A new species of the Tricalysia atherura group (Rubiaceae) from southwestern Cameroon

Une nouvelle espèce de Tricalysia (Rubiaceae) du Cameroun, Tricalysia achoundongiana Robbr., Sonké & Kenfack est décrite et illustrée ici. La nouvelle espèce appartient au groupe de Tricalysia atherura (section Tricalysia) et semble très proche de T. pangolina et de T. vadensis. MOTS CLÉS Groupe de Tricalysia atherura, Rubiaceae, Parc National de Korup, Cameroun.

INTRODUCTION The Korup National Park is a protected lowland rain forest area in the southwest of Cameroon. The park extends 04°54’– 05°28’N and 08°42’– 09°16’E and occupies a total of 125,900 hectares (GARTLAN 1989). The 50 ha Korup Forest Dynamics Plot was established in the park from 1997 to 1999, as a part of a world network of long-term biological and socio-economic research within tropical forest coordinated by the Center for Tropical Forest Science (CONDIT 1998).

Field methods included tagging, mapping, identifying and measuring all stems of free-standing trees and shrubs with diameter at breast height (1.3 m) greater or equal to 1 cm. The aim of identifying to species every single of above 328,000 plant individuals led to some interesting botanical findings, despite previous works in this part of the park (NEWBERY & GARTL AN 1996; THOMAS & GEREAU 1993; GARTLAN et al. 1986). During the field work, one of us (DK) drew attention to a remarkable Tricalysia from the area.

Subsequent collection of flowering and fruiting material from the park and its surroundings allowed a complete morphological examination, and comparison with herbarium specimens at SCA, YA, BR, K and WAG revealed that a new species was at hand, which is described and named in this paper. We dedicate the new species to Dr. ACHOUNDONG, director of the Cameroon National Herbarium, who has always advised and encouraged us (BS and DK).

When in fruit, Tricalysia achoundongiana is immediately identified as a member of the Tricalysia atherura group. This group of section Tricalysia can be recognized by the character combination fruits exceeding 15 mm in diameter and having ± sclerified walls / bracteoles opposite and fused to calyculi. Most species of the genus have the same bracteole type but smaller fruits with fleshy walls.

The Tricalysia atherura group and its features are discussed at length in another current contribution (SONKÉ et al. 2002). The densely hairy twigs of T. achoundongiana compare with only two other members of the Tricalysia atherura group, T. pangolina and T. vadensis. The former differs from the new species by its long-pedicellate flowers and fruits, the latter by its 4-merous flowers and ribbed fruits.

In addition, the calyx of T. achoundongiana withers in fruit and hence differs from all other species of the Tricalysia atherura group. According to ROBBRECHT’ s scheme (1987: 64, table 1) of the relationships in Tricalysia subgen. Tricalysia, Tricalysia achoundongiana takes a position intermediate between T. pangolina and T. vadensis (Fig. 1); it follows the trend of reduction (in flower size and merousness, and number of ovules) shown by T. vadensis, though in a less extreme way.