Stachytarpheta lajedicola (Verbenaceae), a new species from the Caatinga of Minas Gerais state, Brazil: the first report of the genus in limestone outcrops

ABSTRACT

Stachytarpheta is one of the largest genera of Verbenaceae, found mainly in the campos rupestres of Brazil. We describe a new species, Stachytarpheta lajedicola, and report for the first time the occurrence of the genus in limestone outcrops. The new species is endemic to the Caatinga domain, northern Minas Gerais state, growing in an open karst formation of the municipality of Matias Cardoso, locally known as Lajedão. Its main diagnostic characteristics are the branches with well-developed longitudinal edges, leaves with attenuate to caudate apex, and inflorescences with linear bracts, exceeding the calyx in length. We provide a detailed description, illustrations, photographs, a geographic occurrence map, and comparisons with the morphologically similar species. Stachytarpheta lajedicola may be considered Critically Endangered (CR), and the biological importance of Lajedão and the conservation of the new species are discussed.

Keywords:
conservation; Duranteae; endemism; gervão; karst

Introduction

Stachytarpheta Vahl is a monophyletic genus recognized within Verbenaceae by the terminal inflorescences, androecium with two fertile stamens and two staminodes, capitate stigma, pollen with verrucose exine, and fruit divided into two cluses (Atkins 2005; Marx . 2010; O’Leary . 2012). The genus is the largest within the tribe Duranteae, with ca. 120 species distributed in the Neotropical Region (Cardoso . 2021). Stachytarpheta species are popularly known as “Gervão”, “Gervão-Roxo”, “Bluetop”, “Snake Weed”, “Rattail”, “Blue Snake Weed”, and “Bastard Vervain” (Munir 1992; Atkins 2005).

The distribution of Stachytarpheta species is more similar to Lippia L. (Lantaneae) than to the other four genera of Duranteae. The Neotropical genera Bouchea Cham., Duranta L., and Recordia Moldenke are centered in humid forests, while Chascanum E.Mey. is endemic to the Old World (Marx . 2010). Lippia and Stachytarpheta have diversified in the South American Dry Diagonal, mainly in the Cerrado domain (Marx . 2010; Salimena . 2020; Cardoso . 2021). Both are important components of the Brazilian flora, and their species are notably well-represented in campos rupestres (rocky outcrops) and cerrados (savannas), especially in the Espinhaço Range, states of Bahia and Minas Gerais (Atkins 2005; Cardoso & Salimena 2020; Salimena & Cardoso 2020).

During the ongoing taxonomic work on the Brazilian Stachytarpheta, while examining several herbarium collections, a new species was discovered in limestone outcrops of the Caatinga domain. It is here described, illustrated, and compared with its morphologically most similar congeners. The Caatinga is exclusive to Brazil, with an original area of 863,752 km², and is one of the world's largest and most biodiverse tropical drylands; however, its biological richness has been neglected (Albuquerque . 2012; de Queiroz . 2017; Silva . 2017; Araújo . 2022). Furthermore, Bystriakova . (2019) emphasize the need for further studies documenting the plant diversity on limestone outcrops in South America, considering the vulnerability of karst vegetation to threats such as mining and climate change. In this context, the discovery of the new Stachytarpheta species expands our knowledge of plant diversity in limestone outcrops, as no species of the genus had been documented in this type of environment (Atkins 2005; Cardoso & Salimena 2020), while also highlighting the floristic importance of the heterogeneous, rich, and interesting Brazilian Caatinga (Leal . 2005; Albuquerque . 2012; Araújo . 2022)

Materials and methods

The new species was discovered based on the examination of dried specimens deposited at CESJ, HUEFS, and HRCB herbaria, analysis of the relevant taxonomic literature, and comparisons with morphologically similar species. Its circumscription followed the morphological species concept (Cronquist 1978; Zachos 2016). For comparisons between similar species, we examined several Stachytarpheta specimens housed at BHCB, CEN, CESJ, HUEFS, R, RB, SP, SPF, and UB herbaria, as well as high-quality digital images available online in the databases of REFLORA (reflora.jbrj.gov.br/) speciesLink (https://www.splink.org.br/), and JSTOR Global Plants (https://plants.jstor.org/). Acronyms of the herbaria follow Thiers (2023, continuously updated). Verbenaceae species found in the same area as the new species were identified through specialized literature (Moroni & O’Leary 2019; Salimena & Cardoso 2020) and consultation with an expert taxonomist (F. Salimena).

Morphological characters were examined by careful observation of the specimens with the help of a stereomicroscope. The descriptive terminology follows Harris and Harris (2003), Atkins (2005), and Gonçalves and Lorenzi (2007). The measurements are presented whenever possible as minimums and maximums values. Data on habitat, altitude, and fertile period were obtained from exsiccate labels. The geographic distribution map of the new species was prepared based on the specimen occurrence spreadsheets using the software QGIS ver. 3.8. Preliminary conservation status assessment follows the IUCN (2012, 2022), with an Area of Occupancy (AOO) based on 2 × 2 km grids.

Results

Taxonomy

Stachytarpheta lajedicola P.H.Cardoso sp. nov. (Figs. 1, 4C).

Figure 1
Stachytarpheta lajedicola. A- Branch with inflorescence. B- Detail of the branch, showing the well-developed edge. C- Leaf abaxial surface, showing margin and veins. D- Leaf adaxial surface, detail of the pubescence. E- Leaf abaxial surface, detail of the pubescence. F- Detail of the inflorescence, showing rachis, bracts, and flowers. G- Bract, abaxial surface. H, I- Calyx dissected, showing the teeth, external and internal view, respectively. J- Corolla dissected, showing the androecium and internal pubescence of the tube. K- Gynoecium. L- Detail of the stigma. M- Fruit. (P.H.A. Melo . 4786, illustrated by Klei Sousa).

Type: Brasil, Minas Gerais, Matias Cardoso, Horizonte do Gama, "Lajedão", 14°53'03"S - 43°45'27"W, 12 March 2016, P.H.A. Melo . 4786 (holotype CESJ; isotypes HRCB, HUEFS).

Diagnosis: The new species is most similar to Stachytarpheta bicolor Hook.f. and S. quadrangula Nees & Mart. but differs by its branches with well-developed light brown edges, lanceolate leaves, attenuate or caudate at apex, and linear bracts, longer than the calyx.

Description:Shrubs ca. 2 m tall; multi-branched, erect, branches striate, with well-developed longitudinal edges, light brown, tetragonal, strigose, puberulent to glabrescent, nodes hirsute. Leaves decussate, sometimes with smaller leaves in the same axil as adult leaves, petioles 0.5-0.7 cm long, strigose; leaf blade 5.5-9 × 1.6-2.4 cm, lanceolate, conduplicate, sub-coriaceous, slightly discolorous, apex attenuate to caudate, base cuneate, decurrent into petiole, sessile glands present, margin serrate, ciliate, slightly revolute, adaxial and abaxial surfaces strigose, trichomes leaving evident scars when shed, veins prominent abaxially. Spikes 17-30 cm long, 2-2.4 cm wide including the open corollas, 0.4-0.6 cm wide excluding the open corollas, terminal, cylindrical, rachis visible, strigose, puberulent to glabrescent; floral bracts 1.6-2 cm long, linear, membranous, longer than the calyx, green, apex acuminate or caudate, abaxial surface strigose or puberulent, adaxial surface glabrous, margin ciliate. Flowers sessile, spirally arranged; calyx 1.4-1.7 cm long, slightly embedded in the rachis, tubular, erect, green, externally puberulent, along the veins strigose, 4-toothed adaxially, with 1 very long sinus abaxially, teeth equal, tips acuminate, up to 0.2 cm long; corolla blue or blue-purplish, infundibular, zygomorphic, tube 3.3-3.5 cm long, straight or slightly curved, with glandular-pedicellate trichomes externally, limb 5-lobed, lobes equal, ca. 0.2 cm in diam., glandular-pedicellate trichomes in the upper part, throat densely strigose, with a dense and long ring of trichomes just above the ovary, where the tube is very narrowed; androecium composed by 2 fertile stamens and 2 staminodes, inserted at the top of the corolla tube, included, thecae divergent; gynoecium reduced to a single functional carpel, 2-locular, 1 ovule per locule, ovary ca. 0.5 cm long, style filiform, exerted, stigma capitate. Fruit 0.6-0.8 cm long, narrow obloid, lightly brown, apex slightly beaked, stylopodium deciduous, with attachment scar, outer surface smooth to slightly striate, with a thin and flat commissure, totally covered by the persistent calyx, splitting into two cluses when ripe.

Paratype: Brasil, Minas Gerais, Matias Cardoso, Lajedão, Sítio de Raimundo Gato, 14°52'59"S - 43°45'30"W, 23 June 2017, M.M.T. Cota 958 (HUEFS).

Distribution and ecology: Stachytarpheta lajedicola occurs in the municipality of Matias Cardoso, northern Minas Gerais state, in the Caatinga domain, found in a limestone outcrop locally known as Lajedão (Fig. 2). It grows in an open karst formation, in 489-505 m altitude, and was collected with flowers and fruits in March and June.

Figure 2
Occurrence geographic map of Stachytarpheta lajedicola in Brazil.

The Lajedão constitutes a karst area of the Bambuí Group (IEF 2008). In these outcrop islands, the rock is either exposed or covered by shallow, poor, alkaline soils, in an environment with high temperatures, intense solar incidence, and strong water restriction (IEF 2008). These rocks can reach up to 30 m high, forming a deeply rugged terrain, with steep columns, deep crevices and fissures, usually associated with underground drains, and small labyrinthine caves (IEF 2008). The crevices accumulate sand in the dry season and water in the rainy season (IEF 2008). Different types of vegetation are found, such as forest, open karst formation, and aquatic vegetation (IBGE 2012; Melo . 2013). The deciduous forest can be observed on the limestone massifs, related to the presence of epikarst, that is, places where there is soil covering the rock or filling its gaps (Fig. 3A-C); open karst formation occurs in parts where no soil covers or fills limestone interstices (Fig. 3D-E); and aquatic vegetation refers to hydrophytic species established in water environments without a current, such as sinkholes or seasonally flooded crevices (Fig. 3F).

Figure 3
Phytophysiognomies of the Lajedão of Matias Cardoso, Caatinga of Minas Gerais, Brazil. A-C- Transitional zones between limestone outcrops and forest. D, E - Open karst formation. F- Aquatic vegetation. Photos by Pablo Hendrigo Alves de Melo.

In the Lajedão of Matias Cardoso, two other Verbenaceae species typical of limestone outcrops are found, Bouchea agrestis Schauer (P.H.A. Melo 6672), and Lippia subracemosa Mansf. (P.H.A. Melo 4780) (Fig. 4 A and B), collected in 2016 and 2017, although the first species does not have its occurrence recorded for the state of Minas Gerais (Moroni & O’Leary 2019).

Figure 4
Species of Verbenaceae found in the Lajedão of Matias Cardoso, Minas Gerais, Brazil: A- Bouchea agrestis. B- Lippia subracemosa. C- Stachytarpheta lajedicola. Photos by Pablo Hendrigo Alves de Melo.

Preliminary conservation assessment: The Matias Cardoso municipality is located in a region of great biological importance, the so-called Polígono das Secas (Drought Polygon), a semi-arid region known for the fragility of its ecosystems (Drummond . 2005; Magalhães . 2009; Fonseca . 2017). Three strictly protected areas (Lagoa do Cajueiro State Park, Verde Grande State Park, and Jaíba Biological Reserve), and an area managed for sustainable use (Lajedão Environmental Protection Area) were created in the region, which are home to different types of habitats, to protect the local biodiversity of the municipality (IEF 2008; Anjos 2016).

Around the Lajedão Environmental Protection Area, where the new species occurs, there are many rural properties with cattle raising, rock extraction, and removal of plants for ornamental cultivation, in addition to the human-induced fire that frequently devastate the region (IEF 2008; Anjos 2016). Stachytarpheta lajedicola is confined to a small area, AOO < 10 km², and is threatened by human activities that reduce habitat quality, while also facing the possibility of extinction by stochastic events. Thus, according to the parameters of the IUCN (2012, 2022), it can be considered Critically Endangered (CR) B2ab(i, ii, iii).

Etymology: The epithet is a reference to the exclusive occurrence of the new species in the limestone outcrops, called lajedos by the local population of Matias Cardoso municipality. The word lajedo results in the name of the area (Lajedão, or “big lajedo”) and the Environmental Protection Area.

Morphological affinities: Stachytarpheta lajedicola has inflorescences 0.4-0.6 wide excluding the open corollas, calyx slightly embedded in the rachis, and corolla tube longer than 3 cm. These characteristics are shared with S. bicolor, S. glandulosa S.Atkins, S. quadrangula, and S. scaberrima Cham., all of which occur in the Caatinga domain (Cardoso & Salimena 2020). However, S. lajedicola differs by the leaves with attenuate to caudate apex and the linear bracts, 1.6-2 cm long, longer than the calyx. Table 1 summarizes the main differences among these species.

Table 1
Comparison between Stachytarpheta lajedicola and morphologically similar species.

Feature	S. lajedicola	S. bicolor	S. glandulosa	S. quadrangula	S. scaberrima
Edges of the branches	well-developed, with light brown edge	undeveloped	undeveloped	undeveloped, with black edge	undeveloped, sometimes with black edge
Petiole length	shorter than 1 cm	shorter than 1 cm	1-2.2 cm	shorter than 1 cm	shorter than 1 cm
Sessile glands in the petiole	absent	absent	numerous (>30)	absent	absent
Leave shape	lanceolate	ovate	ovate	ovate	ovate
Leave apex	attenuate to caudate	acute	obtuse	acute or obtuse	acute or obtuse
Lave base	cuneate	cuneate or attenuate	truncate or obtuse	cuneate or attenuate	cuneate or attenuate
Bract lenght	1.6-2 cm	1-1.6 cm	0.15-0.3 cm	0.4-1 cm	1.2-1.7 cm
Bract shape	linear	triangular	ovate-triangular	triangular	oblong or narrowly triangular
Length of the bract in relation to the calyx	longer	shorter	shorter	shorter	shorter
Calyx lenght	1.4-1.7 cm	1.2-1.8 cm	0.8-1 cm	1.2-1.7 cm	1.6‒2 cm
Corolla tube lenght	3.3-3.5 cm	3-3.5 cm	2‒2.3 cm	1.7-2.5 cm	3‒4.2 cm
Corolla collor	blue or blue-purplish	blue or blue-purplish	blue-purplish	blue or blue-purplish	red
Distribution (Brazilian states)	Minas Gerais	Alagoas and Bahia	Bahia	Bahia and Minas Gerais	Bahia and Minas Gerais

Walpers (1845) was the first to subdivide Stachytarpheta into two sections. Stachytarpheta sect. Stachytarpheta is defined by the narrow inflorescences with calyces deeply embedded in the rachis, while S. sect. Melasanthus (Pohl) Walpers by the more robust inflorescences with calyces not embedded in the rachis (Walpers 1845). Stachytarpheta lajedicola, S. bicolor, S. glandulosa, S. quadrangula, and S. scaberrima have calyx slightly embedded in the rachis, and narrow inflorescences, but excluding the open corollas, which are very long. These characteristics demonstrate a gradient between S. sect. Stachytarpheta and S. sect. Melasanthus. Therefore, their position within these groups is difficult, although S. quadrandula and S. scaberrima were included in S. sect. Stachytarpheta by Walpers (1845).

Regarding the classification of Atkins (2005), Stachytarpheta lajedicola can be included in the Quadrangula informal group, which does not include S. glandulosa (treated as Incertae sedis), but encompasses S. coccinea Schauer, S. speciosa Pohl ex Schauer, and S. trispicata Nees & Mart. These latter three species have robust inflorescences wider than 1 cm excluding the open corollas, and calyx not embedded in the rachis, thus not resembling the new species (Atkins 2005).

Discussion

Limestone outcrops are known to host a high number of endemic species, which tend to be highly specialized and tolerant to extreme conditions (shallow soils, high temperatures and solar incidence, and low availability of water) (Espírito Santo . 2018, Bystriakova . 2019). However, such species are uncompetitive, and quite vulnerable to environmental disturbances (Espírito Santo . 2018). Approximately 5% of the Brazilian territory is constituted by limestone outcrops, which unfortunately are frequently isolated by urban and agricultural areas and are under immense pressure from commercial mining interests (Espírito Santo . 2018; Marques . 2020; Batista . 2022). Few species of Verbenaceae are found in limestone outcrops in Brazil, including Aloysia virgata (Ruiz & Pav.) Juss., Bouchea agrestis, Lantana achyranthifolia Desf., Lippia origanoides Kunth, Lippia subracemosa (Salimena . 2020). However, there is currently no study that provides a comprehensive and taxonomic verified occurrence list.

The flora of the Lajedão Environmental Protection Area (with an area of ca. 12,000 ha) was considered poorly studied according to the management plan (IEF 2008). Among the collected angiosperm species, some were identified as new geographic distribution records for the state of Minas Gerais, indicating the need for additional collections and floristic studies (IEF 2008). No species of Verbenaceae were recorded in this survey (IEF 2008).

The present contribution reports the first occurrence of Stachytarpheta in limestone outcrops. Stachytarpheta lajedicola was collected recently, in 2016 and 2017. Thus, this discovery highlights the biological importance of the Lajedão, an area that has already revealed another new species, Stemodia perfoliata Scatigna & V.C.Souza, Plantaginaceae (Scatigna . 2018). The presence of these endemic species aggregates information that can help to strengthen environmental protection policies, especially considering that the municipality of Matias Cardoso is part of one of the ten priority areas for conservation in Caatinga of Minas Gerais state (Fonseca . 2017).

Acknowledgments

This study is part of the first author’s doctoral thesis, developed in the Programa de Pós-graduação em Ciências Biológicas (Botânica) of the Universidade Federal do Rio de Janeiro, with a scholarship from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq 141837/2020-9). The authors are grateful to Dr. Nilber Gonçalves da Silva for help with botanical Latin. Special thanks to Dr. Pedro L.R. de Moraes and Dr. Vinícius Dittrich, curator of the HRCB and CESJ herbaria, respectively. M.T. is supported by the Alexander von Humboldt Foundation (BRA/1139098), FAPERJ (E-26/202.708/2019-JCNE), and CNPq (Pq-2 grant, 306758/2019-9).

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Publication Dates

• Publication in this collection
  15 Sept 2023
• Date of issue
  2023

History

• Received
  06 Mar 2023
• Accepted
  27 July 2023