PlantLife Volume 58.6, December 2024. New Bushveld arum lilies (Stylochaeton) endemic to South Africa.

 

New Bushveld arum lilies (Stylochaeton) endemic to South Africa.

 

Madeleen Struwig & Stefan J Siebert

Unit for Environmental Sciences and Management, North-West University, Potchefstroom campus

Inflorescence of Stylochaeton glaucophyllum

 

Bushveld arum lilies (bosveldvarkore), scientifically known as Stylochaeton, are a genus of the Araceae (arum lilies) that is distributed throughout tropical and southeastern tropical Africa. Typical of Araceae, the inflorescence is a spadix, consisting of a fleshy axis on which a column of small male flowers occur above the larger female flowers, surrounded by a vase-shaped modified leaf (spathe) with its base fully closed. Until recently, it was suggested that only one species of Stylochaeton existed in South Africa, namely S. natalense (Figure 1). This species is distributed in the northeastern regions of South Africa (Northwest, Gauteng, Limpopo, Mpumalanga, and KwaZulu-Natal), and it is especially prolific in the subtropical savannah vegetation of the north-eastern escarpment, including the Sekhukhuneland region.

The Sekhukhuneland region is part of the Sekhukhuneland Center of Endemism, a small geographical area located approximately 200 km northeast of Pretoria around the mining town of Steelpoort in the Limpopo and Mpumalanga provinces of South Africa. The core area is more or less congruent with surface outcrops of the Rustenburg Layered Suite, one of the stratigraphic units of the eastern Bushveld Complex. It is a treasure trove of plants and animals new to science, with 15 new plant species described over the past 20 years. Therefore, it was not a surprise when after further investigation, plants thought to be juvenile S. natalense turned out to be two new species of Stylochaeton!

 

Figure 1: Photographs showing (A) hastate leaves and (B) yellowish-greenish spathe of Stylochaeton natalense (Photographs: A, G. Nichols; B, R.G.C. Boon).

For many years, ecologists conducting botanical surveys in Sekhukhuneland recorded juvenile S. natalense, until closer inspection of the plants revealed underground reproductive structures (geocarpy) that are not characteristic of S. natalense. The two new species were named Stylochaeton glaucophyllum (Blue bushveld arum, bloubosveldvarkoor), after its characteristic blue-green leaves, and Stylochaeton sekhukhuniense (Sekhukhune bushveld arum, sekhukhunebosveldvarkoor), after Sekhukhuneland, the geographical region where the species occurs. Both species are endemic to the Sekhukhuneland Centre of Endemism and are classified as Endangered according to the IUCN Red List categories.

Stylochaeton glaucophyllum and S. sekhukhuniense are geophytes with vertical rhizomes and stout roots. The petioles of S. glaucophyllum are 200–300 mm long and a third of the petiole and entire petiolar sheaths are covered with white and/or purple-green transverse bands. The leaves are long (112–332 mm), narrowly sagittate, and blue-green in colour. The apical and basal lobes of the leaf blades are linear in shape. The inflorescences appear before the leaves and are semi-subterranean. The spathe is cream-coloured or brownish, which causes the inflorescence to blend with the surrounding dry grass, rocks, and soil. The spadix protrudes somewhat from the spathe. The white or yellowish berries are carried in a globose infructescence, which is also semi-subterranean. The ripe berries omit a scent reminiscent of a ripe mango, suggesting that the seeds might be dispersed by an animal (Figure 2).

 

Figure 2: Photographs showingtylochaeton glaucophyllum (A) with mature fruit partly below the ground; (B) mature inflorescence with brownish and cream-coloured spathe. (Photographs A, A.E. van Wyk; B, S.J. Siebert).

S. glaucophyllum is restricted to mafic (Igneous rock that is relatively high in magnesium and iron content) and ultramafic (Igneous rock composed of mafic minerals, and a silica content <45%) rocks. It grows in slightly acidic (pH 5.5) ultramafic soil (Ca/Mg ratio 0.68) rich in metals such as Al and Fe, and to a lesser extent Cr, Mn, Ni and Ti. Soils are nutrient-rich, but a low electrical conductivity (40 mS/m) suggests that nutrients are not readily available leading to a “serpentine soil (Weathered ultramafic rock such as peridotite and its metamorphic derivatives such as serpentinite) effect” typical of ultramafic areas.

S. glaucophyllum populations are small and predominantly localised on pyroxenite and norite outcrops and hillsides in the semi-shade of small trees such as Combretum apiculatum, Kirkia wilmsii and Senegalia nigrescens. It is found in herbaceous communities characterised by Corbichonia decumbens, Hibiscus coddii subsp. barnardii, Cyphocarpa angustifolia and Melhania prostrata.

 

Figure 3: Photographs showing Stylochaeton sekhukhuniense (A) with its blue-green leaves; (B) characteristic slender, curling elongations at the tip of the petiolar sheath, and (C) the olive-green spathe partly below the ground. (Photographs: A, S.J. Siebert; B & C, A.E. van Wyk).

The petioles of S. sekhukhuniense are 100–300 mm long, and the lower third of the petiole is covered with transverse purplish and greenish white speckles or bands. The apex of the petiolar sheath characteristically extends into two slender, curling elongations (ligulae) ca. 30 × 3 mm. The leaves are shorter than S. glaucophyllum (44–180 mm), sagittate, and green in colour. The apical and basal lobes of the leaf blade are elliptical in shape. The spathe is cream-coloured on the inside, but brownish olive-green on the outside. The spadix does not protrude from the spathe. The fruits are not known, but based on the position of the inflorescence, they are suspected to be semi-subterranean also (Figure 3).

S. sekhukhuniense is found in rocky areas on the edges of deep valleys and restricted to the same geology as the sister species. It grows in well-drained stony, clay soil along gentle to moderate slopes in the semi-shade of small trees such as Dichrostachys cinerea, Searsia engleri and Terminalia prunioides. Co-occurring forbs include Justicia flava, Sansevieria hyacinthoides and Waltheria indica.

S. sekhukhuniense prefers similar soil conditions to S. glaucophyllum that are associated with norite and ultramafic hills. However, its associated soil seems to be richer in nutrients with lower calcium concentrations, with the “serpentine soil effect” largely maintained.

Stylochaeton natalense differs from these newly described species in that its leaf shape varies from ovate to triangular to sagittate-hastate or hastate-cordate (Figure 1A), while those of S. glaucophyllum and S. sekhukhuniense are sagittate (Figure 2A and Figure 3A). The leaves of S. natalense are also longer and wider (50–310 × 18–260 mm) while those of S. glaucophyllum and S. sekhukhuniense are shorter and narrower (50–172 × 3–30mm). The spathe of S. natalense is greenish to yellowish or purplish (Figure 1B), while in S. glaucophyllum it is cream-coloured or brownish with paler longitudinal stripes (Figure 2B) and in S. sekhukhuniense it is brownish olive-green (Figure 3C) (Table 1).

Together with Zantedeschia jucunda and Z. pentlandii, there are now four species of Aroideae endemic to the Sekhukhuneland Centre of Endemism.

Table 1: Differences between the three Stylochaeton species growing in South Africa.

	S. glaucophyllum	S. sekhukhuniense	S. natalense
Leaves shape	Narrowly sagittate	Sagittate	Ovate, triangular, sagittate, sagittate-hastate, cordate-sagittate
Leaf size:	112–172 × 4–27 mm	55–80 × 3–30 mm	50–310 × 18–260 mm
Spathe colour (outside)	Brownish, yellowish brown or purplish brown	Brownish olive-green	Greenish to yellowish or purplish.

 

Photographs used were first published in Struwig et. al (2023), Phytotaxa

620(1): 1–17. This article, with all references, is available free from

https://doi.org/10.11646/phytotaxa.620.1.1

 

 About the authors: 

Madeleen Struwig is a lecturer at the North-West University on the Mahikeng Campus with plant taxonomy and lichens her research focus.

Stefan Siebert is a professor in botany at the North-West University. He is a geoecologist with  a special interest in edaphic plant specialists that are restricted to harsh soils.