Browsing by Author "Hawkins, Heidi-J"

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    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.
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    Phosphorus sensitivity in species of Proteaceae (Protea obtusifolia, Leucadendron coniferum and Leucadendron salignum) from different soil habitats : possible candidates for growth on former agricultural soils high in P
    (2006) Harris, Sue-Re; Cramer, Michael D; Hawkins, Heidi-J
    The phosphorus (P) uptake capacities and tolerance to high P of three Proteaceae species from acid (Leucadenron coniferum, Leucadendron salignum) and alkaline (Protea obtusifolia, Leucadedndron meridianum) soils in the Cape Floristic Region (CFR) were compared. These species were also investigated as potential candidates for growth on nutrient enriched, post-agricultural soils. In parallel, two crop plant species (Lupinus albus, with cluster roots; and L. angustifolius, without cluster roots) were also compared. The cluster-root forming species (L. albus) was more sensitive to higher P levels and showed toxicity in terms of photosynthesis, efficiency of PSII, transpiration and stomatal conductance. Growth, gaseous exchange and efficiency of photosystem II (as a measure of stress) were determined at 1, 10 and 100 μg P g⁻¹ sand for both lupins and Proteaceae. Kinetics of P uptake (in hydroponics) was determined at a range ( 4 to 2000 μM P) of P concentrations. Phosphorus sensitivity was relatively high in Ld. coniferum as evident from reduced growth, net photosynthetic rate, stomata} conductance and transpiration rates. Protea obtusifolia showed the least sensitivity to increased phosphorus concentrations under acidic and native alkaline conditions. The inability to regulate P uptake rates at elevated phosphorus concentrations and excessive P accumulation were major determinants of P sensitivity. The relatively higher P sensitivity of Lupinus albus compared to L. angustifolius is likely related to the efficient P uptake by cluster roots in the former.
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    The secretion of acid phosphatase by proteaceae as an adaptation to limited phosphate availability
    (2010) Hunter, Roland; Cramer, Michael D; Hawkins, Heidi-J
    The Proteaceae are a distinctive component of South Africa’s Fynbos and Australia’s Kwongan vegetation and display a number of adaptations to the soils of these nutrientpoor regions, where availability of phosphate (P) is particularly limited. The Proteaceae are notable for the virtually family-wide ability to form cluster roots.