Effects of herbivores, fire and harvesting on the population dynamics of Acacia drepanolobium sjoestedt in Laikipia, Kenya.
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Date
2007
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Abstract
Effects of herbivory, fire, and tree harvesting on Acacia drepanolobium were studied using
plant population dynamics as the philosophical basis of research. Specifically, growth
rates, chrono-sequence of re-growth, biomass and charcoal yield, herbivory, flowering,
seed production, germination, mortality and the ants of Acacia drepanolobium were
studied in the black cotton ecosystem of Mpala Research Centre, Laikipia, Kenya,
between September 1995 and December 2000.
Acacia drepanolobium was the most abundant tree or shrub with densities ranging from
80% to 98% of all the overstorey species, but it was the least browsed of all the trees and
shrubs in the black cotton ecosystem, ranging from a mean of 7.2% to 9% of the
individuals browsed. The tree is inhabited by four Acacia ant species, Crematogaster
mimosae, Crematogaster sjoestedti, Crematogaster nigriceps, and Tetraponera penzigi,
which are believed to be obligate, and which probably play a role in the low browsing rates
observed.
Six herbivore treatments replicated three times (no herbivores - O; only cattle - C, all
herbivores allowed - MWC {control}, mega-herbivores {elephants and giraffe} and wildlife
{W} – MW only, wildlife – W - only {all wildlife except mega-herbivores} and wildlife and
cattle only - WC) was the main experimental design used in understanding the dynamics
of the tree species under influence of different herbivores.
Mean annual height growth of A. drepanolobium trees was 24.9 cm yr-1, while the mean
Relative Growth Rates ranged from 14.6 x 10 –3 cm cm-1 yr-1 to 18.7 x 10 –3 cm cm-1 yr-1.
Growth rates were different among the herbivore treatments and between seasons.
Shoots of the tree grew by a mean range of 6.8 cm to 9.1 cm, were similar among the
treatments but differed among the seasons.
Canopy volume increased over time although it fluctuated with seasons, suggesting an
increase in bushiness of A. drepanolobium in the ecosystem. Trees occupied by different
ant species showed differences in shoot density (number of new shoots per twig), being
greater in Crematogaster nigriceps occupied trees compared with the other ant species.
Swollen thorn (gall) density per unit of twig length was greatest in treatments with megaherbivores;
these galls were significantly larger on trees occupied by the ant
Crematogaster nigriceps.
Treatments with herbivores were more spinescent than the total exclusion treatment.
Spine lengths ranged from 0.8 to 2.4 cm, and recorded a progressive reduction of up to
36.36.7% in treatments without browsers suggesting a relaxation of induced defence in A.
drepanolobium.
Flowering in A. drepanolobium was low and staggered over the study period ranging from
0.8% to 2.0% of the trees with no differences among the treatments suggesting that the
level of herbivory was not sufficient to influence reproduction of the tree in the
experimental site. Consequently, seedling recruitment was very low within the
experimental site. However, a nearby site recorded flowering of between 22.7% and
93.5%. Mean pod production, mean number of seeds per tree and mean weight of pods
and seeds had a positive linear relationship with tree density (R2=0.77, 0.81 and 0.81
respectively). Trees occupied by Crematogaster mimosae were the most likely to flower
(68%) compared with C. nigriceps (5.8%), again suggesting that ants had an effect on the
tree’s reproduction.
Mortality of A. drepanolobium trees averaged 0.9% to 4.2% over the study period, being
significantly greater in treatments with mega-herbivores. Seedling survival ranged from
42% to 75%, being greatest in the cattle only treatment. Between 30% and 100% (mean
67.2%), of A. drepanolobium seeds were attacked by a bruchid beetle (Bruchus sp.).
Seeds attacked by bruchid beetles had significantly lower germination rates. Similarly,
seeds passed through a fire also recoded significantly low germination rates compared
with normal seeds. Fire (3.4%) and bruchid beetles (20.7%) germination compared with
(control) undamaged seeds (84%) play an important role in the population dynamics of A.
drepanolobium. Fewer A. drepanolobium seeds (33%) were recovered from the surface
compared with buried (72%) seeds after a fire, indicating seed loss from the effect of fire
and predation.
In the burn experiment, fire top-killed 16% of A. drepanolobium trees but no tree or
seedling was killed. On the other hand, fire significantly reduced the density of non-A.
drepanolobium trees by between 50% and 100%, with none of them showing signs of
coppicing after the fire unlike top-killed A. drepanolobium trees.
Woody biomass from A. drepanolobium was strongly related to stem diameter (Y = 3.77x +
1.17, R2 = 0.96, P < 0.001). Mean charcoal production from earthen kilns was 2.83 Mg ha-
1. Height and stem diameter in coppicing stands increased at a mean rate of 28.6 cm yr-1
and 0.7 cm yr-1 respectively. Biomass in coppicing stands accumulated at a mean rate of
1.3 Mg ha-1 yr-1 in a 14-year period, yielding dry biomass of 18.26 Mg ha-1 useable wood
that can produce a minimum of 3.0 Mg ha-1 of charcoal.
This study shows that Acacia drepanolobium populations are affected by several factors
including herbivory, fire and ants. The population dynamics of this tree shows that it can be
harvested for sustainable charcoal yield over a 14-year cycle.
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2007.
Keywords
Acacia--Ecology--Kenya., Population biology--Kenya., Herbivores--Kenya., Rangelands--Fire management--Kenya., Charcoal--Kenya., Range management--Kenya., Crematogaster--Kenya., Theses--Grassland science.