Phytochemistry of the mopane,Colophospermum mopane

The polyphenolic pool of the heartwood of the mopane, Colophospermum mopane Kirk ex J. Leonard, exhibits extreme diversity and complexity. It comprises a variety of monomeric flavonoids, e.g. flavan-3-ols, flavan-3,4-diols including the mopanols and peltogynols, flavonols, dimeric proanthocyanidins, e.g. proguibourtinidins, profisetinidins, promopanidins, propeltogynidins, and a variety of profisetinidin-type triflavanoids. The di- and tri-meric proanthocyanidins are accompanied by several functionalized tetrahydropyrano- and hexahydrodipyrano-chromenes (phlobatannins) that originate from the bi- and tri-flavanoids, respectively, via rearrangement of the pyran heterocycle(s). Owing to the predominance of the 5-deoxy (A-ring) flavan-3-ols, the chain terminating moieties in the biosynthesis of oligo- and poly-meric proanthocyanidins, the di- and tri-meric analogs also exhibit diversity as far as interflavanyl bonding positions are concerned. Such heterogeneity results from the reduced nucleophilicity of the A-rings of 5-deoxy flavan-3-ols, compared to the A-rings of the 5-oxy analogs (catechins), hence permitting alternative centers to participate in proanthocyanidin formation. Biomimetic-type syntheses were extensively utilized to unequivocally establish constitution and absolute stereochemistry of both the conventional and pyran ring rearranged-type di- and tri-meric compounds. Comprehension of the intricate mechanistic and stereochemical course of the pyran ring rearrangement reactions also contributed significantly to unambiguous structure elucidations. The aerial parts of the mopane are rich in essential oils that comprise mainly alpha-pinene and limonene, which are presumably responsible for the strong turpentine odor of the pods. The leaves also contain significant concentrations of beta-sitosterol and stigmasterol which are apparently the source of sterols in various organs of the mopane moth, Gonimbrasia belina. Three diterpenes, dihydrogrindelic acid, labd-13E-en-15-oate and dihydrogrindelaldehyde are present in the bark and seeds, the latter compound exhibiting significant cytotoxicity against a human breast cancer cell line.     

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.     

Diterpenes and sesquiterpenes from the bark of Taxus yunnanensis

Two taxane-type diterpenes, 10beta-acetoxy-2alpha,5alpha,7beta,9alpha-tetrahydroxytaxa-4(20),11-dien-13-one and 2alpha-acetoxy-9alpha-benzoyloxy-5alpha,7beta,10beta,15-tetrahydroxy-11(15-->1)- abeotaxa-4(20),11-dien-13-one, and two new drimane-type sesquiterpenes, 1beta-acetoxy-7-drimen-11alpha-ol-12,11-lactone and 1beta-acetoxy-11,12-epoxy-6-drimen-8alpha,11alpha-diol, were isolated from the bark of Taxus yunnanensis together with 35 known taxane-type diterpenes, a known drimane-type sesquiterpene and a known flavanone.     

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.     

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.     

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.     

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.     

The influence of large mammalian herbivores on growth form and utilization of mopane trees, Colophospermum mopane, in Botswana's Northern Tuli Game Reserve

Mopane veld is of great value to the general ungulate spectrum in times of drought, and it is capable of retaining its dominance in a community even in the presence of extremely heavy browsing pressure imposed by large browsing mammals. Scrub mopane (hedges) has been regarded as resulting from excessive browsing pressure by large mammals, especially elephants. Both the nutrient and chemical composition of mopane twig bark were investigated, the seasonal results being related back to the seasonal utilization of branches by large mammals. Mopane twigs were most palatable in winter. Eland feed on mopane throughout the year irrespective of palatability. Elephants were rarely present in the scrub mopane area before the onset of spring rains, when the major mopane leaf flush occurs independently of rainfall. The impact of both species was not excessive and recruitment of mopane seedlings does occur. Herbivore browsing is responsible for a mopane morph which buds early and continues to produce accessible, nutritious leaves even when heavily browsed. Many browsing ungulates are reliant on this resource during the stressful transition from spring to summer in south-eastern Botswana.     

Diterpenes of Podocarpus ferrugineus bark

The following oxygenated diterpenes have been isolated from the bark of Podocarpus ferrugineus D. Don: isopimarol, isopimaric acid, sandaracopimaric acid, ferruginol, sugiol, sugiyl methyl ether, xanthoperol, royleanone, 6-dehydroroyleanone, cryptojaponol, 5β-hydroxy-6-oxasugiyl methyl ether, 2-ketoferruginol and 2β-acetoxysugiyl methyl ether.     

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.     

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.     

Influence of Colophospermum mopane canopy cover on litter decomposition and nutrient dynamics in a semi‐arid African savannah

The objective of this study was to investigate the influence of mopane canopy cover on litter decomposition in a semi‐arid African savannah. We used a randomized block design with five blocks of 100 × 100 m demarcated in a 10‐ha pocket of open mopane woodland. Litterbags were placed beneath large (8.3 m crown diameter) and small mopane trees (2.7 m crown diameter) and in the intercanopy area. Decomposition was fastest in the intercanopy area exposed to solar radiation (k = 0.35 year^?1), intermediate beneath small trees (k = 0.28 year^?1) and slowest beneath large trees (k = 0.23 year^?1). Soil temperatures beneath small and large trees were 3–5 and 6–9°C lower than in the intercanopy area, respectively. Bacterial and fungal counts were significantly higher (P < 0.05) beneath large than small trees and in the intercanopy area. The amount of N and P released did not vary significantly among sampling sites. Soil moisture in the dry season was similar among sampling sites but rainy‐season soil moisture was significantly greater (P < 0.05) beneath large than small trees and in the intecanopy area. Mopane canopy cover retarded litter decomposition suggesting that photodegradation could be an important factor controlling carbon turnover in semi‐arid African savannahs.     

Diterpenes from Koanophyllon species

The investigation of two Koanophyllon species afforded, in addition to known compounds, several new diterpenes, two new geranylgeraniol derivatives and four diterpenes, most probably closely related to clerodanes; their structures, however, could not be deduced with certainty. The chemotaxonomic situation of this genus is discussed briefly.