Seed and seedling dynamics of certain acacia species as affected by herbivory, grass competition, fire, and grazing system.
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Date
2001
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Abstract
The influence of herbivory, grass competition and grazing system on emergence, growth and
survival of Acacia seedlings in burnt and unburnt areas was investigated in their first growing
season, from September 1997 to May 1998. The study was aimed at determining possible
reasons for the increase in woody plant density in semi-arid savannas, specifically the effect
of excluding small-mouthed herbivores from domestic and livestock systems, and switching
from continuous grazing to rotational grazing systems. The effect of fire intensity and
maximum fire temperature on seed viability, germination and mortality, specifically with
regard to back versus head fires and seed size , were investigated.
Variability among species in the number of seedlings emerging generally resembled
differences in viability. Emergence of Acacia karroo and Acacia tortilis was poorer than that
of Acacia nilotica under grass competition. Emergence was lower in burnt and open areas,
the latter depending on Acacia species. Herbivory did not affect seedling emergence, in both domestic and wildlife systems.
Seedling survival and growth was adversely affected by small-mouthed herbivores in
both domestic and livestock systems, whilst large-mouthed herbivores exerted no direct
effects on woody seedlings, except to a small degree by trampling. Seedlings showed better
growth and survival under low grass competition, which also resulted in greater leaf-to-height
ratios , indicating that grass interference with irradiance affects woody seedlings. This effect
appeared to be greater for cattle and rotational grazing, and for burning, in domestic and
wildlife systems respectively. Woody seedling establishment was also better in burnt areas.
Whilst seedling growth was better under rotational than continuous grazing, survival was not
significantly different. Grass competition influenced seedling establishment to a greater
extent than herbivory, whilst burning made a greater impact than grass competition.
Seed mortality in response to maximum fire temperature was inversely related to seed
size, and trends in seed germination and mortality, although very variable, appeared to be
influenced by threshold fire intensities and maximum temperatures. Back fires had hotter
maximum temperatures and fire intensities at ground level than head fires, which result in greater seed mortality following fire.
Based on the current study it is likely that the removal of grass competition, burning,
and a change from continuous to rotational grazing systems, and small-mouthed to large-mouthed
herbivores, will result in an increase in woody seedling establishment.
Description
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2001.
Keywords
Acacia., Acacia--Seeds., Acacia--KwaZulu-Natal., Seedlings--Growth., Herbivores., Animal-plant relationships., Competition (Biology), Theses--Grassland science.