Mimosa debilis Humb. & Bonpl. ex Willd

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 : 213-214

18. Mimosa debilis Humb. & Bonpl. ex Willd var. debilis var. vestita (Benth.) Barneby var. parapitiensis (Burkart) Barneby

The M. debilis / M. nuda alliance is geographically widespread and often very abundant in open cerrado, campos rupestres and especially in unstable ruderal habitats across large parts of tropical and subtropical South America. It encompasses great morphological diversity and complexity. Furthermore, these species seed prolifically and appear to be adapted for zoochorous seed dispersal, as well as being extensively polyploid across several elements ( Dahmer et al. 2011; Morales et al. 2010, 2012). Barneby (1991) acknowledged the considerable difficulties associated with delimiting these two species and defining their numerous named infraspecific varieties, as well as the pragmatic nature of his classification, suggesting that the alliance is ‘impervious to organization into discrete units’ ( Barneby 1991: 548) and that ‘during several years of study leading to this account of M. nuda , I have found myself passing from one alternative taxonomy to another without finding satisfaction in any way’ ( Barneby 1991: 552). These difficulties prompted Morales et al. (2010) to suggest treating M. nuda and M. debilis as a single species, a proposal that is both pragmatic but also controversial, given the spectacular morphological diversity that would then be encompassed within a single species. These complexities have been further investigated with new chromosome counts confirming extensive polyploidy across the alliance (Morales et al. 2010), addition of new species ( Morales et al. 2012), and re-circumscription of infraspecific taxa ( Morales & Fortunato 2010). Recent field collections have added abundant new material of these species from eastern Bolivia and this has confirmed Barneby’s (1991) observation that the morphological diversity of the M. debilis / M. nuda complex is especially pronounced and poorly understood in this area, with particularly erratic variation in vesture, armature and quantitative leaf traits. Amongst this morass of apparently random variation, new collections have revealed several striking morphological variants that clearly belong within the M. debilis / M. nuda complex, but which show previously un-described combinations of morphological characters that potentially merit description as new infraspecific taxa. However, it is our view that it would be premature and unhelpful to add new names (especially given the uncertainties surrounding whether these should be assigned to M. debilis or to M. nuda ), pending a comprehensive range-wide morphological synthesis of this alliance, ideally including a detailed molecular analysis. Current attempts to define these ample- and small-leaved populations are defeated by random occurrence, intermediate states and lack of any independent corroborating molecular evidence.

Variant 1. This unusual variant is readily distinguished from all other known variants of M. debilis and M. nuda most notably by its conspicuous silvery lanate indumentum composed of basally plumose hairs, an indumentum type unknown in any other species within subseries Mimosa . In addition it forms an upright shrub to 2 m, a habit unusual in this group, and has significantly larger flowers than any of the M. debilis or M. nuda variants. Finally, the combination of pubescent corolla lobes and secondary leaflet venation confluent with the margin, runs counter to the combinations of these two traits currently used to distinguish M. nuda from M. debilis . Variant 1 is restricted to Prov. Germán Busch, known from a handful of localities in low serranías south-west of the Pantanal around Rincón del Tigre. It occurs in small colonies where it can be locally abundant, in areas with a complex mosaic of different vegetation types including open campo, cerrado and distinctive cerrado –Chiquitano dry forest transition formations, sometimes on or immediately surrounding rocky ‘lajas’ with skeletal soils, these often dominated by Anadenanthera with Comiphora, Maytenus , Tabebuia and abundant Pitcairnia , Manihot and Arachis . Santa Cruz: Germán Busch, Rincón del Tigre, vicinity of the Potosi property, 18°06´20´´ S, 58°14´30´´ W, 243 m, 27 April 2008, (fl), J.R.I. Wood J.R.I. et al. 24509 (K!, LPB!, UB, USZ!). Santa Cruz: Germán Busch, 55 km from Carmen Rivero Torrez towards Rincón del Tigre, 18°23´57´´ S, 58°20´05´´ W, 329 m, 29 November 2009, (fl)., J.R.I. Wood et al. 26556 (K!, LPB!, UB!, USZ!). Santa Cruz, Germán Busch, 1–2 km along side road towards Santa Rosa de Bocaina, road from Carmen Rivero Torrez towards Rincón del Tigre, 18°18´02´´ S, 58°16´44´´ W, 366 m, 29 Nov. 2009, (fl), J.R.I. Wood et al. 26558 (K!, LPB!, UB, USZ!).

Variant 2. A diminutive-leaved, prostrate variant of M. debilis / M. nuda , with leaf stalks 1.6–2.5 cm long, the pinnular rachis 3–4 mm, all four leaflets more or less equal, the interfoliar segments 10–14 mm, the leaflets 9–12 x 4.5–5 mm, i.e. much smaller than any known variants of M. nuda or M. debilis . Known from the southern end of the Serranía Huanchaca in Parque Nacional Noel Kempff Mercado: J.R.I. Wood et al. 26735 (LPB, K, USZ), on trail from Huanchaca 2 camp to ‘la Piscina’, 14°30´36´´ S, 60°44´58´´, 730 m.