A 9,13-epoxylabdane from Colophospermum mopane

A new diterpene, 8(S),13(S)-dihydrogrindelic acid (2) was isolated from the seeds of Colophospermum mopane along with its previously described C-13 epimer (1). The structure of 2 was established primarily by NMR which required some revisions on the NMR shift assignments of 1. The importance of 1 and 2 as the missing links in the biosynthesis of 9,13-epoxylabdanes is briefly discussed.     

Elephant utilization of Colophospermum mopane: possible benefits of hedging

The pattern of utilization of Colophospermum mopane by elephant was investigated in a semi‐arid savanna, the Venetia‐Limpopo Nature Reserve, South Africa, using an animal‐based approach. C. mopane is a staple food item in the diet of elephant, with most individual trees being utilized lightly (< 10% of biomass removed). Utilization of branch, foliage and main stems accounted for most of the utilization of trees < 4 m high whilst utilization of bark accounted for the majority of utilization of trees > 4 m high. The most preferred height of utilization of branches was < 1 m above ground level, reflecting the short stature of C. mopane in the study area. Elephant selected C. mopane trees < 2 m in height that had been previously utilized by them; specifically trees where the terminal part of the main stem had been previously broken and had been coppiced (hedging). The preferred size range of branches utilized by elephant was 0.946–1.718 cm in diameter; hedged C. mopane trees in the 1–2 m category had, on average, significantly more branches within this size range than non‐hedged trees. This hedging of C. mopane appears to have increased the availability of a preferred food item.     

The structure of lerps: Carbohydrate,lipid, and protein components

Chemical analyses and studies by scanning electron microscopy were made of the lerps of Lasiopsylla rotundipennis and of Cardiaspina albitextura. Unlike the open, lace-like structure of the lerp of C. albitextura, the lerp of L. rotundipennis is a solid shell with a surface structure of repeated parallel arcs. Lerps of both species consist mainly of starch, but also contain protein and lipid. In addition, the lerp of L. rotundipennis is associated with proteinaceous spiral filaments secreted by glands on the ventral abdomen.     

Observations of tree growth, woodland structure and elephant damage on Colophospermum mopane in Luangwa Valley, Zambia

Six samples of tagged Colophospermum mopane were monitored for five years in locations with varying soil characteristics but with similar elephant densities. Physiognomic variation among the samples was related to soil differences, which also correlated with different browsing habits by elephants. The impact of elephant browsing further influenced both the physiognomy and demography of C. mopane. Results from this study suggest that the influence of soils and elephants on C. mopane alter successional transitions from grassland to woodland. Soils that promote coppicing of C. mopane yield less stable woodlands when associated with elephants than soils promoting woodlands with large bolus, non-coppicing trees. The dynamics of the latter are determined more by tree recruitment as influenced by such agents as other browsers or frequency and seasonality of bush fires. Implications for forest/elephant management are discussed.     

Evidence for facultative deciduousness in Colophospermum mopane in semi‐arid African savannas

Leaf phenology dictates the time available for carbon assimilation, transpiration and nutrient uptake in plants. Understanding the environmental cues that control phenology is therefore vital for predicting climate‐related changes to plant and ecosystem function. In contrast to temperate systems, and to a lesser degree, tropical forest systems, the cues initiating leaf drop in tropical savannas are poorly studied. We investigated the cues for leaf fall in a tropical monodominant arid savanna species, Colophospermum mopane, using an irrigation experiment. We tracked soil moisture, solar radiation, air temperature, leaf water status, leaf health and leaf carbon balance through the dry season in both irrigated and control plants. Water was the primary cue driving leaf loss of C. mopane rather than temperature or light. Trees watered throughout the dry season retained their canopies. These leaves remained functional and continued photosynthesis throughout the dry season. Leaf carbon acquisition rates did not decline with leaf age but were affected by soil moisture availability and temperature. Leaf loss did not occur when leaf carbon gain was zero, or when a particular leaf carbon threshold was reached. Colophospermum mopane is facultatively deciduous as water availability determines leaf drop in this widespread arid savanna species. Obligate deciduosity is not the only successful strategy in climates with a long dry season.     

Determinants of savanna vegetation structure: Insights from Colophospermum mopane

Abstract Savannas are structurally heterogeneous at the local, community‐level scale due to fine‐scale floristic heterogeneity as well as the responses of individual species to underlying environmental variation. The structure of mopane woodland, an arid savanna of southern Africa, is dictated largely by local variation in the relative dominance of tall, single‐stemmed and shorter, multi‐stemmed forms of the dominant tree species, Colophospermum mopane (Kirk ex Benth) Léonhard. Here we evaluate the hypothesis that the existence of these alternative forms of C. mopane, as well as the factors that dictate their distribution at a local scale, are driven by fine‐scale environmental variability in available water. We surveyed trees at four sites in the Kruger National Park of South Africa, in each instance surveying both forms of the species, from both riparian and non‐riparian zones. A survey of genetic variation across our sample (n = 80 individuals), using inter‐simple sequence repeat (ISSR) amplification profiles, indicates that the two forms are not genetically distinct, instead being environmentally determined. While measurements of xylem pressure potentials, determined using a Scholander pressure chamber, show a significant difference between riparian and non‐riparian zones, there is no significant difference between the two growth forms. Although this seems paradoxical in view of the prevalence of tree and shrub form mopane at riparian and non‐riparian sites, respectively, we speculate that such a pattern may emerge through the interaction of moisture stress and top‐down controls, such as those imposed by large mammal browsing and fire.     

An investigation into the relationship between physical damage and fungal infection in Colophospermum mopane

Colophospermum mopane trees in Zambia's North Luangwa National Park and an adjacent Game Management Area, were examined for evidence of physical damage caused by browsers, humans, weather, fire, etc. The same trees were assessed for visual signs of fungal colonization, with the aim of investigating any possible relationship between physical damage and attack by fungi. The results of this study suggest that the severity of physical damage had a relatively small influence on the activity of fungi colonizing and utilizing mopane heartwood. The majority of fungi isolated from infected trees were saprophytes, and the common occurrence of these agents in healthy trees suggests that heartwood degradation is not detrimental to the health of the tree. ‘Internal roots’ produced from the cambium inside the hollow trunk, which apparently utilize the detritus accumulated there, is probably an adaptation through which C. mopane can recycle minerals and nutrients.     

Patch dieback of Colophospermum mopane in a dysfunctional semi‐arid African savanna

Abstract Patch dieback occurred in an almost monospecific Colophospermum mopane (Kirk ex Benth.) Kirk ex J. Léonard woodland in the Northern Province, South Africa, following severe droughts in 1988–1989 and 1991–1992. Discrete patches of dieback and adjacent paired areas of ‘healthy’ vegetation lost an average of 87 and 13% of basal area to mortality, respectively. Whole trees mostly died on ‘dead’ plots, while single‐stem mortality prevailed on ‘live’ plots. Tree mortality decreased with increasing stem number per tree. Patch dieback did not occur on sandy soils. On fine‐textured soils, variation in soil type, topography or slope did not affect dieback. Dieback was influenced by vegetation structure, soil surface condition and soil chemistry. Intense intertree competition, shown by self‐thinning occurring prior to dieback, was a precondition for dieback. Intertree competition had heightened during the 30 years prior to dieback because of an increase in woody cover. Dieback patches had changed from functioning as sinks of water and sediment to sources of these as a result of loss of perennial herbaceous cover, decreased water retention on bared surfaces, and accelerated erosion during 50 years of livestock ranching. Vegetation had thus become increasingly drought‐prone, exacerbated in places by soils with a high sodium concentration. Dieback had occurred because the water requirements of C. mopane could no longer be met during drought years on the dysfunctional patches.     

Non‐lethal foraging by bell miners on a herbivorous insect: Potential implications for forest health

Tree health is often negatively linked with the localized abundance of parasitic invertebrates. One group, the sap‐sucking psyllid insects (Homoptera: Psyllidae) are well known for their negative impact upon vegetation, an impact that often culminates in the defoliation and even death of hosts. In Australia, psyllid‐infested forest in poor health is also frequently occupied by a native honeyeater, the bell miner (Manorina melanophrys; Meliphagidae), so much so that the phenomenon has been dubbed ‘bell miner‐associated dieback’ (BMAD). Bell miners are thought to be the causative agent behind BMAD, in part because the species may selectively forage only upon the outer covering (lerp) exuded by psyllid nymphs, leaving the insect underneath to continue parasitizing hosts. As bell miners also aggressively exclude all other avian psyllid predators from occupied areas, these behavioural traits may favour increases in psyllid populations. We examined bell miner foraging behaviour to determine if non‐lethal foraging upon psyllid nymphs occurred more often than in a congener, the noisy miner (M. melanocephala; Meliphagidae). This was indeed the case, with bell miners significantly more likely to remove only the lerp covering during feeding, leaving the insect intact underneath. This arose from bell miners using their tongue to pry off the lerp cases, whereas noisy miners used their mandibles to snap at both the lerp and insect underneath. Furthermore, psyllids left behind following a bell miner foraging event were significantly more likely to be viable and regrow a lerp covering than those exposed by noisy miners. Together, this behaviour supports the theory that non‐lethal foraging behaviour of bell miners may contribute to high psyllid abundance, consistent with the mechanisms by which BMAD is thought to develop.     

Effects of pruning on the concentration of macronutrients in Colophospermum mopane leaves

Despite the nutritional value of Colophospermum mopane leaves to browser's diets, there is limited knowledge on the effect of browsers on the concentration of macronutrients in mopane leaves. A field experiment was conducted in Musina Nature Reserve, Limpopo Province, South Africa to determine the effect of pruning, a proxy for browsing, on the concentration of macronutrients in mopane leaves. Samples were analysed for calcium (Ca), potassium (K), nitrogen (N), phosphorus (P), sulphur (S), chlorine (Cl), magnesium (Mg), nitrate (NO₃), sodium (Na), protein and fibre, using inductively coupled plasma atomic emission spectrometry techniques. The effect of pruning on the monthly concentration of macronutrients was tested using a two‐tailed t test: two‐sample assuming equal variance. The seasonal and annual effect of pruning on the concentration of macronutrients was tested using a one‐way ANOVA. Results showed that the amount of Ca, K, N, P, S, Cl, Na, protein and fibre increased during leaf flush in October and then declined as the leaves matured and aged. However, the concentration of Mg and NO₃ increased when the leaves reached maturity in June, particularly during senescence stage, and declined thereafter. The concentration of macronutrients between the control and pruned trees was statistically insignificant for most samples.     

Fire season affects size and architecture of Colophospermum mopane in southern African savannas

Wildfires may be started naturally by lightning or artificially by humans. In the savanna regions of southern Africa, lightning fires tend to occur at the start of the wet season, during October and November, while anthropogenic fires are usually started during the dry season, between July and August. A long-term field manipulation experiment initiated in the Kruger National Park in 1952 was used to explore whether this seasonal divergence affects tree abundance, spatial pattern, size and architecture. After 44 years of prescribed burning treatments that simulated the seasonal incidence of lightning and anthropogenic fires, mean densities of the locally-dominant shrub, Colophospermum mopane, were 638 and 500 trees ha^–1 respectively. Trees in burnt plots had aggregated distributions while trees in unburnt plots had random distributions. Significant differences (p < 0.001) were recorded in a range of morphological parameters including tree height, canopy diameter, mean stem circumference and number of stems. The incidence of resprouting also differed significantly between treatments, with burnt trees containing a high proportion of coppiced stems. The differences in tree size and architecture between the mid-dry season and early-wet season burning plots suggest that anthropogenic fires applied during July and August cannot substitute for a natural lightning fire regime. Anthropogenic fire yields a landscape that is shorter, more scrubby and populated by numerous coppiced shrubs than the landscape generated by natural lightning fire conditions.     

Seasonal diet changes in elephant and impala in mopane woodland

Elephant and impala as intermediate feeders, having a mixed diet of grass and browse, respond to seasonal fluctuations of forage quality by changing their diet composition. We tested the hypotheses that (1) the decrease in forage quality is accompanied by a change in diet from more monocots in the wet season to more dicots in the dry season and that that change is more pronounced and faster in impala than in elephant; (2) mopane (Colophospermum mopane), the most abundant dicot species, is the most important species in the elephant diet in mopane woodland, whereas impala feed relatively less on mopane due to the high condensed tannin concentration; and (3) impala on nutrient-rich soils have a diet consisting of more grass and change later to diet of more browse than impala on nutrient-poor soils. The phosphorus content and in vitro digestibility of monocots decreased and the NDF content increased significantly towards the end of the wet season, whereas in dicots no significant trend could be detected. We argue that this decreasing monocot quality caused elephant and impala to consume more dicots in the dry season. Elephant changed their diet gradually over a 16-week period from 70% to 25% monocots, whereas impala changed diets rapidly (2?C4?weeks) from 95% to 70% monocots. For both elephants and impala, there was a positive correlation between percentage of monocots and dicots in the diet and the in vitro digestibility of these forage items. Mopane was the most important dicot species in the elephant diet and its contribution to the diet increased significantly in the dry season, whereas impala selected other dicot species. On nutrient-rich gabbroic soils, impala ate significantly more monocots than impala from nutrient-poor granitic soils, which was related to the higher in vitro digestibility of the monocots on gabbroic soil. Digestibility of food items appears to be an important determinant of diet change from the wet to the dry season in impala and elephants.     

Effects of pruning on the concentration of trace elements in Colophospermum mopane leaves

Despite the nutritional value of Colophospermum mopane to browser's diets, there is still insufficient knowledge on the effect of browsers on concentrations of these trace elements. A field experiment was conducted in Musina Nature Reserve, Limpopo Province, South Africa, to determine the effect of pruning on the concentration of trace elements in mopane leaves. Samples were analysed for iron (Fe), manganese (Mn), boron (B), molybdenum (Mo), copper (Cu), zinc (Zn), cobalt (Co), fluoride (F) and selenium (Se) using the inductively coupled plasma atomic emission spectrometry technique. The effect of pruning was tested using the two‐tailed t‐test: two‐sample assuming equal variance and two‐tailed Mann–Whitney U‐test. Results showed that the concentration of trace elements in the control and pruned trees varies slightly through the year. Fe, Mn, Mo, Cu, Zn and Se are higher during leaf flush, but declined as the leaves matured and aged. This study concluded that simulated browsing had no significant effect on the concentration of trace elements in the mopane leaves. Seasonal variation in the amount of trace elements has implications on the distribution of browsers in the mopane woodland.     

Influence of soil fertility on the physiognomy of the African savanna tree Colophospermum mopane

This study investigated the influence of soil fertility on the physiognomy of Colophospermum mopane, one of the dominant tree or shrub species in savannas of central and southern Africa. The study was conducted in Gonarezhou National Park (GNP) in the south‐east of Zimbabwe. Mopane stands at the study site were divided into three physiognomic forms or mopane ‘types’: short mopane (SM, 1–2 m in height); medium mopane (MM, 8–12 m) and tall mopane (TM, >15 m). In each mopane type, soil samples were collected up to a depth of 15 cm to investigate the level of soil nutrient concentrations. SM stands were associated with low phosphorus and high levels of calcium and sodium relative to MM and TM. The low levels of phosphorus in SM relative to other mopane types suggest that phosphorus may be an important nutrient limiting growth in dwarf mopane. Alternatively, plant nutrients may be limiting because of interaction with factors such as the high soil sodicity present in the dwarf mopane habitats that may affect nutrient uptake or plant nutrient demand.     

Development of Glycaspis brimblecombei Moore (Hemiptera: Aphalaridae) on Eucalyptus camaldulensis Dehnh. and Eucalyptus dunnii Maiden

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Inducing condensed tannin production in Colophospermum mopane: Absence of response to soil N and P fertility and physical damage

The effects of available soil N and P and the effect of simulated browsing (leave removal) on foliar condensed tannin (CT) concentration were tested on young Colophospermum mopane (J. Kirk ex Benth) J. Leonard (Mopane) plants. Although clear differences in growth occurred between different levels of soil N, no differences in foliar CT concentration were found. Changes in available soil P and physical damage did not affect the plant growth or chemical composition. The complete absence of response of CT concentration to physical damage and soil nutrients may be related to the age of the trees in this study. It is proposed that existing theories on the interaction between soil properties and carbon based defences in trees are expanded, to include the potential responses of young trees to different soil nutrient levels.     

Increasing temperatures can improve seedling establishment in arid-adapted savanna trees

Plant species are shifting their ranges in response to global climate change, thus intensifying the need to predict such changes accurately. As the environmental requirements controlling plant distribution act differently at each developmental stage, there is a need to acquire a demographic-specific understanding of the factors which determine these distributions. Here we investigated the germination niche of two common savanna species Acacia nigrescens and Colophospermum mopane, with the aims to disentangle the direct and indirect effects of temperature on seed germination and establishment and to explore the impact of higher temperatures on the establishment success of savanna trees. Under laboratory conditions, we used thermal gradient plates to determine the thermal germination niche of both species, and a water stress experiment was conducted on C. mopane to account for water–temperature interactions. Using these data we parameterised a soil-moisture model to determine germination and establishment success under field conditions at current and future temperatures (+4 °C). Based on this model, higher future temperatures will not limit germination directly, but they will reduce the number of germination events by reducing the time window of suitable available soil water. Conversely, warmer conditions will accelerate the rate of radicle extension and increase the frequency of seedling establishment events. An additional advantage of higher temperatures is that fewer seeds will germinate, resulting in slower seed bank depletion when successful seedling establishment events do occur.     

The influence of litter quality and micro-habitat on litter decomposition and soil properties in a silvopasture system

Studies to understand litter processes and soil properties are useful for maintaining pastureland productivity as animal husbandry is the dominant occupation in the hot arid region. We aimed to quantify how micro-habitats and combinations of litters of the introduced leguminous tree Colophospermum mopane with the grasses Cenchrus ciliaris or Lasiurus sindicus influence decomposition rate and soil nutrient changes in a hot desert silvopasture system. Litter bags with tree litter alone (T), tree + C. ciliaris in 1:1 ratio (TCC) and tree + L. sindicus 1:1 ratio (TLS) litter were placed inside and outside of the C. mopane canopy and at the surface, 3–7 cm and 8–12 cm soil depths. We examined litter loss, soil fauna abundance, organic carbon (SOC), total (TN), ammonium (NH₄–N) and nitrate (NO₃–N) nitrogen, phosphorus (PO₄–P), soil respiration (SR) and dehydrogenase activity (DHA) in soil adjacent to each litter bag. After 12 months exposure, the mean residual litter was 40.2% of the initial value and annual decomposition rate constant (k) was 0.98 (0.49–1.80). Highest (p < 0.01) litter loss was in the first four months, when faunal abundance, SR, DHA and humidity were highest but it decreased with time. These variables and k were highest under the tree canopies. The litter loss and k were highest (p < 0.01) in TLS under the tree canopy, but the reverse trend was found for litter outside the canopy. Faunal abundance, litter loss, k, nutrient release and biochemical activities were highest (p < 0.01) in the 3–7 cm soil layer. Positive correlations of litter loss and soil fauna abundance with soil nutrients, SR and DHA demonstrated the interactions of litter quality and micro-habitats together with soil fauna on increased soil fertility. These results suggest that a Colophospermum mopane and L. sindicus silvopasture system best promotes faunal abundance, litter decomposition and soil fertility. The properties of these species and the associated faunal resources may be utilised as an ecosystem-restoration strategy in designing a silvopasture system. This may help to control land degradation and increase productivity sustainably in this environment.     

EARLY LEAF ONTOGENY AND THE SHOOT APEX OF COLOPHOSPERMUM MOPANE (LEGUMINOSAE)

Shoot tips of Colophospermum mopane (Kirk ex Benth.) Kirk ex Léonard produce leaves which at maturity are bifoliate and devoid of stipules. Investigation of their early ontogeny, however, shows that these leaves begin as trifoliate structures partially enclosed by their stipules. The latter are fused along their mid regions, forming a tongue-like “connector.” The lower chamber of this stipule pair harbors the apical meristem while the upper compartment enfolds the two lateral leaflets. The terminal leaflet, histologically resembling the stipules, also fulfills a similar function by covering the top portion of its sister leaflets. Anatomically, the shoot apex displays a pendulum symmetry, with rather steep elevation of that internode portion which subtends the newly formed primordium. Some comparisons with the shoot apex of Hymenaea are drawn.