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Tree pruning is a management tool in agroforestry systems for reducing shade, enhancing nutrient cycling or providing fodder. However, little information is available on the effect of pruning management on plant growth, non-structural carbohydrate (NSC) levels in the roots and N2 fixation of Sesbania sesban. A glasshouse experiment was conducted to assess the effect of pruning frequency on biomass production, NSC levels and N2 fixation of Sesbania sesban. Treatments included pruning at (i) 3 months only, (ii) 3 and 6 months, and (iii) 3, 6 and 9 months, with each pruning removing shoot biomass above 50% of the initial height. Increased pruning frequency decreased above- and belowground dry matter (DM), and root NSC levels. The decrease in aboveground DM correlated with reduced levels of starch, sugar and total non-structural carbohydrates. Pruning more frequently significantly decreased nodulation, the percentage N derived from the atmosphere and N2 fixed. It could be concluded that more frequent prunings decreases biomass and NSC levels which could reduce the regeneration capacity of trees since they rely on NSCs to regrow. The suppressive effects of increased pruning frequency on N2 fixation could decrease the desirable benefits of legume trees on soil N fertility improvement.

The occurrence of Dalbergia obovata, which is typically associated with forests, in grasslands within conservation areas in a metropolitan municipality was investigated to assess whether it was encroaching and what the possible causes might be. Relevant legislation has not regarded D. obovata as an encroacher species in grasslands. The abundance and distribution patterns of D. obovata in four conservation areas were mapped in the field and captured as GIS layers to facilitate calculation of the proportion of each grassland area deemed to have been encroached over two decades. Management records for 17 conservation areas were analysed to determine whether there were associations between different management practices and D. obovata encroachment. We found that more than one third of grasslands present in 1999 were lost to encroachment by D. obovata and that some grassland areas were almost completely (99%) encroached. Results of a Chi-squared test and principal components analysis of management records indicated that the absence of large herbivores was significantly associated with D. obovate encroachment into grasslands. We conclude that D. obovata should be listed as a woody encroacher species and that some conventional management practises applied to control woody encroachers, such as the use of fire, are ineffective against D. obovata. Keywords: fire; herbivory; savanna; tropical grassy biomes; woody encroachment

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Condensed tannins are ecologically important compounds in plants for several reasons, e.g. as herbivore deterrents or protecting photosynthetic tissues against ultraviolet radiation. The hand-held Dualex Series 4 instrument was originally developed for indirect determination of flavonol concentrations in crops. Flavonols are low-molecular-weight phenolics from which condensed tannins are synthesised. Therefore, it was expected that the Dualex’s ‘flavonol index’ (range 0.00–3.00) could be used to predict condensed tannin concentrations, providing a rapid, cost-effective, non-destructive method to sample more species and more plants per treatment than destructive methods used for wet-chemistry tannin assays. Thirteen broad-leaved woody species were sampled at the Nkuhlu and Letaba exclosures in Kruger National Park during December 2011. Four plants were sampled per species by collecting 100 leaves per plant, measuring the flavonol index of each leaf and pooling them in classes of 0.10 flavonol units. Condensed tannin concentrations were determined according to the acid-butanol assay with Sorghum tannin as standard. Regression models (condensed tannin = a*flavonol index – b) were significant for only four species: Acacia nigrescens (Senegalia nigrescens), Euclea divinorum, Spirostachys africana and Combretum zeyheri. Other than high variation in chemistry among plants, poor calibration results were likely caused by high chemical variation within leaves.Keywords: Dualex, flavonol, herbivore, Multiplex, secondary metabolite African Journal of Range & Forage Science 2014, 31(1): 55–58