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Titre : A one-year, postfire record of element deposition and cycling in the Kogelberg sandstone fynbos mountain ecosystem of the Western Cape, South Africa

Auteur : Bergh, Eugene W

Université de soutenance : University of Cape Town.

Grade : Master of Science (MS) in Geology 2012

Résumé
The Kogelberg Sandstone Fynbos biome of the Harold Porter National Botanical Gardens (HPNBG) and Kogelberg Biosphere Reserve (KBR) in the southwestern Cape of South Africa thrives on thin soils derived from sandstone bedrock and constitutes a biodiversity ‘hotspot’ with approximately 1650 plant species of which 77 species are endemic to the Kogelberg region. The sandstone bedrock substrate with >97 wt% SiO2 (quartz) provides few nutrients to the overlying thin (2 to 20 cm), acidic soils. Additional nutrients enter the ecosystem through marine and mineral (dust) aerosols. Rainwater, stream and mountain seep water, soil and bedrock samples were collected over a oneyear period in an attempt to understand the dynamics of nutrient sources to the fynbos ecosystem. Fire is a critical component of the fynbos ecosystem and this study documents the macronutrient (Cl, Na, SO4, Mg, Ca and K) dynamics of the fynbos for one year following a major fire event on 3 June 2010. Chloride and sodium in rainwater and stream water were found to be primarily derived from a marine aerosol source. The outflux of the ions Cl, Na and K was found to be greater than the influx for these ions. Sulphate, magnesium, calcium and potassium reflected a mixed source of marine and mineral dust aerosols. The influx of SO4 increased at sites where all the other ions recorded a decreasing influx. The influx of SO4 was greater at sites to the north relative to sites closer to a marine source suggesting a strong influence from a terrestrial source for SO4. Rainout of chloride and sodium ions decreases away from the coast and net evaporative loss from rainfall to streams is around 38% over the year assuming Cl ion behaves conservatively. Stream concentrations were lowest during warm and dry summer months when salts accumulate in soils. Summer accumulated salts are then flushed out into streams during the cool, wet winter months. Sulphate, magnesium, calcium and potassium concentrations in rainwater and stream water increased after the June 2010 fire event and then slowly decreased with time. For example, elevated rainwater and stream water SO4/Cl molar ratios returned to near marine values 8 months after the June 2010 fire event. Magnesium and calcium appear to have a strong mineral/ash source and may have been influenced by a fire that burnt a region bordering the study area in March 2011. Minor amounts of mica minerals occur in the sandstone bedrock and clay minerals within the soils include minor amounts of illite/mica and trace amounts of smectite and kaolinite. The clay minerals in the soil reflect a mixed source of bedrock derived and windblown mica and feldspar minerals. Mica in the bedrock and clay mineral deposition in the soil may provide additional ions, particularly K and Ca, by chemical weathering. Potassium showed the largest bedrock reservoir and has the greatest weathering flux to the overlying soil. Windblown inputs may include mineral and organic matter dust from the continental interior and ash particles from fires in the region. Afromontane vegetation along the sheltered ravines and stream banks was largely unaffected by the fire, but nearly all above ground fynbos vegetation was destroyed on the surrounding slopes. Vegetation slowly increased within the first year after the fire event, but did not recover sufficiently to take up all of the nutrients available which were lost through stream flow from the ecosystem. Plant regrowth was slower on steep slopes than valley floors reflecting the importance of hydrology and soil thickness. Partially burnt plant debris was washed down slope to accumulate on lower slopes which fed additional nutrients into streams where plant regrowth and total biomass are higher. The regrowth of vegetation is dependant on both macro- and micro-nutrient (N and P) supply. Nitrogen may limit regrowth as there was in general an over supply of macronutrients to the ecosystem. Excess nutrient loss was reduced to pre-fire levels as the vegetation mass increased with future vegetation growth limited by the nutrient influx from a mixture of marine and mineral aerosol sources.

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