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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

EUROPEAN ANNUAL GRASSES IN HIGHLY fragmented natural ecosystems along the South African west coast are displacing wildflowers, which form the basis of a growing lucrative, nature-based tourist industry. We examined the cost-effectiveness of different labour-intensive strategies linked to a national poverty relief programme for controlling invasive annual grasses in renosterveld. The treatments tested involved combinations of grass mowing, hand-clearing, light and intense burning and pre-emergent herbicide application randomized over forty-eight 100-m² plots in the Tienie Versfeld Wildflower Reserve. Springtime vegetation responses were monitored over two successive years; labour, capital equipment and consumable costs were audited. Total clearing costs associated with intense burning of uncut grass (R415/ha), grass mowing (R924/ha) and light intensity burning of mowed grass (R1338/ha) were all less than those (up to R1927/ha) reported for clearing dense stands of woody aliens. However, costs of hand-clearing of grass (R6 743/ha) and pre-emergent herbicide application (R13 380/ha) were much greater. Intense burning, the cheapest strategy overall, was ineffective as this promoted recruitment of both alien invasive annual and perennial grasses and inhibited recruitment of native geophytes. We conclude that mowing of grass-infested renosterveld prior to grass seed maturation, and the removal of the cut grass biomass for use as fodder in restricted feed lots to offset clearing costs, provides the most credible strategy for controlling the annual grass populations to conserve native floral diversity over the short term. [ABSTRACT FROM AUTHOR]

Geographical and temporal trends in clear-sky, biologically active ultraviolet-B (UV-B, 280-315 nm) radiation were modelled from 23-yr satellite ozone records across latitudes 22°S to 35°S and Iongitudes16°E to 34°E in southern Africa. Modelled values were calibrated against broadband and spectral instrument-based measurements and mapped at 2.6-km² resolution in accordance with the internationally accepted UV-index scale. Statistically significant latitudinal, longitudinal and intra-annual UV-index gradients were evident, the latitudinal gradients especially displaying large seasonal anomalies ranging from a 4.5% increase in UV-index per degree reduction in latitude during winter to a 0.5% increase in summer. Inter-annual UV-index gradients seemingly corresponded with the 11-yr cycle of extraterrestrial solar activity in which peak UV-indices (up to 13% deviation from 23-yr averages) appeared associated with years of least solar activity, though there were exceptions. These UV-index peaks were most prominent between midwinter and early spring in July, August and September. They also displayed geographical anomalies, the largest deviations in UV-indices mostly apparent at high latitudes and low longitudes. Inter-annual UV-index gradients mostly exceed UV-index increases induced by anthropogenic emissions of ozone-depleting trace gases over the 23-yr assessment period. The latter increases, though relatively modest (up to 9% between 1979 and 2001), have caused substantial geographical alteration in the distribution of indices across South Africa, with midday clear-sky UV-indices of magnitude 13 and greater corresponding to 72% of the country's interior in December 2001 compared with only 2% under comparable levels of solar activity in December 1979. Such geographical shifts are expected to intensify with continuing ozone depletion, especially at the next solar cycle minimum anticipated in 2007/08. [ABSTRACT FROM AUTHOR]

Broadband solid-state detectors, which are robust, relatively inexpensive and uncomplicated to operate, are particularly suitable for routine measurements of biologically damaging solar ultraviolet-B (UV-B: 280-320 nm) radiation, especially in developing countries with limited financial resources and human technical capacity. Specific tables of conversion factors provided by manufacturers of these instruments correct for their dependence on solar zenith angle (SZA) but not on total column ozone; this dependence is due to slight wavelength mismatches between the instruments' spectral responses and biological action spectra. An entirely instrument-based calibration procedure has been described but is applicable only at sites with clear skies and abnormally Iow atmospheric pollution. We have investigated the practicality of calibrating such equipment customarily operating under all weather conditions with typical atmospheric pollution by comparing satellite observation of total column ozone with simultaneous broadband pyranometer and spectroradiometer measurements of erythemal-weighted solar UV-B irradiance over a 42-month monitoring period. The numerous (2657) measurements taken were inadequate to quantify, within statistically acceptable limits, the divergence of ozone-related observations by pyranometer and spectroradiometer of UV-B irradiance at different solar zenith angles. However, a general linear regression that quantified differences in UV-B irradiance measured by broadband instruments as a function of total column ozone inclusive of all SZA values conformed to statistically acceptable criteria. Application of the regression slope and intercept coefficients in a remedial equation reduced the total number of broadband instrument errors in measurement of UV-B irradiance by 40% but by small margins only; these averaged 2.40 ± 0.44%. The considerable time and expenditure entailed for such small corrections are not readily justified. [ABSTRACT FROM AUTHOR]

The interactive effects of increased carbon dioxide (CO2) concentration and ultraviolet-B (UV-B, 280-320 nm) radiation on Acacia karroo Hayne, a C3 tree, and Themeda triandra Forsk., a C4 grass, were investigated. We tested the hypothesis that A. karroo would show greater CO2-induced growth stimulation than T. triandra, which would partially explain current encroachment of A. karroo into C4 grasslands, but that increased UV-B could mitigate this advantage. Seedlings were grown in open-top chambers in a greenhouse in ambient (360 μmol mol-1) and elevated (650 μmol mol-1) CO2, combined with ambient (1.56 to 8.66 kJ m-2 day-1) or increased (2.22 to 11.93 kJ m-2 day-1) biologically effective (weighted) UV-B irradiances. After 30 weeks, elevated CO2 had no effect on biomass of A. karroo, despite increased net CO2 assimilation rates. Interaction between UV-B and CO2 on stomatal conductance was found, with conductances decreasing only where elevated CO2 and UV-B were supplied separately. Increases in water use efficiencies, foliar starch concentrations, root nodule numbers and total nodule mass were measured in elevated CO2. Elevated UV-B caused only an increase in foliar carbon concentrations. In T. triandra, net CO2 assimilation rates were unaffected in elevated CO2, but stomatal conductances and foliar nitrogen concentrations decreased, and water use efficiencies increased. Biomass of all vegetative fractions, particularly leaf sheaths, was increased in elevated CO2, and was accompanied by increased leaf blade lengths and individual leaf and leaf sheath masses. However, tiller numbers were reduced in elevated CO2. Significantly moderating effects of elevated UV-B were apparent only in individual masses of leaf blades and sheaths, and in total sheath and shoot biomass. The direct CO2-induced growth responses of the species therefore do not support the hypothesis of CO2-driven woody encroachment of C4 grasslands. Rather, differential changes in resource use efficiency between grass and woody species, or morphological responses of grass species, could alter the competitive balance. Increased UV-B radiation is unlikely to substantially alter the CO2 response of these species. [ABSTRACT FROM AUTHOR]

Purely symbiotic and NO3-fed nodulated seedlings of Virgilia oroboides (Bergius) T.M. Salter, Cyclopia maculata (L.) Vent and Podalyria calyptrata Willd. were exposed to biologically effective ultraviolet-B radiation (UV-B) to assess the effects of above- and below-ambient UV-B on growth, symbiotic function and metabolite concentrations. Seedlings were grown outdoors either on tables under ambient or 34 or 66% above-ambient UV-B conditions (UV-B100 control, UV-B134 and UV-B166, respectively), or in chambers providing below-ambient (22% of ambient) UV-B (UV-B22) along with a UV-A control and a photosynthetically active radiation (PAR) control. Exposure of seedlings to UV-B166 radiation reduced (P < or = 0.05) leaf and stem dry mass by 34 and 39%, respectively, in C. maculata, and reduced leaf nitrogen concentration (%N) by 12% in V. oroboides. Nodule %N in C. maculata and stem %N in P. calyptrata also decreased (P < or = 0.05) in response to UV-B22 radiation compared with the UV-A control, but not compared with the PAR control. Concentrations of flavonoids, soluble sugars and starch were unaltered by the UV-B treatments. Application of 1 mM NO3 to UV-B166-treated seedlings increased whole-plant dry mass of V. oroboides and P. calyptrata by 47 and 52%, respectively. Dry mass of organs, nodule %N and total N concentration of these species also increased with NO3 application. However, NO3 supply decreased (P < or = 0.05) nodule dry mass, stem %N and leaf %N as well as root and leaf anthocyanin concentrations in C. maculata. In terms of UV-B x N interactions, dry mass of stems, roots, nodules and total biomass of NO3-fed C. maculata seedlings were reduced, and nodule %N, total N and leaf anthocyanins were depressed by the UV-B134 and UV-B166 treatments relative to UV-B100-treated seedlings. Although we found that above-ambient UV-B had no effects on growth and symbiotic function of V. oroboides and P. calyptrata seedlings, feeding NO3 to these species increased (P < or = 0.05) seedling growth. In contrast, purely symbiotic C. maculata seedlings were sensitive to the UV-B166 radiation treatment, and adding NO3 further increased their sensitivity to both the UV-B134 and UV-B166 treatments.