| Summary: | Thesis (MScFor)--Stellenbosch University, 2002. === ENGLISH ABSTRACT: Increases in woody plant density ('bush encroachment') reduce livestock production and biodiversity. By
convention, soil moisture, soil nutrients, fire and herbivory are regarded as the principal factors governing the
tree-grass ratio of savannas. An experiment with a completely-crossed design was employed to investigate
woody seedling (Acacia me/lifera) recruitment near Kimberley, Northern Cape, South Africa, after fire and
under conditions of maximum-recorded rainfall, nitrogen addition and grazing. The field experiment was
repeated as a garden experiment to determine if the two experiments yield comparable results.
Tree germination in the field was extremely low, probably due to below-average natural rainfall in plots that
only received natural rain, and insufficient watering frequency in irrigated plots. As a result of low
germination, none of the treatments (rain/nitrogen/fire/grazing) had a significant effect on tree recruitment in
the field experiment. The duration of the experiment (2000/2001 growing season) was insufficient for the
treatments to affect grass composition, although the high rainfall treatment and grazing exclusion significantly
improved grass cover and height. The garden experiment showed that frequent watering, no nitrogen
addition and grass clipping significantly enhanced tree germination and survival (termed 'recruitment'). There
were also significant interactions among rain, nitrogen and grazing in their effects on tree recruitment. The
effects of rain on tree recruitment were more pronounced under nitrogen supplementation and vice versa.
Similarly, high rain and high nitrogen enhanced the effect of grazing on tree recruitment.
It is inferred that above-average rainfall years with frequent rainfall events are required for mass tree
recruitment. Tree seedlings can further benefit from space and resources which are made available through
grass defoliation. Conversely, nitrogen enrichment improves the competitive ability of the fast-growing
grasses relatively more than that of the N2-fixing tree component, thereby suppressing tree recruitment. In
contrast to conventional wisdom that grazing alone causes encroachment, it is suggested that there are
complex interactions between the abovementioned factors and 'triggering' events such as unusually high
rainfall.
Contrary to many claims that equilibrium models are inappropriate for explaining savanna dynamics, it was
shown that consumer-resource theory has explanatory power for bush-grass dynamics of the savanna
studied. The state-space approach that was used facilitated the understanding of savanna dynamics and
enabled predictions about the system's response to perturbations. The applicability of consumer-resource
theory to semi-arid nutrient-poor savannas confirmed the importance of resource competition in structuring
natural systems. === AFRIKAANSE OPSOMMING: Toenames in die digtheid van houtagtige plante ('bosverdigting') verlaag veeproduksie en biodiversiteit.
Volgens konvensie word grondvog, grondvoedingstowwe, vuur en beweiding beskou as die belangrikste
faktore wat die boom-gras verhouding van savannas bepaal. 'n Volledig-gekruisde ("completely crossed")
eksperiment is ontwerp om boomsaad-ontkieming (Acacia mellifera) naby Kimberley, Noord-Kaap, Suid-
Afrika, te bestudeer na 'n brand en onder toestande van maksimum-aangetekende reënval, stikstof
toevoeging en beweiding. Die veldeskperiment is herhaal as 'n tuineksperiment om vas te stel hoe resultate
van die twee eksperimente vergelyk.
Boomsaad-ontkieming was uiters laag in die veld, waarskynlik weens ondergemiddelde reënval in persele
wat slegs natuurlike reënvalontvang het, en 'n te lae benattingsfrekwensie in die besproeide persele. As
gevolg van lae ontkieming in die veldeksperiment, het geen behandeling (reënval/stikstof/brand/beweiding)
boomsaad-ontkieming beduidend geaffekteer nie. Die duur van die eksperiment (2000/2001-groeiseisoen)
was te kort vir die behandelings om grassamestelling te beïnvloed, alhoewel besproeiing en geen beweiding
die grasbedekking en -hoogte betekenisvol verhoog het. Die tuineksperiment het getoon dat boomsaadontkieming
en vroeë oorlewing (genoem 'vestiging') betekenisvol verhoog is deur gereëlde benatting, geen
stikstof toevoeging en die sny van gras. Daar was ook beduidende interaksies tussen reën, stikstof en
beweiding in hul invloed op boomvestiging. Die reënbehandeling het 'n groter uitwerking op boomvestiging
gehad onder stikstoftoevoeging en vice versa. Hoë reënval en stikstoftoevoeging het ook die invloed van
beweiding op boomvestiging versterk.
Die afleiding is dat bogemiddelde reënvaljare met gereëlde reënbuie 'n vereiste is vir grootskaalse
boomvestiging. Boomsaailinge kan verder voordeel trek uit die spasie en hulpbronne wat beskikbaar raak
wanneer gras ontblaar word. Daarteenoor verhoog stikstofverryking die mededingendheid van die vinniggroeiende
grasse meer as dié van die N2-bindende boomkomponent, met die gevolg dat boomvestiging
onderdruk word. In kontras met die konvensionele veronderstelling dat beweiding opsigself bosverdigting
veroorsaak, word voorgestel dat ingewikkelde interaksies plaasvind tussen die bogenoemde faktore en ander
'sneller-gebeurtenisse', soos buitengewoon hoë reënval.
In teenstelling met baie aansprake dat ekwilibrium modelle ontoepaslik is om savanna-dinamika te verklaar,
is getoon dat die verbruikers-hulpbron teorie ("consumer-resource theory") oor verklaringsvermoë beskik vir
boom-gras dinamika van die savanna wat bestudeer is. Die staat-spasie ("state-space") benadering gebruik,
het begrip van savanna-dinamika bevorder en voorspellings moontlik gemaak aangaande die sisteem se
reaksie op versteurings. Die toepaslikheid van verbruikers-hulpbron teorie vir semi-ariede, nutriënt-arm
savannas bevestig dat kompetisie vir hulpbronne 'n sentrale rol speel in die strukturering van ekologiese
sisteme.
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