The ten tallest individuals of Entandrophragma excelsum ranged from 59.2 to 81.5 m and 1.24 to 2.55 m diameter. Table 1 shows the dimensions of the thirteen tallest measured individuals, all exceeding 50 m in height.
In the lists of the world’s tallest tree species with heights of 80 m and more no African species are mentioned (Tng et al. 2012). Most of the tallest indigenous trees in tropical Africa (with heights of 50–60 m including several species of Entandrophragma) occur in the West African lowland rainforests [about 30 species (FTEA 1952–2012; FWTA 2014)]. Once there was a tall Sydney blue gum (Eucalyptus saligna) with 81.5 m height at Woodbush Forest Reserve, South Africa. However, this tree was a planted non-indigenous species, which fell down in 2006 (http://git-forestry-blog.blogspot.com/2008/07/tallest-tree-in-africa-is-you-guessed.html). On Kilimanjaro the tallest tree species on our 600 forest plots—besides Entandrophragma—are Camphor (Ocotea usambarensis, Lauraceae) with 40 m, Muna tree (Aningeria adolfi-friederici, Sapotaceae) with 45 m, Forest Newtonia (Newtonia buchananii, Mimosaceae) with 45 m, Garcinia tanzaniensis (Clusiaceae) with 47 m, Strombosia (Strombosia scheffleri, Olacaceae) with 47 m and African juniper (Juniperus procera, Juniperaceae) with 45 m. With specimens of over 81 m and 80 tons of woody biomass Entandrophragma excelsum growing at Kilimanjaro has to be included in the hit list of world’s superlative trees (Fig. 1).
Entandrophragma excelsum occurs in east and eastern central Africa in (sub)montane rain forests between (925) 1280–2150 m. It is native to the Congo, Malawi, Tanzania, Uganda, Zambia and its highest recorded heights are 55 m (FTEA 1952–2012).
On Kilimanjaro it grows between 1400 and 2100 m, an elevational range, which belongs following the delineation of East African forest zones of Hemp (2006a) to the submontane and lower montane forest zone, following the universal delineation of Grubb (1977) and Whitmore (1991) to the tropical lower montane forest zone. Entandrophragma is mainly restricted (beside few occurrences in lower montane Cassipourea forests of the northern slope) to Newtonia ravine forest relicts found in the submontane cultivated zone of the southern slope (Hemp 2006a, b). Here it grows on the steep (30°–40°) lower slopes and forms, with few other tree species, the upper canopy typically reaching 50–60 m. With its spreading large branches (Figs. 2, 3), Entandrophragma has the function as the most important epiphyte host tree of submontane forests at Kilimanjaro. Its upper branches are densely covered by ferns such as Basket fern (Drynaria volkensii) and other 50 on this tree recorded epiphyte species (Figs. 2, 3). E. excelsum does not occur in dense stands but is a rare constituent of the upper tree canopy. Based on two 50 × 50 m permanent submontane Newtonia forest plots (Fig. 4), where we did forest inventories we calculated a number of 16 tall (>50 m) specimens of Entandrophragma per ha of undisturbed forest. This may exactly reflect the conditions for tall tree growth: a gorge situation with tall trees calling for especially high height growth, and otherwise good conditions, and little competition between the tallest species.
The submontane zone (lower montane zone in the sense of Grubb (1977) and Whitmore (1991)) of Kilimanjaro’s southern slope displays a favorable climate with high precipitation and warm temperature in combination with a high fertility of the volcanic soils. The climate of the submontane Newtonia forests is perhumid, with a mean annual temperature of 17.5 °C and mean annual precipitation exceeding 1800 mm (Hemp 2006a, b). Global analyses have shown that precipitation is a poor predictor of tree height (King 1991) or biomass (Keith et al. 2009) when forest landscapes are compared. Instead, temperature distribution seems to be the most significant determinant of tall tree growth, supporting the energetic and biomechanical approach to understanding tree size and allometry (Larjavaara 2014). The tallest trees grow in climates with small seasonal temperature variation (Larjavaara 2014). The 17.5 % mean annual temperature and the 5 °C variation at Kilimanjaro are within these climatic conditions. However, according to Whitmore (1991) emergent trees of heights of 60 up to 80 m are characteristic to tropical lowland evergreen forests but not for tropical lower montane rain forests, although Ashton (2014) reports trees of exceptional stature (albeit not precisely measured) at similar elevation and habitat on the slopes of Thailand’s highest mountain.
Trees in fertile soils build a lot of leaf mass and grow faster (Valentine and Makela 2012). The soils of the submontane Newtonia forests are deep humic ferrisols with dark brown humus-rich loamy topsoils and a moderately acid pH of 6.4 (Hemp 2006b; Anderson 1982). Under these conditions trees reach their maximum height and biomass at Kilimanjaro (Rutten et al. 2015; Ensslin et al. 2015). However, the Chagga people, inhabiting the lower slopes of Kilimanjaro since approximately 2000 years (Odner 1971) converted most of the former forests of this belt into agroforests.
Stem wood density of E. excelsum was on average 0.46 g cm−3 and varied between trees from 0.42 to 0.50 g cm−3. On average for mature E. excelsum trees, 4.3 % of total stem and branch volume were bark. Of the twenty trees selected and measured for biomass estimation, the largest total above ground coarse woody biomass (stem, buttress roots and main branches combined) was 80.4 Mg for the 74.2 m tall individual. This tree also had the biggest crown volume estimated to 11,666 m3 with an unusually large crown which contributed 64.0 Mg to the total above ground woody biomass (AGWB). The stem weighted 15.1 Mg and the buttress roots contributed 1.3 Mg.
Cambial cell enlargement in the main stem occurred almost exclusively during the main rainy season from March to June. The annual radial stem increment of the three observed trees ranged from 0.7 to 2.1 mm. Annual increments were very consistent for each individual during the four years observed. E. excelsum at our site did not form specific anatomical ring features every year and did therefore not permit application of classic tree ring studies. We estimated tree age of E. excelsum using the observed actual annual maximum radial growth rate of 2.1 mm annually. Given the DGH values of 63 to 255 cm this indicates that tall individuals may reach at least between 150 to more than 607 years of age. Being conservative in estimate and taking into account that juvenile growth may be faster and assuming 5 mm/year for the first hundred years of growth, estimated maximum age would be still more than 470 years for the largest individual.