Recollections of Acacia confusa in the field

 

Acacia confusa is the only remaining Acacia species native to the Philippines. And I said "only remaining" not because the others already went extinct, but because these have since been transferred to other genera*. To see these trees in their natural state, we had to drive along a newly constructed road and into a dirt road leading to a bridge still in the process of construction. There was one large tree along the aforementioned newly constructed road, but the main population is on the adjoining scrubland which rises just a few meters above a vast river now almost totally covered with lahar from Mt. Pinatubo. No more trees exist along the main Zambales road, the last one having been balled out for relocation to a private home when the roadside where it was growing on for years was slated for road widening by the Department of Public Works and Highways. Road widenings have sharply increased in frequency since 2016, resulting in the killing of innumerable trees across the entire Philippine Archipelago. This lone tree from the Olongapo-Bugallon Road near Apo-apo Bridge in the town of Cabangan, was rescued by horticulturists Ronald Achacoso and Ronald Posadas, but eventually failed to survive relocation. It was apparently planted there by mistake (Ronald Achacoso, pers. comm.) as evidenced by the presence of A. auriculiformis along the entire 2 km. stretch.

Acacia confusa was formally described by Elmer Drew Merrill in 1910 from materials collected by himself 7 years prior in Subig, now Subic. He described the trees as reaching heights of from 6 to 15 meters. And yet, none of the trees we found exceeded 3 meters. In his description of the species, Merrill did not say the reason for his choice of the species name 'confusa', but it may be due to the fact that Philippine plants were formerly understood to be the same as A. richii, until Dr. Charles Budd Robinson "then at the New York Botanical Garden, called my attention to the differences between the Philippine material and the type collection of A. richii, and kindly supplied me with a fragment of the latter, expressing the opinion that two species were represented, an opinion in which I entirely concur."

Above: The highly distinct, sickle-shaped phyllodes of A. confusa are very narrow. The local name is 'ayangile' and 'ualisin', the latter translating to 'to sweep', which is an obvious allusion to the broom-like nature of the stems.

Compared to what may be termed as the (then) 'true' acacias**, A. confusa possesses phyllodes, or specialized and flattened stems that perform the photosynthetic function of leaves. The true mimosoid (pinnate) leaves are shed early on at the seedling stage. A. confusa, like its related species, lack spines. The phyllodes are characteristically narrow and rarely reaching a centimeter in width; this character makes it easy to spot from among the multitude of A. auriculiformis that it coexists with. The glomerules, or the rounded flower clusters that appear like pompoms, appear on the phyllode axils either solitarily or in twos. The flowers are bright yellow. 

Above: Young trees of A. confusa growing on a sandy river bank. Sedges (Cyperaceae) dot the ground. The almost bare mountains of Zambales lurk from behind. Despite what some may think, the mountains here weren't deforested. The toxic substrates have limited the number of plant species that can cope and survive.

Acacia confusa is known only from Taiwan and the Philippines. In the Philippines, the tree is only reliably known to occur in the province of Zambales on western Luzon. Here, the trees grow at low altitudes along slopes or in savannahs and floodplains in poor, gravelly soils with appreciable amounts of silt, from volcanic to clay. These may also be found on the fringes of ultramafic rocks, but are seemingly intolerant enough to avoid it completely. Zambales does have a long dry season, lasting for five months on average.

Above: Acacia mangium and A. auriculiformis growing side by side. Note the silvery appearance of A. mangium, and the broader phyllodes of A. auriculiformis on its right, as compared to A. confusa.

No one knows why the Philippine population of A. confusa is confined only to a few spots within Zambales. The neighboring provinces, Bataan and Pangasinan, also have similar climates and possess the same gravelly beds here and there. However, it mostly lacks the ultramafic rocks that are prevalent in Zambales. Could it be that A. confusa relies on the toxic properties of ultramafics to avoid competition? Possibly. This theory, however, cannot be applied for the Subic subpopulation, as Subic isn't ultramafic. I.C. Nielsen, in his treatment of the Mimosaceae for Flora Malesiana (1992), mentioned that "It is difficult to judge whether this is indigenous, or cultivated and naturalized in the Philippines." But I propose another thought: due to the timber's wide applications as building materials and for furniture, A. confusa may have been aggressively cut long ago, leaving only today's pocket populations in Zambales. The trees, however, have escaped cultivation in Hawaii and are now classified as an invasive species.

It takes not too long to realize that two undeniably foreign species, A. auriculiformis and A. mangium, are much more successful at spreading in these areas. Both have much more numerous recruits (seedlings and young trees), and mature trees decidedly outnumber those of A. confusa. Even along the main road, the two exotics dominate the roadsides and one can even find rows of these trees planted for eventual harvest. By comparison, A. confusa is not at all utilized enough to be afforded equal attention, despite the comparable quality of its timber. But one possible answer lies in A. confusa's apparently slower rate of growth as compared to either A. auriculiformis or A. mangium. The total lack of large specimens of A. confusa may be explained by the trees being harvested years prior, or bulldozed by the construction workers. I should note at this point that Merrill himself mentioned that A. confusa was "common", but this was in 1910, the year he described the species.

Above: The parasitic and dodder-like Cassytha filiformis is seen here pestering an A. auriculiformis. No A. confusa were observed harboring this parasite. 

Above and below: A stream supports the growth of numerous sedges and grasses, surrounded by Pandanus and medium-sized trees. A rare but dense stand of A. confusa recruits colonizes an open, sandy bank.

Despite the proximity of these three species, no apparent hybrids have been observed, though progenies between A. auriculiformis and A. mangium have been reported in Sabah in 1982. But even with no apparent risk of gene dilution, it is clear that A. confusa is already being crowded out by its more aggressive cousins. It certainly does not seem inconceivable that within a lifetime, one may witness how rare these trees will have become, practically lost in a sea of invaders.

Above: A sun-stressed recruit of A confusa begins its life on this gravelly bed. Anthropogenic activities, from quarrying and road works, to the introduction of competitive species, have pushed the species into near oblivion.

*Acacia merrillii, A. palawanensis, A. pluricapitata, A. pluriglandulosa, A. rugata, A. sulitii, and A. tawitawiensis are now in the genus Senegalia. 

**Okay, botany speak. Skip this if you're not so exposed to the multiverse of madness that is botany. The true acacias are primarily African and consist of prickly trees with the typical mimosoid leaves. The type species for the genus is A. nilotica. Acacia was once a massive genus with members distributed from Africa to Malesia (the archipelagic and tropical Southeast Asia) and Australasia. However, when the genus went for revision, it was realized that there were only a few African acacias (with scattered species in Asia) as compared to those from Australia and neighboring regions where there are more than 900 species. What this meant was that name changes for the 'other' non-African acacias would be required, but that would be ridiculously tedious. Botanists, the lazy bastards that they sometimes are, carved out an easier solution: retain Acacia for the Malesian and Australian species because hey, we've got the numbers, right? And then the TRUE acacias from Africa- remember that the type species is African, A. nilotica- will have to be moved into the new genus Vachellia and the new type species for Acacia was bestowed to the Australian A. penninervis. How the world's botanists agreed to this just confounds me. There must be something within their hallowed walls that are worth sniffing.

References

Maslin, B.R., Seigler, D.S., and Ebinger, J. 2013. New Combinations in Senegalia and Vachellia (Leguminosae, Mimosoideae) for Southeast Asia and China. Blumea 58, 39-44

Merrill, E.D. 1910. Philippine Journal of Science C5:27

Nielsen, I.C. 1992. Mimosaceae. Flora Malesiana ser. 1, 11: 1-126