Impact of Acacia tortilis ssp. raddiana tree on wheat and barley yield in the south of Tunisia

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The effects of browsing on the structure of Acacia tortilis (Forssk.) Hayne ssp raddiana (Savi) Brenan along a gradient of water availability in arid zones of Tunisia

In the past, Acacia tortilis colonised thousands of hectares in central and southern Tunisia. Nowadays, A. tortilis is restricted to central Tunisia. The aim of this research was to explore the effects of browsing on the structure of a population of A. tortilis along a gradient of soil water availability. We selected three plant communities differing in soil water availability and pressure of disturbances by livestock (browsing). We explored populations of A. tortilis by measuring dendrometric parameters to check the regeneration potentialities of this tree. We hypothesised that the regeneration of A. tortilis is positively correlated with both soil water content and browsing. The dominance of young trees of A. tortilis demonstrates that this tree definitely shows a natural regeneration mainly in the browsed areas. The variation in water availability regime may have considerable effects on the structure of A. tortilis populations and the regeneration of A. tortilis is positively correlated with water availability. Finally, browsing seems to be a negative factor affecting the general structure of the A. tortilis populations (decreasing in density, canopy cover and canopy diameter) but a positive factor in terms of regeneration (higher abundance of young trees in browsed areas)

Stem growth of woody species at the Nkuhlu exclosures, Kruger National Park: 2006–2010

An important aspect of managing African conservation areas involves understanding how large herbivores affect woody plant growth. Yet, data on growth rates of woody species in savannas are scarce, despite its critical importance for developing models to guide ecosystem management. What effect do browsing and season have on woody stem growth? Assuming no growth happens in the dry season, browsing should reduce stem growth in the wet season only. Secondly, do functional species groups differ in stem growth? For example, assuming fine-leaved, spiny species’ growth is not compromised by carbon-based chemical defences, they should grow faster than broad-leaved, chemically defended species. Dendrometers were fixed at 20 cm in height on the main stems of 244 random plants of six woody species in three plots (all large herbivores excluded, partial exclusion, and control) and observed from late 2006 to early 2010. Average monthly increment (AMI) per dendrometer and season (dry, wet) was calculated and the interaction between plot and season tested per species, controlling for initial stem girth. AMIs of Combretum apiculatum, Dichrostachys cinerea and Grewia flavescens were zero in the dry season, whilst those of Acacia exuvialis, Acacia grandicornuta and Euclea divinorum were either positive or negative in the dry season. Wet-season AMI of D. cinerea and dry-season AMI of G. flavescens tended to be reduced by browser exclusion. Net AMI (sum of the seasonal AMIs) was tested per species, but results suggested that only D. cinerea tended to be affected by browser exclusion. The results also suggested that stem radial growth of some fast-growing species is more prone to reduction by browser exclusion than the growth of other species, potentially reducing their competitiveness and increasing their risk of extirpation. Finally, the usefulness of grouping woody species into simple functional groups (e.g. fine-leaved vs. broad-leaved) for ecosystem management purposes in savannas requires further consideration. Conservation implications: Growth rates of woody plants are important parameters in savanna models, but data are scarce. Monitoring dendrometers in manipulative situations over several years can help fill that gap. Results of such studies can be used to identify species prone to high risk of extirpation