Tree invasion in a semi-arid savanna in Zimbabwe : seedling recruitment of Acacia karroo

Abstract

In this thesis Chirara reports on his study on the competitive interaction between savanna grasses and young tree seedlings of Acacia karroo, from hereon indicated as ' Acacia seedlings' . Acacia is one of the tree species that dominates savanna grassland in situations of overgrazing (bush encroachment). The discussion is whether this invasion is predominantly caused by the decreasing light competition or water competition of grasses in overgrazed situations. So, Chirara studied: -Acacia seedling survival in the field under different grazing intensitie -Acacia seedling survival in an experimental set up with different grass clipping regimes -Water availability in relation to grass clipping regimes and the effect on it for Acacia seedling survival -Interaction between water and light limitation for Acacia seedling survival -Effect of defoliation of Acacia seedlings on survival. Surprisingly, low grass cover as well as a high grass cover facilitated Acacia seedling establishment. Most probably shade of grasses reduces evaporation and hence the water demand of the very young tree seedling. This will effect establishment rather than survival on the long run. Young Acacia seedlings develop very quickly a long thin taproot in search for water deeper in the soil. This enables the seedling to survive during the dry season. In deep shade (less than 6% of full sunlight) seedling growth is very much hampered and independent of soil water level. These situations of less than 6% are not exceptional in the field. However, in the slightly higher light levels, water shortage is much more determining seedling growth. Depressed seedlings due to water shortage develop less quickly their long taproot and that can lower the probability for seedling survival in the dry season or dry spells during the wet season. Grazing up to 90% of the Acacia seedling finally did not effect survival and growth. The seedling recovers very quickly. In conclusion, grass cover effects seedling establishment and growth. However, considering the plasticity of the allocation pattern and especially the ability to produce a deep taproot early in its life cycle, enables the Acacia seedling to accommodate to stress caused by grass competition. Competition is important in suppressing Acacia seedlings, but not the only mechanism.