Geography and distribution
Restricted to central Mozambique (Manica and Sofala Provinces), where it has been found up to 100 m above sea level. The British botanist J. P. M. Brenan described the species as gregarious (growing in open clusters or in pure associations), but a collection made by Kew botanist David Goyder and colleagues in 2006, describes it as forming localised thickets (in the Gorongosa National Park, central Mozambique).
Thorns of Acacia torrei (Image: Bart Wursten)
Overview: A shrub 1–2 m high, with young branchlets with numerous, conspicuous, dark, sessile, pustular glands, sometimes also with a very few inconspicuous hairs up to 0.5 mm long.
Leaves: The stipules (leaf appendages) are spiny, up to 5 cm long, sometimes rather thicker than in related species, brownish to whitish, and often somewhat deflexed. There are no ‘ant-galls’ or other prickles present. The leaves have a small, sessile gland at the junction of the top one or two pairs of pinnae and there is often also a similar gland on the upper side of the petiole (leaf stalk). The leaves also have numerous smaller, dark, scattered glands, and a covering of downy hair. There are usually 7–13 pairs of pinnae and 6–15 pairs of leaflets. The leaflets are normally 3–6.5 x 1–2 mm, with a fringe of hairs on the margins, but otherwise hairless. They are without glands, except perhaps for a few inconspicuous glands on the margin near the apex. The leaflets end abruptly in a short, stiff point or spine. The lateral nerves are not visible on the underside.
Flowers: The golden-yellow flowers are borne in spherical masses (pom-pom like heads). The calyx (whorl of sepals) is about 2 mm long. The corolla (whorl of petals) has only been observed when withered and torn.
Fruits: The sickle-shaped fruit is a dehiscent pod of 3–6 x 0.8–1 cm, and is sometimes constricted (often irregularly) between the seeds, with numerous, conspicuous, dark, sessile, pustular glands, and numerous, spreading, bristle-shaped hairs up to about 1 mm long, over the surface.
Seeds: The seeds are olive-brown, about 5–6 x 5–6 mm, subcircular and compressed. The seeds have a 90% open pleurogram (fracture line) and an areole (flat area within the pleurogram) of 3–3.5 x 2.5–3 mm. The pods are produced in July.
The ‘glandular pod group’ of legumes
Acacia torrei belongs to a group of seven legume species which bear dark, sticky, glandular pods and are restricted to southern Africa. These seven species are all closely related to one another, and also to Acacia karroo. Each species seems to have a different habitat preference, and some think that that the group might have developed within the palaeo-Limpopo Basin over the last few million years from an ancestral form similar to A. karroo. As the basin became more arid, different populations became adapted to specific dry environments, such as black clay soils, gravelly soils and free-draining calcareous outcrops, evolving into different species as their geographical separation became more pronounced. Most of the species in this ‘glandular pod group’ occur in the Transvaal, where A. torrei is not present, it being found only in the Manica e Sofala region of Mozambique.
Fruit of Acacia torrei (Image: Bart Wursten)
A similar group of dark, sticky, glandular pod-bearing legumes is present in very dry areas of the USA and Mexico. Recent molecular analysis has shown that two of the four American species in this group are basal to the neotropical Acacia. This American group was shown to have a mean age of 7.93 million years (My). The African group, of which A. karroo was shown to be a ‘sister’ to the American species, is thought to be older, with a mean age of 10.85 My. The results of this research suggest a possible biogeographical relationship amongst the glandular pod lineages in Africa and America.
Threats and conservation
No major threats to Acacia torrei are known, and there are no known conservation measures in place for this species. It has been recorded as forming localised thickets in the Gorongosa National Park in central Mozambique. It has been recommended that A. torrei seeds should be collected and stored in Kew’s Millennium Seed Bank as an ex situ conservation measure.
Very little is known about this species, and it is difficult to assess its conservation status, which therefore has to be rated as Data Deficient (DD). Further research and fieldwork is required to define its distribution range, habitat preference and population status and to determine whether A. torrei might be of conservation concern.
Acacia torrei growing in the greenhouse of the Botanical Garden in Nelspruit, South Africa (Image: Wolf-Achim Roland)
Conservation assessments carried out by Kew
Acacia torrei is being monitored as part of the Sampled Red List Index Project, which aims to produce conservation assessments for a representative sample of the world’s plant species. This information will then be used to monitor trends in extinction risk and help focus conservation efforts where they are needed most.