The very extensive botanical family was known as Leguminosae (aka Fabaceae) and divided into subfamilies including Papilionaceae, which numbers a total of 19000 species, is divided into 640 different genera. The Acacia genus itself contains more than 100 species...and unsurprisingly members of this prodigious group are found all over the planet in mainly tropical locations, with quite a few aggressively spiny trees and bushes in dry-weathered areas. The one I intend to introduce here does not grow within a lush tropical setting with banana trees and brightly colored vines. If you want to enjoy its rather peculiar aspect, you have to go inland Reunion Island and wait until you reach at least 1000 meters in elevation (3000 feet). There you can see the "tamarin des hauts" (Highland tamarind) enjoying fresh air and generous rains happily growing in pastures or by the road to the volcano.
Acacia heterophylla (former Mimosa heterophylla, syn. Acacia brevipes, Acacia xiphoclada and the more modern though discussed Racosperma heterophyllum) is a tree endemic to Reunion Island, one of the three islands which form the Mascareignes archipelago in the southwest Indian Ocean. Endemism is quite common in islands and isolated areas where plants have grown and evolved until they become unique and found in the wild only in those areas. Our tree is called locally "tamarin des hauts" which translates as "Highland tamarind." The tamarind (Tamarindus indica) which unlike its species name expresses comes from Africa has been grown on Reunion for centuries, both for its fruits and timber. Although both species do belong to the same botanical family they do not look much alike and grow in completely different settings, thus the vernacular name remains surprising. Another name used by timber men was "Bourbon oak" in order to give credit to its good quality wood (Bourbon island was the name by which Reunion island was known in former kings times).
Anyway, let us step out of the car and walk closer to the tree. It is a quite handsome plant, reaching up to 20 or 25 meters (60 to 75 feet high) but its rather shallow root system makes it prone to falling when the cyclones start blowing during austral summer. Unsurprisingly it has adapted to such situation and is able to keep growing after hitting the ground, and as a result, the highland tamarind forests have a canopy no higher than 10 to 15m (30 to 45 feet) with contorted trunks and shattered main branches growing at weird angles. When one hikes down to the Mafate cirque from Salazie, the trail goes through a "tamarinaie" which is often filled with mist or fog and the atmosphere of those foggy lying trees is very peculiar.
The name "heterophylla" describes how the leaves show different shapes when juvenile and adult, a feature very common within local endemic species and true nightmare for the botanist. Young leaves are bipinnate (divided twice) with a typical leguminous look but they change as the tree get older, the blade part becomes smaller and smaller until it disappears while the petiole (the leaf's tail) becomes larger and longer. Those leaves reduced to a sole petiole are called "phyllode," they look like blades with a coriaceous touch and deep grooves lengthwise. Bipinnate young leaves can be produced by an old tree when it produces shoots. Sometimes you can see on a tree both sort of leaves and all the in-betweens you may think of ! The inflorescences look somewhat like Acacia dealbata (silver wattle, blue wattle or mimosa), made of tiny pale yellow spheres a few millimeters wide hosting 30 to 40 flowers and held in small clusters. They turn into blackish pods 8 to 10 cm long which contain 5 to 10 shiny black seeds. Those seeds need sun and heat to germinate; if they fall under heavy vegetation they will remain in the ground for years with no chance to grow.
They have adapted to forest fires which used to be more common when the island was void of men and volcanic activity and lightning strikes would set fire every once in a while and burn everything to the ground. When the seeds suffer from high temperatures and bright light they will pop from the ground, making a forest of dwarves which will turn into giants after many years. Thus when one wants to propagate the tree this dormancy has to be considered and thermal shock has to be applied: the seeds are set in a heat-resistant container, then covered with simmering water and left to soak overnight before sowing. Actually this system is used for various tree species which have adapted to frequent bush fires such as the African baobab (Adansonia digitata), the flame of the forest (Delonix regia) and many Australian species. If untreated the seeds can be kept for several decades.
As this tree is found only in highlands--say between a low end at 1000m and a high end at 2500m (3000 to 7500 feet)--where it enjoys cool weather, 11 to 17°C, and generous rains (at least 1500mm per year) with a high humidity level. It enjoys soils on the acid side (which is what we have here with volcanic grounds) and will grow to timber size after 60 to 100 years. This wood is of good quality and has been used a lot for ship building; it is still enjoyed for local small fishing canoes. It is also used in carpentry and for the traditional "bardeaux" or wood slabs which cover roofs and walls in typical creole houses. Those bardeaux were common use in the past when people had time (and knowledge!) to cut them by hand with a small ax, today there are only two people who make them on the whole island and the cost is quite high!
An interesting point regarding the "tamarin des hauts" is that it closely resembles a tree called koa (Acacia koa) which is endemic to...Hawaii! More than 16,000km or 10,000 miles between the two islands, no sea currents nor winds to connect both place, how is this possible ? As a matter of fact some recent genetic works have shown that both tree species have evolved from a common ancestor (blackwood or Acacia melanoxylon) born in Australia, half-way between Reunion and Hawaii. Seeds were brought over by cyclones, birds stomaches or water and evolved each in its own environments.
So, when will you come over here and enjoy those unique trees ? We could hike to "Le Coin Tranquille," where I know a nice little enchanted forest? I guarantee you'll love it!