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  • 1
    Electronic Resource
    Electronic Resource
    Ecosystem Level Impacts of Invasive Acacia saligna in the South African Fynbos (2004)
    Oxford, UK; Malden , USA : Blackwell Science Inc
    Restoration ecology 12 (2004), S. 0 
    Details
    ISSN: 1526-100X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Recent efforts to clear invasive plants from the fynbos of South Africa forces managers to think about how N2-fixing invasives have altered ecosystem processes and the implications of these changes for community development. This study investigated the changes in nitrogen (N) cycling regimes in fynbos with the invasion of Acacia saligna, the effects of clear-cutting acacia stands on soil microclimate and N cycling, and how altered N resources affected the growth of a weedy grass species. Litterfall, litter quality, soil nutrient pools, and ion exchange resin (IER)-available soil N were measured in uninvaded fynbos, intact acacia, and cleared acacia stands. In addition, a bioassay experiment was used to ascertain whether the changes in soil nutrient availability associated with acacia would enhance the success of a weedy grass species. Acacia plots had greater amounts of litterfall, which had higher concentrations of N. This led to larger quantities of organic matter, total N, and IER-available N in the soil. Clearing acacia stands caused changes in soil moisture and temperature, but did not result in differences in IER-available N. The alteration of N availability by acacias was shown to increase growth rates of the weedy grass Ehrharta calycina, suggesting that secondary invasions by nitrophilous weedy species may occur after clearing N2-fixing alien species in the fynbos. It is suggested that managers use controlled burns, the addition of mulch, and the addition of fynbos seed after clearing to lower the levels of available N in the soil and initiate the return of native vegetation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1061-2971.2004.00289.x
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  • 2
    Electronic Resource
    Electronic Resource
    Distribution of South African C3 and C4 species of Cyperaceae in relation to climate and phylogeny (2004)
    Oxford, UK : Blackwell Science Pty
    Austral ecology 29 (2004), S. 0 
    Details
    ISSN: 1442-9993
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Abstract  In this study the contribution of climatic factors and phylogenetic relationships affecting the geographical distribution of C3 and C4 genera of the Cyperaceae in South Africa was investigated. The δ13C values of herbarium specimens of 68 southern African species from 22 genera and eight tribes were used to assign the species to either the C3 or C4 photosynthetic pathway. Geographical distribution data for the Cyperaceae were used to investigate relationships between climatic factors and the number of species and proportional abundance of C4 species per region. The number of Cyperaceae species per 2° × 2° square across South Africa varied from less than five in the north-western regions to more than 15 in the south-western and north-eastern regions of South Africa where rainfall exceeds 800 mm y-1. Of the 68 species investigated, 28 had C4 photosynthesis and these were scattered among nine genera of four tribes (Cypereae, Scirpeae, Abildgaardieae and Rhyncosporeae). The proportional abundance of C4 species ranged from 14% in the winter rainfall regions of the south-west of South Africa to 67% in the summer rainfall areas of the north-east. The geographical distribution of species was related to their phylogenetic position such that the distributions of C3 and C4 species in Cypereae, Scirpeae and Schoeneae was quite distinct. Linear regression analysis showed that the transition temperatures (equal C3 and C4 species numbers) for the Cyperaceae were different to those obtained for the Poaceae from the same region. No strong relationships were found between the proportional abundance of C4 species and other climate factors such as altitude and rainfall. Our analysis of the current geographical distribution of C4 Cyperaceae in southern Africa in a phylogenetic context suggests that the ecological advantages conferred by the C4 pathway differ amongst the different plant groups.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1442-9993.2004.01368.x
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  • 3
    Electronic Resource
    Electronic Resource
    Impacts of invading N2-fixing Acacia species on patterns of nutrient cycling in two Cape ecosystems: evidence from soil incubation studies and 15N natural abundance values (1995)
    Springer
    Oecologia 101 (1995), S. 375-382 
    Details
    ISSN: 1432-1939
    Keywords: Nitrogen isotope discrimination ; Cape floristic region ; Fynbos ; Strandveld ; Acacia
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract This study examines the impacts of woody, N2-fixing invasive Acacia spp. on the patterns of nutrient cycling in two invaded ecosystems of differing nutrient status in the Cape floristic region. Patterns of soil nutrient mineralization were measured by a field incubation method while the significance of the fixation process in altering nutrient cycling was assessed by the δ15N natural abundance technique. The results confirm earlier reports that invasion by woody shrubs results in organic matter and nutrient enrichment of surface soils of both ecosystems. However, patterns of nutrient availability (phosphorus and nitrogen) were not necessarily enhanced. In the more fertile strandveld both phosphorus and nitrogen (significant at P〈0.10) showed trends towards enhanced annual mineralization rates upon invasion, while in the low nutrient fynbos system only phosphorus followed this trend. It is unclear whether this differential response is a consequence of plant- or soil-derived feedbacks on the decomposition processes in each system. The δ15N values of the soils from the invaded sites of both ecosystems indicated a strong influence of the alien species on the soil nitrogen component. However, as with other studies of natural ecosystems, the contribution of nitrogen from fixation could not be readily quantified with the δ15N natural abundance method because of problems in selecting suitable non-N2-fixing reference plants. A technique of disrupting nodule structure and function, by fumigation with O2, to obtain the δ15N value of a non-N2-fixing speciment of the study species was tried and found to overcome some of the problems associated with the lack of suitable reference plants. With this technique it was possible to detect the almost total dependence of A. saligna on N2-fixation in the fynbos soils with their low nitrogen mineralization rates. In the strandveld ecosystem with much higher soil nitrogen release rates A. cyclops was only partly dependent on fixation (about half) for its nitrogen. The nutrient enrichment of both ecosystems and trends towards enhanced rates of nutrient mineralization could have profound implications on the long-term success of alien invader clearing operations and the restoration of the indigenous flora at these sites.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00328825
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  • 4
    Electronic Resource
    Electronic Resource
    Nonstructural carbohydrate allocation following different frequencies of simulated browsing in three semi-arid shrubs (1995)
    Springer
    Oecologia 102 (1995), S. 238-245 
    Details
    ISSN: 1432-1939
    Keywords: Browsing responses ; Semi-arid shrubs ; Nonstructural carbohydrate accumulation ; Regrowth
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Nonstructural carbohydrate allocation patterns in response to different frequencies of simulated browsing (leaf and twig removal) were studied in the following semi-arid shrubs: Osteospermum sinuatum, a dwarf deciduous shrub, Pteronia pallens, a dwarf evergreen shrub, and Ruschia spinosa, a dwarf leaf-succulent shrub. Simulated browsing at all frequencies resulted in the elevation, or had no effect, on total nonstructural carbohydrate (TNC) concentrations of O. sinuatum plant parts, and resulted in the decrease in TNC concentrations of R. spinosa plant parts. The responses of P. pallens were intermediate with elevations as well as declines in TNC concentrations of plant parts measured in response to various clipping frequencies. At the low frequency of simulated browsing (every 26 weeks) elevations in plant TNC content were measured in the two non-succulent shrubs O. sinuatum and P. pallens. It was concluded that the overcompensation with respect to TNC accumulation observed in the two non-succulent species represents one of the ways in which excess photosynthate is utilized by browsed shrubs with a limited regiowth potential. Simulated browsing was the least detrimental with respect to biomass production to the non-succulent O. sinuatum and P. pallens, and most injurious to the leaf-succulent shrub, R. spinosa. The observed TNC allocation patterns could not adequately explain the variation among species in the production of new growth and it was concluded that some factor(s) other than the carbon resource was limiting regrowth.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00333256
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  • 5
    Electronic Resource
    Electronic Resource
    Bird effects on organic processes in soils from five microhabitats on a nunatak with and without breeding snow petrels in Dronning Maud Land, Antarctica (1998)
    Springer
    Polar biology 20 (1998), S. 112-120 
    Details
    ISSN: 1432-2056
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Five microhabitat types with varying degrees of bird influence were examined. Soils were collected from open polygons, under mosses and bird nests on a nunatak with breeding snow petrels (Pagodroma nivea) and from open polygons and under mosses on a non-bird nunatak. Nutrient levels (total N and P, nitrate, nitrite and ammonia), moisture levels and δ 15N values were determined and the organic processes of nitrogen fixation (acetylene reduction) and soil respiration (CO2 flux) were examined. Nests represented the most favourable microhabitat type for soil respiration having the highest nutrient levels and most favourable temperature and moisture regimes. The soils under mosses were also favourable and appear to act as a nutrient sink for nutrients originating from the nests. The open polygons were the least favourable for biological activity. There was little nitrogen fixation in any of the soils except for the soils under mosses from the non-bird nunatak. Fixation is possibly limited in favourable microhabitat types on the bird nunatak by high nitrogen levels. These results were confirmed by the δ 15N results, which had high values typical of a seabird signal in the soils from the bird nunatak and values near zero, typical of soils containing fixed nitrogen, on the non-bird nunatak.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003000050284
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  • 6
    Electronic Resource
    Electronic Resource
    On the uptake of ornithogenic products by plants on the inland mountains of Dronning Maud Land, Antarctica, using stable isotopes (1998)
    Springer
    Polar biology 20 (1998), S. 107-111 
    Details
    ISSN: 1432-2056
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Breeding snow petrels (Pagodroma nivea) may be a major source of nutrients for the ecosystems on the inland nunataks (mountain outcrops) of continental Antarctica. To test this theory, the δ15N and δ13C values of soil and plants (moss, lichen and algae) from 11 nunataks with and without breeding snow petrels in northwestern Dronning Maud Land were compared. High δ15N values in all samples from nunataks with breeding birds provide a strong indication of the presence of breeding birds, as well as the use of bird-derived nitrogen by the plants of these ecosystems. The δ13C values showed little difference between the nunataks and are not a useful indicator of bird effects on ecosystem development.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003000050283
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  • 7
    Electronic Resource
    Electronic Resource
    Seasonal allocation of dry mass and nitrogen in a fynbos endemic Restionaceae species Thamnochortus punctatus Pill. (1987)
    Springer
    Oecologia 72 (1987), S. 315-320 
    Details
    ISSN: 1432-1939
    Keywords: Allocation patterns ; Restionaceae ; Nitrogen ; Dry mass ; Fynbos
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The annual growth pattern of male plants of the endemic, dioecious fynbos species, Thamnochortus punctatus, revealed sequential growth of the various below-and above-ground organs. Root and rhizome development were favoured in the wet winter months while culm elongation occurred in the warmer but drier spring to summer period. The reproductive phase of development followed in the hot dry summer and autumn. The asynchronous pattern of growth is suggested to enable the plant to maximize utilization of scarce resources within the confines of the environmental controls determined by the mediterranean-climate. Maximum root and rhizome activity in the rainy season enabled the plant to absorb and store nitrogen in mature culms for later above ground organ development. As the absorption of nitrogen from senscing material is essential for this pattern of organ development to assist in alleviating nutrient scarcity the high degree of nitrogen reabsorption, 35 to 70% for branches and culms respectively, shows the potential importance of asynchronous organ growth in overcoming nitrogen limitation of plant growth. Comparisons of dry mass and nitrogen allocation patterns indicate that developmental strategies employed by evergreen, perennial plants in climatically and edaphically constrained environments cannot be deduced from biomass allocation patterns alone as they do not appear to reflect strategies evolved to overcome nutrient limitation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00379284
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  • 8
    Electronic Resource
    Electronic Resource
    Density dependent interactions between VA mycorrhizal fungi and even-aged seedlings of two perennial Fabaceae species (1992)
    Springer
    Oecologia 91 (1992), S. 281-287 
    Details
    ISSN: 1432-1939
    Keywords: Vesicular-arbuscular mycorrhizas ; Densitydependence ; Plant population dynamics ; Resource depletion ; Phosphorus
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The interaction of density and mycorrhizal effects on the growth, mineral nutrition and size distribution of seedlings of two perennial members of the Fabaceae was investigated in pot culture. Seedlings of Otholobium hirtum and Aspalathus linearis were grown at densities of 1, 4, 8 and 16 plants per 13-cm pot with or without vesicular-arbuscular (VA) mycorrhizal inoculum for 120 days. Plant mass, relative growth rates, height and leaf number all decreased with increasing plant density. This was ascribed to the decreasing availability of phosphorus per plant as density increased. O. hirtum was highly dependent on mycorrhizas for P uptake but both mycorrhizal and non-mycorrhizal A. linearis seedlings were able to extract soil P with equal ease. Plant size distribution as measured by the coefficient of variation (CV) of shoot mass was greater at higher densities. CVs of mycorrhizal O. hirtum plants were higher than those of non-mycorrhizal plants. CVs of the facultatively mycorrhizal A. linearis were similar for both mycorrhizal and non-mycorrhizal plants. Higher CVs are attributed to resource preemption by larger individuals. Individuals in populations with high CVs will probably survive stress which would result in the extinction of populations with low CVs. Mass of mycorrhizal plants of both species decreased more rapidly with increasing density than did non-mycorrhizal plant mass. It is concluded that the cost of being mycorrhizal increases as plant density increases, while the benefit decreases. The results suggest that mycorrhizas will influence density-dependent population processes of faculative and obligate mycorrhizal species.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00317797
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  • 9
    Electronic Resource
    Electronic Resource
    Regrowth and tannin production in woody and succulent karoo shrubs in response to simulated browsing (1993)
    Springer
    Oecologia 96 (1993), S. 562-568 
    Details
    ISSN: 1432-1939
    Keywords: Induced defence ; Clipping ; Semi-arid shrubs ; Herbivore-deterrents
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Allocation of carbon to chemical defences has often been suggested to be a direct response to browsing or grazing by herbivores. This study quantitatively compares total polyphenol and tannin production in response to simulated browsing of three karoo shrubs in order to test this induced defence hypothesis. The three species studied responded to browsing either by rapid regrowth or by increasing polyphenol production in the remaining tissues. The patterns did not follow any phylogenetic relationships but were weakly associated with the palatability of each species. The highly palatable deciduous species Osteospermum sinuatum, which is capable of rapid regrowth, showed no or very low levels of constitutive and browsing-induced total polyphenols, condensed tannins and protein-precipitating tannins. The evergreen sclerophyllous species Pteronia pallens showed a limited regrowth capacity and had intermediate levels of polyphenols, while the evergreen succulent species Ruschia spinosa showed almost no regrowth over the study period. R. spinosa contained the highest constitutive and browsing-induced levels of polyphenols, condensed tannins and protein-precipitating tannins. In two of the species more than one anti-herbivore defence feature co-occur. P. pallens foliage contains both hepatotoxins and polyphenols while R. spinosa has both structural (spines) and chemical defences. Responses of karoo shrubs to simulated browsing are interpreted as the result of passive alterations in plant chemistry rather than as an active defence response to herbivores.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00320514
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  • 10
    Electronic Resource
    Electronic Resource
    The costs of leaving home: ants disperse myrmecochorous seeds to low nutrient sites (1989)
    Springer
    Oecologia 81 (1989), S. 412-417 
    Details
    ISSN: 1432-1939
    Keywords: Myrmecochory ; Fynbos ; Regeneration ; Fire ; Nutrient
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Leucospermum conocarpodendron (L.) Buek (Proteaceae) seedlings were excavated several months after a fire in Cape fynbos. Seedlings under burnt parental skeletons had short hypocotyls (mean 25 mm) indicating passive dispersal whereas seedlings in the open were more deeply buried (mean 48 mm) by ants. Soil nutrient concentrations at the site of germination were negatively related to depth of burial and distance from parent. Ant dispersal resulted in seedlings emerging in soils with lower nutrient concentrations than passively dispersed seeds. Tissue analysis supported the soil results with lower P content in seedlings from open (ant dispersed) sites. Seedling survival in the first year of establishment was also lower in open sites, but not significantly so. However seedlings were slightly taller in the open. The results of this study, the first on naturally occurring intraspecific variation in myrmecochory, strongly contradict current explanations for the high incidence of myrmecochory in nutrient poor environments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00377092
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