The effects of browsing on growth, structure and physiological aspects of Acacia grandicornuta and Combretum apiculatum
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Date
2010
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Abstract
Very little is known about plants growing in savannas, especially how woody plants respond to browsing and variations in resources. It is assumed that growth rate and concentrations of defenses are inversely related, but will be affected by resource availability. It can be postulated further that increased growth and photosynthesis would be observed concomitant with reduced tannin concentration when water and nutrients are abundant at the beginning of the wet season. Growth in terms of shoots length, thorn length and plant height would increase in terms of increasing photosynthetic rate on browsed plants compared to unbrowsed plants. Therefore, research was conducted to investigate the effects of browsing on Acacia grandicornuta and Combretum apiculatum.
Research was conducted at the Nkuhlu experimental exclosures (Kruger National Park) on these two tree species. The exclosures were designed so that there were three broad levels of browsing pressure: no mammal herbivores excluded, elephants and giraffes excluded and all mammal herbivores bigger than hares excluded, all of which incorporate the catena from sodic footslope to sandy crests. The focus was on heavily browsed plants between the heights of 0.7-1.7 m. Ten trees per species per treatment (full, partial and no exclosure) were sampled and height and stem circumference were measured in the early and late wet season. Five new shoots per tree were marked; these shoots were re-measured at intervals to monitor their growth. Leaves were harvested from other short trees of both species in all the exclosures. Leaves were dried, milled and analysed for phosphorus (P), nitrogen (N) and condensed tannins (CT). Photosynthetic measurements were also recorded at the same time from short individual trees of C. apiculatum from the full and non exclosure.
The results showed that there were no significant effects of the treatments and no significant growth on shoot and thorn length of A. grandicornuta in all the treatments from October to December 2007. There was significant growth in the shoot lengths of C. apiculatum from October to December 2007 in all the treatments (full exclosure t = -4.65, df = 10.20 and P = 0.001, non exclosure t = - 4.67, df = 7.90 and P = 0.002 and partial exclosure t = - 8.86, df = 7.30 and P < 0.001).
There was a significant effect of the treatments on the shoot lengths of C. apiculatum during December 2007. There was no significant growth in heights and stem circumference of A. grandicornuta from September 2007 and March 2008 (P> 0.05). There was significant growth in the height of C. apiculatum in the full exclosure (t =2.33, df = 17.10 and P = 0.032) and the stem circumference of C. apiculatum was significantly smaller in the partial exclosure (t =3.71, df =8.00 and P = 0.006) in September 2007 compared to March 2008.
The pattern for both A. grandicornuta and C. apiculatum in all the treatments was a decreasing P concentration from October 2007 to March 2008. Nitrogen decreased in A. grandicornuta where mammals were excluded and also decreased in the non exclosure. In the partial exclosure there was no change in N concentration from October 2007 to March 2008. For C. apiculatum, N concentration decreased in all the treatments from October 2007 to March 2008. The CT concentration remained the same for A. grandicornuta in all the treatments and in all the months. The CT concentration of C. apiculatum in the full exclosure significantly increased (t = -7.08, df = 10.60 and P < 0.001), in the non exclosure the CT concentration also increased (t = -5.34, df = 8.00 and P = 0.001). In the partial exclosure, the CT concentration remained the same for C. apiculatum from October 2007 to March 2008. The leaf removal of A. grandicornuta was not correlated with CT concentration and was not correlated for C. apiculatum.
The results from the photosynthesis data showed a steadily decrease in Jmax and Vcmax from November 2007 to March 2008 where the herbivores were excluded. The plants in the non exclosure responded differently compared with the plants in the full exclosure. Jmax and Vcmax decreased form November 2007 to February 2008 but increased again during March 2008 where herbivores were not excluded.
A nursery experiment was conducted to see the effects of browsing as affected by water availability. The interests were in finding out the responses of plants after browsing at different water levels. Potted seedlings of C. apiculatum were allowed to grow in the nursery and watered once after 3 days. The heights, basal circumferences, shoot lengths were measured and plants were given to goats to achieve a range of browsing intensities in September 2007 (0, 30, 60 and 90% intensity).
Plants were weighed before and after browsing to estimate the amount removed, and were separated into three groups that were watered after 3, 7 and 10 days for four months. After four months of growth, the plants were re-measured (heights, basal circumferences, shoot lengths), harvested, separated into morphological parts (roots, shoots and leaves), oven dried and each part weighed. Leaves were then analysed for P, N and CT.
Increased water availability resulted in increased leaf, shoot and root mass. The results from the combined data showed that water treatment had a significant effect on leaf mass (t = -9.39, df = 20.60 and P < 0.001) shoot mass( t = -6.18, df = 34.40 and P < 0.001) and root mass (t = -2.19, df = 31.20 and P = 0.036). Defoliation did not have an effect on leaf and root mass within each water treatment but defoliation had a significant effect on shoot mass.
The data was combined and the results show that water treatment had a significant effects on heights (t = -4.28, df = 38.00 and P < 0.001) shoot length (t = -8.02, df = 29.10 and P < 0.001) and basal circumference of the seedlings after defoliation (t = -7.02, df = 29.00 and P < 0.001). Defoliation had a significant effect on height, shoot length and basal circumference of seedlings after defoliation (P< 0.001).
Water had a significant effect on P, N and CT concentration in the seedlings where P, N and CT concentrations increased with increasing water availability. The chemical concentrations between seedlings which were watered every 10 and 3 days, were significant for phosphorus (t = -2.07, df = 28.20 and P = 0.047) N concentration (t = 2.74, df = 35.90 and P = 0.009) and CT concentration (t = -2.23, df = 20.50 and P = 0.037). There was no effect of defoliation on P and N concentrations on the seedlings which were defoliated at different intensities. Defoliation had an effect on CT concentration on seedlings which were watered after 7 and 3 days.
It was concluded that A. grandicornuta was not affected by treatments or season in terms of growth. The C. apiculatum trees which were exposed to browsing increase their growth rate in the form of shoots length and stem circumference. No growth in terms of height was detected in C. apiculatum trees in browsed plants because the plants were suppressed by browsing. No growth in A. grandicornuta was recorded in all the treatments.
The CT concentration for A. grandicornuta remained constant in all the treatments for all the seasons. The CT concentration in C. apiculatum increased under browsing, which was unexpected. Increased shoot length occurred when browsing occurred during the growth season when resources were abundant and browsing was inevitable. When growth increases, photosynthesis was expected to increase in C. apiculatum trees exposed to browsing. The photosynthetic rate in C. apiculatum exposed to browsing was higher than in unbrowsed C. apiculatum trees.
Heavily defoliated seedlings increased their growth in the form of height, stem diameter and shoot length as a defence strategy as expected. Seedlings with more resources produced more CT in their leaves as a form of defence mechanism which was expected. Seedling parts which allocated more biomass had more water access than seedlings which had less water.
Since very little is known about tree responses to browsing, this research will help improve knowledge and understanding of browse browser interactions in savannas. The knowledge gained from this research is useful for building models of browse-browser interactions in seasonal subtropical zones where browsers are abundant and have the potential to deplete vegetation resources and how to prevent this from happening.
Description
Submitted in partial fulfilment of the academic requirements for the degree
MASTER OF SCIENCE (BOTANY)
In the
University of Zululand
Faculty of Science and Agriculture
Department of Botany, 2010.
Keywords
Acacia grandicornuta, Combretum apiculatum