Hymenostegia viridiflora ( Detarieae , Caesalpinioideae , Leguminosae ) , a new tree species from Cameroon

A new species of Hymenostegia�(Detarieae, Caesalpinioideae, Leguminosae)�isdescribedfromCameroon.� Hymenostegia viridiflora has previously been confused with the type species of the genus, H. floribunda, but differs from it in having pale green to greenish yellow instead of lemon yellow petals, a broader upper part of the stipule, morenumerousreddishveinsonthewhitebracteolesandgenerallymorenumerousandnarrowerleaflets.�Asa� consequence,�thegeographicrangeoftrueH. floribundanolongerincludesCameroon.�Hymenostegia viridiflora isassessedasVulnerableaccordingtothecriteriaofIUCN.

Among the 82 genera of the Detarieae enumerated by Mackinder (2005, subsequently modified to accommodate an addition by Breteler 2011 and a reduction by De la Estrella et al. 2012a) is Hymenostegia s.l., a genus of small to large trees of tropical African forest.Seventeen species are currently assigned to the genus (Mackinder 2005, Wieringa & Mackinder 2012) which is characterised by 1-many-jugate paripinnate leaves; showy persistent paired petaloid bracteoles in the upper part of the pedicel and woody, compressed, usually single seeded dehiscent pods.However, as presently delimited, Hymenostegia is not a natural group (Bruneau et al. 2000, Mackinder et al. 2010).Based on a suite of vegetative characters, only nine species may be congeneric with the type species H. floribunda.Among the herbarium vouchers received on loan as part of our wider revision of Hymenostegia s.l., it was apparent that when placed side by side, those that had been determined as H. floribunda by different legume workers together formed a morphologically diverse set of specimens.Amid that set, gatherings from Cameroon could readily be separated from the type of H. floribunda on the basis of several floral characters which was consistent with some variation in the leaves.We believe that the Cameroonian collections represent a previously unrecognised species of Hymenostegia which we describe here as H. viridiflora.Differences between H. viridiflora and H. floribunda are given in Table 1.
The formal description of H. viridiflora increases the number of Hymenostegia s.str.species to ten but the number of Hymenostegia s.str.species known to be native to Cameroon remains at three, H. viridiflora, H. felicis (A.Chev.)J. Léonard (1951) and H. pellegrinii (A.Chev.)J. Léonard (1951).The latter two were reported by Aubréville (1970) in his account of the caesalpinioid legumes in Flore du Cameroun where he also reported another sensu stricto species H. mundungu (Pellegr.)J. Léonard (1951) as occurring in Cameroon.However, the single specimen on which Aubréville (1970) based that country record (Letouzey 4543) has been identified by us as H. pellegrinii.By transferring the Cameroonian collections of H. floribunda to H. viridiflora, an element of the variation of the H. floribunda specimen set mentioned above has been removed.Nevertheless, some heteromorphy remains which we have yet to disentangle.We think it likely that true H. floribunda is restricted to Gabon and (probably) Equatorial Guinea where it was reported by De la Estrella et al. (2006) although of the few vouchers that we have seen from the latter country, none are adequate to confirm its presence there.
Of the three Cameroon native species only H. viridiflora is endemic.In a study predicting legume diversity patterns in western Central Africa, De la Estrella et al. (2012b) forecast that the locations in Cameroon where this new species occurs should be rich in both terra-firma and particularly in riverine lowland rainforest legume species.That H. viridiflora fits the area-habitat pattern predicted by De la Estrella et al. (2012b) serves as independent corroboration of the findings of the study because Hymenostegia species were not among the taxa modelled.Mackinder & Wieringa, sp.attachment which is narrowly oblong, four to five times longer than wide (not linear to filiform, six to nine times longer than wide).
Etymology.The species epithet refers to the conspicuous pale green to greenish yellow petals, a colour which to our knowledge has not been recorded in any other species of Hymenostegia.Other Hymenostegia species have lemon yellow petals.
Distribution & Ecology -Cameroon, SW and Littoral Provinces.Primary and little disturbed lowland forest, along river banks; 50 -100 m.
Conservation assessment -Hymenostegia viridiflora is assessed here as Vulnerable VU B1ab(i,iv) according to the criteria of IUCN (2001).In IUCN terms, we consider the status of VU as a best estimate given that the assessment was not straightforward and that there is more than one plausible category to which the species might be assigned.The category proposed is based on confirmed (flowering) records from subpopulations into two locations, one in Korup National Park in SW Cameroon and the other near Nkam in the Littoral Province (denoted as closed dots on Map 1).From these records, we calculate a species EOO of 570 km 2 suggesting a category of Endangered and AOO of 1 460 km 2 (at cell size 22 km 2 ) suggesting VU.We have selected the lower category because the SW province population occurs in Korup National Park, a protected area where other than stochastic events, no other threats are known.We are also influenced away from Endangered (E) and towards VU by the several unconfirmed records of the species (sterile or fruiting only collections) which were they confirmed, would result in a designation of Least Concern (LC).However, until we are able to confirm those records we do not consider LC, which is a category that also accommodates pantropical weeds, as an appropriate conservation status for this relatively poorly known species from the Cameroonian forest, a forest that is under considerable threat.We also considered the category of Data Deficient (DD) which is intended for species for which data are inadequate to determine a threat category with confidence.However, we judge that there exists sufficient data to arrive at an assessment, based on the combination of our current understanding of species distribution and knowledge of general levels of threat to forest habitats in Cameroon.
Notes -In the field flowers were noted as weakly scented (van der Burgt 568) or sweetly and strongly scented (L.J. Pearce 11).A colour change in the petals of fallen flowers was also observed in the field (van der Burgt 568) who recorded the fallen petals as 'dark red inside' and 'dirty white outside'.About 10 weeks is needed for flowers to develop into almost mature pods (van der Burgt 598).Once mature, pods audibly dehisce whilst still on the tree and the seeds are dispersed up to 18 m from the edge of the crown (van der Burgt 621).Following dehiscence the valves do not remain on the tree but fall to the forest floor (van der Burgt 621).
Letouzey 11626 (K, P) collected in Cameroon's Central Province, 20 km NW Yaoundé on the Mbam Menkoum Massif consists of a vegetative collection mounted on the herbarium sheet with associated pods gathered from the forest floor in a capsule.The pods are hairy with a winged upper suture.Typically, pods of Hymenostegia s.str.species are glabrous without a winged upper suture.We suggest that Letouzey 11626 may represent a mixed collection with only the foliage belonging to H. viridiflora.We speculate that the pods gathered from the forest floor are a collection of Talbotiella breteleri (Aubrév.)Mackinder & Wieringa (Mackinder et al. 2011) based on the combination of the pod morphology (indicating the genus Talbotiella) and the collecting locality which is the same as that of a known sterile collection of T. breteleri, Letouzey 11635 (YA).No other species of Talbotiella are known from the vicinity.

Table 1
Attributes of morphology and distribution that can assist in distinguishing H. viridiflora from H. floribunda.
Map 1 Distribution of Hymenostegia viridiflora.Confirmed localities denoted by black-filled dots.Unconfirmed localities, denoted by open dots are shown to assist in the discussion of conservation status.