Effect of the intensity and phenophase of defoliation and water stress on the rate of photosynthesis and the recovery of carbohydrate reserves in Acacia karroo Hayne.

A field study was conducted with Acacia karroo plant to determine changes in relative photosynthetic rates, the extent of carbohydrate reserve depletion and the rate reserves take to recover, following defoliation by goats at different intensities and phenophases, at a "wet" and a "dry" site. The rate of photosynthesis of fully expanded leaves increased markedly following defoliation. Light defoliation increased photosynthetic rate the most. Total non-structural carbohydrate levels dropped significantly after defoliation. The magnitude of decrease was directly related to the intensity of defoliation. Following the heavy defoliations, recovery of carbohydrate levels was much faster than after the light defoliations. Rates of recovery were also faster following defoliation in the second half of the growing season, than in the first half. However, the plants that had been heavily defoliated in the second half of the growing season had not fully recovered carbohydrate levels before leaf fall in late autumn. Moisture stress had very little effect on carbohydrate levels in comparison with the defoliation treatments.Keywords: acacia karroo; carbohydrate; defoliation; dohne research station; eastern cape; field study; goats; leaves; moisture stress; phenophases; photosynthesis; photosynthetic rate; recovery; south africa; stutterheim; water; water stres

The rate of consumption of bush and grass by goats in a representative Acacia karroo savanna community in the Eastern Cape.

Consumption of browse and grass in a representative Acacia karroo savanna community was measured. Acacia karroo was preferred to grass. It was selected almost exclusively when available at high leaf densities. Grass was consumed in significant amounts only when approximately 50% of the available A. karroo leaf had been consumed. Maximum daily intake was 78 g.W -0, 75kg.day -1 and decreased with decreasing amount of browse on offer. When approximately 90% of the available A. karroo leaf had been removed, the goats had consumed only 30% of the available grass. Total intake of browse plus grass was lower during the period when the goats changed form consuming mostly grass. Goats did not appear to select a diet according to their nutritional needs, as judge by feeding tables, when this was possible. This may be due to protein indigestibility caused by tannin complexing by A. karroo, luxury consumption of A. karroo as a favoured food, an adaptation by goats to use browse more efficiently than grass, or differences in palatability between A. karroo and grass.Keywords: acacia karroo; browse; bush; consumption; diet; dietary swithcing; eastern cape; feeding; feeding table; foraging ecology; goats; grasses; intake; leaves; palatability; protein; rate of consumption; savanna; south afric

Effect of intensity and frequency of defoliation on aerial growth and carbohydrate reserve levels in Acacia karroo plants.

Acacia karroo trees were defoliated by goats at two intensities and four frequencies; 2, 4, 8 and 12-weekly. Leaf accumulation and carbohydrate reserve levels were compared to a non-defoliated control, and to plants (defoliation controls) which were defoliated for the first time that season each time a frequency treatment was defoliated. These plants are activated by defoliation in such manner that successive defoliations can result in this activation being additive. There is clearly a defoliation level below which they are not activated. Activation appears to be negated to a degree by defoliations at 2 and 4-weekly frequencies, relative to the 8-weekly defoliation frequency. The 12-weekly frequency at heavy defoliation produces less than the same defoliation at 8-weekly frequency. The 2-weekly frequency treatments produced as much leaf as the 4 and 12-weekly defoliations at the same defoliation intensity. The more frequently plants were defoliated, the more carbohydrate reserves dropped. However, plants adjusted to cope with very frequent defoliations. There was no connection between leaf accumulation and carbohydrate reserve levels following the different frequencies and intensities of defoliation.Keywords: acacia karroo; botany; browse; carbohydrate; compensatory growth; defoliation; defoliation frequency; frequency; frequency of defoliation; goats; leaves; tree

The response of Acacia karroo plants to defoliation of the upper or lower canopy.

The response of Acacia karroo trees to defoliation of either the upper or lower canopy only, was compared experimentally with that of plants whose whole canopies had been defoliated at a range of defoliation levels. These plants were very sensitive to defoliation of the upper canopy. A 100% defoliation of the upper canopy only, resulted in the same amount of growth as 100% defoliation of the whole canopy. This was considerably less than the growth of plants defoliated overall, at 25% and 50% leaf removal. In contrast, defoliation the bottom half of the canopy only, stimulated growth in the whole canopy to the same degree as defoliation of the whole canopy at 25-50%. The increases of growth were due largely to increased growth in the top half of the canopy. Plants were very sensitive to defoliation in the early-flush phenophase. This probably masked the positive effects of the partial defoliations applied at this phenophase.Keywords: acacia karroo; browse production; defoliation; eastern cape; goats; growth stimulation; leaves; south africa; university of fort har

Patterns of selection of Acacia karroo by goats and changes in tannin levels and in vitro digestibility following defoliation.

Patterns of browse selection by Boer goats in a representative Acacia karroo community in the eastern Cape were studied. The rate of intake of browse was positively related to the leaf mass per unit length of the shoot. The ease of harvesting leaf material, as determined by the height off the ground, modified the rate of intake. Generally, following browsing, tannin levels increased significantly and in vitro digestibility decreased significantly. These changes in tannin content and digestibility differed in magnitude according to the plant size and age of the shoot and leaf. Generally, the leaf and shoot intake was negatively related to tanning content and positively related to digestibility, thus influencing patterns of selection for different plant parts and size classes of A. karroo. However, some of the results are contradictory.Keywords: acacia karroo; boer goats; browse; browsing; defoliation; digestibility; eastern cape; goats; harvesting efficiency; in vitro digestibility; intake; leaves; plant parts; south afric

Growth patterns and annual growth cycle of Acacia karroo Hayne in relation to water stress. II. The annual use and replenishment of non-structural carbohydrates in relation to phenological development.

The use and replenishment of carbohydrate reserves in Acacia karroo is closely related to the successive events of the annual growth cycle. Acacia karroo displayed the rapid and substantial decline in non-structural carbohydrates typical of deciduous woody species. In addition, each time shoot, leaf or reproductive organs grew rapidly, reserves declined. Replenishment begins soon thereafter when the recently emerged leaves are still very small. Water stress does not change the pattern of use and replenishment. However, increasing water stress does slow replenishment down. This probably delays the beginning of the next growth event as well as the growth of organs already initiated.Keywords: acacia karroo; botany; carbohydrates; development; growth cycle; growth patterns; leaves; reproductive organs; reserves; south africa; water stress; woody specie

The response of Acacia karroo plants to defoliation by hand compared to defoliation by goats.

The leaf and shoot growth of Acacia karroo plants defoliated by goats was compared to that of plants in which leaf only or leaf plus shoot-tips were removed by hand, to simulate goat browsing. There were no differences in leaf or shoot production between plants whose leaf only was removed compared to those that had both leaf and shoot-tips removed. Leaf growth on the plants defoliated by goats at moderate levels was approximately three-fold that on plants defoliated moderately by hand. The plants defoliated by hand produced approximately twice the amount of leaf that the undefoliated plants produced. Heavy defoliation by both hand and goat, produced approximately half the leaf growth that moderately defoliated plants produced, following equivalent defoliation. Shoot production of all plants defoliated by hand was no different to that of the undefoliated control plants and was considerably less than that of the plants defoliated by goats.Keywords: acacia karroo; browse production; browsing; clipping; defoliation; goats; growth stimulation; leaf growth; leaves; shoot growth; shoot productio

Simulating browse production and response of Acacia karroo to defoliation. I. Model discription and sensitivity analyses.

The model is aimed at synthesizing existing data to enhance the understanding of this data and explore the consequences of possible browse management strategies. It gives a good reproduction of the shoot and leaf growth responses used to develop it and a reasonable prediction of leaf growth when tested against independent data. The prediction of shoot growth against independent data was poor. Sensitivity analysis indicate that five parameters have a very strong influence on the output of the model. These are moisture, soil depth, the magnitude and duration of growth stimulation following defoliation, how soon growth is initialized and how favourable growing conditions are in spring. Plant size and the carry-over of growth stimulation from one year to the next had a moderate influence.Keywords: acacia karroo; browse production; defoliation; eastern cape; goats; growth response; growth stimulation; leaf growth; leaves; management strategy; model; sensitivity analysis; shoot growth; simulation model; soil depth; south afric

Effect of intensity of defoliation by goats at different phenophases on leaf and shoot growth of Acacia karroo Hayne.

Defoliation by goats (leaves plus shoots) during the growing phenophases resulted in a considerable stimulation of leaf and shoot growth relative to non-defoliated plants. The response differed considerably depending on the intensity and phenophase of defoliation. Plants were most susceptible to defoliation and young shoot removal during the spring flush when carbohydrate levels were at their lowest. During the rest of the growing season carbohydrate levels were high. At these times moderate to heavy (50% to 75% leaf removal) defoliations resulted in the greatest leaf and shoot growth. In contrast, the initial and continuing impact of avian and insect consumption of leaves only reduced leaf and shoot production. The stimulatory effect of defoliation in one season carried through the dormant season to the following growing season. Defoliations during the dormant season did not have any positive or negative impact on plants relative to non-defoliated plants. Leaf and shoot material removed when the plants were growing, was replaced within weeks of defoliation.Keywords: acacia karroo; alice; carbohydrate; consumption; defoliation; eastern cape; goats; growth; growth stimulation; insect defoliation; leaf growth; leaf mortality; leaf removal; leaves; phenophases; production; shoot growth; shoot production; shoot removal; south africa; university of fort har

Growth patterns and annual growth cycle of Acacia karroo Hayne in relation to water stress I. Leaf and shoot growth.

A field study was conducted to describe the growth patterns of A. karroo at different levels of water stress in different edaphic situations. Shoots are heterophyllous and are formed by green growth. The degree of development of a shoot, relative to others in the canopy, is governed by branch and position in the canopy. At least six phenological phases were identified in the annual growth cycle. The pattern of growth and the phenological cycle are not changed by water stress. Initiation, emergence and development of shoots and leaves are governed by how favourable environmental conditions are. If there is little shoot growth early in the season, these plants can partially compensate by producing more leaf per unit of shoot if environmental conditions improve. Leaf and shoot growth at the beginning of the season took place only if there was sufficient moisture available, and if the minimum temperature had risen above a threshold amount. Where there was insufficient soil moisture, no growth was observed before rains had fallen. The growth strategy of A. karroo differs markedly from that of broad-leaved African savanna tree species. Growth in A. karroo is dominated by current growing conditions, rather than those of the previous season. They are able to make opportunistic growth at any time. Soil depth had a marked influence on plant growth, presumably due to a larger available nutrient and moisture pool.Keywords: acacia karroo; alice; botany; compensatory growth; condition; development; eastern cape; emergence; environmental conditions; field study; growth cycle; growth initiation; growth patterns; growth strategy; leaf growth; plant growth; savanna; shoot growth; soil depth; soil moisture; south africa; university of fort hare; water stres