Elevated temperatures and diminished precipitation amounts accompanying climate warming in arid ecosystems are expected to have adverse effects on the photosynthesis of lichen species sensitive to ...elevated temperature and/or water limitation. This premise was tested by artificially elevating temperatures (increase 2.1-3.8°C) and reducing the amounts of fog and dew precipitation (decrease 30.1-31.9%), in an approximation of future climate warming scenarios, using transparent hexagonal open-top warming chambers placed around natural populations of four lichen species (Xanthoparmelia austroafricana, X. hyporhytida , Xanthoparmelia. sp., Xanthomaculina hottentotta) at a dry inland site and two lichen species (Teloschistes capensis and Ramalina sp.) at a humid coastal site in the arid South African Succulent Karoo Biome. Effective photosynthetic quantum yields (∆F/F′ m ) were measured hourly throughout the day at monthly intervals in pre-hydrated lichens present in the open-top warming chambers and in controls which comprised demarcated plots of equivalent open-top warming chamber dimensions constructed from 5-cmdiameter mesh steel fencing. The cumulative effects of the elevated temperatures and diminished precipitation amounts in the open-top warming chambers resulted in significant decreases in lichen ∆F/F′ m . The decreases were more pronounced in lichens from the dry inland site (decline 34.1—46.1%) than in those from the humid coastal site (decline 11.3-13.7%), most frequent and prominent in lichens at both sites during the dry summer season, and generally of greatest magnitude at or after the solar noon in all seasons. Based on these results, we conclude that climate warming interacting with reduced precipitation will negatively affect carbon balances in endemic lichens by increasing desiccation damage and reducing photosynthetic activity time, leading to increased incidences of mortality.
Bird flight distances for the small Zosterops capensis, the medium-size Pycnonotus capensis and the large Colius striatus were extracted from these birds' initial ring and subsequent recapture ...locations and expressed on equivalent per km bases. The products of the bird-ring recapture records in nine different flight distance categories and daily consumption rates by these birds of seeds of two native (Chrysanthemoides monilifera and Olea europaea spp. africana) and two alien (Lantana camara and Solanum mauritianum) shrubs were used to construct seed dispersal curves. The dispersal distances to which ingested seeds were theoretically restricted were computed from the product of the retention time of seed in the birds' guts and their flight speeds using published functions. All three bird species displayed thin long-tailed seed dispersal curves characterized by peaks at distances below 1 km which declined progressively with increasing distances, the tails extending to distances of up to 400 km. Flight distances corresponding with predicted seed gut retention times were 9.4 km in the small Z. capensis, 17.8 km in the medium size P. capensis and 21.2 km in the large C. striatus. These potential seed dispersal distances were much greater that the frequently reported long distance seed dispersal threshold of 1 km by frugivorous birds in fragmented landscapes.
Alien plants with abundant and nutritious fruits may compete more effectively for avian dispersal services than native shrubs. This premise was examined by comparing daily foraging activity ...(visitation frequency, foraging bird number and foraging period) by four frugivorous bird species of different size on fruits of two native and two alien shrub species co-occurring at four different sites. Also, the quantities of seeds consumed daily by each of the four birds species from fruits of the four shrub species were measured and compared with the numbers of fruits and seeds, and the mass of fruits present in the shrub canopies as well as with their fruit monosaccharide concentrations. The quantities of seed consumed daily by four different size bird species (Columba arquatrix, Colius striatus, Pycnonotus capensis, Zosterops pallidus subsp capensis) were positively correlated with the numbers of seeds per m 2 of canopy area and with fruit mass and fruit monosaccharide content per m 2 of canopy area, as well as with the monosaccharide concentration of individual fruits, except in the C. arquatrix (African olive pigeon). All four bird species displayed the highest daily visitation frequencies on fruits of the alien Solanum mauritianum which were more abundant and nutritious than fruits of the other alien Lantana camara and fruits the natives Olea europaea subsp africana and Chrysanthemoides monilifera. They also all consumed greater quantities of seed daily from fruits of the alien S. mauritianum than from fruits of the other shrub species. These results corroborate proposals that frugivorous birds concentrate their foraging activities on those alien plants with the most abundant and nutritious fruits.
We examined the efficacy of seed enumeration and videographic techniques for determining seed removal by birds from indigenous (Chrysanthemoides monilifera and Olea europaea subsp africana) and alien ...(Lantana camara and Solanum mauritianum) shrubs at different study sites in the Cape Floristic Region. The seed enumeration technique involved counting the numbers of fruits and associated seeds removed monthly by birds, excluding those naturally abscised, from the shrub canopy. The videographic technique involved visual counts from images of the numbers of fruits and associated seeds consumed by birds over specific time intervals captured by a digital camcorder. Daily seed removal rates by all birds, irrespective of species, measured by both techniques were similar with no significant interactions evident between measuring techniques, site and shrub species. Both techniques displayed higher seed removal from tiny‐seeded S. mauritianum than other shrub species; this was also evident among individual bird species. However, the seed enumeration technique was unable to discriminate between foraging organisms, contamination of traps by wind‐blown fruits abscised from neighbouring branches and fruit theft from the canopy and the traps. In contrast, the videographic technique provided permanent visual and time‐lapse records for individual foraging bird species allowing greater measurement precision and interpretation of fruit removal behaviour by birds. We recommend use of the videographic technique over the seed enumeration technique for studying vertebrates’ seed removal in a detailed manner.
Résumé
Nous avons examiné l'efficacité de la technique de dénombrement des semences et de la technique vidéographique pour déterminer le prélèvement, par les oiseaux, de semences provenant de buissons indigènes (Chrysanthemoides monilifera et Olea europaea subsp africana) et exotiques (Lantana camara et Solanum mauritianum) sur différents sites d’études de la Région floristique du Cap. La technique de dénombrement des semences impliquait de compter le nombre de fruits, et des semences associées, prélevés chaque mois par les oiseaux à la canopée des buissons, à l'exclusion de ceux qui sont tombés naturellement. La technique vidéographique impliquait des comptages visuels, à partir d'images captées par un caméscope digital, du nombre de fruits et des semences associées consommés par les oiseaux à intervalles déterminés. Les taux journaliers de prélèvement des semences par tous les oiseaux, quelle que soit leur espèce, mesurés selon les deux techniques, étaient semblables, sans interactions significatives visibles entre les techniques de mesure, les sites et les espèces de buissons. Les deux techniques révélaient un prélèvement plus important de S. mauritanium, aux graines minuscules, que d'autres espèces d'arbustes. Ceci était aussi très clair entre chaque espèce d'oiseaux. Cependant, la technique de dénombrement des semences ne permettait pas de faire la distinction entre les organismes consommateurs, la contamination des pièges par des fruits détachés de branches voisines par le vent, le vol des fruits dans la canopée et les pièges. Par contre, la technique vidéographique fournissait un compte‐rendu visuel régulier et permanent pour chaque espèce d'oiseau consommateur, ce qui permet une plus grande précision et une meilleure interprétation du comportement des oiseaux qui se nourrissent des fruits. Nous recommandons d'utiliser la technique vidéographique plutôt que le dénombrement des semences pour étudier de façon détaillée le prélèvement des semences par des vertébrés.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
This paper provides a synthesis of current knowledge on phytoplankton production, seasonality, and stratification in tropical African lakes and considers the effects of nutrient enrichment and the ...potential impacts of climate warming on phytoplankton production and composition. Tropical African lakes are especially sensitive to climate warming as they experience wide fluctuations in the thermocline over a narrow range of high water temperatures. Recent climate warming has reduced phytoplankton biomass and production in the lakes. A decline in the production of palatable chlorophytes and an increase in cyanobacteria has led to reduced zooplankton production and a consequent decline in fish stocks, all of which can be associated with the elevated water temperatures. This indicates that even moderate climate warming may destabilise phytoplankton dynamics in tropical African lakes, thereby reducing water quality and food resources for planktivorous fish, with consequent negative impacts on human livelihoods.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Two hypotheses—that elevated night-time temperatures due to climate warming would enforce post-fire dormancy of Proteaceae seed due to low moisture, and that periods without rain during summer would ...exceed desiccation periods tolerated by Proteaceae seedlings—were tested empirically. Enforced dormancy, i.e., the inability to germinate due to an environmental restraint, was tested by measuring seed germination in 11 Proteaceae species in experimental mesocosms whose soils were artificially elevated by 1.4 and 3.5 °C above ambient by far-red wavelength filtered infrared lamps. Diminished totality of germination and velocities were observed in 91 and 64 %, respectively, of the Proteaceae species tested. Drought resilience was tested in one-year-old seedlings of 16 Proteaceae species by withholding water from potted plants during summer in a greenhouse. The most drought-resilient Proteaceae species displayed the lowest initial transpiration rates at field capacity, the smallest declines in transpiration rate with decreasing soil water content, and the lowest water losses by transpiration. Projected drought periods leading to the complete cessation of transpiration in all Proteaceae species greatly exceeded the number of days without rain per month during summer in the current distribution ranges of those species. It was therefore concluded that enforced seed dormancy induced by elevated night-time temperatures is the post-fire recruitment stage of Proteaceae that is most sensitive to climate warming.
With a high percentage of endemics along the west coast of South Africa, especially in the family Aizoaceae, the region is considered one of the earth's biodiversity hot spots. It has been suggested ...that the diversity and radiation of the Aizoaceae are coincident with low but predictable rainfall and lack of competition between species. In this study we examine the relationship between water source and the efficiency of PSII photochemistry for representative Aizoaceae and non-Aizoaceae. We do this to determine the extent to which the different genera are adapted to the frequent, low volume, precipitation characteristic of the region and to ascertain the extent to which there is competition for water. Our water isotope results show that the Aizoaceae use shallow surface water while the non-Aizoaceae use a deeper water source. We are however not able to show the extent to which the Aizoaceae utilize fog or dew. Our chlorophyll fluorescence results show that there are no differences in efficiency of PSII photochemistry between the species in the wet season. The decline from wet to dry season for the Aizoaceae is, however, more dramatic than that of the non-Aizoaceae reflecting the differences in rooting depth between the different families. These results suggest that, during the dry season, there is no competition for resources between families but there is competition between species. We conclude that the adaptation to using shallow water, coupled with susceptibility to drought of adult short lived Aizoaceae may be a, mechanism for the diversification of this family.
Predicted elevated temperatures and a shift from a winter to summer rainfall pattern associated with global warming could result in the exposure of hydrated lichens during summer to more numerous ...temperature extremes that exceed their thermal thresholds. This hypothesis was tested by measuring lethal temperature thresholds under laboratory and natural conditions for four epilithic lichen species (Xanthoparmelia austro‐africana, X. hyporhytida, Xanthoparmelia sp., Xanthomaculina hottentotta) occurring on quartz gravel substrates at a hot arid inland site two epigeous lichen species (Teloschistes capensis, Ramalina sp.) occurring on gypsum‐rich topsoil at a warm humid coastal site. Extrapolated lethal temperatures for photosynthetic quantum yield under laboratory conditions were up to 4°C higher for lichens from a dry inland site than those from a humid coastal site. Lethal temperatures extrapolated for photosynthetic quantum yield at a saturating photosynthetic photon flux density of ≥11,000 μmol photons m⁻² s⁻¹ under natural conditions were up to 6°C higher for lichens from the dry inland site than the more humid coastal site. It is concluded that only under atypical conditions of lichen exposure in a hydrated state to temperature extremes at high midday solar irradiances during summer could lethal photosynthetic thresholds in sensitive lichen species be potentially exceeded, but whether the increased frequency of such conditions with climate warming would lead to increased likelihood of lichen mortality is debatable.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
► CAM and C3 modes were determined from foliar organic acid accumulation in six succulent shrubs. ► Stem diameter changes relative to atmospheric vapour pressure deficits were used to distinguish CAM ...and C3 modes. ► These methods were compared at two arid South African sites in distinguishing CAM and C3 photosynthetic modes. ► Foliar acidification concurred with predicted stem diameter variations for CAM and C3 modes. ► Stem diameter changes revealed CAM–C3 changes that were concealed when measuring CAM activity from total foliar acids.
Stem diameter variations relative to atmospheric vapour pressure deficits and the corresponding differences in pre-dawn and late dusk foliar organic acids were measured in 4 evergreen and 2 partly drought deciduous succulent shrubs. These methods were compared at two arid South African sites in distinguishing CAM and C3 photosynthetic modes. Changes in stem diameter were continuously recorded at 2h intervals using linear variable differential transducers, known as stem diameter variation sensors. Annual data collected over varying climatic conditions revealed that rapid changes in the stem diameter in response to vapour pressure deficit concurred with typical foliar acid trends expected for CAM and C3 photosynthetic modes. Nocturnal foliar acidification, which typifies the CAM mode, was predominant in evergreen shrubs Cephalophyllum framesii, Ruschia bolusiae, Ruschia stricta and Zygophyllum cordifolium whilst C3 mode was prevalent in drought deciduous Lycium cinereum and Tripteris sinuata. Diurnal trends in foliar acidification concurred with the predicted patterns in stem diameter variations for CAM and C3 modes, thus confirming the potential application of stem diameter sensors in distinguishing CAM and C3 modes in arid environments. Daily amplitudes in stem diameter were significantly reduced by nocturnal fog and dew precipitation in a C3–CAM shrub. This supports earlier studies that the CAM mode in facultative and CAM-cycling plants does not only conserve water, but may also aid in water absorption. Stem microvariation changes revealed CAM–C3 variations that were concealed when measuring CAM activity from total foliar acids. This technique may be very important considering that even the dependable carbon isotope technique under-estimates the number of CAM-equipped plants in natural systems.
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GEOZS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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