Didelotia korupensis Burgt, 2016

Burgt, X. M. van der, 2016, Didelotia korupensis and Tessmannia korupensis (Leguminosae, Caesalpinioideae), two new tree species from Korup National Park in Cameroon, Blumea 61 (1), pp. 51-58 : 52-55

Didelotia korupensis Burgt , sp. nov. — Fig. 1 , 2

Morphologically comparable to Didelotia afzelii Taub. , but D. korupensis is an understory tree to c. 15 m tall with an often leaning stem to 30(–53) cm diam; the leaflets are (4–) 7–18 cm long; the pedicel is 5–10 mm long and the bracteoles 5 mm long; the pod is 9–15.5 cm long. Didelotia afzelii is a canopy tree to c. 30 m tall with a vertical stem to 91 cm diam; the leaflets are 3–9 cm long; the pedicel is 4–5 mm long and the bracteoles 3–4 mm long; the pod is 6.5–10 cm long. — Type: X. M. van der Burgt 718 (holo K (2 sheets: K000460433, K000460434); iso B, BR, G, LISC, MO, P, PRE, SCA, US, WAG, YA), Cameroon, Southwest Region, Korup National Park, NW plot near P transect, subplot 42XN, N5°00'48.5" E8°46'58.1", 100 m, in flower, 18 Sept. 2004.

Tree, to c. 15 m tall. Stem to 30(–53) cm diam, often leaning, stem base conical. Bark dull dark brown, smooth; lenticels small, corky, concolorous or lighter in colour. Twigs puberulent to glabrescent, hairs erect, yellowish brown, to 0.2 mm long. Stipules in fused pairs, intrapetiolar, caducous, lanceolate, 6–11 by 4–6 mm, with parallel veins; densely puberulent, hairs to 0.2 mm long, distal part inside glabrous; apex bilobed, both lobes acute. Leaves paripinnate, to 35 by 25 cm, with 3–5 pairs of opposite leaflets; petiole 3–10 mm long, 1.5–4 mm diam, puberulent, hairs to 0.2 mm long; a pair of caducous gland-like small basal leaflets inserted laterally on the petiole, 1–2 mm long; leaf rachis (2–) 6–20 cm long, puberulent; petiolules 1–3 mm long on proximal part, 2–4 mm long on distal part, puberulent. Leaflets elliptic, (4–)7–18 by (1.5–) 3–6 cm, base oblique, cuneate to obtuse, apex attenuate, finely emarginated; both sides dull to somewhat glossy, concolorous; upper surface glabrous, lower surface sparsely appressed puberulent, hairs to 0.2 mm long; midrib puberulent above with hairs to 0.3 mm long, below prominent and sparsely puberulent; 10–15 pairs of secondary veins. No glands seen on leaflets. Inflorescence axillary, sometimes terminal, pendant; c. 7 basal bud scales, broadly ovate, progressively becoming larger, to 12 by 9 mm, veins parallel, apex bilobed, densely puberulent outside, golden brown hairs to 0.1 mm long, glabrous inside; main axis of inflorescence light green, (4–) 12–30 cm long by 1–2 mm diam, densely puberulent, golden brown hairs to 0.2 mm long; peduncle 1.5–4 cm long, internodes 1–3 cm long; 5–17 lateral axes, alternate, light green, 2–25 mm long by 2 mm diam, densely puberulent, hairs to 0.2 mm long, 15–18 flowers per cm; bract at base of lateral axis resembling a bud scale, 9–11 by 7–9 mm; 4 colleters inserted outside the bract, deep purple, partly puberulent, c. 2 by 1 by 0.7 mm, the two middle colleters each with a linear appendage 1–3 mm long. Flowers: floral bract caducous, broadly ovate, 3–5 by 2–4 mm, puberulent outside, hairs to 0.2 mm long, glabrous inside; pedicel pink, 5–10 mm long, puberulent, hairs to 0.2 mm long; bracteoles 2, elliptic, greenish light pink on both sides, 5 by 4 mm, outside puberulent, inside glabrous, nerves parallel; receptacle 1 mm high, 3 mm diam at the top, glabrous; disk yellow, 3 mm diam, 1 mm high, glabrous, centre depressed; sepals 5, pink, triangular, 1–2 mm wide by 0.5–1 mm high, glabrous; petals 5, alternate to the sepals, red, linear-lanceolate, 4–6 by 0.3 mm, glabrous; stamens 5, alternate to the petals; filaments red, glabrous, 13–15 mm long; anthers dark red, 2 by 1.2 mm, glabrous; staminodes 5, red, filiform, 3–4 mm long, glabrous, alternate to the stamens; ovary green, 3 by 1.2 by 0.4 mm, margins densely hirsute, sides hirsute, hairs to 0.3 mm long; 5–7 ovules; stipe 0.5 mm long, glabrous; style red, 11–15 mm long, proximal part sparsely hirsute, distal part glabrous; stigma capitate. Infructescence pendulous, to 32 cm long, with 1–4 fruits. Fruits oblong-rectangular, dull, glabrous, 1–7-seeded, 9–15.5 by 3.5–4.5 cm, valve 1.5 mm thick, beak to 1 mm long, sutures not winged; a single longitudinal vein running from the base to the apex, more or less equidistant to both sutures.

Distribution — Endemic to Korup National Park in Cameroon ( Fig. 3 ). The species has only been recorded in and near the permanent plots along the P transect in the southern part of this park.

Habit — Understory tree. The stem is often leaning; the stems of two of the known trees had fallen to the ground, after which two or three stem shoots on each tree grew to a diam of 10–15 cm.

Habitat — Rain forest dominated by trees in the Detarieae tribe of the Legume subfamily Caesalpinioideae , on well-drained sandy and sometimes rocky soil, at 100 m altitude.

Additional material. CAMEROON, Southwest Region, Korup National Park, NW plot near P transect, subplots 42XN, 42WN and 43WN, twigs with fruits, X. M. van der Burgt 952 (BR, G, K, MO, P, SCA, WAG, YA), 28 May 2007 .

Conservation status — Didelotia korupensis is assessed here according to IUCN (2015) criterion D as Endangered (EN D). The new species is only known from an area of rain forest of c. 1600 m by 3000 m (c. 4 km 2), where 51 trees over 10 cm stem diam of D. korupensis have been recorded so far (see above). These 51 trees are mature. Although much of the forest in south Korup remains unexplored for this species, the number of mature trees might be lower than 250 because the species is not common in the area where it is found, and thus the category Endangered applies. IUCN criteria A, B and C were not used to evaluate the species, because there is no evidence of population reduction or decline in the past. Decline in the future is a possibility; see the conservation assessment of the other species in this article.

Notes — The permanent plots along the P transect, inside and near which D. korupensis has been found, have a total area of 155.75 ha. Of the 3 181 trees ≥ 50 cm stem diam in these plots, only one tree, of 53 cm stem diam, was identified as D. korupensis . Trees between 10 and 50 cm diam were registered in 56 randomly located subplots within the plots (area of each subplot 0.25 ha; total area of all 56 subplots 14 ha). Of the 5 755 registered trees between 10 and 50 cm diam, 27 trees (in 12 subplots) were identified as D. korupensis . During random surveying 23 additional trees over 10 cm stem diam were recorded in and near the plots. Therefore, at present a total of 51 D. korupensis trees are known. More trees are un- doubtedly present inside and near the plots; however, D. korupensis trees are absent from most of the forest within the plots (Van der Burgt pers. obs.).

Stem diam measurements are available for 25 trees: four trees were measured in the years 1991 and 2015 (24.7 years apart) and 21 trees in the years 2003 and 2015 (average 12.0 years apart). The average annual diam increment of the 25 trees is 1.13 mm /y. The three fastest growing trees had grown c. 3.0 mm/y. Four trees had grown only c. 0.1 mm /y in c. 12 years; indicating that individual trees of D. korupensis may hardly grow at all for at least 12 years, presumably waiting for better growth conditions. Low increments like these are typical for many understory tree species in Korup National Park (Newbery & Van der Burgt unpubl. data).

The D. korupensis trees grow in small groups, mixed with trees of many other species. None of the groups of D. korupensis trees has been mapped in its entirety, but a group probably consists of c. 5–10 individuals over 10 cm stem diam, in an area of up to 0.5 ha. Many tree species in the Legume tribe Detarieae in Korup occur in co-dominant groups, mostly in the upper story of the forest but sometimes, as in D. korupensis , in the middle story. The D. korupensis groups are small compared to the groups of some other tree species in Korup. Microberlinia bisulcata A.Chev. for example, grows in more or less circular groups of 400–1100 m diam; while two species of Tetraberlinia grow in even larger groups ( Newbery et al. 2004, 2013).

The pods of D. korupensis probably curl up when dry (the pods on the only fruiting collection are immature), which would indicate the presence of ballistic seed dispersal ( Van der Burgt 1997). The maximum seed dispersal distance was not recorded but is probably small, c. 10–20 m, because the fruits are less strong and placed less high above the ground than those of most other species in the Legume tribe Detarieae . The tendency of trees of D. korupensis to grow in groups is probably related to the relatively short and strictly limited maximum dispersal distance of the ballistic seed dispersal method. In addition to ballistic seed dispersal, seeds of D. korupensis may occasionally be subject to some form of long distance dispersal, dispers- ing the seeds far enough for the establishment of a new group of trees, but the dispersal type is unknown.

Figures

Fig. 1 Didelotia korupensis Burgt. a. Twig with two inflorescences; b. twig with infructescence and four fruits; c. emarginate leaflet apex; d. fused pair of stipules, petiole showing scars of a caducous basal leaflet pair; e. leaflet lower surface; f. flower showing five linear petals and five filiform staminodes; g. floral bract; h. part of inflorescence showing a lateral axis with four colleters at the base (most flowers removed); i. caducous bract at base of lateral axis (a, c–f. Van der Burgt 718, WAG; b. Van der Burgt 952, K). — Drawn by Xander van der Burgt.

Fig. 2 Didelotia korupensis Burgt. a. Section of inflorescence; b. infructescence; c. inflorescence and leaves; d. stem of the tree from which the type speci- mens were collected; 14.8 cm diam at 1.3 m above ground (a, c, d. Van der Burgt 718, WAG; b, d. Van der Burgt 952, K). — Photos by Xander van der Burgt.

Fig. 3 Distribution of Didelotia korupensis Burgt (only the upper black circle) and Tessmannia korupensis Burgt (both black circles).