Austral Ecology (2004) 29, 278–286

Leaf chemistry of woody in relation to season, canopy retention and goat browsing in a semiarid subtropical savanna

PETER F. SCOGINGS,1* LUTHANDO E. DZIBA2 AND IAIN J. GORDON3 1Department of Livestock and Pasture Science, University of Fort Hare, Alice, 5700, , 2Department of Forest, Range and Wildlife Sciences, Utah State University, Logan, Utah, USA and 3Macaulay Institute, Craigiebuckler, Aberdeen, UK

Abstract It is assumed that the phytochemistry of browse species protects their biomass and nutrients against herbivory. In this study we were primarily interested in (i) seasonal and phenology-related variations in chemistry, and (ii) chemistry-related variations in the feeding behaviour of domestic goats. Such knowledge would guide management-orientated modelling of browse–browser interactions in seasonal, subtropical zones where goats are abundant. The browse species studied were typical of semiarid savannas in southern Africa: Grewia occidentalis L. (Tiliaceae), myrtina (Burm. f) Kurz (), Diospyros lycioides Desf. ssp. lycioides (Ebenaceae), Rhus longispina Eckl. and Zeyh. (Anacardiaceae), rigida (Thunb.) Druce () and Acacia karroo Hayne (Mimosoideae). Nitrogen (N), phosphorus (P), condensed tannins (CT), protein-precipitating tannins (PPT), total phenols (TP), cellulose and lignin concentrations were estimated for each species during the late dormant and early growing seasons. N, P, CT and TP were elevated during the growth season, while cellulose, lignin and PPT decreased. Unlike cytoplasm contents, which varied seasonally, cell wall and vacuole contents varied both seasonally and among species. Except that seasonal variation in N of deciduous species was greater than that of evergreen species, leaf phenology was not related to variations in forage quality. Short-term intake rates were not related to primary metabolite concentrations, but were positively related to secondary metabolites. Elevated intake rates of putative defences were concluded to be side-effects of attempts by goats to increase nutrient intake rate, indicating tolerance of chemical defences. Without support for the hypothesis that chemical defences are correlated to canopy retention, we propose an alternative hypothesis, that defences are distributed among woody plants in semiarid, subtropical savannas according to shoot morphology because it affects the vulnerability of parts to browsers.

Key words: consumption rate, optimal defence, plant defence, resource availability, southern Africa

INTRODUCTION Variations in chemical characteristics may, in turn, affect the feeding behaviour of herbivores. For exam- The chemical characteristics of woody plants that ple, soluble phenols such as tannins can influence diet interact with browsing ungulates in savannas have choice and food consumption because they either make largely been explained in terms of resource-driven the food taste astringent, or reduce nutrient availability hypotheses of plant defence (O’Connor 1996; Scholes after ingestion (Robbins et·al. 1987; Cooper et·al. 1988; 1997). These hypotheses predict that concentrations of Freeland 1992). The structural carbohydrates (cellu- carbon-rich chemical defences, such as phenols, are lose and hemicellulose) and insoluble phenols (lignin) negatively correlated with the growth rate and sink that constitute plant fibre influence the toughness and strength of plants (Herms & Mattson 1992). Therefore, digestibility of plants, thus affecting intake rates such chemical defences would be less concentrated in (Shipley & Spalinger 1992; Jung & Allen 1995). plants during growth flushes than during phases of However, some generalist herbivores are known to dormancy. In addition, these chemical defences, which tolerate high levels of defensive chemicals in their diets can reduce the nutritional quality of plants for the (Provenza et·al. 1990). herbivores that eat them, are expected to be more In this study, we were primarily interested in whether common in evergreen species than deciduous species (i) nutrient concentrations increase, but chemical (Coley & Aide 1991). defence concentrations decrease, when plants are growing (summer) compared to when they are dormant (winter) (ii) nutrient concentrations are high, *Corresponding author. Present address Department of but chemical defence concentrations low, in deciduous Agriculture, University of Zululand, Private Bag X1001, KwaDlangezwa 3886, South Africa species compared to evergreen species, and (iii) con- (Email: [email protected]). sumption rates by generalist browsers are affected by Accepted for publication August 2003. chemical defences that cause either astringency or

BROWSE–GOAT INTERACTIONS IN SAVANNAS 279 toughness. Ultimately, we hoped the investigation of goats (Capra hircus L) are abundant and therefore have these questions could develop our understanding of the the potential to either compete with wild browsers for ecology of browsing resistance among woody plants the same resources (Haschick & Kerley 1997), or act in semiarid, subtropical savannas. Such knowledge as agents of biological control of invasive woody plants would be useful for guiding management-orientated (Trollope et·al. 1989). modelling of browse–browser interactions in seasonal, subtropical zones in southern Africa where domestic METHODS

Study area and species

The study was conducted at the Fort Hare Research Farm (3247S, 2653E) in the Eastern Cape Province of South Africa. The research farm, at an altitude of 500–600 m above mean sea level, is situated 80 km inland from the Eastern Cape coastline in a bush- clump savanna described by Acocks (1975) as ‘False Thornveld of the Eastern Cape’. The savanna consists of subtropical thicket vegetation on soils derived mainly from shales and mudstones and is dominated by deciduous woody shrubs shorter than 1.5 m, although the woody layer reaches 5 m (Scogings et·al. 1996). The vegetation of the study area is characteristic of the transitional region between the humid subtropics of the south-eastern seaboard of South Africa and the sub- tropical arid shrublands of the westward hinterland (Schultz 1995). Average annual rainfall from 1970 to 1998 was 617 mm year–1 (range: 324–991), with a coefficient of variation of 22%. During the decade 1991–2000, 75–93% of the annual rain fell during the seven months from October to April, while 3–20% fell during the three coldest months of June to August. The climate is therefore seasonal (Schultz 1995), but more so in some years than others. Domestic goats, particularly of the Boer and Nguni breeds, are abundant browsers in the Eastern Cape Province, where approximately half of the 6.8 million goats in South Africa are kept (National Department of Agriculture 1998). Six woody species constitute the main browse resource in the semiarid rangelands of the Eastern Cape (Raats et·al. 1996). The species are Grewia occidentalis L. (Tiliaceae), Scutia myrtina (Burm. f) Kurz (Rhamnaceae), Diospyros lycioides Desf. ssp. lycioides (Ebenaceae), Rhus longispina Eckl. and Zeyh. (Anacardiaceae), Ehretia rigida (Thunb.) Druce (Boraginaceae) and Acacia karroo Hayne (Mimosoideae). Four of the species are facultatively deciduous in response to either frost or drought, while two, S.·myrtina and R.·longispina are evergreen. There are no published data for leaf lifespans of any of the species, but S.·myrtina live for approximately 2 years, while R.·longispina leaves live for less than Fig. 1. Illustrations of the six woody species studied in the 2 years (P. F. Scogings pers. obs.). · · False Thornveld of the Eastern Cape, South Africa: (a) Three of the six species (G. occidentalis, D. lycioides Diospyros lycioides; (b) Grewia occidentalis; (c) Scutia mytlina; and S.·myrtina) produce all their new leaves on new, (d) Acacia karroo; (e) Ehretia rigida; (f) Rhus longispina. growing shoots (long-shoots sensu Bell 1991), which

280 P. F. SCOGINGS ET AL. bear one or two leaves per node (Fig. 1). Hence, we acid standard. All concentrations were expressed on refer to them as shoot-dominated species. The the basis of dry matter. Differences in leaf chemical remaining species are shoot-limited. They produce characteristics among species and between seasons most of their new leaves in discrete clusters (whorls were determined with ANOVA. Because each sample or tufts) of larger primary leaves and smaller was a pseudoreplicate, results should be regarded with secondary leaves at the nodes of thick, lignified shoots caution and should be seen as first approximations, or branches (Fig. 1). Such an arrangement of leaves is which are useful for generating hypotheses. a morphological state known as heterophylly, which To investigate relationships between intake rate and may be characterized by the nodes being raised into chemical characteristics, short-term intake rate (STIR) short-shoots or spurs (Bell 1991). The shoot-limited data from Dziba et·al. (2003) were used (Fig. 2). While plant architecture is characteristic of semiarid and S.·myrtina, R.·longispina and A.·karroo are spinescent, temperate environments (Hallé et·al. 1978; Fisher STIR is not strictly affected by the spines (Dziba et·al. 1986). 2003). Although STIR may be less closely related to chemistry than overall intake is, STIR has been found to be closely related to digestion and rate of passage, Data collection and analysis which affect intake (Romney & Gill 2000). Relation- ships between chemical characteristics and intake rates Chemical characteristics of the plant species were within each season were investigated by means of determined from foliage samples collected from three Pearson’s correlation analysis. Significant correlations unbrowsed trees per species during the late dormant were modelled with regression analysis using the season (winter) and early growing season (summer). species means as individual data points to overcome The samples were oven-dried at 60C for 24 h and pseudoreplication. Because of severe multicollinearity were analysed for ash, nitrogen (N), phosphorus (P), among the variables, which could not be overcome cellulose, lignin, condensed tannins (CT), protein- precipitating tannins (PPT) and total phenols (TP). Ash, Kjeldahl-N and P were determined according to AOAC (1984). Cellulose and lignin were determined according to Goering and van Soest (1970). Con- densed tannin concentrations were determined by the acid-butanol proanthocyanidin assay (Porter et·al. 1986), while protein-precipitating tannin concentra- tions were determined by the radial diffusion assay (Hagerman 1987), with sorghum tannin standards. Total phenol concentrations were determined by the Prussian blue assay (Price & Butler 1977), with a gallic

Fig. 3. Mean (a) nitrogen and (b) phosporus concentra- tions ( SE) of the six woody species studied during the late dormant and early growing seasons in the False Thornveld of the Eastern Cape, South Africa. Letters above bars indicate Fig. 2. Mean short-term intake rates ( SE) of goats for significant differences among species, while asterisks indicate the six woody species studied in the False Thornveld of the significant differences between seasons, based on confidence Eastern Cape, South Africa. Letters above bars indicate limits (n = 3; P < 0.05): AK, Acacia karroo; DL, Diospyros significant differences among means, compared with the lycioides; ER, Ehretia rigida; GO, Grewia occidentalis; RL, Tukey test (P < 0.05) (From Dziba et·al. 2003). Rhus longispina; SM, Scutia myrtina.

BROWSE–GOAT INTERACTIONS IN SAVANNAS 281 by appropriate techniques (Philippi 1993), multiple A. karroo, G. occidentalis and R. longispina had the regression analysis was not done. lowest (Fig. 4). All the species showed significant declines of 15–77% in either cellulose or lignin from when they were growing, to when they were dormant. RESULTS The biggest decline in lignin content was in E. rigida, while the smallest decline in cellulose content was in Nitrogen (F5,35 = 18.55; P < 0.001), cellulose (F5,35 = A. karroo. During the growing season, G. occidentalis · 8.45; P < 0.001) and lignin (F5,35 = 6.09; P < 0.001) and S. myrtina had 4.5 times more cellulose than concentrations varied among plant species, but phos- phorus content did not (P > 0.05). During dormancy, A.·karroo had significantly higher N concentration than all other species, but it exceeded only D.·lycioides during the growing season (Fig. 3). Except for S.·myrtina, N concentration was 8–56% higher during the growing · season (F1,35 = 43.63; P < 0.001), with the greatest increase in G.·occidentalis. All species except D.·lycioides, had significantly higher P (30–75%) during the growing season (F1,35 = 59.02; P < 0.001), with the greatest increase being in E. rigida (Fig. 3). During dormancy, G. occidentalis had significantly more cellulose than all other species, while A. karroo and E. rigida had significantly less than three others. Scutia myrtina had the highest lignin content, while

Fig. 5. Mean (a) total phenol concentrations (b) con- densed tannins and (c) protein precipitating tannins ( SE) Fig. 4. Mean (a) cellulose and (b) lignin concentrations of the six woody species studied during the late dormant and ( SE) of the six woody species studied during the late early growing seasons in the False Thornveld of the Eastern dormant and early growing seasons in the False Thornveld of Cape, South Africa. Letters above bars indicate significant the Eastern Cape, South Africa. Letters above bars indicate differences among species, while asterisks indicate significant significant differences among species, while asterisks indicate differences between seasons, based on confidence limits significant differences between seasons, based on confidence (n = 3; P < 0.05): GAE, gallic acid equivalents; STE, limits (n = 3; P < 0.05): AK, Acacia karroo; DL, Diospyros sorghum tannin equivalents; AK, Acacia karroo; DL, lycioides; ER, Ehretia rigida; GO, Grewia occidentalis; RL, Diospyros lycioides; ER, Ehretia rigida; GO, Grewia occiden- Rhus longispina; SM, Scutia myrtina. talis; RL, Rhus longispina; SM, Scutia myrtina. 282 P. F. SCOGINGS ET AL.

E. rigida, but S. myrtina and D. lycioides had 4.9 (summer), compared to when they were dormant times more lignin than E. rigida, G. occidentalis and (winter). The magnitude of seasonal change broadly R. longispina (Fig. 4). approximated the 30% change reported by Bergström Condensed tannin (F5,35 = 402.29; P < 0.001), pro- (1992) for savannas. However, when canopy retention tein-precipitating tannin (F5,35 = 492.56; P < 0.001) pattern was considered, the magnitude of change in and total phenol (F5,35 = 118.24; P < 0.001) concen- deciduous species (9–56%) was up to 19 times greater trations varied among plant species. S. myrtina had than that in evergreen species (3–8%). Phosphorus was significantly higher total phenol, condensed tannin and generally higher during growth than during dormancy, protein-precipitating tannin concentrations than all but the changes were not evidently associated with other species during dormancy, while the lowest canopy retention. Also consistent with expectations, protein-precipitating tannin concentrations were in cellulose and lignin showed the reverse trends of nutri- E. rigida and R. longispina (Fig. 5). Both S. myrtina and ents, in that they tended to be lower during growth than A. karroo had significantly higher total phenol and during dormancy. There was no overall pattern of condensed tannin concentrations than other species deciduous species having higher nutrient and lower during the growing season. In addition, condensed fibre concentrations than evergreen species, among six tannin (F1,35 = 282.22; P < 0.001), protein-precipi- species typical of semiarid, subtropical savannas in tating tannin (F1,35 = 157.03; P < 0.001) and total southern Africa. · phenol (F1,35 = 228.64; P < 0.001) concentrations The high cellulose content of G. occidentalis can be varied between seasons. Most species showed signifi- attributed to its habit of growing in bush-clumps, cantly higher condensed tannin and total phenol which requires fast growth to a tall height and offers concentrations during the growing season (Fig. 5). the benefit of escape from browsers and fire The total phenol and condensed tannin contents of (Scogings 1998; Watson 1999). The high lignin A. karroo were three-fold and four-fold higher, respec- content in S. myrtina and D. lycioides is a possible cause tively, but seldom more than double in the other of their low consumption by goats over the long-term species. In contrast, protein-precipitating tannin levels (P. F. Scogings unpubl. data; Mbuti et·al. 1996; Raats in most of the species were 8–100% lower during et·al. 1996; Bjelland & Grøva 1997; Gøthesen 1997). the growing seaon, with the greatest decline being Thus, D. lycioides may benefit from lignin as a defence in R. longispina and E. rigida. mechanism, although lignin has other beneficial Cellulose was positively correlated with STIR functions such as drought tolerance (Huston 1994). during the growth season (r = 0.79; n = 6; P <·0.05) Likewise, there is evidence to suggest that S. myrtina (Table 1). Conversely, condensed tannins (r = 0.90; has defences other than lignin, such as high tannin n = 6; P < 0.05), protein-precipitating tannins (r = content and hooked spines (Haschick & Kerley 1997; 0.89; n = 6; P < 0.05) and total phenols (r = 0.76; Scogings 1998). n = 6; P < 0.05) were positively correlated with STIR Like the polysaccharide and insoluble phenol com- during dormancy (Table 1). Nitrogen and phosphorus ponents of the foliar cell walls, the soluble phenol were not correlated with STIR (P > 0.05) in either components of the foliar vacuoles varied markedly season. between seasons and among plant species. Against expectations, condensed tannin and total phenol concentrations were higher in all the plant species during the growth season. Because these concen- DISCUSSION trations were not diluted by growth, their absolute amounts must have increased substantially. We Variations in leaf chemistry interpret this observation as indicative of an optimal defence strategy that potentially reduces the risk of Consistent with expectations, the N content of all the losing active apical buds to browsers (Rhoades 1979). plant species increased when they were growing Consistent with expectations, however, protein-

Table 1. Parameters ( SE) of the linear regression models (y = a + bx) relating chemical concentrations (x) to short-term intake rates of goats (y) during the late dormant season and early growing season in the False Thornveld of the Eastern Cape, South Africa

Season xabnr2 P

Late dormant Total phenols (mg g–1) 0.119 (0.0913) 0.004 (0.0016) 6 0.58 0.077 Condensed tannins (mg g–1) 0.184 (0.0409) 0.005 (0.0012) 6 0.81 0.015 Protein-precip. tannins (mg g–1) 0.114 (0.0592) 0.002 (0.0004) 6 0.79 0.019 Early growing Cellulose (%) 0.171 (0.1589) 0.053 (0.0211) 6 0.62 0.064 BROWSE–GOAT INTERACTIONS IN SAVANNAS 283 precipitating tannins decreased with growth. Assuming 2002). These observations in the subtropical savannas the decline was caused by dilution, the absolute of the Eastern Cape and southern Texas contradict amount of protein-precipitating tannins could be notions derived from temperate and tropical systems expected to have remained constant during the year, that associate either canopy retention or leaf lifespan which would imply that they were less mobile than with physiological attributes underpinning general the condensed tannins. Nevertheless, the lower models of plant defence, such as the carbon-nutrient protein-precipitating tannin concentrations during the balance and growth rate hypotheses (Coley 1988; growth season, together with the generally observed Bryant et·al. 1991). increase in N content, would result in higher N avail- ability for goats in summer. Digestion trials, which did not form part of this study, are needed to test the Forage quality and browsing effect that this combination of conditions has on N availability. The absence of positive correlations between nutrient As was previously found in this group of six woody concentrations and STIR was most likely caused by the species (Scogings 1998), S. myrtina had the highest absence of detectable variation in P and N among plant levels of tannins and phenols, while E. rigida had the species (except for A. karroo), combined with the clear lowest. Likewise, low levels of tannins and phenols variation among species in terms of dry matter intake have been previously observed in G. occidentalis and rates by goats (Dziba et·al. 2003). For example, while R. longispina (Scogings 1998). High levels of phenols E. rigida had the same N and P content as G. occiden- in A. karroo, as observed in the current study, have talis, goats achieved higher intake rates from the latter been previously observed among other Acacia species species. Likewise, although D. lycioides had lower N (Owen-Smith 1993). Thus, no clear evidence exists to content than A. karroo, goats achieved higher intake indicate that evergreen species have consistently higher rates from D. lycioides. The increases in nutrients in the concentrations of tannins and phenols compared with plants during the growth season, however, coincided deciduous species typical of semiarid, subtropical with increases in STIR, which would lead to increased savannas. nutrient intake in summer. The variation in nutrient concentrations between Short-term intake rate was positively, rather than seasons can be explained by the noticeable intra-annual negatively, related to foliar cell wall components and and interannual variation in conditions for growth that vacuole contents, indicating that putative chemical prevail in semiarid, seasonal systems (Schultz 1995), defences that enhance either toughness or astringency especially where rainfall is unpredictable and droughts do not limit intake rates of goats, as would be predicted. that last more than a year can occur every few decades. However, goats restrict their long-term consumption of As plant growth in subtropical systems commonly species that have high concentrations of tannin and occurs in recurrent flushes during the year and is lignin, despite their ability to achieve high STIR from closely coupled to rainfall (Larcher 1995), nutrient those species (Mbuti et·al. 1996; Raats et·al. 1996; concentrations within species would be expected to Bjelland & Grøva 1997; Gøthesen 1997; Dziba et·al. fluctuate significantly and in relation to the growth 2003). It is therefore reasonable to expect postingestive flushes. That the nutrient concentrations were found feedback to occur, but it is delayed, indicating that it is to be consistent among the species, except where linked to digestion processes. The positive relationships nitrogen-fixation elevated concentrations, indicates that between STIR and putative defensive chemical con- growth rates and photosynthetic rates among the centrations can be explained as a consequence of goats species are likely to be the same (Lambers et·al. 1998), optimizing their rates of nutrient intake, regardless of at least under field conditions. Such expectations have the prospect of postingestive feedback sometime later. been verified in other work on the same group of Being generalist herbivores, however, goats can tolerate species in the same environment when growth rates high intake of defensive chemicals and therefore they were measured in the field and found to be the same can feasibly increase their nutrient intake by ingesting across all six species, except A.·karroo (Scogings 1998). chemically defended species for limited time periods Growth rates, however, may be different among the six (Provenza et·al. 1990; Tixier et·al. 1997). species when affected by browsing or fire (Scogings Important implications for the conservation and 1998). In addition, specific leaf area does not vary management of savannas arise from the observation across the species (mean = 46.25 cm2 g–1; SE = that goats tolerate the putative chemical defences of 1.521; F5,54 = 0.90; P > 0.05; P. F. Scogings unpubl. woody plants growing in semiarid, subtropical data). Although leaf lifespans can be highly variable savannas. If their tolerance of tannin and lignin is within groups of evergreen and deciduous species in sufficient to allow goats to survive on poor-quality semiarid subtropical savannas, leaf lifespan is also not vegetation, then it is understandable that they can correlated with growth response, photosynthetic rate, control invasive woody plants in rangelands altered by N concentration and specific leaf area (Nelson et·al. heavy grazing that has reduced herbaceous cover and 284 P. F. SCOGINGS ET AL. increased woody cover, for example in semiarid grass- tended to be less in the shoot-dominated (0–20%) than lands of the Eastern Cape (Trollope et·al. 1989). In the shoot-limited (–100–294%) species. In addition, contrast, tolerance of tannins would allow goats to lignin and tannin in D.·lycioides and S.·myrtina must be utilize shrub-dominated communities whose native effective deterrents because, although goats achieve browsers exert less detrimental impacts on the woody high STIR of these two species, they generally avoid vegetation, resulting in either biodiversity depletion or them over the long-term (Mbuti et·al. 1996; Raats et·al. enhanced resource-competition with native browsers, 1996; Bjelland & Grøva 1997; Gøthesen 1997; Dziba for example in the subtropical thicket of the Eastern et·al. 2003). In the case of G.·occidentalis, avoidance is Cape (Haschick & Kerley 1997). postulated to be in the form of escape into bush-clump refugia that provide a physical barrier to browsers (Watson 1999), and fire. The soil beneath bush-clumps Role of apical bud activity in browse–goat is moister and more fertile than that between clumps, interactions and irradiance is lower (Jarvel & O’Connor 1999), forcing G. occidentalis to grow tall rapidly so that it can Without support for the hypothesis that putative emerge above the canopy of the clump and expose its defence traits are correlated to canopy retention leaves to full sunlight. Hence the high cellulose content, patterns, an alternative hypothesis is required to which is typical of tall woody plants (Wilson 1994). explain the variations in putative defences among Whether or not these traits are ultimately effective as woody plants in semiarid, subtropical savannas. The defences against goats can only be determined through species that had higher fibre concentrations in their bioassays aimed at determining the effects of the traits leaves consist of both deciduous (D. lycioides and on goat performance. G. occidentalis) and evergreen species (S. myrtina). In Because leaves are actively growing for limited time terms of the measured phenols, the high-fibre group periods during the growth season, and switch from tended to have higher concentrations of lignin and being resource sinks to sources early in their lives, they tannin, but the trend needs further investigation to can be less costly to produce and maintain than verify its generality. Nevertheless, the variation in foliar shoots (Larcher 1995). In addition, when grown on cell wall concentrations can be related to variation in old, lignified branches, leaves are less vulnerable to bud activity, which is affected by resource availability browsers, as illustrated by restricted intake rates of the and ultimately determines plant architecture (Herms & shoot-limited species (Dziba et·al. 2003). The species Mattson 1992; Bell et·al. 1999). with low bud activity therefore do not need to depend Because active meristematic tissues require large on mechanisms for preventing the loss of valuable quantities of nutrients and water to drive cell division tissues to herbivores. Instead, we postulate that the and growth, growing shoots constitute strong resource shoot-limited species tolerate the loss of leaves by sinks (Herms & Mattson 1992). Mature leaves, how- rapidly replacing the lost tissues through regrowth. The ever, are strong sources (Larcher 1995). Therefore, low, often undetectable concentrations of tannins and shoot-dominated species should have higher concen- phenols in E. rigida and R. longispina, and the well- trations of nutrients, but lower concentrations of documented, strong regrowth ability of Acacia species chemical defences, than shoot-limited species. How- (Scogings 2003), provide evidence in support thereof. ever, this is not the case for the group of six woody Furthermore, we postulate more broadly that, because species studied. Two of the shoot-dominated species bud activity is both affected by resource availability (D. lycioides and S. myrtina) had high concentrations of and interacts with the community of herbivores, the tannin and lignin. Because of their nutrient richness interaction between woody plants and browsers in and apical position, the active meristems of growing semiarid, subtropical savannas is both resource-driven shoots are both valuable to plants and vulnerable to and herbivore-driven. herbivores (Rhoades 1979), especially if shoot growth is for prolonged time periods during the growth season, as is common in subtropical environments (Larcher ACKNOWLEDGEMENTS 1995). The vulnerability of the apical meristems of the shoot-dominated species was demonstrated by The National Research Foundation (South Africa) and Dziba et·al. (2003). It follows therefore that the shoot- Royal Society of London (UK) funded the research. dominated species require mechanisms of avoiding The Agricultural Research Council (South Africa) herbivory (Rhoades 1979). In the case of D. lycioides provided additional financial support to L. E. Dziba, and S. myrtina, we postulate that the putative defences while I. J. Gordon’s participation was funded by the are in the form of deterrent chemicals, in particular the Scottish Executive Rural Affairs Department. Alan phenols (lignin or tannins), although there could be Duncan, Juan Villalba, Robin Pakeman and two anony- other defensive chemicals. Supporting the postulation mous reviewers gave helpful comments on earlier drafts is the observation that the seasonal variation in phenols of the manuscript. BROWSE–GOAT INTERACTIONS IN SAVANNAS 285

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