Tree species in Tanzania 613 Methods
To evaluate the abundance and distribution of G. macrosi- phon and C. porphyrantha in the East UsambaraMountains, we used 15 randomly placed 20 × 20 m vegetation plots to survey the isolated, 17 ha forest fragment that harbours both species (Fig. 1). Additionally, we surveyed 32 randomly placed vegetation plots, of the same size, in seven neigh- bouring forest fragments (Fig. 1), assuming that similar ed- aphic factors would increase the likelihood of the presence of the two species. The areas of the seven other adjacent for- est fragments we surveyed are 1.3, 4.6, 36.1, 98.3, 114.7, 135.4 and 153.0 ha. To ensure adequate coverage of the forest frag- ments, we also walked transects through all of the sites, in various directions. Transects were of varying lengths de- pending on fragment size and shape, and the width of each transect was 5meither side of the centre line. The vegetation surveys were primarily in May and June 2022 and in January, May and September–October 2023. We made observations of fruiting and flowering of both species and of leaf shed- ding of the deciduous G. macrosiphon on a monthly basis during January 2022–December 2023.We alsomade fruiting and flowering observations in our ad hoc visits to the site for other research purposes. These observations are from 2000 onwards. We identified all trees, shrubs and seedlings in all 47
plots, and we measured the diameter at breast height and height of all individuals of the two target species (i.e. seed- lings, saplings and mature trees); we recorded root collar diameter for all individuals , 2 m in height. We also re- corded any anthropogenic impacts, such as tree cutting or harvesting, in these plots. We recorded the location of all individuals of the two species .5 cm diameter at breast height, with a GPS, for future monitoring.
Results
We located C. porphyrantha and G. macrosiphon almost exclusively in Bulwa, the same forest fragment in which they were previously known. We recorded G. macrosiphon and C. porphyrantha in three and six plots, respectively, of the 15 plots in the forest fragment; both species were found together in a localized area on a steep and rocky slope. We found a total of 47 G. macrosiphon, mostly seedlings and small saplings, with several larger saplings and five mature trees (Fig. 2b). We found a total of 153 C. porphyrantha in four of the plots, with 18 mature trees and 38 large saplings, and abundant seedlings and small saplings (Fig. 2a). We recorded a further eight saplings in another two plots: four saplings were at a mean of 120 m to the south-east and four at a mean of 130 m to the north-west from the edges of the clump of mature trees. Although we traversed the entire forest fragment, we did not find G. macrosiphon and C. porphyrantha elsewhere.
FIG. 2 Size class distribution of individuals of (a) Cola porphyrantha and (b) Gigasiphon macrosiphon in the East Usambara Mountains, Tanzania (Fig. 1). For C. porphyrantha,data were pooled from four 20 × 20 mplots,and for G. macrosiphon from three plots. Size classes are based on diameter at breast height for individuals .1.5m tall and root collar diameter for individuals below this height. Note the y-axis scales are different.
We surveyed a total 46.6 km of transects through the
eight fragments (area in ha/distance in km: 1.3/0.6, 4.6/1.2, 17.0/5.3, 36.1/4.8, 98.3/10.6, 114.7/10.5, 135.4/8.1, 153.0/5.5), and surveyed additional transects on the upper parts of the Dodwe River (2.1 km) and along the Vungwe River at Kwezitu (1.6 km). In all of these surveys we found no C. por- phyrantha and only a single fruiting tree of G. macrosiphon, in rocky terrain near Kwezitu in October 2023. In the forest fragment that harboured both species, an-
thropogenic impacts appeared to be low in the five plots in which mature trees of the two species were found, with only two small saplings of C. porphyrantha having been cut. However, in the other 10 plots, which were on more gently sloping to flatter terrain, we observed cutting of saplings and poles in all of the plots, and larger trees had been felled in six of the plots, including the two plots where we found saplings of C. porphyrantha (all cutting and felling was of species other than G. macrosiphon and C. porphyrantha). Flowering and fruiting of both species are variable across
the various years of observation (Table 1). Leaf-shedding in G. macrosiphon appears to precede the hot, dry season (Table 1). Seed dispersal of C. porphyrantha by one of three resident primate species, the blue monkey Cercopithecus mitis, was observed opportunistically in 2001, 2021 and 2022. The flowers and fruits of C. porphyrantha are shown in Plate 1.
Oryx, 2024, 58(5), 611–617 © The Author(s), 2024. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605324000462
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