The influence of tree thinning and subhabitat differentiation on the reproductive dynamics of acacia mellifera subsp. detinens

[摘要] The study was conducted in an area described as Kalahari Thomveld in the vicinity ofthe towns of Bray and Pomfret in the North-West Province, where Acacia mel/iterasubsp. detinens is the dominant woody species. The soils of the area are deep sand toloamy soils described as Kalahari sand with an extremely low organic matter andmineral element content.The study area consisted six 0.5 ha plots (50 m x 100 m), where the trees were thinnedduring November 1989 to different densities, ranging from a totally cleared plot (0%) toplots thinned to the equivalent of 10%, 20%, 30%, and 50% of the tree density of acontrol plot (100%) of about 2 000 tree equivalents (TE) ha-l. The plots were locatedadjacently on a homogeneous area of 3.0 ha. Treatments were allocated randomly tothe plots and not in numerical order. A permanent transect of 20 m x 100 m wasestablished in the center of each plot for phenological observations and harvesting ofseeds. The remainders of the experimental plots were used for soil sample collectionand to determine seed distribution within the defined subhabitats. The spatial canopiesof 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 numberof Evapotranspiration Tree Equivalents (ETTE) ha-l calculated, using the BECVOLmodel.Estimates of the browsing capacities were also made from the leaf dry matterestimates.For the study of the phenology, seed harvesting and leaf biomass estimates of individualtrees, fifty (50) A. mel/itera trees (10 sample trees/plot) were randomly selected. Onlyone phenological observation was done at the onset of flowering (August 2000) and allthe 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 seedproduction over the tree density gradient did not follow a specific trend, and differenceswere observed between the seed production of individual trees. Significant correlationsbetween ETTE ha and leaf dry mass (kg ha) and total seed production ha wereestablished. Although the correlation coefficients were low, there are indications thatseed production of individual trees increase with an increase in ETTE tree-land leaf drymass tree.For the determination of seed mass and seed dimensions ripe A. mellifera seeds wererandomly selected from each sample tree (n = 10 and 50, respectively) after which theywere weighed and measured. Seeds harvested from the high tree density plot werelonger, wider and thicker than those from the lower tree density plots. The average seeddry mass production tree-1 showed no distinct pattern and was not correlated with thetree density, but the total seed dry mass ha-l increased in relation to an increase in treedensity. Leaf biomass of the A. mellifera trees per unit area, expressed as ETTE haand 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 nopersisting seed bank exists. The percentage of viable fresh A. mellifera seeds wasobserved to decrease with the increase of the time and exposure to high temperatureand moisture fluctuations.For the study of seed distribution within defined subhabitats (stem base area, undercanopy spread and open area), six A. mellifera trees in each tree density plot wererandomly selected. Small rectangular plastic containers were inserted. in the threesubhabitats of all marked trees (in two opposing directions). In all subhabitats a largernumber of seeds were blown toward the prevailing wind direction. The number of seedsaccumulated in the open subhabitats in both wind directions was also higher incomparison to the canopied subhabitats. A very low, non-significant correlation wasobserved between ETTE ha-l and seed distribution along the tree density gradient of thethree subhabitats.For the assessment of germination potential, 50 normal and 20 bruchid beetle infestedseeds were randomly selected from each plot. Germination tests were conducted at thefacilities of the Department of Agronomy, UFS, and root and coleoptile lengths andgrowth rates were also measured. Thinning of A. mellifera trees had no effect on thegermination potential of the seeds from the various tree density plots and an extremelyhigh germination potential of the fresh A. mellifera seeds were found. Though somedamage was caused, bruchid beetle infested seeds exhibited a low but fast germinationrate, possibly due to faster imbibition of moisture. No marked difference in root lengthand root growth rates were observed between seeds of the various plots. However,seeds harvested from the lower tree density plots developed a higher coleoptile lengthwith a faster coleoptile growth rate than the control (100%) plot.Soil from the canopied and uncanopied subhabitats were analyzed for soil nutrientstatus in order to evaluate seedling growth. Soils excavated from the canopiedsubhabitats were more acidic and displayed higher concentrations of P, total Nandorganic matter than the uncanopied (open) subhabitat. However, no marked differencesin exchangeable cautions were demonstrated between the subhabitats, except Ca thatdisplayed higher concentrations in the canopied subhabitat.No allelopathic effect that inhibits the growth of seedlings was found to be present in soilfrom the canopied subhabitats of A. mellifera. Marked differences in growth parametersof the seedlings grown in the soils from around the stem base area in comparison to theother two subhabitats were observed. The relatively higher soil nutrient status observedin 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 highergrowth rates than those from the control (100%) plot. A denser root system, as reflectedby higher total root lengths and root dry mass, was observed in seedlings grown in soilfrom the low tree density plot and declined linearly in soils collected along the treedensity gradient. In general, the subhabitat differentiation had a more pronounced effecton seedling growth than soil differences associated with the tree density gradient.

[发布机构] University of the Free State