Acacia chiapensis Safford

David S. Seigler & John E. Ebinger, 1995, Taxonomic Revision of the Ant-Acacias (Fabaceae, Mimosoideae, Acacia, Series Gummiferae) of the New World, Annals of the Missouri Botanical Garden 82, pp. 117-138 : 122-123

2. Acacia chiapensis Safford , J. Wash. Acad. Sei. 5: 356. 1915. TYPE: Mexico. Chiapas: near San Fernando, between Tuxtla and Chicoasen , 12 Jan. 1907, G. N. Collins 164 (holotype, US ; photo, F) .

Shrub or small tree to 10 m tall, young twigs brown to reddish brown, usually densely puberulent. Stipular spines (Fig. 1 A) usually dark brown to black, smooth, terete, usually densely puberulent (rarely glabrous), symmetrical, V-shaped with an angle of 70-100°, straight to slightly reflexed near the apex, 30-80 mm long, 4-7 mm thick near the base. Leaves 100-250 mm long; pinnae 12-30 pairs per leaf, 30-70 mm long, 5-12 mm between pinna pairs; rachis grooved, densely puberulent, a columnar gland commonly present between each pinna pair (sometimes absent); petiole grooved, densely puberulent, 7-17 mm long. Petiolar glands (Fig. 1 E) columnar, densely puberulent, striate, apex 0.5-0.8 mm across, base 0.6-1.2 mm across, usually 6-13 scattered along the petiole. Leaflets 22-50 pairs per pinna, glabrous, linear, 3-8 mm long, 0.7-1.6 mm wide, lateral veins not obvious, only one vein from the base, apex acute to obtuse. Inflorescence a densely flowered globose head 5-9 mm across, in clusters of 10-40 in the axil of slightly reduced leaves, or in clusters of 10-40 in the axil of much reduced leaves on axillary branches; peduncles glabrous to lightly puberulent, 10-35 mm long, 0.6-1.1 mm thick, nearly the same thickness throughout; involucre located near the middle of the peduncle, glabrous to puberulent, 4-lobed. Floral bracts peltate, apex circular, stalk 0.8-1.5 mm long. Flowers sessile; calyx 5-lobed, glabrous, 1.0- 1.3 mm long; corolla 5-lobed, glabrous, pale yellow, 1.6-1.9 mm long. Legume nearly straight, flattened, 65-90 mm long, 8-10 mm wide, glabrous, longitudinally striate, black to dark brown, tardily dehiscent, stipe less than 5 mm long, the apex narrowing to a very short spinelike beak, margins ridged. Flowering January-May.

Distribution. Usually rare in dry lowland sites, and along and near watercourses in heavily disturbed vegetation in extreme southern Mexico.

Representative specimens. MEXICO. Chiapas: steep slope with montane rainforest, 18-20 km N of Ocozocoautla along road to Mal Paso , 800 m, Breedlove 23862 ( MEX, MICH, MO, NY, TEX) . Oaxaca: gravel pit, second growth vegetation, 1 mi. W of Temascal , Janzen 185 ( F, MEX, MO) . Veracruz: Dos Ríos, Mell 506 ( NY) .

Acacia chiapensis appears to be most closely related to A. globulifera in its morphology and habit. These two species are easily separated, because in A. chiapensis there are usually 6-13 columnar petiolar glands scattered along the petiole, the globose inflorescences are in axillary clusters of 10-40, and the legume has a distinct marginal ridge. Acacia globulifera , in contrast, usually has only 3-6 narrow, volcano-shaped petiolar glands on a petiole, the globose inflorescences rarely exceed 8 in a cluster, and the legume lacks marginal ridges. The only other species with which A. chiapensis could be sometimes confused is A. collinsii , but this species has elongated inflorescences, fewer petiolar glands, and its leaflets have obvious secondary veins.

Acacia chiapensis is usually shrubby, but can reach a height of 10 m ( Janzen, 1967b). When repeatedly cut it commonly forms a dense bush, but rarely produces root sprouts. In many respects this species has a life form similar to that of non-ant-acacias such as A. farnesiana and A. macracantha . It is similar to these species in seed dispersal, seedling ecology, low Beltian body production, and the fact that many leaves on elongated lateral branches are not subtended by swollen spines ( Janzen, 1974).

It appears that A. chiapensis is a marginal host for obligate acacia-ants. Janzen (1974) indicated that this species can sometimes survive in the absence of ants. Most specimens of this species retain cyanogenic glycosides in the leaves, which probably limits herbivory ( Seigler & Ebinger, 1987). Low Beltian body production in this species may also indicate that it is a marginal host for acacia-ants. Beltian bodies of this species are generally less than 0.6 mm long, commonly are not well developed, and usually occur only on the lower 3-9 pairs of leaflets on some of the pinnae.

According to Janzen (1974), Acacia chiapensis occasionally hybridizes with A. cornigera and with non-ant-acacias such as A. macracantha . During the present study, no specimens were found that would indicate hybridization involving these species. Hybrids involving A. pennatula and A. chiapensis , however, have been observed.