Austral Ecology (2004) 29, 278–286 Leaf chemistry of woody plants 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, South Africa, 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 leaf 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), 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). 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 plant 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 (32Њ47ЈS, 26Њ53ЈE) 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 leaves 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