Dawood Hattas

Differential effects of defoliation by mopane caterpillars and pruning by African elephants on the regrowth of Colophospermum mopane foliage

Plant responses to herbivory vary depending on herbivory type, yet the comparative effects of defoliation (e.g. by insects) and pruning (e.g. by large mammals) on a single tree species are poorly documented. We investigated this in the Northern Province of South Africa by comparing the regrowth of Colophospermum mopane trees previously defoliated by caterpillars or pruned by elephants, the two main browsers of C. mopane foliage. Shoots were up to 160% and 125% longer after natural (elephant) and simulated pruning and leaves ∼25% longer in regrowth after natural pruning (n = 13-15 trees per treatment). Shoot density and chemical defences in leaves (tannin:protein ratio and total polyphenolic concentration) were, however, no different from control trees. Simulated defoliation resulted in statisticallyinsignificantchangestoregrowthintermsofleafandshootsize(bothslightlydecreased)andshootdensity (slightly increased). Natural (caterpillar) defoliation, however, resulted in regrowth with significantly decreased shoot and leaf size (about 50% and 20% of control lengths, respectively), as well as decreased leaf chemical defence. Shoot and leaf length were longer on trees flushing for the first time after pruning and late-season defoliation had a greater negative impact than mid-season defoliation. Despite the differences in regrowth characteristics after pruning and defoliation, mopane plants showed no apparent trade-off in investment between tolerance and resistance after either herbivory type, as neither regrowth nor chemical defence occurred at the expense of the other.

Differential phenolic profiles in six African savanna woody species in relation to antiherbivore defense

Low molecular weight phenolics are suggested to have a role in mediating diet selection in mammalian herbivores. However, very little is known about low molecular weight phenolic profiles of African savanna woody species. We determined low molecular weight phenolic profiles of six woody species with different life history, morphological and functional traits. We investigated interspecific phytochemical variation between species and found that: (1) related Acacia species were chemically dissimilar; (2) similarity percentage analysis revealed that Acacia grandicornuta was most dissimilar from other species and that the evergreen and unpalatable Euclea divinorum had a qualitatively similar chemical profile to the deciduous and palatable Acacia exuvialis and Combretum apiculatum; (3) C. apiculatum had the highest chemical diversity; (4) relative to spineless plants, spinescent plants contained significantly less HPLC phenolics and condensed tannins; and (5) the major quantitative difference between the evergreen and unpalatable E. divinorum and other species was its high myricitrin concentration.

Intraspecific host preferences of mopane moths (Imbrasia belina) in mopane (Colophospermum mopane) woodland

Colophospermum mopane trees are a common larval food plant of the mopane moth (Imbrasia belina), which commonly reaches outbreak proportions. Our study investigated factors determining host tree choice by ovipositing mopane moths within a single host species (C. mopane). Tree size was the primary determinant of oviposition at both the habitat (tall riverine-, medium height woodland- and short-shrub mopane) and individual tree scale, with an increase in the number of egg masses with increasing tree size (estimated by height and canopy volume). Preference for larger trees was only evident at the habitat scale, however, as individual trees were utilized as expected according to the availability of their canopy size class. More detailed tree characteristics, such as leaf size, shoot size, stem number and even leaf chemistry (protein:tannin ratio and total polyphenols) had no influence on host choice. Host choice was based on the most obvious measure of resource abundance, namely tree size, as expected for an outbreak species, since resource availability rather than nutritional quality is likely to be the primary determinant of larval survival.

Does the Growth Differentiation Balance Hypothesis Explain Allocation to Secondary Metabolites in Combretum apiculatum , an African Savanna Woody Species?

The growth differentiation balance hypothesis (GDBH) provides a framework that predicts a trade-off between costs of secondary metabolites (SMs) relative to the demand for photosynthate by growth. However, this hypothesis was developed using empirical evidence from plant species in northern boreal and temperate systems, leaving its applicability to species under different abiotic and biotic conditions questionable and generalizations problematic. The objective of this study was to investigate whether the GDBH explains allocation to SMs in the deciduous African savanna woody species C. apiculatum along a 6-point N gradient. The cornerstone prediction of the GDBH, i.e., the parabolic response in SMs along the N gradient, was not observed, with secondary metabolism showing compound-specific responses. Quercetin, myricetin, and kaempferol glycoside concentrations, all produced via the same pathway, responded differently across the N gradient. Flavonol glycoside, cinnamic acid, and quercetin glycoside concentrations decreased as N increased, which provides partial support for the carbon nutrient balance hypothesis. Simulated herbivory had no effect on photosynthesis, decreased foliar N and consequently increased C:N ratio, but did not induce an increase in SMs, with condensed tannins and flavonol glycosides being unaffected. Defoliated plants at low N concentration compensated for lost biomass, which suggests a tolerance response, but as predicted by the limiting resource model, plants at higher N concentration were evidently C limited and thus unable to compensate. Our results show that the GDBH does not explain allocation to SMs in C. apiculatum, and suggest that mechanistic explanations of plant allocation should consider the integrative defensive effect of changed SMs.

Effect of organic cultivation of rooibos tea plants (Aspalathus linearis) on soil nutrient status in Nieuwoudtville, South Africa

The shoots of rooibos (Aspalathus linearis (Burm.f.) R.Dahlgren) plants, cultivated organically by small-scale farmers in Nieuwoudtville, are harvested for the production of tea. These practices could lead to decreasing soil fertility. It was hypothesised that soil from cultivated rooibos plots will have lower nutrient concentrations than soil from adjacent uncultivated plots. Soil and shoot samples were collected in December 2005, 2006 and 2009 from cultivated fields of increasing plot age and from adjacent uncultivated plots on three farms, and analysed for nutrient concentration. Compared with the uncultivated plots, no measured soil nutrients including concentrations of phosphorus (P), exchangeable potassium (K), magnesium (Mg) and calcium (Ca), and soil carbon (C) decreased in cultivated plots over the five-year period of assessment. Soil C correlated positively with concentrations of soil exchangeable K, Mg and Ca, and sodium (P<0.001), indicating that soil C is an important indicator of soil fertility. Foliar P increased, and consequently the nitrogen:P ratio decreased in cultivated relative to uncultivated plants, implying higher P-uptake by cultivated plants. Overall, organic rooibos cultivation in Nieuwoudtville appears to be sustainable in terms of maintaining soil nutrition because soil nutrient status did not decrease over the five-year period.

Seasonal regulation of condensed tannin consumption by free-ranging goats in a semi-arid savanna

Although condensed tannins (CTs) are known to reduce forage intake by mammalian herbivores in controlled experiments, few studies have tested these effects in the field. Thus the role of CTs on foraging ecology of free-ranging herbivores is inadequately understood. To investigate the effects of CTs under natural savanna conditions, we pre-dosed groups of goats with polyethylene glycol (PEG, a CT-neutralising chemical), CT powder or water before observing their foraging behaviour. While accounting for the effects of season and time of the day, we tested the hypothesis that herbivores forage in ways that reduce the intake rate (g DM per minute) of CTs. We expected pre-dosing goats with CTs to reduce CT intake rates by (1) consuming diets low in CTs, (2) reducing bite rates, (3) increasing the number of foraging bouts, or (4) reducing the length of foraging bouts. Lastly, (5) expected CT to have no influence the number of dietary forage species. In both wet and dry seasons, pre-dosing goats with CTs resulted in lower CT consumption rates compared to PEG goats which seemed relieved from the stress associated with CT consumption. During dry season, the number of dietary forage species was similar across treatments, although goats that were dosed with PEG significantly increased this number in the wet season. Dosing goats with PEG increased the number and length of browsing bouts compared to goats from the other treatments. Pre-loading goats with PEG also tended to increase bite rates on browse forages, which contributed to increased consumption rates of CTs. Based on the behavioural adjustments made by goats in this study and within the constraints imposed by chemical complexity in savanna systems, we concluded that herbivores under natural conditions foraged in ways that minimised CTs consumption. More research should further elucidate the mechanism through which CTs regulated feeding behaviour.