Is there a trade-off in resource allocation to condensed tannins and thorn defences in Vachellia (Acacia) species?

Acacia species invest in condensed tannins (CT) and thorn defences among others. Plant defences are considered costly because they use resources that would otherwise be available for growth and reproduction. To reduce plant defence costs it has been argued that there are trade-offs between different types of defences. I tested for a trade-off between CT and thorn defences in Vachellia (Acacia) species. Thorn defences were expressed as thorn biomass, thorn density and thorn length. The relationship between defences was tested using Pearson correlation in addition to that with leaf area. My results support a trade-off between CT and thorn biomass and thorn density but not thorn length. Vachellia karroo (Acacia karroo) had high CT but low thorn biomass and thorn density, whereas V. gerrardii (A. gerrardii) had low CT but high thorn biomass and thorn density. Thorn length was positively correlated with CT and negatively related to thorn biomass and thorn density. Leaf area had a strong positive correlation with thorn biomass and thorn density, a weak negative relationship with CT, and a non-significant correlation with thorn length. I conclude that CT has a strong trade-off with thorn biomass and thorn density but not thorn length.     

Predictive modelling of the potential future distribution of Vachellia nilotica within the KwaZulu-Natal province of South Africa

Many regions of South Africa are prone to woody plant thickening. This can have an ecologically detrimental effect on the open savannas and grasslands. KwaZulu-Natal, a province on the east coast of South Africa, is currently experiencing an increase in both the density and distribution of Vachellia nilotica. This research aims to gain better insight into the potential distribution of this plant and to determine some of the main environmental conditions that promote its thickening. Using the Maxent programme to determine the potential distribution, a map was developed to illustrate the possible extent of V. nilotica within KwaZulu-Natal. It is estimated that a possible 800 968 ha (8.5%) of the province has a greater than 50% distribution probability, whereas in 26.9% of the province there is 25%–50% probability of V. nilotica inhabiting these areas. Using Maxent, it was determined that geology and altitude were key determinants for V. nilotica habitat selection. This model-based map will be particularly useful for conservation and rangeland planning for future management and control of the plant through being able to predict which areas of the province are more likely to be high potential regions for the thickening of V. nilotica.