Siphocampylus siberiensis is similar to Siphocampylus boliviensis, but with a corolla tube lacking a constriction at its base and a hemispherical hypanthium.

Bolivia. Cochabamba: Carrasco, en la entrada para Sunchal, cerca al rótulo “Unidad Educativa Manuela Gandarillas”, 17°47'267"S, 064°47'669"W, 2668 m, 18 December 2011 (fl & fr), L. Lagomarsino, D. Santamaría & J. M. Mendoza 241 (holotype: A!; isotype: LPB, USZ).

Multi-stemmed shrub 3–4 m tall, branched, all branches arising from a single point at ground level, with soft wood, the bark suberose; branches 0.4–0.6 cm in diameter, fistulose, the youngest parts purple, brown when mature (greyish to whitish in dry material), glabrescent to tomentose; internodes 0.9–1.7 cm long; latex white. Leaves spirally arranged and generally clustered at the apex of branches, leaving prominent leaf scar after falling; petiole 0.3–0.7 cm long, glabrescent to tomentose with whitish trichomes, winged, adaxially canaliculate, abaxially more or less triangular with two ribs; leaf blade 10.5–19.5 × 2.9–4.6 cm, oblanceolate, adaxially tomentose and abaxially densely pubescent, the pubescence principally on the veins, trichomes simple, the base decurrent; apex acuminate; margin doubly dentate and ciliate, 75–95+ teeth per side, the teeth triangular; venation reticulate with 16–21 pairs of lateral nerves, lightly ascendant, impressed adaxially and elevated abaxially. Flowers solitary, axillary, generally towards branch apex; pedicel 4.5–9.4 cm long, straight, cylindrical, densely pubescent; bracteoles absent; hypanthium 0.5–0.8 × 0.3–0.4 cm, hemispherical, tomentose; calyx lobes 5, (0.8–) 1.0–1.4 × (0.2–) 0.3 cm, narrowly triangular, the margins ciliate, entire, pubescent on both surfaces, erect or recurved, the apex acuminate; corolla 3.5–3.8 cm long, completely pink, diminutively pubescent on both surfaces; corolla tube 0.9–2.2 × 0.7–0.8 cm, cylindrical for its entire length, a little wider apically than basally, straight at anthesis; corolla lobes 5, narrowly triangular, slightly falcate, the margins ciliate, the apex acute to acuminate, the two dorsal lobes 1.5 cm long, the two lateral lobes 1.4 cm long, the ventral lobe 1.4 cm long, staminal tube 3.0–3.8 × 0.1–0.2 cm, straight, sparsely pubescent, pink, exserted between the two dorsal lobes; anther tube 0.8–0.9 × 0.2–0.3 cm, gray in living material, glabrous except in the sutures between anthers, which are densely pubescent, the trichomes white, ventral anthers 0.6–0.8 cm long, penicillate at the apex, the trichomes white, the dorsal anthers 0.7–1.0 cm long, penicillate at the apex, the trichomes white. Fruit capsule, 5.0 × 1.0–1.2 cm, ca. 15-lobed, with external ridges, the calyx persistent; seeds not seen.

Siphocampylus siberiensis is endemic to Bolivia, where it has been collected at the edge of the road at ca. 2700–2900 m in elevation in cloud forest.

Individuals were collected in flower and fruit in December and in flower only in April; the rest of the phenology of this species remains unknown.

The specific epithet of this species refers to the type locality, the Serranía de Siberia, a mountain range at the limit between the Cochabamba and Santa Cruz departments in Bolivia.

Siphocampylus siberiensis is known only from a single population in Serranía de Siberia in central Bolivia; this population is represented by the two cited collections. This species appears to be locally rare, and only one individual was encountered during our fieldwork. Due to its small area of occurrence and the threat of future deforestation in its habitat, we tentatively consider this species to be Vulnerable (IUCN 2014). Its vulnerable status is further justified by its roadside occurrence in montane cloud forest, a habitat type known to be particularly sensitive to human encroachment.

Siphocampylus siberiensis can be recognized by its shrubby habit with multiple stems arising from a single point; leaves aggregated at the apex of branches (Fig. 4A); conspicuous venation (especially on the abaxial leaf surface) (Figs 4A, 5C); solitary flowers borne in the axil of leaves (Fig. 4A); light pink corolla with a tube that is cylindrical for its entire length (i.e., not basally constricted) (Figs 4B, D, 5F); anther tube that is densely pubescent in the sutures between individual anthers (Figs 4E, 5F); and fruits that are both ribbed and lobed (Figs 4F, 5E).

Molecular phylogenetic analysis places Siphocampylus siberiensis in a clade that includes Siphocampylus tunarensis Zahlbr., Siphocampylus tunicatus Zahlbr., and Siphocampylus umbellatus (Kunth) G. Don; this clade is closely related to Siphocampylus boliviensis Zahlbr. and Siphocampylus sparsipilus E. Wimm. (Lagomarsino et al. 2014) (Figs 5, 6). These species are all restricted to the Central Andes of Peru and Bolivia, with the exception of Siphocampylus umbellatus, whose range also extends to Brazil. This clade is composed of robust shrubs or trees that are exceptionally tall for the centropogonid clade (Figs 5A, 6E–F), or rarely scandent subshrubs (Siphocampylus sparsipilus and some collections of Siphocampylus boliviensis), with ebracteolate pedicels, a shallow, hemispherical hypanthium (turbinate in Siphocampylus tunarensis), and leaves that leave prominent scars after abscission (Fig. 4A) and have dentate margins and reticulate venation that is conspicuous on both surfaces, but especially the abaxial surface (Figs 4A, 5C). Both bright pink (Siphocampylus siberiensis, Siphocampylus boliviensis, Siphocampylus sparsipilus) and dull colored (Siphocampylus tunarensis, Siphocampylus tunicatus, Siphocampylus umbellatus) corollas are represented in this clade. This color variation, which is associated with different gross corolla morphologies, likely reflects adaptation to different pollinators (hummingbirds and bats, respectively) (Figs 5F–G, 6A–D).

Even though they are not the most closely related species, the pink, narrow flowers of Siphocampylus siberiensis most closely resemble those of Siphocampylus boliviensis and Siphocampylus sparsipilus. However, the latter two species can be easily distinguished by their corollas that are constricted at the base (vs. not constricted) and much rounder hypanthium (vs. flattened at top). The other species in the immediate clade that includes Siphocampylus siberiensis differ in their dull reddish (Siphocampylus tunarensis [Fig. 6B]) or whitish-green (Siphocampylus tunicatus [Fig. 6C], Siphocampylus umbellatus [Fig. 6D]) corollas (vs. bright pink in Siphocampylus siberiensis). Siphocampylus tunicatus and Siphocampylus siberiensis are sister species that are vegetatively very similar, though their flowers are markedly different (Figs 4, 5, 6C, F). In addition to its green corolla, the former can be distinguished by its longer sepals (2.0–2.8 cm [Fig. 6C] vs. [0.8–] 1.0–1.4 cm [Figs 4B, D, 5F]) that are leaf-like (vs. not leaf-like) and its wider hypanthium (1.5–2.0 vs. 0.3–0.4 cm) (Fig. 6C). Siphocampylus tunarensis can be separated by its linear, revolute corolla lobes (Fig. 6B) (vs. narrowly triangular and not revolute [Figs 4B, 5F–G]) and short sepals (0.2–0.4 cm [Fig. 6B] vs. 1.0–1.4 cm [Fig. 5F]). Furthermore, while Siphocampylus siberiensis is a robust shrub 3–4 m tall, Siphocampylus tunarensis can grow to be a very tall tree (>10 m) with a diameter of more than 30 cm and is possibly one of the largest species of Campanulaceae in the Americas (Fig. 6E).

The species that is most superficially similar to Siphocampylus siberiensis, Siphocampylus boliviensis, is placed in the same couplet as Siphocampylus macrostemon A. DC. in the dichotomous key to the members of the genus in Wimmer (1953). This markedly different species, which has not yet been sampled in molecular phylogenetic analysis, can be distinguished by its subsessile leaves (vs. pedicels 0.3–0.7 cm long) that are smaller (5–8 × 1.31.5 cm vs. 10.5–19.5 × 2.9–4.6 cm) and sparsely pubescent on the adaxial surface (vs. tomentose), minutely dentate leaf margins (vs. doubly dentate and ciliate), shorter pedicels (2.6–4.2 cm vs. 4.5–9.4 cm) that are bracteolate (vs. ebracteolate), and glabrous corolla (vs. pubescent). The other species most closely related to Siphocampylus siberiensis fall into many disparate taxonomic units within the current classification scheme of the genus. This makes it difficult to place this new species in the context of Wimmer’s taxonomy; this is likely due to this treatment’s reliance on single, often arbitrary characters to designate groups.

The measurements of the calyx and corolla in parentheses correspond to E. Fernández et al. 3583 (MO). This specimen apparently has a white corolla, but otherwise corresponds to the species concept for Siphocampylus sibieriensis presented here.

Bolivia: Cochabamba, Carrasco, Siberia, 17°48'11"S, 064°46'12"W, 2900 m, 16 April 2005 (fl), E. Fernández et al. 3583 (MO).