The influence of tree thinning and subhabitat differentiation on the reproductive dynamics of acacia mellifera subsp. detinens
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Date
2001-11
Authors
Hagos, Mesghena Ghilay
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
The study was conducted in an area described as "Kalahari Thomveld" in the vicinity of
the towns of Bray and Pomfret in the North-West Province, where Acacia mel/itera
subsp. detinens is the dominant woody species. The soils of the area are deep sand to
loamy soils described as Kalahari sand with an extremely low organic matter and
mineral element content.
The study area consisted six 0.5 ha plots (50 m x 100 m), where the trees were thinned
during November 1989 to different densities, ranging from a totally cleared plot (0%) to
plots thinned to the equivalent of 10%, 20%, 30%, and 50% of the tree density of a
control plot (100%) of about 2 000 tree equivalents (TE) ha-l. The plots were located
adjacently on a homogeneous area of 3.0 ha. Treatments were allocated randomly to
the plots and not in numerical order. A permanent transect of 20 m x 100 m was
established in the center of each plot for phenological observations and harvesting of
seeds. The remainders of the experimental plots were used for soil sample collection
and to determine seed distribution within the defined subhabitats. The spatial canopies
of all rooted live A. mel/itera trees encountered in the fixed transects (5 m x 100 m),
located in the middle of each of the experimental plots, were measured and the number
of Evapotranspiration Tree Equivalents (ETTE) ha-l calculated, using the BECVOLmodel.
Estimates of the browsing capacities were also made from the leaf dry matter
estimates.
For the study of the phenology, seed harvesting and leaf biomass estimates of individual
trees, fifty (50) A. mel/itera trees (10 sample trees/plot) were randomly selected. Only
one phenological observation was done at the onset of flowering (August 2000) and all
the pods and seed from the marked trees were harvested during late November 2000.
Tree thinning brought about ear1yflowering of A. mel/itera in the lower tree density plots,
possibly as a result of reduced inter-tree competition. However, the mean seed
production over the tree density gradient did not follow a specific trend, and differences
were observed between the seed production of individual trees. Significant correlations
between ETTE ha" and leaf dry mass (kg ha") and total seed production ha" were
established. Although the correlation coefficients were low, there are indications that
seed production of individual trees increase with an increase in ETTE tree-land leaf dry
mass tree".
For the determination of seed mass and seed dimensions ripe A. mellifera seeds were
randomly selected from each sample tree (n = 10 and 50, respectively) after which they
were weighed and measured. Seeds harvested from the high tree density plot were
longer, wider and thicker than those from the lower tree density plots. The average seed
dry mass production tree-1 showed no distinct pattern and was not correlated with the
tree density, but the total seed dry mass ha-l increased in relation to an increase in tree
density. Leaf biomass of the A. mellifera trees per unit area, expressed as ETTE ha"
and leaf dry mass ha", was highly correlated with the seed dry mass ha-l
.
There is no carry over of A. mellifera seed from one season to another and thus no
persisting seed bank exists. The percentage of viable fresh A. mellifera seeds was
observed to decrease with the increase of the time and exposure to high temperature
and moisture fluctuations.
For the study of seed distribution within defined subhabitats (stem base area, under
canopy spread and open area), six A. mellifera trees in each tree density plot were
randomly selected. Small rectangular plastic containers were inserted. in the three
subhabitats of all marked trees (in two opposing directions). In all subhabitats a larger
number of seeds were blown toward the prevailing wind direction. The number of seeds
accumulated in the open subhabitats in both wind directions was also higher in
comparison to the canopied subhabitats. A very low, non-significant correlation was
observed between ETTE ha-l and seed distribution along the tree density gradient of the
three subhabitats.
For the assessment of germination potential, 50 normal and 20 bruchid beetle infested
seeds were randomly selected from each plot. Germination tests were conducted at the
facilities of the Department of Agronomy, UFS, and root and coleoptile lengths and
growth rates were also measured. Thinning of A. mellifera trees had no effect on the
germination potential of the seeds from the various tree density plots and an extremely
high germination potential of the fresh A. mellifera seeds were found. Though some
damage was caused, bruchid beetle infested seeds exhibited a low but fast germination
rate, possibly due to faster imbibition of moisture. No marked difference in root length
and root growth rates were observed between seeds of the various plots. However,
seeds harvested from the lower tree density plots developed a higher coleoptile length
with a faster coleoptile growth rate than the control (100%) plot.
Soil from the canopied and uncanopied subhabitats were analyzed for soil nutrient
status in order to evaluate seedling growth. Soils excavated from the canopied
subhabitats were more acidic and displayed higher concentrations of P, total Nand
organic matter than the uncanopied (open) subhabitat. However, no marked differences
in exchangeable cautions were demonstrated between the subhabitats, except Ca that
displayed higher concentrations in the canopied subhabitat.
No allelopathic effect that inhibits the growth of seedlings was found to be present in soil
from the canopied subhabitats of A. mellifera. Marked differences in growth parameters
of the seedlings grown in the soils from around the stem base area in comparison to the
other two subhabitats were observed. The relatively higher soil nutrient status observed
in soils of this subhabitats is considered responsible for this increased growth. In general,
A. mellifera seedlings grown in the soil from the lower tree density plots exhibited higher
growth rates than those from the control (100%) plot. A denser root system, as reflected
by higher total root lengths and root dry mass, was observed in seedlings grown in soil
from the low tree density plot and declined linearly in soils collected along the tree
density gradient. In general, the subhabitat differentiation had a more pronounced effect
on seedling growth than soil differences associated with the tree density gradient.
Description
Keywords
Acacia mellifera -- Breeding, Acacia -- Yields, Multipurpose trees -- South Africa, Dissertation (M.Sc.Agric. (Grassland Science))--University of the Free State, 2002