Mahajanga’s ‘Grand Baobab’

The ‘Grand Baobab’ of Mahajanga (Majunga), the old port on the west coast of the island, is an important symbol of the city and the region. Mahajanga has been a major commercial port since 1745 and was likely an important port for Arab dhow traffic long before that. The Grand Baobab is situated where two wide waterfront boulevards meet a major avenue and functions as the centre of a large traffic roundabout. It appears to be a major focus of the city with people almost always sitting under it or sitting on the sea wall watching the changing light in the cool afternoon sea breeze. One afternoon we saw three wedding parties noisily honking their way down the avenue and around the tree, stopping for what appeared to be obligatory photos in front of the tree. A symbol of longevity?

Sarah and I measured the circumference in 1979 as 20.6 m (breast height). Mischa and I measured it again this December as 20.7 m. Not much growth in 32 years! Is there a maximum size for such a tree under given climate conditions or does the pavement (blacktop) and auto traffic tell part of the story? We found an old postcard in the museum which recorded the diameter in 1898 as 14.6 m, so there had been significant growth in the 80 odd years before our first visit.

The ‘Grand Baobab’ is said to be about 700 years old, though I doubt that anyone has scientifically aged it. It is not the only large baobab in Mahajanga. Another impressive Adansonia digitata is to be found in a private yard behind a bank in the commercial centre of town. It was in full bloom when we were there, the flowers busily visited by honey bees for both pollen and nectar. Other trees of different ages can be found at various locations around town. They all appear to be the African species, A. digitata. Analysis of the chloroplast DNA indicates that these Malagache digitata are genetically identical to those of eastern and southern Africa, so they were probably brought by seafaring traders across the Mozambique Channel.

There are apparently some much larger and older A. digitata farther north on the Analabe peninsula that carry a mutation which indicates that they originated from baobabs whose fruit floated over from Africa before the island was populated by Homo sapiens. This would indicate that there have been at least three introductions of baobabs to Madagascar over the millennia. I would love to see these remarkable trees in their setting which is apparently largely undisturbed. Perhaps I’ll be able to organize it on some future visit to Madagascar.

Chloroplast genetics work conducted by CIRAD scientists strongly support the hypothesis that baobabs originated in West Africa and spread across the continent when suitable climatic conditions existed, evolving slightly different sub groups as populations became isolated by unsuitable climatic conditions. At some point the ancestor or ancestors of the Madagascar baobabs drifted over the channel and, in the new island environment, were subject to the phenomenon of adaptive radiation that lead to the 6 indigenous species found there today. I’ll discuss the whole question of island biogeography and baobabs in a future contribution.

Photos

‘Le Grand Baobab de Mahajanga’

Social life at the Grand Baobab

Distribution of A. digitata chloroplast DNA halplotypes (from Pock Sty et al.)

The baobabs of northern Madagascar

The rarest baobab tree in the world is Adansonia perrieri. Satellite mapping by CIRAD indicates that there are fewer than 200 of these trees known to exist. We hiked for 2 1/2 hours on slippery leech infested trails in Amber Mountain National Park to see two trees beside the Antomboka River in the humid tropical montane forest. The very last place one would expect to find a tree whose relatives grow in the dry savanna of Africa. We were fortunate to find one of these trees in bloom. Large yellow flowers that look just like a banana peel (not the tasteless large bananas in North American markets, but the small sweet ones that people eat in the tropics). The staminal tube is very long and the anthers quite short. Clearly a member of the longitubae group. We found only two fruit – floating in the river. Is this because very few fruit ever set or because they are picked up (illegally) by local farmers taking a shortcut through the park. In spite of growing in a natural, undisturbed forest environment we could find no evidence of regeneration in this species. Was it a dead-end evolutionary side track? So few individuals spread over a wide area doesn’t give much hope for a sex life!

We were able to see one other perrieri just north of Ambondrofehy between Anatalaka and Anivoranu on Route 6 not far from Akarana National Park. This one is in a highly degraded tropical forest environment on a river bank. Again, no sign of fruit or of regeneration. A large injury on the lower trunk which has rotted about halfway through. Not much longer a living tree, I suspect.

Often recorded as the most widespread baobab of Madagascar is the appropriately named Adansonia madagascarensis. Without seeing the flower, it is difficult to distinguish from A. za, which is in fact the only species found from north to south on the west coast. We saw healthy reproducing populations of A. madagascarensis in natural tropical deciduous forest in Ankarana Park and in degraded forest on French Mountain. The substrate was always coarse broken limestone. Since the deciduous forest was freshly in leafy profusion, it was difficult to photograph the tall baobabs which towered over the diversity of surrounding trees. None was blooming when we were there.

The other species confined to the north of the country is A. suarezensis, named presumably after the colonial naval port city of Diego Suarez, now known as Antsirarana. This species has very restricted distribution in the north of the country. We were unable to visit the sites west of Antsirarana where it is supposed to be most numerous due to the rains making the roads impassible in January. “Well, we might be able to get there, but then not be able to get back.” We found a few individuals of suarezensis at French Mountain in the degraded forest next to madagascarensis and on the same coarse substrate. There was no sign of regeneration of this species there except where it had been planted in front of the now closed King Lodge resort. Since the large botanical garden planted by the owners was also closed, we were unable to see how successful the attempts were to re-establish a healthy population there. Suarezensis is a really beautiful tree with a smooth golden red bark and regal stature. It’s a shame we didn’t get to see it in a happier environment.

A. perrieri on Antomboka River

Flower of A. perrieri

madagascarensis in Akarana forest

suarezensis at French Mountain

The Morondava Baobabs

Three species of baobab are found in the dry forest north of Morandava on the west coast of Madagascar. We entered this area from the north, having taken pirogue (dug-out canoe) down the Tsiribihinia River from Mirandrivazo. On the third day on the river we entered baobab country. The locals call this tree ‘za’ and it was given the scientific name Adansonia za. What a change! To be using an indigenous name rather than the name of some foreign botanist. The tree stands tall above all of the other forest vegetation, though this is probably because the indigenous forest has been so degraded by years of slash and burn agriculture along the river, a major transportation route. The agriculturalists appear to leave the large baobabs alone. They are a lot of work to cut and the wood is soft and spongy so of no value for timber or charcoal. Their thick bark also appears to resist fire well. I presume that these conditions would not be conducive to regeneration, however. There are no young baobabs evident.

After coming off the river we spent a night at a ‘hotel’ of little thatched bungalows in the town of Antsiraraka. The town is on a baobab strewn hill surrounded by rice paddies. The hotel bar is built around the base of a baobab which was in full bloom every night. Beautiful yellow sweet smelling flowers which fell off in the morning, providing enticing munchies for the foraging goats. These gorgeous one night wonders are probably pollinated by long-tongued hawk moths, though I was not able to see any activity by moonlight. The fallen flower consists of a 8 to 9 cm staminal tube with about 100 attached anthers and the curled back sepals. There was often still copious sweet nectar at the base. The Madagascar baobabs are divided into two groups, longitubae and brevitube, based on the length of the staminal tube. A. za is clearly a longitubae. Perhaps an example of punctuated equilibrium (more on the theory later).

The next baobab we encountered was Adansonia grandidieri, named after a French botanist who documented a lot of the flora of Madagascar. The locals call it reniala or renala. It is a brevitubae with flowers that look like a shaving brush with many hundreds (a thousand?) of stamens. It is apparently visited by many mammals (including lemurs and bats) as well as insects such as moths and bees. Research is underway to identify which of these visits actually result in pollination. Graduate students climb the trees during flowering and spend the night watching the nocturnal visitors. This is the remarkable tall baobab of the ‘Avenue des Baobab’ and the tourism pamphlets of Madagascar. It’s fruit is for sale in the market and along the roadside.

Growing very much in the same area as the reniala is the ‘fony’, known scientifically as Adansonia rubrostipa, another longitubae species, but finished flowering recently. We were fortunate to make contact with Sehenu, a graduate student studying the dispersal of baobabs who took us into a protected forest study area where we saw all ages of this species, including lots of promising regeneration. The question of seed dispersal in Malagache baobabs has long been an area for botanical/ecological speculation since there appear to be no animals (other than recently arrived humans) large enough to carry the fruit. Sehenu is trying to get some facts. Imagine feeding baobab fruit to a giant tourtoise and then checking the scat daily for two weeks until the seeds finally came out the other end. Then collecting the seeds and planting them to see if they germinate. Lots of other animals get fed baobab fruit by Sehenu. But perhaps the fruit doesn’t have to be carried anywhere. If they break open and the seeds are consumed by smaller animals, dispersal could happen.

All three species were fully leafed out in a forest of leafless trees.The dearth period in the tropics is often the dry season, when plants lose their leaves so as not to suffer water deficit. Winter of the tropics, so to speak. The rains are late this year and the forest is a very hot, dry place to be. The baobabs, however, leaf out in anticipation of the coming rains – a head start start using water stored in the enormous spongy trunk.

Next stop, the northern baobabs.

Adansonia za
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Adansonia za flower

Adansonia grandidieri at ‘Allee des Baobab’ (no forest left)

Adansonia rubrostipa in natural forest (dry season)

Natural regeneration of A. rubrostipa

Sometimes called the ‘upside-down tree’, the baobab tree is usually identified with the continent of Africa. One species, Adansonia digitata, is wide-spread on the African continent. Another species, Adansonia gibbosa is found in a limited area of Northern Australia. The island of Madagascar has six indigenous species (A. madagascarensis, A. perrieri, A. suarezensis, A. grandidieri, A. rubrostipa, and A. za) as well as the mainland African one.

The obvious question for a biologist is ‘Why?’ Why does the ecologically diverse continent of Africa have only one species while the comparatively small island of Madagascar has six unique species. It is easy to explain the existence of one species in Australia – somehow, probably by ocean currents, a baobab fruit came ashore millennia ago and, isolated from others of its species and exposed to unique selection forces, evolved into a new one. It is easy to explain the existence of A. digitata species in Madagascar; it was almost certainly taken by seafaring traders as it was to many other places, including India. But how did six different species evolve so close to each other in Madagascar? What were the isolating mechanisms that triggered this speciation?

It is a question that has baffled biologists for ages and has kept me musing since I first went to Madagascar in 1974. At that time I was studying the honeybees of Madagascar. (People look at you in a quizzical way when you wander around the market catching bees on the wing and putting them into a little vial – weird foreigners! When the bees suddenly quit coming to collect flour it is time to leave lest you be accused of magic.)

Another issue that has interested me as a community resource planner and an instructor of environmental studies is how to involve the residents of an area in the conservation of species. What are the values, economic or cultural, which would cause people to protect their baobab trees? What is the role of parks and conservation areas in protecting these rare species?

After nine years in Africa, I returned to Canada in 1980 and became tied up as an educator in the academic world. Now retired, I am free to follow up on my baobab fancies again.

There is a lot more information available on baobabs today. We are finally clear on just how many species there are. We have a much better picture of their distribution. Data on their chromosomes give us some clues to their evolutionary beginnings.

In Iate 2011 I will make my third trip to Madagascar. I plan to see all of the baobab species, work with researchers in the country, and post blog updates including photos.

Here is a photo of Sarah and I in 1979 at ‘Le grand baobab’ in Majunga and a photo of some of these marvelous tree in the south of Madagascar.