves, in relation to soil P concentrations, mean annual precipitation and temperature. Due to all 6 of these species being invasive within South Africa we compared their distribution of these species both within Australia and South Africa to determine wither species were following similar trends. In this study we determined that phyllodinous Acacia spp. were occurring on low P soils at signiﬁcantly higher frequencies to species bearing compound leaves. The reverse relationship however was also recorded for areas of high P with compound leaf bearing species being more dominant. Species followed similar trends between continents, however due to these species being invasive in SA it was assumed that species had not reach their ideal or potential distribution ranges which may mean that trends may become stronger with time. Temperature and rainfall did not show any relationship to foliage type and it was there for concluded that phosphorus concentrations have been the main evolutionary driver of the phyllode.
Open Access
Ecological differentiation and the evolution and maintenance of fynbos diversity
(2011) Slingsby, Jasper; Verboom, George Anthony; Cramer, Michael D; Midgley, Jeremy J
Preserving the structure and functioning of ecosystems and the benefits they provide to society depends on our understanding of how biodiversity influences ecosystem function and which processes determine the composition and diversity of biological assemblages. Ecological, evolutionary and biogeographical processes interact to determine the genesis, maintenance and spatio-temporal arrangement of biodiversity. Here I investigate these processes in the Cape fynbos of South Africa by examining the diversification, biogeography and community assembly of a clade of reticulate-sheathed Tetraria species (Cyperaceae:Schoeneae).
Open Access
Ecosystem functioning, ecosystem services and rooibos production as affected by connectivity to natural vegetation and agrochemical use in rooibos tea (Aspalathus linearis) farming
(2011) Herbst, Marianté; Cramer, Michael D; Hawkins, Heidi-J; Seymour, Colleen
Globally, increasing land-use intensity has led to more intensive farming practices at the local scale and the loss of non-crop habitats at the landscape scale which may affect various ecosystem services. Insect pollination by wild pollinators is especially affected, but their relative impact and possible interactions have been relatively unexplored. There is also considerable evidence for the negative impacts of agricultural activities and agrochemical use on ecosystem services delivered by natural vegetation, but these impacts have not been assessed for the indigenous crop Aspalathus linearis (rooibos). The study was performed on 13 sites in Nieuwoudtville and Clanwilliam. I found evidence that rooibos pollination is dependent on flying pollinators and that the natural environment is an important provider of ecosystem services to rooibos farmers.
Open Access
Evaluating the importance of root abscission versus efflux to plant N-loss: consequences for plant N-isotope composition
(2018) Packer, Kirsten F; Cramer, Michael D
The common observation that plant δ¹⁵N values are lower than those of associated soil is generally attributed to transporter-facilitated efflux of ¹⁵N-enriched N. N efflux tends to occur under specific conditions, for instance, when the external N concentration is high, when the external medium is acidic and when roots experience mechanical stress. While efflux is presumed to act as a regulator of cytoplasmic N concentrations, it is energetically costly for plants to take up N only to release it back into the rhizosphere. A link between root tissue loss (e.g. root turnover or rhizodeposition) and plant δ¹⁵N has not been suggested, although root abscission is likely to be more ubiquitous than N efflux. This thesis questions the extent to which N efflux and root abscission contribute to plant N-loss and plant δ¹⁵N values. I hypothesized that: (1) plants supplied with more N would have more negative δ¹⁵N relative to the source, and greater root abscission from a relatively larger root biomass (2) the aeration necessary for hydroponic culture can act as a mechanical stressor on roots, accentuating plant N-loss through root abscission and N efflux. Wheat was grown in sand with NO₃- supplied at five relative addition rates (RAR) and in hydroponics with three physical disturbance regimes (direct aeration, aeration constrained within a pipe and circulation of nutrient solution through sand). The δ¹⁵N of roots and shoots, as well as the plant-derived N accumulation in both growth mediums, were determined. When the N supply matched the plant N demand, as determined by the relative growth rate, there was no discrimination between plant and source δ¹⁵N. N-loss here, although negligible, was in the organic form, which implies root abscission. By contrast, when N supply exceeded plant N demand, plant δ¹⁵N values decreased (e.g. after 47 d, plant δ¹⁵N of RAR 0.075 d⁻¹ was 0.4‰ but was −4.1‰ at RAR 0.175 d⁻¹) because they lost ¹⁵N-enriched N. This N was largely inorganic and presumably lost through efflux. In disturbed hydroponic conditions (i.e. direct and pipe treatments), root 'fragments' were a major biomass- (six-fold greater than root dry weight) and N-loss (two-fold greater than plant net N uptake) pathway. Plants from all treatments lost more N within root fragments than through efflux, although the cumulative N-loss was significantly smaller from plants grown in relatively undisturbed hydroponic conditions (i.e. sand). This suggests that root abscission is likely to be an important N-loss pathway for plants and thus contributes to the global offset between plant and soil δ¹⁵N values. Moreover, efforts to improve nitrogen use efficiency of crop plants, though reduced efflux, need to take cognizance of root abscission because it is an unavoidable artefact of root growth.
Open Access
Fire and herbivory effects on soil organic carbon stocks and soil greenhouse gas emissions from South African grasslands and savannas: Implications for global change
(2022) Chikomba, Farai; Hawkins, Heidi-Jane; Cramer, Michael D; Swanepoel, Pieter A
Understanding drivers of soil organic C (SOC) stocks and soil greenhouse gas emissions in grassland and savanna ecosystems is critical for climate change mitigation considering that soils are large C reservoirs while functioning as sinks and sources of the principal soil greenhouse gases, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). We however, have limited understanding of how wildlife herbivory, fire and pyric-herbivory influence SOC stocks and soil greenhouse gas emissions in these ecosystems. We hypothesised that, in low nutrient soils, wild herbivores would shift vegetation towards low quality (high C:N) foliage and litter that are resistant to microbial decomposition. This may contribute to the stable SOC pool, while reducing fluxes of all soil greenhouse gases. By contrast, fire was expected to reduce soil N pools by volatilisation, but to increase the stable SOC pool by addition of mostly recalcitrant pyrogenic organic C, thereby lowering CO2 and N2O fluxes. CH4 fluxes would be increased by the fire induced death of methanotrophic bacteria. To explore this, we examined SOC stocks, soil CO2, CH4 and N2O fluxes including other soil and environmental variables from Long-Term Ecological Research sites within a South African grassland and savanna. Four fire treatments distinguished by fire frequency were used (annual, biennial, triennial and no fire) while the presence or absence of wild herbivores represented the two herbivory treatments. We found that grassland SOC stocks were highest with biennial fires (20.47 ± 2.04 g C m-2, p < 0.001) compared to other treatments. In the same site, soil CO2 fluxes were highest with herbivory plus annual fires (2.28 ± 0.72 µmol m-2 s-1, p < 0.001). In the savanna, neither SOC stocks nor soil CO2 fluxes were affected by treatments. Generally, lower frequency fires (biennial or triennial) increased soil CH4 uptake while annual fires reduced soil CH4 uptake. From this, we estimated that South African grassland and savanna soils could consume about 0.47% per m2 of the CH4 released by global wetlands. These native grassy sites with wildlife exhibited very low fluxes of N2O whereas tropical grasslands and savannas that include livestock populated rangelands are thought to contribute about 16% to the global terrestrial N2O emissions. Linear mixed-effects models for SOC stocks showed total soil N was the strongest predictor rather than plant detritus inputs. Thus, we conclude that fire and herbivory drove SOC stocks via effects on soil N. After pooling the data for the grassland and savanna to compare with local and global figures, we conclude that South African grasslands and savannas have a small but significant potential for SOC sequestration and reduction of soil greenhouse emissions given that they constitute ca. 57% of the country's terrestrial ecosystems.
Open Access
Flowering in protea : a molecular and physiological study.
(2012) Smart, Mariette; Roden, Laura; Cramer, Michael D
Proteas have been extensively cultivated and are grown as floricultural crop plants in many parts of the world, including South Africa. However, the factors that influence the initiation of flowering in Protea have not been identified. From data gathered by the Protea Atlas Project it is evident that Protea spp. have greatly varying flowering times. Furthermore, flowering times between Protea spp. and their hybrid cultivars are also very different. Towards a better understanding of the factors involved in floral initiation in this cultivated crop, three aspects of flowering were investigated in this study. The carbon input into Protea inflorescence development was determined by measuring respiration rates and weights of developing structures. By manipulating source-sink ratios in plants, the carbon assimilatory capacities to support inflorescences were investigated in three cultivars and one wild-grown species of Protea which develop different sized flowers. As some Proteas flower in response to seasonal change, an orthologue of the floral inducer FLOWERING LOCUS T (FT), ProteaFT (ProFT), was isolated from ‘Carnival’ (P. compacta x P. neriifolia) and its expression pattern followed diurnally and seasonally. Finally, the functions of paralogous genes of Protea LEAFY (ProLFY) from ‘Carnival’ displaying sequence similarity to the meristem identity gene LEAFY from Arabidopsis thaliana, were investigated through heterologous expression studies in A. thaliana.
Open Access
Functional role of plant water fluxes in nutrient acquisition
(2014) Matimati, Ignatious; Cramer, Michael D; Verboom, George Anthony
Transpiration is inevitable during photosynthesis; however, it may also function to cool leaves, transport nutrients and drive nutrient acquisition via mass-flow. In addition to transpiration, plants water fluxes occur through hydraulic redistribution (HR). I hypothesized that an important function of plant water fluxes is to drive mass-flow nutrient acquisition, with flux rates positively correlated with nutrient limitation but not deficiency. To test whether nutrient availability regulates mass-flow, Phaseolus vulgaris was grown with N placed at one of six distances behind a root-impenetrable mesh whilst control plants intercepted the N-source. In plants forced to acquire N through mass-flow transpiration rates were 2.9-fold higher and P and K accumulation was greater compared to control plants. The contribution of nocturnal transpiration and HR to nutrient acquisition was assessed by supplying Aspalathus linearis (N-fixer) with no fertilizer or Na¹⁵NO₃ and CaP/FePO₄ either above or below-ground with varying rates of below-ground irrigation. ²H₂O was used to trace HR. HR by A. linearis accounted for the bulk of surface soil moisture at dawn and responded positively to surface fertilization. In contrast, plants supplied below-ground fertilizer exhibited both HR and nocturnal transpiration with increased ¹⁵N and P acquisition. Finally, to establish whether clay fraction moderates mass-flow P availability, Triticum aestivum was grown with 0, 1, 5 or 10% (w/w) clay combined with either Ca-P, Fe-P or inositol-P. Transpiration and nutrient accumulation were monitored. Plants acquired P through massflow and diffusion. The acquisition of N and P increased from 0 to 5% clay (w/w) due to enhanced moisture retention, but further additions (10%) reduced P-availability (Inositol-P > Fe-P > Ca-P). Overall, this thesis explored and confirmed the relatively novel idea that nocturnal and diurnal transpiration by plants are not merely the consequence of stomatal opening for CO₂ acquisition. Rather nocturnal and diurnal transpiration are regulated by nutritional requirements and serves as a driving force for nutrient transport to roots. Likewise, hydraulic redistribution serves to draw water from deep and wet soil layers to the upper layers, which serves as a means to enable uptake of nutrients from the rich, but often dry, upper soil. Plants may thus be opportunistic in their water uptake, taking it up when it is available in order to improve the acquisition of nutrients through mass-flow delivery. Plants in low nutrient substrates elevated their water fluxes for mass-flow nutrient acquisition. Consequently, plants growing in mesic climates with low clay soils are likely to display greater dependence on mass-flow nutrient acquisition. This might vary between C₃ and C₄ plants, which differ in WUE. Plants may also increase mass-flow nutrient acquisition during inter-specific competition thus reducing investment in root proliferation for nutrient interception. Plants growing in elevated atmospheric [CO₂] with suppressed transpiration could show limited mass-flow nutrient acquisition.
Open Access
The functional, ecological and evolutionary significance of culm structures in the Cape Floristic Region
(2011) Yates, Megan J; Cramer, Michael D; Verboom, G Anthony; Linder, H Peter
The Cape Floristic Region lies in the southwestern Cape, South Africa, and is known for its high species richness and endemism. This Mediterranean-type climate region receives winter rainfall that averages between 250 and 650 mm annually over most of the region but reaches an average of more than 1000 mm in the mountains of the southwest and southern Cape.
Open Access
Investigating the functional and adaptive significance of leaf size and shape variation in Jamesbrittenia (Scrophulariaceae (s.s). tribe Manulae) : an experimental and comparative approach
(2007) Moore, Timothy E; Verboom, George Anthony; Cramer, Michael D
The adaptive significance of variety in leaf forms remains a mystery for many plant groups. This study aimed, using a combined experimental and comparative approach, to investigate the functional significance of leaf size and shape variation, as well as the patterns of leaf form variation in the genus Jamesbrittenia within the context of phylogenetic history. Leaf sizes 63-measured Jamesbrittenia varied between 0.006cm² (in J. microphylla) and 6.52cm² (in J. megaphylla). Correlations between leaf form and the environment suggest leaf size and shape are primarily adapted to water availability, with only leaf dimension being significantly associated with temperature, while, soil fertility shows no relationship with leaf size. Life history is important, however, as broader leaved annuals and species with shorter-lived leaves are associated with more arid habitats. Results of the experimental trials suggest that the primary function leaf size and shape reduction is to reduce water loss, and not to increase heat shedding. While larger leaves transpire more on a leaf-by-leaf basis, transpiration may be higher in broader leaved species at the whole-plant due to higher total plant transpiration. Thus, it is suggested a reduction in leaf size and dimension in Jamesbrittenia is an adaptation to more arid environments. Alternatively, a change in life history may enable a plant to escape harsh periods and capitalise on favourable times. A small- to intermediate-leaved, perennial ancestor is inferred for Jamesbrittenia, which was associated with arid regions in either the summer or winter rainfall regions of southern Africa. Shifts to an annual life history in Jamesbrittenia are associated with a shift to drier habitats, particularly in the arid winter rainfall region of South Africa.