Soil, herbaceous and woody responses to different methods of bush control in a mesic eastern Cape savanna.
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Date
2000
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Abstract
Bush encroachment is a major problem for the savannas supporting livestock in the
Eastern Cape. Farmers employ chemical poisoning and mechanical clearing of woody
vegetation to improve grass production. This thesis addressed the following questions.
1. Does soil fertility and hence, herbaceous production and/or quality increase
beneath or between former bushclumps following woody clearing or poisoning?
2. Are chemical or mechanical control methods effective for all woody species?
3. Does bush density and height decline with increasing competition from the
herbaceous layer?
4. Can fire and goats retard or revert woody re-establishment, thereby keeping these savannas open?
A trial for assessing different methods of controlling bush was conducted near Kei Road. Initial treatments included chemical poisoning, mechanical clearing and a control,
each replicated eight times in 0.36 hectare plots. Follow-up treatments were control, fire,
goats, and the combination of fire and goats each replicated twice per primary treatment.
Two additional mechanically cleared plots were oversown with Chloris gayana seeds.
Herbaceous production, species composition, foliage quality and soil fertility, and
mortality, recruitment, height increase and density of woody individuals were monitored
for five seasons.
Mechanical clearing of the woody vegetation increased soil fertility, except total
nitrogen, and that explained the dramatic response in grass production that endured for
the first four seasons since clearing. The periphery and ex-bushclump zones were
characterised by increased colonization of Panicum maximum while there were no
changes in frequencies of other key herbaceous species in all vegetation zones.
Acacia karroo and woody "weeds" (Solanum incanum, Berkheya bipinnatifida)
established from seed while all other woody species recruited mainly from coppicing.
Seedling recruitment and resprouting resulted in high densities of woody stems and
individuals by the second season after clearing when compared with the pre-clearing
levels. Oversowing cleared areas with pasture grasses did not only increase grass
production but also reduced the density of coppicing woody plants and "weeds".
Chemical treatments mirrored the controls in terms of grass production, except
during a very wet season, and species composition. Although encroaching woody species
(e.g. Scutia myrtina, Maytenus heterophylla and Trimeria trinervus) were susceptible to
poisoning, woody plant density was not reduced. Multi-stemmed woody individuals were
resistant to poisoning. Fire and goats kept most coppicing woody plants short, less than
half a metre, after three seasons of browsing and also improved grass production in the
dense bushclumps suggesting that these clumps were being opened up. However, high
browsing pressure forced goats to graze more hence this effect was masked.
This study indicates that chemical and mechanical control of bush are
economically unsustainable for beef and mutton production, at least in the medium term.
Fire and goats are appropriate as a follow-up strategy for retarding woody regrowth,
keeping bushclumps open, improving grass production and economic viability of
mechanical clearing.
Description
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2000.
Keywords
Range Management--Eastern Cape., Savanna Ecology--Eastern Cape., Bush Encroachment--Eastern Cape., Theses--Grassland science.