Pachypodium habitats
The study of detailed scattered, specialized, micro-environmental habitats of Pachypodium species are often "indifferent" to the greater, regional ecological, biotic zone of vegetation. This "indifference" explains Pachypodium morphology and the architecture. The large scale vegetation zones are in a lot of cases, though not all, irrelevant to the micro-environments of Pachypodium species within continental Southern Africa and Madagascar.
Most Pachypodium species occur on rocky outcrops, steep hills, and on "inselbergs." An inselberg is a mountain or rocky mass that has resisted erosion and stands isolated in an essentially level area, sometimes above a forest canopy. Rocky outcrops, steep hills, and inselbergs create microclimate conditions that are quite different from the more general climate of the region.
These habitats of Pachypodium are thought as "arid islands," even when they occur in "prehumid zonobiomes" because the taxon's topographic position and microclimate conditions differ significantly from the context of the greater ecological conditions within the landscape. The "prehumid zonobiomes" are humid zones of regional biotic community characterized chiefly by the dominant forms of plant life and prevailing climate, such as forests or montanes.
On these "arid islands" the flora significantly differs from the greater surrounding zones of vegetation, where a smaller immediate area under the influence of a micro-environmental condition is defined. In larger areas of vegetation, the isolation of these plants in these "arid lands" become very intense during prolonged periods without rain.
The plants on outcrops, steep hills, and inselbergs are subjected to fluctuating moisture, high winds, and extremes in temperatures. This exposure occurs because, in the case or outcrops and inselbergs, all plants typically are growing in fissures that very often, but not always, have shallow soil and a exposed situation within the landscape.
Only plants with special adaptations to extreme drought can survive exposed rocky habitats. Plants with special morphological adaptations produce plants that can bear the intensity of these arid, xeric, exposed micro-environments. Pachypodium employ two mechanisms to adapt to these conditions:
- Spinescent: The presence of spines that collect moisture in fogs and dews and point downward so that moisture falls to the immediate ground surrounding the plants.
- Pachycaule Trunks: The use of an abnormally thickened trunk or stem of various shapes to store water in times of drought.
In some instances, the pachycaule trunks are geophytic and exist underground as is the case for Pachypodium bispinosum and Pachypodium succulentum.
On outcrops and inselbergs, Pachypodium species root in the cleft, fissures, and crevices of these rocky formations. The non-succulent roots penetrate deeply into the acuminated 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 acuminated fine soil in the cracks, which, in turn, retain water. In these conditions, Pachypodium can store enough water in their trunk to easily withstand a dry period of five or more months.
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 exception 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 of 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 either deep or shallow sand with laterite host 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 it is taken up easily and there is less evaporation except for the top layer. Very deep sand bodies present yet another problem--seepage. Whereas shallow sandy soils do not dry out easily, if water acuminates on deeper yet 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.
Rapanarivo et al sampled only half of the sites in Madagascar for pH level reading. That is not to say that only half the species in Madagascar were tested. The results show no significant difference in the soil type pH Level recorded in literature.
Pachypodium have a pH range from strictly acid soils with a pH Level of 3.5 to 5 to neutral to Alkaline soils at a pH level of 7 to 8. Species growing on gneiss, granite, and quartzite adapt to acidic soils. Species preferring a pH level of 3.5 to 5 are Pachypodium brevicaule, P. cactipes, P. densiflorum, P. eburneum, and P. rosulatum. The species growing on calcareous, limestone, for instance, adapt to a basic substrate. Species growing in acid to almost basic soil that have a pH level between 4.5 to 7 are P. lamerei and P. rutenbergainum. P. meridionale grows in neutral soils. And, some species tolerate both acidic and basic soil conditions. P. sofiense can be found in either soil condition. (For species that grow in only one type of soil pH condition maintaining that "simulacrum" of acidity or alkalinity is crucial to success in cultivation.)
Within the "arid islands" or micro-environments, the difference between vegetative type acidity and immediate acidity of the surrounding biotopes of Pachypodium seem to be a signification factor in defining habitat type.
There is no "fossil" records of Pachypodium. This does exclude an analysis to determine common ancestry and current relationships between taxa.
Yet certain conclusions can be drawn from the geology of the landscape itself as to the past natural history of Pachypodium.
Geological history safely demonstrates that Pachypodium and other genera like Aloe, Euphorbia, Cissus, Sesamothamnus, Kalanchoe, and Adansonia existed before the separation of Madagascar from continental Southern Africa. Pachypodium and these other genera, for instance, are represented on both the Island and the Mainland, suggested that their populations were once continuous within the landscape before the separation of Madagascar form Africa about 65 million years ago in the Cretaceous period.
The diversity of Pachypodium in Madagascar, as noted, is the result of accelerated evolution that occurs in xeric climates and dry landscapes. Three factors attribute to the acceleration:
- In dry climates, the diversity of geology and topology is thought to have a greater effect upon plants than in areas with high rainfall.
- The broken geological formations of locally xeric landscapes tend to break up populations into smaller groups so that each group can initially
interbreed but with time new genotypes, taxa, or species develop.
- Taxa develop specialized xeromorphoric structures at some architectural level for which the alliance "succulents" are a good example; and where dew and fog dripping spines are another example at the level of an organ.
Therefore, the exceptional micro-endemism (native or confined to a certain habitat) occur in Madagascar as a result of isolation of flora in very different climates, landscapes, or environments at a exceptionally small scale. Pachypodium have proven to be no different. The scale is so small that it is thought that, in some instances, the resolution of speciation of this flora is limited to just a single outcrop of granite, for instance.
Efforts at maintaining possible habitats must be weighed with the potential for the growth of the Malagasy people. Conservation is a high priority, dependent upon an accurate catalogue of species and equally an understanding of the potential habitats of Pachypodiums yet to be discovered in Madagascar.
External Resources
- Rapanarivo, S.H.J.V., Lavranos, J.J., Leeuwenberg, A.J.M., and Röösli, W. Pachypodium (Apocynaceae): Taxonomy, habitats and cultivation "Taxonomic revision of the genus Pachypodium", S.H.J.V. Rapanarivo and J.J. Lavranos; "The habitats of Pachypodium species" S.H.J.V. Rapanarivo; "Cultivation" W. Röösli. (A.A. Balkema: Rotterdam, Brookfield, 1999) [Rapanarivo et al.]
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