Soils and Rooting Characteristics
On outcrops and inselbergs, Pachypodium species root in the clefts, fissures, and crevices of these rocky formations. The non-succulent roots penetrate deeply into the accumulated soil and humus in these crevices. On these geological formations, cracks in the rocks will fill quickly with water that can penetrate quite deeply. Under these conditions, there is very little evaporation so that almost all the collected water remains. Therefore, rocky substrates provide moisture in the habitat, so long as there is not appreciable runoff from the rock surfaces and there is plenty of accumulated fine soil in the cracks, which, in turn, retains water. In these conditions, Pachypodiums can store enough water in their trunks to easily withstand a dry period of five or more months.
Inselbergs are quite common in Angola extending into Namibia, Zimbabwe, and western Madagascar. Inselbergs have been demonstrated to support a plant community dramatically different from their surroundings (Rapanarivo,1999a). What little is known about seed dispersal of Pachypodium supports a scattering effect confined to sets of outcrops and inselberg landscapes. Pachypodium seed dispersal is carried out by wind, suggesting the scattering target as outcrops and inselbergs. Yet, according to Rapanarivo et al., this type of dispersal might not be the best method for the genus. The Rapanarivo study suggested, instead, that seedlings tended to emerge around "mother" plants that presumably have been well-established, rather than on distant outcrops or inselbergs. Evidence for this conclusion is found in the occurrence of P. densiflorum from Kandreho to Zazafotsy where seeds have dropped in between inselbergs and outcrops. Other examples include P. eburneum, P. windsorii, P. inopinatum, and P. decaryi where in all cases seed distribution is restricted; because the wind does not always carry seeds very far from the host plant.
The substrate plays a critical role in the creation of micro-environmental "arid islands." It has been recorded, for instance, that vegetation on rocks exposed to the sun may reach temperatures of 50°C (122°F) to 60°C (140°F), an almost lethal exposure. A black colored rocky substrate tends to be the hottest in these micro-environments. Yet, even sandstone is not immune to this thermal condition as it, too, can reach 60°C (140°F) by day. This factor in the micro-environmental conditions of Pachypodium causes many plants to occupy fissures on these rocks where soil and humus has collected.
Other substrates encountered by Pachypodium include: (in Madagascar) Mesozoic limestone, granite, gneiss, sandstone, quartzite, sand, schist, Tertiary calcareous, sandy loam, basalt, and sandy soil; And (in continental southern Southern Africa) quartzite, sandstone, clay, gravel, sandy soil, dolomite, granite schist, basalt, limestone, rhylite, sand and stone, and dolerite.
A substrate of variable depth sand with laterite hosts a number of Pachypodium, such as P. rutenbergainum, P. bispinosum, P. geayi, P. lamerei, P. namaquanum, P. rosulatum, P. saundersii, and P. succulentum. Laterite is a red residual soil in tropical and subtropical regions that is leached of soluble minerals, aluminum hydroxides and silica, but still contains concentrations of iron oxides and iron hydroxides.
Sand can readily store water because of its high percolation rate. Very deep sand bodies present yet another issue: seepage. If water accumulates within deeper impermeable substrate, Pachypodium can gain a footing in the sandy soil type. Yet in a sense both shallow and deep sand substrates has water available to Pachypodium. With shallow sand substrates, Pachypodium grow on sand dunes close to the sea. Examples include Pachypodium geayi near Tuléar, Madagascar and northwest and west coastal regions for Pachypodium rutenbergianum. Often in shallow sandy areas, the water table is high so that Pachypodium send out long roots systems.
Where water is in a deep, sandy substrate, Pachypodium grow on sand over laterite red soil. Laterite is relative impermeable and thus traps water. Provided that the sand is not too deep, a water source is available to Pachypodium rosulatum and Pachypodium rutenbergainum near Antsohihy and Ankarafantsika, Madagascar. In Anjajavy Forest and other sites within the Madagascar dry deciduous forests Pachypodium thrive growing above the limestone tsingy.
Read more about this topic: Pachypodium Habitats
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