Celtis Africana Research Articles

Regeneration niches and functional traits of three common species in subtropical dune forest

South African coastal dune forests are young, highly disturbed subtropical communities where conventional models of forest dynamics may be challenged. We tested predictions from the gap-phase regeneration model by comparing seedlings of three common species representing contrasting regeneration strategies: Acacia karroo as a ruderal, Celtis africana as a coloniser of forest gaps, and Diospyros natalensis as a late-successional species. We grew seedlings under contrasting light and nitrogen levels in a greenhouse and in the field for 1 year to compare their growth and survival rates, allocation and photosynthetic traits. Species’ growth rates generally followed the expected order: Acacia>Celtis>Diospyros, but Acacia responded strongly to light and Celtis responded strongly to nitrogen, leading to cross-overs in growth rates. The plasticity of allocation and photosynthesis did not clearly differentiate the strategies, although it was greater in the light-demanding species. Acacia and Celtis tended to survive better in Acacia stands than in forest plots. Leaf-level light compensation points (LCPs) were similar for the three species in most conditions, but auxiliary data suggest Diospyros has a lower whole-plant LCP than Acacia. Growth rates and LCPs were lower than most of those reported for primary-forest species in the literature, suggesting an unusual degree of shade-tolerance in this habitat. We discuss reasons why variation in shade-tolerance may be less important here than in the prevailing model for forest regeneration and suggest other biotic factors that may help differentiate regeneration niches.

Cambial growth dynamics and climatic control of different tree life forms in tropical mountain forest in Ethiopia

Munessa Forest is a mountain forest in south-eastern Ethiopia experiencing seasonal rainfall variation. We investigated seasonal cambial activity and dormancy from increment rates of four different tree species belonging to varying life forms, namely, evergreen native conifer (Podocarpus falcatus), evergreen introduced conifer (Pinus patula), evergreen broadleaved tree (Prunus africana) and deciduous broadleaved tree (Celtis africana). Measurements of stem radius fluctuations were registered with the help of high-resolution electronic dendrometers. Daily amplitudes of stem diameter variations and daily and monthly net growth rates were determined and related to climatic variables measured at local climate stations. Thin sections of wood collected with a microcorer every 3–6 weeks allowed a visual control of newly formed wood cells during consecutive time intervals. Lack of water availability during the long dry season induced cambial dormancy of 5–7 months depending on life forms. After the onset of the short rainy season, stem swelling started quite synchronously with a variation of only single days in all studied species. Evergreen tree species were able to initiate wood formation during the short rainy season, whereas growth in the deciduous broadleaved species started in the long rainy season. The acquired data provide a basis for delineating the species-specific growth boundaries and the duration of the cambial growing season.

Ecological filtering by a dominant herb selects for shade tolerance in the tree seedling community of coastal dune forest

The regeneration niche is commonly partitioned along a gradient from shade-tolerant to shade-intolerant species to explain plant community assembly in forests. We examined the shade tolerance of tree seedlings in a subtropical coastal forest to determine whether the ecological filtering effect of a dominant, synchronously monocarpic herb (Isoglossa woodii) selects for species at either end of the light response continuum during the herb's vegetative and reproductive phases. Photosynthetic characteristics of seedlings of 20 common tree species and the herb were measured. Seedlings were grown in the greenhouse at 12-14% irradiance, and their light compensation points measured using an open-flow gas exchange system. The light compensation points for the tree species were low, falling within a narrow range from 2.1 ± 0.8 μmol m(-2) s(-1) in Celtis africana to 6.4 ± 0.7 μmol m(-2) s(-1) in Allophylus natalensis, indicating general shade tolerance, consistent with a high and narrow range of apparent quantum yield among species (0.078 ± 0.002 mol CO(2) mol(-1) photon). Rates of dark respiration were significantly lower in a generalist pioneer species (Acacia karroo) than in a forest pioneer (C. africana), or in late successional phase forest species. We argue that the general shade tolerance, and phenotypic clustering of shade tolerance, in many tree species from several families in this system, is a result of ecological filtering by the prevailing low light levels beneath the I. woodii understorey, which excludes most light-demanding species from the seedling community.

Antifungal efficacy of ten selected South African plant species against Cryptococcus neoformans

Fungal infections are a major threat to public health care. Cryptococcosis in humans and animals, caused by Cryptococcus neoformans, is a life-threatening disease. In a random antifungal screening of acetone leaf extracts of 400 tree species against Cryptococcus neoformans, the following plant species had good activity: Zanthoxylum capense (Thunb.) Harv. (Rutaceae), Morusmesozygia Stapf (Moraceae), Calodendrum capense (L.f.) Thunb. (Rutaceae), Catha transvaalensis Codd (Celastraceae), Cussonia zuluensis Strey (Araliaceae), Ochna natalitia (Meisn.) Walp. (Ochnaceae), Croton sylvaticus Hochst. ex C. Krauss (Euphorbiaceae), Maytenus undata (Thunb.) Blakelock (Celastraceae), Celtis africana Burm.f. (Ulmaceae), and Cassine aethiopica Thunb. (Celastraceae). Hexane, dichloromethane, acetone, and methanol extracts of these 10 plants were tested against Cryptococcus neoformans using bioautography and microdilution assays. Acetone extracted the highest quantity of plant material. Dichloromethane and hexane extracts of Maytenus undata showed clear bands in bioautography while the other species did not produce good results in bioautography. Maytenus undata extracts had promising antifungal activity against C. neoformans, with average minimum inhibitory concentration (MIC) of 0.09 mg/mL after 24 h and 0.18 mg/mL after 48 h incubation. Croton sylvaticus and Catha transvaalensis extracts also had good activity, with average MIC values of 0.07 mg/mL and 0.09 mg/mL, respectively. Because of the clear bands on bioautograms and low MIC values compared to the other plant species investigated, M. undata was identified as a good candidate for further studies.

Plant community distribution and variation along the Awash river corridor in the main Ethiopian rift

AbstractThe vegetation along the Awash River (1200‐km long) in the main Ethiopian Rift, and its relationship with environmental factors was studied. Seven plant communities were described from the study area: (1) Acacia nilotica subsp. leiocarpa. – Carissa edulis type; (2) Acacia robusta subsp. usambarensis – Acokanthera schimperi type; (3) Celtis africana – Mimusops laurifolia type; (4) Acacia senegal – Acacia mellifera – Dobera glabra type; (5) Acacia nilotica subsp. indica – Ficus capreaefolia type; (6) Lannea schimperi–Glycine wightii type; and (7) Tamarix nilotica – Acacia hocki community type. It has been shown that the plant communities along the river follow an altitudinal gradient. The vegetation of the Awash River is mainly the result of the interactions between edaphic factors, the hydrology, altitude, slope and climate.

Regeneration of seven indigenous tree species in a dry Afromontane forest, southern Ethiopia

Regeneration of seven indigenous tree species having significant ecological and economic importance was investigated in the Munessa-Shashemene dry Afromontane forest (MSF), southern Ethiopia. Densities and distributions of seedlings, saplings and trees were assessed along gradients of altitude, light and disturbance using quadrat sizes of 10×5 m (for seedlings) and 20×20 m (saplings and trees) following line transects. The number of individuals, frequency and height of the study species were recorded in the quadrats at every 100 m drop in altitude. Seedling densities varied markedly among the species and altitudes. Mean densities (number of individuals ha −1) of seedlings ranged from zero ( Polyscias fulva) to 5334 ( Prunus africana), and from three ( Polyscias fulva) to 102 ( Podocarpus falcatus) for trees and saplings. Canonical Correspondence Analysis revealed that seedlings of Celtis africana and Croton macrostachyus were highly favored by disturbance and, hence, were concentrated in canopy gaps within the forest. Based on their population structures, the study species could be categorized into three groups: (1) Species that showed the highest proportion of individuals in the lowest height class and with a gradual decrease towards the upper height classes, which suggests good regeneration; Podocarpus falcatus, C. africana, C. macrostachys and P. africana belonged to this group. (2) Species that showed higher proportions of individuals in the lowest height class and with missing individuals in the subsequent middle height classes, indicative of hampered regeneration; Syzygium guineense and Pouteria adolfi-friederici belonged to this group. (3) Species with no individuals in the lowest and middle height classes but represented by individuals in upper height classes; P. fulva belonged to this group. The species categorized in the last two groups exhibited hampered regeneration, and P. fulva is in the verge of local extermination. High seedling densities (e.g. C. africana and P. africana) and/or adaptive defense mechanisms to herbivory (e.g. P. falcatus and C. macrostachyus) were common attributes of species, which exhibited good regeneration. Regeneration problems were largely attributed to human disturbance, lack of suitable habitat for seed germination or problems associated to seed set (seed predation or abortion). Our study indicated that P. fulva, P. adolfi-friederici and S. guineense require the highest immediate attention for conservation in the MSF.

Floristic composition and environmental factors characterizing coffee forests in southwest Ethiopia

Afromontane rainforest stands in Ethiopia with a high frequency of wild populations of Coffea arabica are commonly known as ‘coffee forests’. These forests are important for the conservation of the genetic diversity of wild Arabica coffee, and for subsistence coffee production. This study analyses the floristic composition and environmental factors of such coffee forests, using the Yayu forest as a case. Fifty-eight plots of 20 m × 20 m were assessed, and a total of 220 plant species, representing 73 families, were recorded, of which 71 were trees, 28 shrubs, 27 climbers and 94 herbs. A cluster analysis, coupled with indicator species analysis, revealed three plant community types, which can designated as C. arabica– Cassipourea malosana, Argomuellera macrophylla– Celtis africana and Dracaena fragrans– Teclea noblis communities. The C. arabica– C. malosana and D. fragrans– T. noblis communities occur at relatively higher altitudes (1450 and 1435 m on average) on gentle and steep slopes, respectively, while the A. macrophylla– C. africana community occurs on steep slopes at lower altitudes (1380 m on average). C. arabica is one of the most frequent species in the Yayu forest, and its abundance is strongly negatively correlated with slope. These habitat differences, together with the predominance of Coffea in the understorey, support the floristic circumscription of a coffee forest. Forests commonly known as “coffee forests” have, therefore, to be viewed as a complex mosaic of different plant communities, which needs to be taken into consideration in the designation of conservation areas and differential management planning. The plant species composition of Yayu forest is predominantly Afromontane rainforest type, with several dry peripheral semi-deciduous Guineo-Congolian forest type species. Hence, Yayu forest can best be described as transitional between these lowland and montane forest types.

A comparative study of regeneration under live and dead canopy trees in a tropical rain forest ecosystem of Kibale National Park, Uganda

AbstractPatterns of forest regeneration and recruitment are vital aspects in forest ecosystem maintenance. In Kibale, some tree species are experiencing unusual mortality and have been monitored for several years. This study was conducted in order to investigate the regeneration of five canopy tree species that were experiencing high mortality rates among adult individuals. The five canopy tree species, some of which are important timber species, include: Pouteria altissima, Celtis africana, Lovoa swynnertonii, Mimusops bagshawei and Newtonia buchananii. The study was conducted in the mature forest tract which was never logged mechanically (K‐30). The objective of the study was to establish whether these tree species influence regeneration of any of the other species including their own and the role of small gaps created by tree deaths in forest recruitment. The study species did not appear to regenerate beneath their own canopies. There was a tendency for higher survival rate away from the conspecific adult tree species. However, gaps created after the death of the mature trees were found to be sites of high forest regeneration level. They supported significant densities of the juveniles of the five study species where N. buchananii clearly demonstrates gap specialization.

Pollen Grain Morphology of Selected Allergenic Species Native to Southern South America

Pollen grains of Celtis tala (Celtidaceae), Phytolacca dioica (Phytolaccaceae), Schinopsis balansae (Anacardiaceae) and Solidago chilensis (Asteraceae) are examined with light and scanning electron microscopy, described, and illustrated. These four unrelated species are native to southern South America and considered an important source of skin prick allergy tests. Most relevant pollen characters are compared to those of their closest relatives. Celtis tala (3-, 4-, or 5- porate, verrucate) shares characters with the Celtis africana type in lacking a sunken area around the ora and in having the sculptural elements regularly arranged; Phytolacca dioica (3-colpate) with the Phytolacceae type in being microechinate and microperforate; Schinopsis balansae (3-colporate, striato-reticulate) with some members of the Lithraea molleoides subtype (Anacardiaceae) in having high muri and long striae; Solidago chilensis (3-colporate, echinate) with the Solidago type in being caveate. This study contributes to a better understanding of pollen morphology in species that are especially common to southern South America.

A comparison of the airspora at two sites in Cape Town

The airspora, comprising pollen and fungal spores, was recorded in Cape Town during 1993 using a Burkard sampler. Two sites were monitored, the first located in Parow and the second at Kirstenbosch. The results do not clearly reflect the indigenous vegetation, but rather, exotic elements that are either naturalized or cultivated. The only indigenous pollen that was recorded in any numbers as a single taxon was Poaceae, of which Cynodon dactylodon (L.) Pers. was probably the major contributor, and Celtis africana Burm. f. All pollen types recorded have been implicated as allergens. The most serious of these were Cupressaceae and Quercus L. Incidence of both pollen and fungal spores was greater at Kirstenbosch than Parow, attributable to localized differences in vegetation.

Airspora of Durban: a sub-tropical, coastal South African city

Abstract The atmospheric pollen of Durban has been monitored since 1989 using a Burkard sampler. Data are presented here for the three-year period ending in 1991. The results do not reflect the indigenous sub-tropical vegetation of the area, but rather exotic elements that are either naturalised (Morus) or cultivated (Cannabis and Poaceae). The only indigenous pollen that was recorded in any number was that of the lawn grass Cynodon dactylodon and the tree Celtis africana. However, these two plant types are found country-wide, and are not restricted to sub-tropical regions. All major pollen types recorded have been implicated as allergens.

Tree mortality in the Kibale Forest, Uganda: A case study of dieback in a tropical rain forest adjacent to exotic conifer plantations

We report the dieback of three out of five unrelated species of rain-forest canopy trees in a very restricted part of the Kibale Forest, W. Uganda that were monitored since 1977. High mortality rates began about 1978 and continued through 1986, reaching levels of 5–50% year −1. Within the restricted area of mortality, all adults of Newtonia buchananii were dead in 1984, while nearly 90% of Lovoa swynnertonii and 45% of Aningeria altissima were dead in 1986. In contrast, Mimusops bagshawei and Celtis africana experienced relatively low (0.5%) and moderate (1.6%) levels of annual mortality, respectively. Regeneration of the seedling and sapling stages was high for all five species, while the pole class was poorly represented. The single most important variable associated with the dieback was downslope proximity to the conifer plantations. Based on this case study, it is strongly recommended that exotic trees, particularly conifers, should not be planted near natural forests in the tropics. Furthermore, those conifer plantations already existing near the natural forests should be harvested in a manner which encourages the regeneration of native trees and shrubs, thereby replacing exotic plantations with indigenous forest.