Ph.D., Faculty of Science, University of the Witwatersrand, 2011 / 1
Investigating the life history strategy of an African savanna
tree, Sclerocarya birrea subsp. caffra (marula)
Chantal Vinisia Helm
Abstract
Lack of understanding of the life history attributes and responses of savanna woody
plants to disturbances, as well as the observation of unstable population structures in a
keystone, savanna tree, Sclerocarya birrea subsp. caffra (marula), prompted this study.
This study employed a combination of empirical, experimental and model formulation
techniques, aimed at achieving its ultimate purpose of understanding the life history
strategy of marula in the face of disturbance. Four main population structures were
identified for marula in the low altitude savannas of South Africa: 1) adult dominated, 2)
juvenile dominated, 3) with a “missing size class” and 4) stable (negative J-shaped).
Spatial variability in structure indicated different drivers affecting different populations.
High annual mortality rates of up to 4.6% in adult trees, no recruitment out of the fire trap
and little regeneration were observed in the Kruger National Park (KNP) between 2001
and 2010, and consequently even greater instability in the structure of these populations
already observed earlier in the decade.
Growth rates of saplings between 2 and 8 m in height and 2 and 30 cm in stem diameter
in the field were monitored between 2007 and 2010. Annual growth rates of up to 11 mm
in diameter and up to 22 cm in height were observed. Annual relative growth rates ranged
between 1.9 and 4.8% across sites. Growth rates were positively linked with rainfall and
plant size. Growth rates, biomass allocation patterns, as well as storage and defence
allocation in 3 to 28 month old marula seedlings were assessed under glasshouse
conditions. Relative growth rates were highest directly after germination (20%), but did
not exceed 5% thereafter. Allocation to roots (already 65% of the overall biomass at 3
months of age and >80% when older) was high regardless of soil type or provenance.
Provenance affected height gain, and plants germinating from seeds collected at higher
rainfall sites had faster height growth rates than those from seeds collected at lower
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rainfall sites. Allocation to storage in the form of root starch peaked at 35%, while
allocation to defence in the form of phenolics in the leaves peaked at 18%, being
relatively higher than other species. No trade-off between growth and defence allocation
was observed. However, in the second growing season, growth at the start coincided with
a 50% decrease in starch reserves in the roots. Reproductive maturity was found to occur
after 46 years and escape from the fire trap after 12 years in a disturbance free
environment. Marula trees appear to be able to live for up to 300 years of age.
High temporal variability in fruit production was observed, marginally linked to rainfall.
Only 2% of seeds persisted for more than one year, and hence marula relied mostly on the
current season’s fruit crop for input of new germinants. Fruit production was highly
synchronous across trees at a site. Very high levels of seed predation were observed.
Marula seeds can remain dormant for at least 10 years when stored in the laboratory.
Germination takes place after 3 mm of rainfall every four days for two weeks and is
enhanced by acid digestion and high temperatures. Germination percentages are relatively
low (<50% of the endocarps).
Marula seedlings appear highly adapted to fire, with high allocation to below-ground
biomass and starch storage, as well as very thick bark from very small stem diameters,
including a well developed resprouting response from very young. Marula stems were
able to resist fire from 3.4 cm in stem diameter, and were completely resistant above 7
cm. Stem diameter growth was prioritised above stem height growth, indicating that in
marula, diameter gain is more important than height gain in escaping the fire trap.
Topkilled marula saplings are able to regain their prefire height within one season.
However, rainfall patterns may have an overriding effect on these growth patterns. Adult
trees appear to be made vulnerable to fire through bark stripping, toppling and pollarding
and the subsequent invasion of the soft wood by borers.
On nutrient-poor granite soils, marula has a resistant strategy to herbivory, however on
nutrient-rich basalt soils, marula overcompensates for herbivory even at very low levels.
This may explain why marulas are more vulnerable on basalt soils in the KNP, having
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already been extirpated from the northern arid basaltic plains. Marula seedlings are
extremely drought resistant through fast root penetration rates and high root: shoot ratios.
A simple demographic model was developed which predicted that marula populations are
unlikely to survive given the current elephant impact in the KNP and if the fire interval is
less than once every seven years. Even though marula is highly resilient to damage from
herbivory or fire alone, the combination of frequent fire and heavy utilisation is proving
fatal for marula populations in the KNP and elsewhere.
In terms of other savanna tree species, marula is an outlier in its life history strategy,
being extremely well adapted to the effects of fire with very thick bark, extensive
resprouting ability and fast growth rates, combined with very high allocation to root
mass, and levels of storage and chemical defence, as well as having very drought tolerant
seedlings. Its main weakness as an adult, appears to be its soft wood, which is susceptible
to wood borer attack. The perplexing lack of recruitment at some sites in spite of the
extraordinary ability of marula seedlings to resprout from an early age, withstand
extensive drought, have fast root penetration rates, extremely high root reserve storage
and resistance to fire at small stem diameters, combined with high levels of fruit
production and low water requirements for germination, is probably due to a combination
of the lack of a dense persistent seed bank, high inter-annual variability in fruit
production, low germination percentages, high seed and /or seedling predation rates and
possibly dispersal of seeds away from suitable habitats. Overall, the unstable population
structures observed in the low altitude savannas of South Africa, specifically in the KNP,
do not bode well for the future persistence of marula as a dominant canopy tree species.
Keywords: elephant, fire, growth, mortality, recruitment, regeneration
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/10808 |
| Date | 18 November 2011 |
| Creators | Helm, Chantal Vinisia |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf |
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