PERU – Amazonas department • Chachapoyas, Leymebamba, Laguna de los Cóndores track, antes de Abra el Rayo, scrubland vegetation, very humid; 6°46’06”S, 77°46’37”W; 2990 m; 21 Sep. 2023; D.B. Montesinos, C.E.L. Dasher & E. Chávez 9695; holotype: HUT [HUT-64624]; isotype: B [B 10 1330351].

Arcytophyllum leymebambense sp. nov. can be recognized within the genus by the presence of two hooks protecting the ovary, a unique characteristic for this taxon and by the combination of acuminate leaf apex and hirsute-lanuginose seeds; it differs from the also herbaceous and mat-forming Arcytophyllum filiforme in leaf shape (ovate-lanceolate vs linear in A. filiforme) and sepal apex shape (acuminate vs acute or aristate in A. filiforme), the absence of long digitiform projections (present in A. filiforme), stipule length (1.3–2.0 vs 0.3–1.0 mm in A. filiforme), and seed ornamentation (hirsute-lanuginose vs finely scaly in A. filiforme). It also differs from A. muticum, another mat forming species by its sepal apex shape (oblong in A. muticum), corolla tube length (1.2–1.5 vs 3.0–4.5 mm long in A. muticum), and seed ornamentation (hirsute-lanuginose vs glabrous seeds in A. muticum).

Perennial herb with a decumbent, prostrate, or erect growth habit, forming a dense and ramified mat measuring approximately 5–10 cm in diameter. Roots subwoody-fibrous. Stems ranging from 5 to 8 cm in length, display frequent branching and are often concealed by older leaves; bark dark red hue, young shoots exhibit shades of dark green to pale red. Internodes short (2.0–3.5 mm) and narrow (0.2–0.3 mm), scarcely covered by irregular brown protuberances less than 0.05 mm in length, striate, deeply sulcate with 9–11 ribs. Stipules oblong-lanceolate, translucid, 1.3–2 × 0.8–1.8 mm, acuminate apex, irregular dentate margins, base truncate. Leaves opposite, ovate-lanceolate, 1.9–2.7 × 0.5–0.7 mm, sessile, apically subrevolute, slightly bulging midnerve, lamina dark green, thick, densely covered by sericeous trichomes on the underside and margins, sparsely and glandulous on the upperside, margin trichomes are whitish, measuring 0.10–0.25 mm long, multicellular, 18 to 40 per margin side, leaf apex largely acuminate, hirsute, hooked, with a long linear to sublinear ciliate hair, 0.5–0.8 mm long that tends to turn yellowish red with age, persisting on older foliage, leaf base truncate, narrow, densely hirsute on both sides and margins. Flowers solitary, axillary or terminal, tubular to shortly campanulate, tube 1.2–1.5 mm long, sessile or pedicellate, pedicels 0.1–0.2 mm long, sepals 4, ovate-lanceolate, 2.0–2.3 × 0.5–0.8 mm, covered with a thin translucid ovate stipule, densely hirsute on margins and upper surface, bearing a whitish acumen of approximately the same size as in the leaves, 2.0–2.2 mm long; petals 4, entire, white translucid, with a unique lobule, ovate-lanceolate, 1.8–2.0 × 0.8–1.0 mm, glabrous, hidden by the concave sepal, and fused with the stamens at the base. Androecium with 10 stamens, spreading and slightly curved filaments, 1.2–1.4 mm including the roundish and reddish anthers, which exceed the size of the ovary. Gynoecium 1 mm long, ovary ovate, 0.9–0.8 × 0.4–0.6 mm, with a slightly hirsute surface, ovary protected by two hooks. Fruits capsular, roundish, persistent, sparsely hirsute, otherwise glandulose, 1.1–1.3 mm in diameter, five carpels. Seeds about 6 to 8 per capsule, ovate to triangular, 0.4–0.6 mm in diameter, densely pubescent, trichomes dark maroon coloured and glandulose.

This species is exclusively documented in the Amazonas department, Peru. This area is situated within the Amotape–Huancabamba Zone, a major biodiversity hotspot extending from the Pacific coast, across the Andean cordilleras, to the tropical lowlands of southern Ecuador and much of northern Peru (Weigend 2002, 2004) with many narrowly endemic species concentrated (e.g. Weigend 2002; Weigend et al. 2005; Henning et al. 2019). It primarily thrives in the Jalca ecosystems, which are characterized by persistent humidity in forest patches amidst wet grasslands. These ecosystems are typically found at elevations ranging from approximately 2,990 to 3,791 m. Despite extensive surveys conducted by the first author across all Peruvian herbaria, no herbarium collections or additional specimens have been found, underscoring its status as an endemic species in northern Peru. The Jalca region, which borders dense montane tropical forests and transitions to lowland tropical forests, receives continuous rainfall year-round. Notably, the highest point of the collection site, around 3,800 m in Pampa del Rayo on the route to the Laguna de los Cóndores, receives an estimated 2,500 mm of precipitation annually.

The new entity exhibits a flexible phenological pattern finely tuned to the extreme rainfall conditions of the northern Jalca in Peru, an ecosystem characterized by persistent and intense precipitation. Flowering and fruiting periods are highly variable, primarily influenced by the amount and timing of rainfall. Typically, flowering begins as early as August, coinciding with the onset of consistent rains, and may extend into October, depending on the intensity and duration of the wet season. This variability suggests a strong ecological adaptation to the region’s erratic precipitation patterns, enabling the species to optimize reproductive success during favourable moisture conditions.

“Atunina”, name used by the locals in the communities of Atuén and Dos de Mayo of the Leymebamba district.

Named after the Leymebamba district, several populations were identified in the vicinity of Laguna de los Cóndores. Although the lagoon itself lies just within San Martín (province of Huallaga) the type specimen’s collection site is clearly located in Amazonas in the Amotape–Huancabamba Zone as defined by Casper et al. (2020). This area represents a significant habitat for the species, emphasizing its ecological importance and potential for further research. The diverse environments found in the Jalca ecosystems in the uppermost west topographical areas of the lagoon may contribute to the species unique adaptations and distribution patterns.

The species is given a Red List status of Critically Endangered [CR B1ab(iii)+2ab(iii)]. So far, Arcytophyllum leymebambense sp. nov. has been identified in only three locations, making it a relatively uncommon species. The extent of occurrence (EOO) is of 1.2 km² and the area of occupancy (AOO) is of 8 km². Both EOO and AOO fall within the limits of the Critically Endangered (CR) category under subcriteria B1 and B2. According to the IUCN criteria and categories (IUCN 2024), it meets the conditions for the CR category. The preferred habitats for Arcytophyllum leymebambense sp. nov. are the summit slopes of Jalca ecosystems, adjacent to lowland tropical ecosystems in northern Peru. These mountain slopes face vulnerabilities from factors such as overgrazing, changes in annual rainfall, and fire, all of which could reduce the distribution area of this species. During a field visit by the first author in 2023, extensive burning of these ecosystems was observed. Despite their natural resistance to fire, rapid environmental changes threaten biodiversity in the region. Furthermore, recent fires in September 2024 have devastated much of the habitat where this species grows, leading to concerns about its potential disappearance unless immediate conservation protocols are implemented by the state.

PERU – Amazonas Department • Chachapoyas Province, Leymebamba District, Camino a Laguna Yanacocha, humid grasslands, forest patches, continuously humid; 3791 m; 23 Sep. 2023; D.B. Montesinos, C.E.L. Dasher & E. Chávez 9706; HUT [HUT-64623], B [B 10 1330350] • Jalca ecosystem, highly moist, dense scrubland with tussock grasses; 3760 m; 22 Jul. 2024; D.B. Montesinos, C.E.L. Dasher & E. Chávez 10400; AMAZ, B [B 10 1330352].

Arcytophyllum was retrieved as monophyletic and highly supported either in combined analysis (PP = 1.00) and in each individual molecular marker analysis (results not shown). Both samples of A. leymebambense sp. nov. form a clade with high Bayesian posterior probabilities in the combined analysis (PP = 1.00) and in the ITS region (PP = 1.00) (results not shown), only one sample amplified for the rps16 marker. Furthermore, the phylogeny also shows that A. leymebambense sp. nov. is closest to a clade formed by several Andean species, however, the sampling of both characters and terminals prevents establishing a biogeographic hypothesis and this molecular analysis only confirms the existence of A. leymebambense sp. nov. as a unique lineage within Arcytophyllum.