Albizia forbesii
Broad-pod False-thorn, Breëpeulvalsdoring
Botany in the bush can be a hazardous occupation!
The species name 'forbesii' honours the botanist John Forbes, who died of malaria and privation at the age of 25 in 1823 while on a plant collecting journey up the Zambesi river.
The Broad-pod False-thorn is a large deciduous tree that may grow to 20 metres. We usually find this tree on deep sands in dry woodland and in alluvial soils along river banks.
The bark is grey to blackish, thick and rough, as protection against fire damage. Young branchlets are grey and densely pubescent.
These trees are not armed, which is a characteristic of the Albizia genus.
Note leaflet venation!
These leaflets have an easily-visible feature that points toward the tree's identity. Note the off-centre midrib running up to the leaflet apex. In addition, a prominent side vein curves up the opposite side of the leaflet. Spot those two veins, and the tree may well be Albizia forbesii.
The apex of the leaflet is rounded and asymmetric, with a small tip. The two terminal leaflets are obovate in shape.
The leaf has up to 7 pairs of pinna along the rachis, and each pinna may hold up to 14 pairs of closely-packed leaflets.
A mature entire leaf, including the petiole, is about 6 cm long.
A round, raised gland sits on the petiole. This is an extrafloral nectary that provides food for ants and other insects. Ants are valuable to the tree, protecting the leaflets from unwanted browsers and leaf-chomping insects, and the tree therefore feeds the ants, giving them good reason to remain. This is a win-win relationship.
Notice, too, the densely pubescent stem and petiole, and the close ranking of the leaflets.
Stipules drop early.
A fresh, young flower has opened, but notice all the unopened flowers, each protected by a green calyx. The image illustrates the staggered flowering tactic of the tree, which enhances reproductive success.
Note too, the half-dome of creamy-white stamens. This flower design is characteristic of the Albizia genus.
The anthers are peltate (shield shaped) with theca (pollen sacs) opening outwards.With all those pollen-laden stamens, how does the flower avoid self-pollination?
Self-pollination is an undesirable outcome for the flower.
The answer lies in a fascinating two-phase tactic.
Prolific pollen production occurs in the first phase, while the stigma and style situated well below the dome of stamens. In this phase, pollen is carried by pollinators to other flowers now in the second stage.
Once pollen production dies down in a flower, the second stage emerges. The thin style, carrying the stigma, grows upward, beyond the stamens, to collect pollen from visiting pollinators, and fertilise the ovary.
The plant's advertising strategy is simple. Use a mass of stamens to attract pollinators. Don't waste valuable energy on producing big, colourful petals. Petals do exist in these species. They are white, and no more than 8 mm long.
The flowering season may begin before the emergence of leaves in spring and and continue well into the leaf-bearing season.
The Broad-pod Albizia is well named . . . its pods are wider than those of most other Albizia species.
The pods in the image above are young, and are starting to show the russet-brown colour characteristic of mature pods.
The pod does not have bumps along its length indicating the locations of seeds.
Note the width and colouring of the pod. The seeds are slightly flattened, elliptic and about 12 mm long and 5 mm wide.
The blurred background tells us that this tree is growing in the fringing forest of a drainage line.
Early keys to the tree's identity are these reddish- brown, wide, flat pods, bipinnate leaves with the leaflet venation described above, and the absence of thorns or spines.
Distribution of this species is limited in Kruger. Trees can be spotted from Shingwedzi northwards to the Limpopo, and from Olifants southwards to the Crocodile river, in appropriate soils. This species does not occur in the sourveld of south-western Kruger.
The species can tolerate a range of soil types, from river and stream banks to thorn scrub and edges of forests. Overall, Albizia forbesii has a preference for well-draining loam and deep sandy soils, avoiding highly compacted ground.
A. forbesii shares many similarities with Albizia harveryi , but the leaflets of A. harveryi are sickle-shaped, with a pointed tip. Learning the differences in leaflet shapes is a quick and easy way to separate these two Albizia species.
The genus name Albizia comes from Filippo degli Albizzi, a Florentine nobleman who lived in the mid-18th century, and introduced Albizia julibrissin, called the Persian Silk Tree, from Constantinople to Europe.
