Tessmannia princeps A.Bianchi, Tomasi, Mwakisoma, Barbieri & Q.Luke, 2025

Bianchi, Andrea, Tomasi, Laura, Mwakisoma, Aloyce, Barbieri, Matteo & Luke, Quentin, 2025, Tessmannia princeps (Fabaceae), a new rainforest tree from the Udzungwa Mountains, Tanzania, Phytotaxa 694 (2), pp. 109-118 : 110-116

Tessmannia princeps A.Bianchi, Tomasi, Mwakisoma, Barbieri & Q.Luke , sp. nov. ( Figs. 1–5 )

Type:— TANZANIA. Boma la Mzinga Village Land Forest Reserve , Udzungwa Mountains, Afromontane rainforest (sensu Lovett et al. 1988), 1416 m a.s.l., - 8.19900°S 36.08371°E, 6 December 2019. Bianchi & Mwakisoma 153 (holotype NHT!; isotypes EA!, K!) .

Diagnosis:— Superficially comparable to Tessmannia densiflora , but Tessmannia princeps is a taller tree, growing up to 40 m in height; leaves bear 18–24 pairs of leaflets on a rachis 68–94 mm long; leaflets are opposite to alternate, sub-rectangular or oblong in shape; flowers are white, with sepals and tepals up to 5 mm in length. Tessmannia densiflora is a smaller tree, reaching 20–25 m in height; leaves bear 8–13 pairs of leaflets on a rachis 30–60 mm long; leaflets are sub-opposite to alternate and may be oblong, lanceolate, ovate-oblong, or ovate-lanceolate in shape; flowers are red, with sepals and tepals measuring 10–20 mm in length.

Description:— Large, unarmed tree 35(–40) m tall, usually canopy emergent. Bole straight, cylindrical, unbranched for 15–20 m, up to 2.7 m in diam. Buttresses up to 15 m tall, radiating for up to 1.5 m from bole, sometimes extending along first branches. Bark greyish-light brown, with lenticels (1–)2–4(–6) mm wide, projecting ca. 1 mm from bark surface. Older twigs sparsely hairy, young twigs (less than 1 year old) pubescent, with short <0.1 mm long, brown hair, curved. Stipules not seen on adult trees, but whitish, linear, 8 × 1 mm, early caducous on young, copperycoloured leaves. Leaves 7.5–10.0 cm long, 2.0– 2.5 cm wide, rachis densely pubescent, with short brown hairs, <0.1 mm, curved towards leaf tip. Leaf rachis 68–94 mm long, caniculate on upper surface. Petiole slightly wider in diam. than leaf rachis, 1–3 mm long, 1 mm in diam. Leaflets sessile, 18–24 pairs, opposite (in young plants and proximally) to alternate (distally), often with single odd leaflet at leaf tip; sub-rectangular/oblong, 11–13 mm long, 3–4 mm wide, margin slightly revolute; mid-vein asymmetrical, with smaller part of leaf being proximal; base trapezoidal, with angle closer to leaf attachment being acute, while distal one is obtuse; margin sometimes with 1–3 hairs, similar in shape and dimensions to ones found on leaf rachis, but transparent/very light green; tip obtuse, midvein almost symmetrical; surface shiny adaxially, dotted with pellucid translucent dots, and 2–7 raised glands. Inflorescences axillary, raceme and panicles arising from same bud, racemes with up to 2 branches, each 5–10-flowered; up to 4 raceme/panicles arising from bud, pubescent; inflorescences carried on regular branchlets and sparsely distributed; or also carried on short, 10–15 cm branchlets, densely covered in flowers with leaves then reflexed. Flowers: bracts and bracteoles not noticeable; sweetly scented. Sepals 4, outside very light green, inside white, unequal, one, elliptic, 5 × 4 mm, other three lanceolate, 5 × 2 mm; glabrous on both surfaces, but having an extremely thin ‘skirt’ on margin, projecting inwards, <1 mm wide. Petals 5, alternate to sepals, pure white, 5 × 1.0– 1.5 mm, lanceolate; with noticeable midvein; sparsely pubescent in inside, with very thin 0.5–1.0 mm long hairs. Stamens 10, in two whorls, one whorl alternate to petals and one whorl opposite; filaments white, 10 mm long in outer whorl and 5 mm long in inner, all filaments glabrous; anthers, of same size in both whorls, yellow, 0.5 × 0.8 mm. Pistil: ovary pure white, 2 × 1 mm, but only × 0.5 mm at base, glabrous. Style white, 3 mm long, glabrous. Stigma not differentiated. Fruits obovate, dark beige/tan, smooth, glabrous, woody, 45–49 mm long, 30 mm wide; valves 1–2 mm thick, being thicker in dorsal suture part; beak 1–2 mm long, sharp and woody; 1–2(–3)-seeded, explosively dehiscent. Seeds flat, hard, with small basil aril, shape variable, elliptic to sub-rectangular, 16–19 mm long, 9–12 mm wide, 2.1–3.0 mm thick, dark glossy brown, tending towards olive, pure brown close to aril; aril triangular, dull yellow-brown, 2–4 mm wide, 1–3 mm long. Seedlings with permanent cotyledons, first two true leaves opposite. Wood of fallen trees dark red/brown with very compact fibres and visible vessels, very heavy and durable, 1040 kg /m 3 at 12% moisture content.

Etymology:— The specific epithet derivation is from the Latin ‘ princeps ’ for ‘the first, the foremost, the most eminent’, in relation to the canopy emergent habit of Tessmannia princeps . Phenology:— Found in flower in December, seeds dispersed in October–November. Distribution and ecology:— Found in Afromontane rainforest (sensu Lovett et al. 1988) associated with Allanblackia stuhlmannii Engler (1897: 249) , A. ulugurensis Engler (1900: 435) , Anthocleista grandiflora Gilg (1893:

582), Cassipourea malosana ( Baker 1897: 267) Alston (1925: 258) , Cryptocarya liebertiana Engler (1899: 390) , Entandrophragma excelsum (Dave & Sprague 1906: 511) Sprague (1910: 80) , Kuloa usambarensis ( Engler 1894: 51) Trofimov & Rohwer (2020: 528) , Newtonia buchananii ( Baker 1894: 354) Gilbert & Boutique (1952: 213) , and Parinari excelsa Sabine (1824: 451) . Tessmannia princeps seems to be restricted to montane rainforest in Mhanga and Mngeta valleys, Tanzania, between 1300 and 1500 m a.s.l. It is found in two (sub)populations, a larger one in Boma La Mzinga Village Land Forest Reserve and a smaller in Uluti Village Land Forest Reserve. A single individual has been recorded in a small patch of riverine forest (100 m × 20 m) close to Boma La Mzinga. Extensive and systematic research (transects and permanent plots) in the neighbouring Uzungwa Scarp Nature Forest Reserve and Mwanihana Forest (a forest ca. 150 km to the north with strong floristic affinities to the research area) have failed to locate it. Local knowledge (Aloyce & Ruben Mwakisoma, pers. com.) also strongly suggests that this species may be a narrow endemic restricted to the two adjacent valleys.

Taxonomy:— Although Tessmannia princeps has a very narrow distribution and ecology, making the possibility of misidentification of this species in the field virtually nil, it is deemed relevant to update the key of East African Tessmannia species to be able to distinguish this species from herbarium samples alone. Tessmannia princeps is readily distinguishable from other species even from sterile specimens due to the numerous leaflets that separate it from any other species known from East Africa.

Tessmannia princeps also exhibits morphological affinities with T. anomala . However, there are distinct morphological differences that readily separate the two species. Unlike T. princeps , T. anomala has an emarginate leaflet apex, fewer leaflets (up to 30, compared to a maximum of 48 in T. princeps ), somewhat elliptic pods, and flowers with large, spatulate, pink petals.

It is noteworthy that, in contrast with other species in the genus Tessmannia , which have cardboard-like pods that do not curl up when dry and thus not constructed for ballistic dispersal, T. princeps has, as recently described for T. korupensis , hard pods that are explosively dehiscent.

Tessmannia densiflora , the most closely related species to T. princeps is a tree found on sandy ground between 200– 600 m in elevation, whilst T. princeps is restricted to a narrow altitudinal range between 1300–1500 m in elevation.

Biogeography:— The present distribution of Tessmannia princeps , which is very limited, is interesting and most likely caused by a series of factors. The historical past of the area in which the tree is found, has likely had a major impact in the decline of the habitat of this species. The vast majority of montane forests in the immediate vicinity have been deforested in the last 120 years. It is important to note, however, that the species is totally absent from forest patches, even quite large ones, in the vicinity of the known two subpopulations.

At the same time, the Uluti subpopulation is located in a small Village Land Forest Reserve bordering the larger Uzungwa Scarp Nature Forest Reserve (USNFR), which covers more than 200 km 2 of closed canopy forest. Nevertheless, permanent plots, transects, and casual observations failed to locate this species in USNFR. Uzungwa Scarp is a nature forest reserve, from which the majority of valuable timbers, such as Kuloa usambarensis , Khaya anthotheca Candolle (1878: 721) , and Entandophragma excelsum , had been selectively extracted in the past on a systematic, large scale, and, to this day, on a much smaller one. Is it possible that Tessmania princeps has been completely eradicated from Uzungwa Scarp and was left only in two less accessible village land forest reserves? The observed total lack of regeneration in Uzungwa Scarp and the curious fact that this species is not felled in village land forest reserves (where other trees such as Newtonia buchananii and Kuloa usambarensis are still being logged) would seem to make this hypothesis invalid. As a matter of fact, the other species extracted in Uzungwa scarp in the past show a timid regeneration (vegetatively in Kuloa and sexually for the other three species).

Although Tessmannia is primarily a genus of plants adapted to perhumid environments, the three species described so far for East Africa grow in more or less arid environments. Further research is needed to investigate whether T. princeps is indeed, genetically, more closely related to the species already known for East Africa or more to the species of West and Central Africa instead, an interesting feature that would prove, once again, the link that the humid forests of the Eastern Arc have with those of West Africa, as demonstrated, for example, with the genera Octoknema Pierre (1897: 1290) , Schefflerodendron Harms (1901: 87) , and Ternstroemia Mutis ex Linnaeus (1782: 264) .

Lastly, it is interesting how the seeds of a canopy-emergent species are small, orthodox, and dispersed abiotically. Most of the other canopy-dominant or emergent species (excluding the pioneer species) growing alongside T. princeps have large, recalcitrant seeds, found in fruits or drupes palatable by a wide variety of animals that disperse the seeds. Exceptions are Newtonia buchananii and Entandophragma excelsum , which possess long, thin, flattened seeds dispersed by the wind. These seeds, however, are highly recalcitrant, cannot tolerate dehydration, and their germination rate decreases exponentially as early as two weeks after dispersal.

Conservation status:— Tessmannia princeps is provisionally assessed as Vulnerable (VU) based on the IUCN 3.1 criteria ( IUCN 2012), specifically meeting criteria D1, population size estimated to number fewer than 1000 mature individuals (a thorough survey of the forest patches indicates 101 mature individuals). Both T. princeps (sub)populations occur within the Udzungwa Corridor, a 75 km 2 forest restoration project, which has a payment for ecosystem services scheme and protects remaining patches of forest. If the project were to cease, T. princeps would be immediately threatened with habitat loss and would then need to be assessed as Critically Endangered (CR) based on criteria B1a (severely fragmented or known to exist at only a single location) and B1b(v) (continued decline of mature individuals). In fact, the species is known from only two (sub)populations, that are very close, and fall within the same location.

Additional specimens examined:— Tessmannia densiflora : TANZANIA. Kichi Hill Forest Reserve, NW of Nawanje village, Miselu/Mkinga Peak, Tree 13 m tall × 18 cm dbh, Bark grey, smooth, dry fruit brown, Kindengereko name: “Muundei”, 27 th April 2001, Y.S. Abeid 899 (NHT!); Matumbi Hills, Kiwengoma F./R., NE of WWF—camping site, Tree 7 m tall, sterile, 29 th April 2001, Y.S. Abeid 924 (NHT!).— Tessmannia sp. : TANZANIA. Kanga Mtn, Mvomero district, Submontane forest, Tree 4 meters, leaflets alternate, emarginate w ‘windows’—sterile, 10 th May 2007, Festo 2854 (NHT!); Udzungwa Mountains NP, Pt 228, 7.7500°S, 36.83333°E, Alt 1400 m, Sub-montane forest, Tree 7 m, 29 th September 2001, Luke WRQ 7982 (NHT!).

Figures

FIGURE 1. Tessmannia princeps. A. Mature tree. B. Branchlet and leaves. C. Leaflet adaxial lamina (top) and abaxial lamina (bottom) showing glands. D, E. Flower. F. Diagram of flower. G. Mature pod. H. Valve of pod after seed dispersal. I. Seeds. Illustration by: Laura Tomasi.

FIGURE 2. Bole of a very large individual of Tessmannia princeps, showing the numerous buttresses. Photo by: Andrea Bianchi.

FIGURE 3. Morphological details and general habit of Tessmannia princeps. A. Close-up of a flower, note the dense inflorescence and a spent, presumably pollinated flower on the left with a young, beaked pod starting its long maturation process.B. Flowering branchlet, with leaves naturally reflexed. C. Adaxial lamina, note a mature pod in the upper right corner. D. Abaxial lamina. E. A mature individual of T. princeps emerges in a closed, dense canopy of Parinari excelsa. F. Canopy of T. princeps as seen from the forest floor. Not all individuals show canopy shyness as this specimen. Photos by: Andrea Bianchi.

FIGURE 4. Stereomicroscope pictures of Tessmannia princeps. A. Abaxial lamina. B. Adaxial lamina. C. Hairy rachis. D. Short hairs on leaflet margin. Photos by: Andrea Bianchi.

FIGURE 5. Morphological details of Tessmannia princeps. A. Buttresses. B. Bark with numerous lenticels. C. Branching pattern. Photos by:Andrea Bianchi.