OpenUCT :: Browsing by Author "West, Adam"

Browsing by Author "West, Adam"

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Open Access
Assessing safety margins and hydraulic strategies in Restionaceae: evaluating xylem hydraulic traits in two Elegia species
(2021) Atkins, Kayla; West, Adam; Skelton, Robert
The movement of water through a plant is responsible for acquiring nutrients, avoiding desiccation and enabling gas exchange. Therefore, access to soil water is vital in determining vegetation structure in a landscape, which suggests that plant hydraulic strategies are an important ecological driver of responses to seasonality and intensity of drought events, especially in the face of climate and habitat change. Wide-spread mortality has been observed over recent decades as a result of drought stress. Many plant species have exhibited hydraulic niche segregation as a result of a trade-offs between stomatal regulation and xylem integrity, and the associated physiological adaptations. This is concerning in the face of climate change predictions of more frequent and intense drought and flooding events because predicted habitat water availability changes may exceed the physiological tolerance ranges of many species. The hydraulic safety margin (difference between xylem vulnerability (P50; MPa) to cavitation and point at which stomata are considered closed (turgor loss point; MPa)) displays the tolerance margins based on plant physiological capacity. Restionaceae have shown extensive evidence of hydraulic niche segregation and are a key demographic of fynbos vegetation but not much is known about their hydraulic strategies. The optical vulnerability method was used to construct vulnerability curves and the turgor loss point (ΨTLP) was acquired from pressure-volume curves. E. fenestrata, a localized seep species, was more vulnerable (P12 = −0.52 MPa; P50 = −1.07 MPa; ΨTLP = −1.57 MPa; negative safety margin = −0.5 MPa) than E. tectorum, a widespread, dry habitat species (P12 = −0.99 MPa; P50 = −1.6 MPa; ΨTLP = −1.64 MPa; slightly negative safety margin = -0.04 MPa). The Cape Floristic region is predicted to receive less rainfall and become drier with ongoing climate change. We expect that this overall drying trend will have a profound impact on the Restionaceae, particularly E. fenestrata that does not have the physiological capacity to deal with severe drought stress.
Open Access
Behavioural and isotope ecology of marine-foraging chacma baboons (Papio ursinus) on the Cape Peninsula, South Africa
(2015) Lewis, Matthew Charles; O'Riain, M Justin; West, Adam
The dominant vegetation type on the Cape Peninsula, South Africa, is an oligotrophic shrub land that supports low numbers of medium-sized and large terrestrial mammals. Of these, only the adaptable and dextrous chacma baboon (Papio ursinus) has learnt to supplement its diet with protein-rich foods from the marine intertidal zone. There are qualitative descriptions of this behaviour in the literature, but the relative contribution of marine foods to baboon diet, the influence of lunar cycles on exploitation and the impacts of marine foraging on ranging behaviour and activity budgets through different seasons have yet to be quantified. Furthermore, all previous studies included data from troops that had access to nutrient-rich exotic foods, which may have reduced their reliance on marine organisms. For this thesis I collected behavioural data on ranging patterns, activity budgets and diet of a free-ranging, natural-foraging troop through full lunar tidal cycles over consecutive seasons. I supplemented the behavioural data with estimates of the troop's diet composition modelled from stable isotope ratios of foods, faeces and hair samples. The troop ranged over 45.262km2 and travelled an average of 6.044 km per day over the study period. The troop's activity budget was dominated by walking and feeding behaviour, both of which peaked during the hot, dry summer months. Both spatial and behavioural data suggest that the study troop is nutrient-stressed relative to other troops in this population, and hence it was surprising that they only consumed small amounts of marine foods during all four seasons. Models incorporating δ13C and δ15N values of baboon faeces and hair confirmed that marine foods were not major dietary items for these baboons, whilst generalized additive models revealed that a range of abiotic factors negatively affect the exploitation of marine foods. Both the probability and intensity of marine foraging within a given hour declined with increasing tide height and swell height, and fluctuated depending on wind speed and direction. Intensity of marine foraging varied through seasons (it was highest in autumn and lowest in spring), and was higher on the east coast than on the west. Together, these results suggest that exploitation of nutrient-rich foods in the intertidal zone is limited by rapid, unpredictable changes inaccessibility. The levels of deviance left unexplained by these models however imply that other as yet unknown factors (e.g. alkaloids in mussels and limpets) may also limit the troop's exploitation of marine foods. In conclusion, this thesis represents the first in-depth study of marine foraging, a behaviour which exemplifies baboons' remarkable behavioural and dietary flexibility. That said, the temporal unpredictability of ease of access, and potential dangers associated with harvesting this resource, appear to limit how much of this high nutrient food resource baboons are able utilise.
Open Access
Bryophytes in the hydrological cycle and climate change implications : A case study of La Réunion cloud forest
(2012) Cardoso, Anabelle Williamson; Hedderson, Terry A; Ah-Peng, Claudine; Flores, Olivier; West, Adam
Bryophytes are able to intercept atmospheric water over the entire surface of their shoot and, once intercepted, this water forms a vital part of the hydrological cycle of their surrounding ecosystems. To investigate the role of bryophytes in the hydrological cycle, our study, conducted in the biodiversity hotspot of the tropical montane cloud forest of La Réunion, focused on two leafy liverwort species, Mastigophora diclados and Bazzania decrescens. We evaluated liverwort biomass, water storage capacity, atmospheric or cloud water interception, and photosynthetic response to desiccation. We found that B. decrescens stored approximately double the mean and maximum litres of water per hectare despite occupying less than half the volume of M. diclados. Despite this decreased water storage capacity, we found that M. diclados had a greater ability to intercept atmospheric moisture than B. decrescens, which had similar interception ability to the control. These interception abilities affected water flux in the two liverwort species. We found that this variation in water flux had an effect on photosynthesis. Both species displayed a significant relationship between photosynthesis and water content. We found that both species showed a loss of photosynthesis at very low and very high water contents with the optimal water content for photosynthesis corresponding to the in situ water content of the liverworts. The abundance of both species and their cloud water interception ability together with the wide range of photosynthetic tolerance of M. diclados and the large water storage capacity and slow desiccation rate of B. decrescens make both liverwort species ecologically important in the forest's hydrological cycle. Anthropogenic climate change threatens this ecosystem as the cloud that these species are so dependent on is predicted to lift. Our findings tie the liverworts very closely to their environment and therefore show support for the idea that bryophytes are excellent early warning signals for predicted climate changes.
Open Access
Classification and dynamics of the forest vegetation of Hluhluwe Nature Reserve, Kwa Zulu-Natal
(1996) West, Adam; Midgley, J J
The forests of Hluhluwe Nature Reserve, Kwa Zulu-Natal, were examined to determine composition, grain, classification, dynamics and conservation value. Field sampling took the form of recording DBH for all species in twenty-one 0.04 ha circular plots, walking 5-metre wide transects in mature and marginal forest to record size-classes and analysis of twenty gaps. Data were analysed by the multivariate programs, TWINSPAN and CANOCO. The composition of the canopy and subcanopy is distinct. Four canopy and two subcanopy communities can be detected. Community succession in the mature forest takes the form of almost random replacement of canopy trees through two subcanopy communities. Size-class distributions indicate a prevalence of shade-intolerant species. This forest is deemed to be coarse-grained and can be classified as coastal scarp forest. Comparisons with other forests shows Hluhluwe to have slightly above average species richness. A removal of anthropogenic disturbance by the creation of the reserve may have resulted in present forest canopy structure. Management for conservation of species diversity should encourage disturbance. There is potential for harvesting of certain shadetolerant species by local communities. This forest is a pristine fragment of a rapidly diminishing vegetation type and efforts should be made to conserve its entire expanse, within Hluhluwe Nature Reserve, in order to prevent species loss.
Open Access
Classification and dynamics of the forest vegetation of Hluhluwe Nature Reserve, Kwa Zulu-Natal
(1996) West, Adam
Open Access
Climatic specialisation: an explanation for the range size and distribution of bryophytes on a tropical island elevational gradient
(2020) Figenschou, Laura Kristina; Hedderson,Terry Albert; West, Adam; Ah-Peng, Claudine
Are species physiologically specialised to the specific climatic conditions in their habitats and can this explain their distributions? This thesis addresses this question using a very specific system: bryophytes on the elevational gradient of Piton des Neiges, Réunion Island. Bryophytes are expected to be specialised to fairly narrow conditions of drought, temperature and insolation, which restrict their geographic range. However, little is known about the mechanisms that connect bryophytes with climatic factors. In this thesis I test the idea of ecophysiological specialisation using reciprocal transplant experiments, along with direct laboratory measurements of species' responses to desiccation and temperature. In these experiments I use species restricted to low, mid or high elevation, as well as widespread species found along most of the gradient. The transplant experiment revealed a trend of upslope survival of restricted species, with species from all sites performing best at their elevation of origin and the site above, and badly at lower elevations. Despite macroclimate being found as an important factor shaping bryophyte range size and distribution, the effect of microhabitat could not be ignored. This was especially true for the widespread species, which showed a strong effect of microhabitat placement in the transplant experiment. Desiccation tolerance was found to increase with elevation in range-restricted species, but widespread species showed little difference in their sensitivity to desiccation, regardless of elevation of origin. Range-restricted species from low elevation were more sensitive to low temperatures and had higher optimum temperatures for photosynthesis than mid- and high-elevation species. Widespread species had narrower ranges of temperature tolerances than range-restricted species, and did not differ in their response to temperature, regardless of elevation of origin. The results of these experiments corresponded well with the climatic conditions that these species habitually experience – with extremes at the gradient peripheries and intermediate conditions in between. This thesis showed that specialisation to both macro- and microclimatic conditions can be attributed as a main driver of bryophyte range size and distribution on the elevational gradient of Piton des Neiges, Réunion Island. This research adds to the body of knowledge on the physiological responses of tropical bryophytes, which is important for species' distribution modelling. Furthermore, it provides insight into the factors that shape bryophyte distribution, critical for biodiversity management under climate change scenarios.
Open Access
Cycad forensics: tracing the origin of poached cycads using stable isotapes, trace element concentrations and radiocarbon dating techniques
(2013) Retief, Kirsten; West, Adam; Pfab, Michele
In South Africa, the greatest threat to cycad populations is the illegal removal of wild plants for landscaping purposes and collector's gardens, resulting in declining populations that are already threatened with extinction. Radiocarbon dating in conjunction with stable isotopes and trace element concentrations could provide the essential evidence needed in court when prosecuting poachers and collectors. The aim of this study was to determine the feasibility of using radiocarbon dating, stable isotopes and trace element concentrations to identify specimens removed from the wild. To test this method, we sampled two individuals of Encephalartos lebomboensis and E. arenarius, with known wild origins and relocation histories (1946 and 1992 respectively), and compared these to individuals from the same wild location that had not been relocated (controls). Vascular rings were sampled using an increment corer from the upper, middle and lower sections of the cycads, which were cut into sections so that the inner, middle and outer rings for each core could be sampled. These samples were reduced to cellulose for δ18O and δ13C isotope analysis. Leaf bases were sampled from the upper, middle and lower sections of the cycad stem using a hammer and chisel, and petioles were cut from the cycad to represent the most recent growth. These samples were analysed for the following stable isotopes δ13C, δ15N, δ34S, 87Sr/86Sr, 206Pb/207Pb and 208Pb/207Pb, and the concentration of trace elements were analysed for the leaf bases. The means and variance were compared between the relocated and control specimens and a principal component analysis was done. The radiocarbon ages of E. lebomboensis tissues were more recent than expected, dating from 2006 to 1979, suggesting that this method would only be appropriate for specimens relocated within the past 6 to 30 years. Consistent with this, our δ18O and δ13C in cellulose, as well as the δ34S,87Sr/86Sr ratio and principal component analysis indicated that the relocated E. lebomboensis specimen (relocated 66 years ago) was significantly different from the control, implying that the stem of the relocated specimen did not grow in the wild. In contrast, the relocated E.arenarius specimen (relocated 20 years ago) was isotopically indistiguishable from the control in older tissues indicating a common origin, but was significantly different for the upper leaf bases, as well as a shift in δ18O cellulose for the upper vascular rings, indicating that the plant has been relocated. Thus, the use of radiocarbon dating and stable isotopes showed great potential as a forensic technique for cycads and suggestions for future development of this technique are given.
Open Access
The effects of cloud moisture on Restions, Ericas and Proteas in the Cape Floristic region
(2012) Gibson, Myfannwyn; West, Adam
Recent studies on the interception and utilization of occult precipitation (fog, cloud-borne mist and dew) have revealed that the direct wetting of foliage provides a water subsidy to plants of various ecosystem types. In this study, we investigate the presence of foliar uptake, and the effects of misting on the plant water potential of species representing diverse functional types, namely ericoids, proteoids and restioids in Fynbos species occurring within the Cape Fold mist belt. In this study, foliar uptake after 180-min submergence in distilled water was demonstrated by five of the seven species investigated. These species included all the restioids and ericoids investigated in this study. By contrast, the proteoids L. conocarpodendron and L. laureolum were found to show no significant amount of foliar uptake or increased leaf water content (%). There was an increase in the average, normalized leaf water content in individuals subjected to misting treatments in both proteoids, L. laureolum and L. conocarpodendron. Similarly, there was also an overall increase in plant water status, as shown by the increased water potential in individuals that were subjected to the misting treatment. It was found that control individuals showed a decrease in plant water potential (i.e. lost water) during the day, as can be expected when soil water is not replenished. All species showed significant stomatal conductance, during both night and day. Results indicate that misting events have a significant effect on the overall plant water status in all functional types and the presence of foliar uptake in both ericoids and restioids; thus indicating that cloud events may have an important effect on the vulnerability of these species to drought, under the precepts of global climate change.
Open Access
Germination success and drought response in Erica coccinea
(2015) Leonard, Jenny; West, Adam; Ojeda, Fernando
Resprouters and seeders are two common phenotypes found in fire-prone ecosystems. Although the distribution of the two forms is usually attributed to fire frequency, it has been proposed that the distribution of resprouter and seeder Erica in the Cape Floristic Region (CFR) of South Africa is determined more by water availability. Erica seeders are predicted to withstand the mild droughts of the southwest CFR better than Erica resprouters, which would account for the abundance of seeders in this region. This thesis tested the assumptions that 1) seeders germinate more quickly and successfully than resprouters and 2) seeders survive mild drought better than resprouters. A germination experiment (Chapter 2) and a drought experiment (Chapter 3) were conducted using Erica coccinea, a common Erica species in the CFR, which contains both a resprouter and a seeder form. Germination success was also tested for a third form of E. coccinea found only in fire refugia. I predicted that this form would not require smoke as a cue for germination. Results indicated that (1) resprouters germinated faster than seeders in the presence of smoke, (2) seeders had better germination success than resprouters in the absence of smoke, (3) the "pyrofuge" form did not require smoke to germinate and (4) seeders had lower survival than resprouters during drought. Overall, these results refuted the proposition that E. coccinea seeders have improved germination and drought tolerance. However, variation between populations within the seeder form indicated that more populations should be tested to verify that these results represent the species as a whole. Due to the lack of variation between populations of the "pyrofuges", it is clear that this form has adapted to its' fire-free environment by allowing for germination in the absence of fire.
Open Access
How do trees die following low intensity fires: Exploring the hydraulic death hypothesis
(2014) Nel, Jacques Adriaan; West, Adam
The mechanism by which trees die following a low intensity fire is poorly understood. Traditionally, cambial necrosis (Carbon starvation hypothesis) has been used to explain post-fire tree mortality, however, this does not explain why some trees die within days following a fire. To address this, the hydraulic death hypothesis argues that post-fire tree mortality is a result of a reduction in hydraulic conductance rather than the necrosis of cambium. There are a number of proposed mechanisms by which hydraulic failure can occur during a fire: firstly, plume-induced cavitation as a result of high vapor pressure deficit (VPD) in a fire-plume has been shown to reduce hydraulic conductance. Secondly, changing surface tension as water is heated has also been shown to increase the chance of cavitation. The final mechanism is a reduction in conductance as a result of direct xylem vessel deformation due to the visco-elastic properties of xylem walls (lignin). In order to determine the relative importance of each proposed mechanism, stems of Kiggelaria africana and Eucalyptus cladocalyx were exposed to 70 and 100„aC in two treatments designed to isolate the effect of each mechanism. An oven treatment was used as a surrogate for a fire-plume in order to demonstrate VPD-induced cavitation and a water bath treatment (transpiration inhibited) was used to demonstrate xylem deformation (along with microscopy). This was possible because post-treatment flushing was indicative of the initial cause of the reduction as cavitation is reversible while deformation is permanent. The data was then explored using a Hydraulic Death model we created based on a xylem conductance model from literature. The results showed that VPD-induced cavitation as well as deformation are able to reduce hydraulic conductance in trees exposed to fire, however, E. cladocalyx showed higher loss of conductance at 65„aC than K. africana and deformation was only seen to occur in water bath treatments and only in K. africana. Here we propose that a chain of events provides a mechanism for slowing the rate of heating in branches exposed to a fire-plume and that cavitation plays a protective role. Model exploration implied that vulnerability segmentation is responsible for preventing fire-plume induced runaway cavitation. This is in agreement with the ¡§safety valve hypothesis¡¨, however, rather than drought stress, it is a fire-plume which causes the cavitation. It was also found that E. cladocalyx was able to prevent deformation because of xylem vessel characteristics (thick vessel walls) and not bark properties. We propose that the necrosis of cambium and phloem leading to the inability to refill cavitated vessels is the actual cause of mortality in trees exposed to low intensity fires. The ability to refill is dependent on water availability and carbohydrate content, which is highly sensitive to drought. As resprouters store water and carbohydrates in lignotubers and stems, they are less sensitive to pre-fire conditions. However, the survival of cambium and phloem is essential to the refilling process and thus the mechanism for reducing heat transfer, bark properties as well as xylem characteristics work in combination to determine persistence after a fire.
Open Access
Hunting for humans in forest ecosystems : are the traces of Iron-age people detectable? : an investigation into the importance of Iron-age slash-an-burn agriculture in KwaZulu-Natal forests using compositional and demographic data and carbon isotope techniques
(1999) West, Adam; Hall, Martin
To what extent are humans responsible for the biological landscapes that we see today? We relate to recent phenomena such as urban environments and commercial farmlands as anthropogenically created landscapes, however historic anthropogenic influence may have been a lot more extensive than previously accepted (Gomez-Pompa & Kaus 1992, Bird 1995, Motzkin et a/1996). In southern Africa we are surrounded by landscapes influenced by humans to some degree (Hoffman 1997). It is now accepted that even wilderness landscapes previously labelled as "pristine" or "natural" are subject to constant change (Botkin 1990) and could well have been generated, or at least influenced, by humans in the past (Gomez-Pompa & Kaus 1992, Foster et a/1996, Bird & Cali 1998). This is certainly the case for many forest systems (Binford eta/ 1987, Balee 1989, Northrop & Horn 1996, Noble & Dirzo 1997, Ogden eta/ 1998, Lindbladh & Bradshaw 1998, Foster et a/1999). This thesis attempts to answer, for forest ecosystems, the question posed almost 20 years ago by Feely (1980): "Did Iron Age Man have a role in the history of Zululand's wilderness landscapes?" In doing so, I hoped to address the larger issue of "ecosystem virginity" and to what extent landscapes with a lengthy history of human habitation are dependant on human-ge·nerated disturbance.
Open Access
A long-term record of the physiological plasticity of stomatal condactance in Proteaceae
(2013) Cronin, Kate; West, Adam
Bioclimatic models suggest that Protea species are likely to be severely affected by predicted increases in temperature and reductions in rainfall in the Western Cape. However, throughout their 400 million year history, land plants have been exposed to considerable climatic variations that have driven physiological adaptations promoting long-term resilience. A principal response of plants to increases in atmospheric CO2 concentration and changing rainfall regimes is to minimise water loss by reducing stomatal conductance (gs) and simultaneously increasing intrinsic water-use efficiency (iWUE). Plasticity in these two physiological traits may enhance the ability of plants to survive future climate change. There is considerable evidence in the literature for major changes in gs and iWUE in response to both short-term and long-term changes in environmental conditions. However, to date, the effects of decadal climate change on plant physiology are still largely uncertain. In this study, the responses of gs and iWUE to perturbations in rainfall and atmospheric CO2 are reconstructed from a chronological sequence of herbarium and modern Protea specimens. The results indicate that in the two high-altitude study species, P.cryophila (t= -2.44, df=7, p= 0.045) and P.venusta (t=3.08, df=5, p=0.027), stomatal conductance has significantly declined in response to increasing atmospheric CO2 concentrations over the past century. The low altitude species appears to maintain fixed stomatal conductance trajectories as a response to more xeric conditions. It is difficult to draw inferences about stomatal physiological plasticity from iWUE data as it is not possible to distinguish between the effects of physiological reductions in stomatal conductance and biochemical enhancement of photosynthesis. Microhabitat sensitivity in iWUE is a further factor confounding interpretation. This highlights the importance of using multiple parameters as analytical tools for assessing long-term physiological change. A key contribution of this study is that it has confirmed the value of using archival material and highlights methodologies that may aid future herbarium-based interpretations. In addition, despite some methodological limitations, the study has identified interesting trends at climate change-relevant timescales that point the way for further research to understand relative vulnerabilities and inform conservation strategies. Specifically, it suggests that future research and conservation efforts may need to be focused on species occurring at low altitudes because of their apparently more limited physiological plasticity.
Open Access
Rodents and Restionaceae : sex-specific plant-animal interactions in dioecious plants
(2016) Van Blerk, Justin; West, Adam; Midgley, Jeremy J
Differing plant canopy structures between the sexes of the dioecious Cannomois congesta (Restionaceae) were found to be the result of the destructive pre-dispersal seed predation methods used by the four-striped grass mouse (Rhabdomys). Mature, reproductive culms on female plants were felled down (mid way along the culm) in large numbers by Rhabdomys in order for it to access the seeds held terminally on culms. Male plants were largely unaffected due to their lack of seeds. Seeds acquired through culm-felling by Rhabdomys were all eaten shortly after their discovery, leading to significant seed mortality each year and reducing the rate of seed dispersal by ants (myrmecochory). I studied the physiological effects of culm-felling by Rhabdomys in C. congesta. Firstly I investigated nutrient resorption from senescing culms and considered the effects of reproductive effort (higher in females) and sex-specific nutrient losses. I found that nutrient resorption from mature culms was highly efficient in both sexes. Culm-felling by Rhabdomys lead to the loss of pre-senescent culm tissue in females and therefore lead to the loss of a significant proportion of potentially resorbed nutrients. Nutrient resorption was found not to be flexible as a strategy to compensate for higher reproductive costs/nutrient-losses in females. This was possibly due to nutrient resorption occurring near is physiological maximum. Plant photosynthetic performance was also investigated using stable isotopes and gas exchange and was found to be similar in both C. congesta sexes throughout the year. The influence of culm-felling by Rhabdomys on female micro-climate also did not appear to have a significant influence on photosynthetic performance between sexes. There was a general lack of evidence to suggest compensatory physiological changes or reductions in fitness (photosynthetic capacity/growth/reproduction) in females due to higher reproductive costs/nutrient-losses. The lack of physiological differences and the fact that females supported a greater number of culms relative to males suggested that other compensatory mechanisms could exist. Phenological differences were discovered and could be important in allowing flexibility between sexes to best use available resources for their specific sexual requirements. Another possibility is that compensatory growth occurred in females at the cost of rhizome-stored nutrients. This has been shown to occur in graminoids after high levels of herbivory (Bryant et al 1983).
Open Access
The role of hydraulic strategies in understanding the response of fynbos to drought
(2014) Skelton, Robert Paul; West, Adam
This dissertation is an investigation into the relevance of the hydraulic framework (sensu McDowell et al., 2008) in understanding the response of diverse communities to drought. This framework distinguishes species that respond to dehydration by rapidly decreasing stomatal conductance (gs) (isohydric), thereby maintaining relatively constant midday minimum water potential, from those that maintain relatively high gs (anisohydric), thereby maintaining carbon assimilation albeit at the cost of hydraulic dysfunction. However, currently, the importance of the hydraulic framework in explaining drought response in diverse communities is poorly understood. Drought-related plant mortality is of particular concern in South Africa’s fynbos biome, a global biodiversity hotspot. This Mediterranean-type region is predicted to experience an increase in drought, with unknown consequences for the endemic flora. We asked whether drought affected this diverse ecosystem in a predictable manner and if these patterns fit with existing frameworks of plant drought-mortality mechanisms.
Open Access
Substrate dependant survivorship of Ecklonia maxima, in southern African kelp communities
(1996) West, Adam; Anderson, Robert J
Several south-western Cape Ecklonia maxima communities were examined to determine their basic demography and the effects of substrate selection. Overall percentage substrate composition, stipe lengths and substrate attachment for all kelps were recorded at four sites off the Cape Peninsula. Strength of attachment to different substrates was measured. Selection for substrate type was analysed using Jacobs' index. A substrate of Pyura stolonifera was the most common for juveniles, whereas substrates of rock and conspecific holdfasts were dominant in the larger individuals. No significant difference in strength of attachment was found for kelps less than 50 cm. An analysis of the frequency of single- and multi-stiped holdfasts indicates that survival on rock is greater than on that of holdfasts. Size-class distributions indicate that E. maxima is a shade-tolerant species showing advanced regeneration. Establishment on rock may occur through episodic recruitment escaping grazing pressure. In areas with low grazing intensity, recovery from catastrophic storms may be limited by competition with understorey algae.
Open Access
The effect of wind run on transpiration and leaf temperature of Leucospermum concarpodendron on the Cape Peninsula
(2012) Karpul, Rebecca; Karpul, Rebecca; West, Adam
Parts of the Cape Floristic Region are subject to exceptionally elevated levels of wind in certain seasons, however it has recently been recorded that the Western Cape has experienced a reduction of 25.7% in wind run between the years 1974 - 2005. Shifts in wind run across the globe may significantly affect whole plant physiology through changes in transpiration which can inﬂuence mass-flow mediated movement of nutrients in winter as well as leaf temperature. The objectives of this study were to establish whether (a) decreased wind run could reduce transpiration in winter when vapour pressure deficit is low and (b)leaf temperature approaches damaging levels during low wind run conditions in Leucospermum conocarpodendron on the Cape Peninsula over a given time period. High levels of vapour pressure deficit (>l.5 kPa.w4hr⁻¹) did not occur simultaneously with high levels of wind run (>250 m.24hr⁻¹) and therefore the effects that these factors have on transpiration could be separated. Variable rates of transpiration corresponded with reduced wind run under low vapour pressure deficit conditions in winter, therefore decreased wind run may have the potential to reduce transpiration. However, transpiration is highly sensitive to minor ﬂuctuations in vapour pressure deficit during low wind run conditions. Variable leaf temperatures corresponded with low wind run and low vapour pressure deficit conditions, therefore decreased wind run may have the potential to increase leaf temperatures. However, leaf temperature is highly sensitive to low wind run and low vapour pressure deficit conditions. Low wind run conditions, currently predicted for the future, does not pose a threat to transpiration, nutrient uptake and leaf temperature of Leucospermum conocarpodendron plants on the Cape Peninsula.

Browsing by Author "West, Adam"

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