A synopsis of Stenostephanus Nees (Acanthaceae) in Bolivia

12 species of Stenostephanus Nees occur in Bolivia, two of which, S. madidiensis J.R.I.Wood and S. sehuencasii J.R.I.Wood are described as new. Ten species are endemic to the country with all growing on the humid eastern slopes of the Andes. The distribution of all Bolivian species is described and mapped and implications for conservation are discussed. A key to the species found in Bolivia and a table of their principal distinguishing characteristics are provided. The new species are illustrated with line drawings and photographs.

Stenostephanus Nees is a neotropical genus of Acanthaceae consisting of about 80 species. It is distributed along the mountain chains from south central Mexico through Central America and then along the Andes from Venezuela south to the Santa Cruz region in central Bolivia. A single species, S. lobeliiformis Nees occurs in the Atlantic Forest of southeastern Brazil. The greatest diversity is found in Colombia from where 29 species are known (Wood 2016). All species are perennial herbs, usually weakly ascending in habit and with flowers arranged in a racemose or open, paniculate thyrse. The corolla is quite varied in form, commonly prominently 2-lipped or with four, often short, subequal lobes. There are only two stamens and the often exserted anthers are monothecous. The capsule is 4-seeded. Also distinctive is the pollen, which is biporate, spherical, somewhat compressed, usually with an encircling band and resembling an aeroplane wheel.
The Bolivian species of Stenostephanus were treated fully by Wasshausen & Wood (2004), but an additional species, S. suburceolatus J.R.I.Wood was described in 2015 (Wood 2015) and a further two species are described in the current paper bringing the total for Bolivia to 12. This is the same total as for Peru (Wood 2019) although there is a greater likelihood of discovering additional species in Peru. The opportunity is here taken to update information on the distribution of species in Bolivia and at the same time to clarify differences between species based on the greater number of specimens that have become available this century.
This paper is based on the study of herbarium specimens of Stenostephanus at BOLV, K and LPB. This has been supplemented by photographs and field notes by collectors, together with the author's knowledge of the genus in the field in Bolivia.
In Bolivia Stenostephanus is a genus of moist evergreen forest "Bosque humedo del pie de monte" and "Bosque Montano Humedo de las Yungas" (Killeen et al. 1993;Jorgensen et al. 2015) on the eastern slopes of the Andes north of latitude 18 degrees south, growing from around 400 m in the Andean foothills up to about 2800 m in the cloud forests of the Cochabamba region (Map 1). Of the 12 species known from Bolivia, ten are endemic to the country. Of the two exceptions, one S. longistamineus (Ruiz & Pav.) V.M.Baum is only known from a single collection and has not been seen in Bolivia for a hundred years. It is, however, the commonest species of Stenostephanus in Peru and, probably, in Brazil (Wood 2019). The other species, S. davidsonii Wassh., is scattered over a wide area of northern Bolivia and Peru and this distribution has been mapped by Wood (2019). Of the ten species endemic to Bolivia much the most commonly collected is S. crenulatus (Britton ex Rusby) Wassh., which is present in many of the cloud forests in La Paz Department. All other species are very localised in their distribution occurring in a single restricted area (S. cochabambensis Wassh., This pattern of isolated localised populations is characteristic of the genus in South America. As noted above, species of Stenostephanus in Bolivia are characteristic of moist Andean hill forest, being found from lowland rainforest in the Andean foothills (S. spicatus, S. tenellus) up to the moist montane cloud forest at higher altitudes up to about 2800 m. The species with a distinctly 2-lipped corolla (S. cochabambensis, S. davidsonii, S. pyramidalis, S. sehuencasii) occur at higher altitudes (mostly above 2000 m) than the species with a subequally lobed corolla, which are found below 2000 m. All species grow in primary forest but may be favoured by a degree of disturbance being found often near tracks or along wooded quebradas. They sometimes survive in partially cleared forest.
Although only three species, Stenostephanus cochabambensis, S. krukoffii and S. spicatus have been formally assessed using IUCN guidelines (Ministerio de Medio Ambiente y Agua 2012), all species occur-ring in Bolivia face similar threats. All except S. crenulatus and S. davidsonii are very restricted in their ranges and all occur in forest, which is to a greater or lesser degree subject to logging or casual disturbance by cattle or encroachment from cultivated areas. On the positive side, all species grow at least in part of their range within legally protected areas, at least five entirely so. Most of the habitats where they grow are relatively inaccessible and poorly explored. A good example is the Serranía de Bellavista lying between Caranavi and Alto Beni which is crossed by the main road from La Paz to the Beni and where three species, S. crenulatus, S. krukoffii and S. lymansmithii grow. Unprotected, superficially easily accessi-Map 1. Distribution of Stenostephanus in Bolivia, which is restricted to the Eastern slopes of the Andes north of 18°south.
ble, the hillsides are precipitous and only on the rare days when the cloud and mist disperse, do they merit their name. Inevitably, all known populations are from within a few metres of the road, and the mountain range which extends many kilometres both north and south remains botanically unexplored and could harbour extensive population of all three species. Slopes are rocky, the summits perpetually wet and soil limited to gullies so it is not easily or profitably exploited for logging or cultivation. The situation of S. sehuencasii and S. spicatus is not dissimilar with both restricted to the Carrasco National Park. Both grow near dirt tracks passing through forest and no serious search has ever been made away from these trails. In sum, therefore, any conservation assessment is highly speculative and must await thorough exploration of the very inaccessible areas where most species grow.
Another factor that needs to be considered in assessing the status of Stenostephanus species is the irregular flowering pattern of many species. There is no reliable evidence for this phenomenon in Bolivia but it has been noted elsewhere (Wood 1988) and at least one species from Mexico is known to be plietesial (Daniel 2006). Consequently, the absence of flowering specimens does not necessarily indicate the absence of the plant itself.
A key to all 12 species occurring in Bolivia is provided here as well as a table of the principal differences between the species. The key and the table add to and refine elements of the account by Wasshausen & Wood (2004) as more specimens and information have gradually become available as a result of more intensive collecting in Bolivia. However, the very localised distribution of most species is clear (Maps 2, 3, 4 and 5) with only two species extending to Peru.
The species can be distinguished by the following key. It should be noted that specimens are difficult to identify in the absence of corollas as their distinctive shape and colouring are usually diagnostic. The principal differences between species are shown in Table 1.
Key to species of Stenostephanus known from Bolivia DISTRIBUTION. Restricted to between 2200 and 2500 m in the area around Tablas Monte, Corani Pampa and Incachaca in Chapare Province of Cochabamba Department. Map 2.