Mimosa chiquitaniensis Atahuachi & C.E. Hughes, 2016

Atahuachi, Margoth, Bent, M. Leontien Van Der, Wood, John R. I., Lewis, Gwilym P. & Hughes, Colin E., 2016, Bolivian Mimosa (Leguminosae, Mimosoideae): three new species and a species checklist, Phytotaxa 260 (3), pp. 201-222 : 202-206

Mimosa chiquitaniensis Atahuachi & C.E. Hughes , sp. nov. ( Fig. 2)

Similar to M. pudica Linnaeus (1753: 518) , but consistently unarmed rather than armed with a pair of aculei shortly below all nodes, with strictly globose as opposed to ellipsoid capitula, strictly 1-jugate rather than 2-jugate leaves, and longer (4.5–6 cm compared to 1–3 cm) peduncles.

Type:— BOLIVIA. Santa Cruz: Chiquitos, vicinity of Santiago de Chiquitos , on roadsides and in disturbed Cerrado on sandy soils, 18°20´03´´ S 59°35´22´´ W, 600 m, 24 October 2007, fl, J. R. I. Wood et al. 23707 (holotype LPB!; isotypes FHO!, K!, USZ) .

Functionally herbaceous, unarmed, perennial herb with several humifuse, 20–40 cm-long stems arising from a stout, woody root or short caudex, the capitula held on ± erect peduncles in axils of coevally developing leaves on actively growing shoots, and rising amongst and above foliage, the stems, leaf stalks and peduncles densely pilose and short white puberulent, with slender horizontally spreading or weakly downwardly pointing 2 (–3) mm long setae. Stipules persistent, narrow or triangular lanceolate, 7 × 1.3 mm, variably (0–) 3–4 (–5)-nerved, and densely setulose on margins, the setae slender to 2 mm. Leaves sensitive, bipinnate, consistently 1-jugate, the leaf stalks 2.5–3.5 cm long ending in a slender spicule, the pinnae 3–4.5 cm, with a pair of minute narrowly lanceolate paraphyllidia to 2 mm, and (12–) 13–15 (–17) pairs of leaflets per pinna, these linear, acute at apex, 7 (–12) × 1.3 (–3.5) mm, glabrous above, densely but unevenly pilose, with 2–3 mm-long appressed trichomes below, only the strongly asymmetric midrib prominulous below, secondary veins barely visible. Flowers in globose capitula, often singly or occasionally in pairs in leaf axils held on slender 4.5–6.5 cm-long peduncles, cone-like, the narrow linear or subulate, apically setulose, 2 mm long bracts exserted in bud; calyx minute, a diminutive campanulate, incipiently pappiform somewhat asymmetric ring to 0.4 mm; corolla 4-merous, the petals 2–2.5 × 0.5 mm, 1-nerved, the lobes densely puberulent; 8-androus, the pink filaments exserted 6–7 mm beyond the corolla. Fruits in dense clusters of 12–15 per capitulum, linear-oblong, asymmetrically acute at apex, round at base, 10–15 × 3–4 (–5) mm, (3–) 4-seeded, the valves and replum pilose with slender 1 mm setae, readily breaking up into one-seeded articles to leave a persistent replum, this splitting at the apex after the articles are shed.

Distribution and Ecology: — Mimosa chiquitaniensis is known only from the type locality where it grows in disturbed, grazed and periodically burned scrubby cerrado on sandy soils at the edge of the village of Santiago de Chiquitos amongst grasses with Mimosa nuda Bentham (1841: 362) , Eriosema crinitum ( Kunth 1823: 421–422) Don (1832: 348) , Indigofera asperifolia Bong. ex Bentham (1839: 431) , Tephrosia adunca Bentham (1839: 432) , and scattered trees of Dimorphandra gardneriana Tulasne (1844:185) and Stryphnodendron obovatum Bentham (1875: 374) .

Additional specimens examined: — BOLIVIA. Santa Cruz: Chiquitos, just below the town of Santiago de Chiquitos on trail to La Cueva, 642 m, 18°20´39´´ S, 59°35´31´´ W, 30 December 2002, (fl), J.R.I. Wood et al. 18818 (FHO!, K!, LPB, USZ). Santa Cruz: Chiquitos, at the edge of the town of Santiago de Chiquitos on trail to La Cueva and El Arco, 600 m, 18°21´S, 59°36´W, 23 February 2006, (fl), J.R.I. Wood et al. 22258 (FHO!, K!, LPB, USZ).

Etymology: —Nowhere else in Bolivia is the density of species of Mimosa greater than in the Serranía de Santiago in Chiquitos Province in the Department of Santa Cruz, and this is especially true of the area around the village of Santiago de Chiquitos where M. chiquitaniensis is locally endemic.

Phenology: —Flowering from October to January in the southern hemisphere s ummer, mainly after fire.

Conservation status: — Mimosa chiquitaniensis is known only from the type locality where a single population covering ca. 2 ha and including ca. 400 individuals is found on the edge of the small town of Santiago de Chiquitos in an area that is clearly threatened as the town expands and new houses are constructed. Indeed the majority of the species population is on plots already demarcated and fenced for such use. For these reasons, the species was categorised as Critically Endangered (CR) by Mamani et al. (2010) under criteria B2 ab (iii) within the IUCN system. No further populations have been found and the future of this species would seem extremely precarious.

Notes: — Mimosa chiquitaniensis fits morphologically, ecologically and geographically within either subseries Pedunculosae or subseries Pudicae of ser. Mimosa , sect. Mimosa , and keys out to subseries Pedunculosae in Barneby’s (1991) key. However, placement in neither of these subseries is borne out by phylogenetic analysis of trnD -trnT sequences of two accessions of M. chiquitaniensis (one of which— J.R.I. Wood 23707 —was included, as Mimosa sp. , in the study of Simon et al. 2011), which are not placed with other members of subseries Pedunculosae (Clade U in Simon et al. (2011)), or subseries Pudicae (Clades P and S) ( Fig. 1). Instead, M. chiquitaniensis , whilst clearly a member of sect. Mimosa ser. Mimosa , forms an isolated lineage within that section, outwith Clades P, U, or S recognized by Simon et al. (2011) ( Fig. 1), adding weight to delimitation of M. chiquitaniensis as a new species. Despite the uncertain phylogenetic placement of M. chiquitaniensis and the non-monophyly of subseries Pudicae (split between Clades P and S), we have assigned this species to subseries Pudicae , pending realization of a new infrageneric classification of Mimosa .

Barneby (1991) was well aware of the morphological heterogeneity and potential non-monophyly of subseries Pudicae , recognizing two subgroups, one around M. pudica and M. polydactyla Humb. & Bonpland ex Willdenow (1806: 1033) , and the other allied to M. verecunda Bentham (1841: 368) and M. xanthocentra Martius (1838: 50) , and this is borne out by the molecular phylogeny of Simon et al. (2011) with placement of these two groups in their Clades P and S. Mimosa chiquitaniensis resembles M. pudica in habit, but may be distinguished from that species in being consistently unarmed ( M. pudica is usually armed with a pair of aculei shortly below all nodes), by the globose as opposed to ellipsoid capitula, strictly 1-jugate as opposed to generally 2-jugate leaves, and 4.5–6.5 cm, as opposed to 1–3 (–3.5) cm-long peduncles. From M. xanthocentra , M. chiquitaniensis can be distinguished by 3–4.5 cm as opposed to 6–12 cm long pinnae, 13–15 versus 30–82 pairs of leaflets per pinna, and 4.5–6.5 cm versus 5–20 mm long peduncles.

Amongst the ten species of subseries Pedunculosae which occur in similar campo habitats on hills of the Paraná and Paraguay basins in southern Paraguay and adjacent Brazil and Argentina, M. chiquitaniensis most closely resembles M. pedunculosa Micheli (1883: 55) , and to lesser degrees M. brevipetiolata Burkart (1948: 176) and M. alleniana Morong (1893: 98) . From M. pedunculosa it differs in larger 7 mm as opposed to 2–3 mm long stipules, a dense indumentum of spreading setae compared to the essentially glabrous M. pedunculosa , pinnae 3–4.5 cm as opposed to 1–2.5 cm, 1- as opposed to 3–4-veined leaflets and shorter peduncles. From M. brevipetiolata it differs in the spreading as opposed to appressed setae, 25–35 mm as opposed to 10–15 mm petioles, moriform as opposed to cone-like capitula and lack of visible secondary leaflet venation. Finally, from M. alleniana , M. chiquitaniensis differs most notably in leaflet venation with just the midrib visible below, as opposed to (2–) 3-nerved from the pulvinule and with coarsely V-shaped secondary veins within the margin in M. alleniana .