Pachypodium habitats
Encyclopedia
Pachypodium habitats consist of isolated, specialized, micro
–environmental
niche
s, generally xeric, rocky, frost-free areas within parts of western Madagascar
and southern Africa. Pachypodium
species
are often indifferent to the regional ecological
, biotic
zone of vegetation
, a fact which explains some of Pachypodium morphology
and architecture
. The large scale vegetation zones are in some cases irrelevant to the micro-environments of Pachypodium, in the sense that the xeric niches may be embedded in larger mesic
biome
s.
Most Pachypodium are rupicolous species occur on rocky outcrops, steep hills, and on inselbergs or kopjes, land or rocky masses which have resisted erosion
and stand isolated in level or gently sloping terrain, sometimes above a forest canopy. Rocky outcrops, steep hills, and inselbergs create microclimate
conditions that may be different from the general climate
of a region.
The habitats of Pachypodium are thought as arid
ecological, 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 mountains. On these ecological 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 islands" become very conspicuous during prolonged periods without rain.
and are exposed within the landscape. Pachypodium typically grows in frost-free areas with the exception of P. bispinosum and P. succulentum. The latter two species can sustain winter temperatures reaching into -10°C).
The regions where Pachypodium biotypes exist are (a) situated between sea-level and 2000 meters and (b) are defined by the effects of temperatures. Without exception, all Pachypodium share in a preference for sunny, xeric habitats. Their exclusion from humid forests is almost entirely the case except where "inselbergs" surge above the humid, forest canopy: an example of Pachypodium response to micro-environments than larger vegetative types.
Only plants with special adaptation
s to extreme drought can survive exposed rocky habitats. Plants with special morphological adaptations produce offspring that can bear the intensity of these arid, xeric, exposed micro-environments. Pachypodium employs two mechanisms to adapt to these conditions:
In some instances, the pachycaule trunks are geophytic and exist underground as is the case for Pachypodium bispinosum and Pachypodium succulentum.
so that almost all the collected water remains. Therefore, rocky substrate
s 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.
s. In some cases, some species inhabit only one substrate. Pachypodium ambongense, P. decaryi, and P. windsorii grow exclusive on calcareous rocks
. P. brevicaule and P eburneum are contained to quartzite
. Other cases demonstrate that species can grow in multiple substrates. P. cactipes, P. densiflorum, and P. horombense dwell mainly on gneiss
and granite
. P. rutenbergianum grows in various substrates, but particularly on sand
and laterite
. Sand provides a substrate for P. geayi and P. rosulatum. In African, P. namaquanum can grow on three substrates: granite, quartzite, and sand. The other species likewise can be found on myriad, various substrates: limestone
, granite
, gneiss
, sandstone
, quartzite
, sand
, schist
, tert calcareous, sandy soils, basalt
, and sand
y loam
in Madagascar. In Africa, the substrate ranges from: quartzite, sandstone, clay
, gravel, sandy soil, dolomite
, granite, schist, basalt, limestone, rhylite, and dolerite. The more adaptable the taxon is to substrate seems to be indication of how specialized the species is within its habitat in the landscape and climate.
level reading. 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 significant factor in defining habitat type.
levels for the soil
in half the sites of Pachypodium endemic to Madagascar. The pH ranges approximately from 3.5 to 7. Strictly acid
soil
s that had a pH level of 3.5 to 5 were preferred by Pachypodium brevicaule, P. cactiples, P. densiflorum, P.eburneum, and P. rosulatum. The next level, acid to basic soils, with a pH between 4.5 and 7 were suitable for P. lamerei and P. rutenbergianum. On soils with a neutral pH
level of 7, P. meridionale inhabit. P. sofiense grows on calcareous soils but also in acid soils with a pH level of 4. P. lealii grows on outcrops of granite in fairly fertile
, acid soil.
are known to inhabit areas from sea level for the species
P. bispinosum to 1600 meters for P. lealii. In Madagascar, the range is roughly the same with P. cactiples, P. geayi, and P. rutenbergainum inhabiting at sea level to P. brevicaule reaching an altitude of 1900 meters, virtually the uppermost limit for the genus
.
with a heat regime resembling greater amplitude
s than those of Madagascar. There, in the central part where most species come from, frost occasionally occurs. One station there has recorded a temperature of -2.6°C (27°F) whereas another has recorded a low of -6.3°C (21°F). In the sub–arid regions of Madagascar, the maximum temperature recorded can climb up to 40°C (104°).
receives annually less than 100 mm (3.94 inches) but the fog coming off the Atlantic Ocean plays a significant role in maintaining plant diversity
. Madagascar, on the other hand, can receive precipitation from 344 mm (13.54 inches) annually for the regional habitats of P. geayi, P. lamerei, and P. meridionale to 1985 mm (78.15 inches) for P. baronii, P rosulatum, and P. rutenbergianum. These lower values of precipitation apply to the spiny desert and Madagascar dry deciduous forests
while the higher rainfall regimes apply to the (eastern) lowland rainforests. There is little commonality in precipitation regimes for Pahypodium. Having said that, a precipitation regime for a species of Pachypodium depends upon a habitat's location relative to the influences of the Atlantic and India Oceans and the various mountain ranges and open expanses of southern continental Africa and the island of Madagascar.
One common thread holds true for the genus: all Pachypodium are succulent plants making use of their trunks and their spines to inhabit dry, xeric climates.
flora of continental Africa and Madagascar. The genus grows in areas where there are significant periods of dry months. This dry period can vary from 5 months for Pachypodium brevicaule to 10 or more months for P. cactipes, P. geayi, P. lamerei, and P. meridionale. It would seem likely that the Atlantic and India Oceans pay a big role in the creation of weather conducive to rainfall. The effect of mountains might also affect the localized conditions of the climate for Pachypodium.
records of Pachypodium, a fact that does exclude analysis to determine common ancestry and current relationships between taxa. Yet certain conclusions can be drawn from the geology
of the landscape itself to the past natural history
of Pachypodium. Geological history 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 Madagascar and the mainland, suggesting that their populations were once continuous within the landscape before the Gondwanaland contental separation 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 contribute to the acceleration:
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 an 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 economic development of the Malagasy people. Conservation may become 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.
Micro
Micro is a prefix in the metric system denoting a factor of 10-6 . Confirmed in 1960, the prefix comes from the Greek , meaning "small".The symbol for the prefix is the Greek letter μ...
–environmental
Natural environment
The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....
niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
s, generally xeric, rocky, frost-free areas within parts of western Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
and southern Africa. Pachypodium
Pachypodium
Pachypodium is a genus of succulent spine-bearing trees and shrubs, native to Africa. It belongs to the dogbane family, Apocynaceae. Pachypodium comes from a Latin form from Greek pachus and podion , hence meaning thick-footed.-Genus characteristics:All Pachypodium are succulent plants that...
species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
are often indifferent to the regional ecological
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
, biotic
Biotic component
Biotic components are the living things that shape an ecosystem. A biotic factor is any living component that affects another organism, including animals that consume the organism in question, and the living food that the organism consumes. Each biotic factor needs energy to do work and food for...
zone of vegetation
Vegetation
Vegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader...
, a fact which explains some of Pachypodium morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
and architecture
Architecture
Architecture is both the process and product of planning, designing and construction. Architectural works, in the material form of buildings, are often perceived as cultural and political symbols and as works of art...
. The large scale vegetation zones are in some cases irrelevant to the micro-environments of Pachypodium, in the sense that the xeric niches may be embedded in larger mesic
Mesic
Mesic may refer to:* Mesic, North Carolina, a town in the United States* Mesic habitat, a type of habitat...
biome
Biome
Biomes are climatically and geographically defined as similar climatic conditions on the Earth, such as communities of plants, animals, and soil organisms, and are often referred to as ecosystems. Some parts of the earth have more or less the same kind of abiotic and biotic factors spread over a...
s.
Most Pachypodium are rupicolous species occur on rocky outcrops, steep hills, and on inselbergs or kopjes, land or rocky masses which have resisted erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
and stand isolated in level or gently sloping terrain, sometimes above a forest canopy. Rocky outcrops, steep hills, and inselbergs create microclimate
Microclimate
A microclimate is a local atmospheric zone where the climate differs from the surrounding area. The term may refer to areas as small as a few square feet or as large as many square miles...
conditions that may be different from the general climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
of a region.
The habitats of Pachypodium are thought as arid
Arid
A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant and animal life...
ecological, even when they occur in prehumid zonobiomes because the taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
's topographic
Topology
Topology is a major area of mathematics concerned with properties that are preserved under continuous deformations of objects, such as deformations that involve stretching, but no tearing or gluing...
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 mountains. On these ecological islands the flora significantly differs from the greater surrounding zones of vegetation
Vegetation
Vegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader...
, 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 islands" become very conspicuous during prolonged periods without rain.
Environmental conditions
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 often have shallow soilSoil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
and are exposed within the landscape. Pachypodium typically grows in frost-free areas with the exception of P. bispinosum and P. succulentum. The latter two species can sustain winter temperatures reaching into -10°C).
The regions where Pachypodium biotypes exist are (a) situated between sea-level and 2000 meters and (b) are defined by the effects of temperatures. Without exception, all Pachypodium share in a preference for sunny, xeric habitats. Their exclusion from humid forests is almost entirely the case except where "inselbergs" surge above the humid, forest canopy: an example of Pachypodium response to micro-environments than larger vegetative types.
Only plants with special adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
s to extreme drought can survive exposed rocky habitats. Plants with special morphological adaptations produce offspring that can bear the intensity of these arid, xeric, exposed micro-environments. Pachypodium employs two mechanisms to adapt to these conditions:
- SpinescentThorns, spines, and pricklesIn botanical morphology, thorns, spines, and prickles are hard structures with sharp, or at least pointed, ends. In spite of this common feature, they differ in their growth and development on the plant; they are modified versions of different plant organs, stems, stipules, leaf veins, or hairs...
: The presence of spines that collect moisture in fogs and dews and point downward so that moisture falls to the ground at the base of 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.
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 evaporationEvaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
so that almost all the collected water remains. Therefore, rocky substrate
Substrate (biology)
In biology a substrate is the surface a plant or animal lives upon and grows on. A substrate can include biotic or abiotic materials and animals. For example, encrusting algae that lives on a rock can be substrate for another animal that lives on top of the algae. See also substrate .-External...
s 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
Angola
Angola, officially the Republic of Angola , is a country in south-central Africa bordered by Namibia on the south, the Democratic Republic of the Congo on the north, and Zambia on the east; its west coast is on the Atlantic Ocean with Luanda as its capital city...
extending into Namibia
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
, Zimbabwe
Zimbabwe
Zimbabwe is a landlocked country located in the southern part of the African continent, between the Zambezi and Limpopo rivers. It is bordered by South Africa to the south, Botswana to the southwest, Zambia and a tip of Namibia to the northwest and Mozambique to the east. Zimbabwe has three...
, and western Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
. Inselbergs have been demonstrated to support a plant community dramatically different from their surroundings (Rapanarivo,1999a). What little is known about seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
dispersal of Pachypodium supports a scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
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
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
, granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
, gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
, sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
, quartzite
Quartzite
Quartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...
, sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
, Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
calcareous
Calcareous
Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...
, sandy loam
Loam
Loam is soil composed of sand, silt, and clay in relatively even concentration . Loam soils generally contain more nutrients and humus than sandy soils, have better infiltration and drainage than silty soils, and are easier to till than clay soils...
, basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
, and sandy soil; And (in continental southern Southern Africa) quartzite, sandstone, clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
, gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
, sandy soil, dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
, granite schist, basalt, limestone, rhylite, sand and stone, and dolerite.
A substrate of variable depth sand with laterite
Laterite
Laterites are soil types rich in iron and aluminium, formed in hot and wet tropical areas. Nearly all laterites are rusty-red because of iron oxides. They develop by intensive and long-lasting weathering of the underlying parent rock...
hosts a number of Pachypodium, such as P. rutenbergainum, P. bispinosum, P. geayi
Pachypodium geayi
Pachypodium geayi is a species of Pachypodium originated from Southwest Madagascar. It has a metallic grey pachycaule trunk. Leaves are thin and grey-green, with a bright pink mid-rib. The plant has white flowers. Pachypodium geayi is one of the largest of the Madagascar species....
, P. lamerei
Pachypodium lamerei
Pachypodium lamerei is a species of Pachypodium. It has large thorns and leaves mostly just at the top of the plant. It is a stem succulent and comes from the island Madagascar. The plant bears large, fragrant flowers. The species has become one of the best known pachypodiums in cultivation,...
, P. namaquanum
Pachypodium namaquanum
Pachypodium namaquanum Welw. is a succulent single-stemmed plant growing to 4 metres tall in the arid, rocky mountains of the Richtersveld in the Northern Cape and southern Namibia. The warty trunk, thickset at the base and tapering to the top, is densely covered in sharp spines. Where damaged,...
, P. rosulatum
Pachypodium rosulatum
Pachypodium rosulatum, common name Elephant's Foot Plant, belongs to the dogbane family Apocynaceae.-Description:Pachypodium rosulatum is a shrubby perennial caudiciform plant with a bottle-shaped trunk, brownnish silver and almost spineless, about wide and about tall. From the caudex depart many...
, P. saundersii
Pachypodium saundersii
Pachypodium saundersii, the Kudu Lily, is a succulent plant belonging to the Apocynaceae family.It is found naturally in Southern Africa, on the Lebombo Mountains and other areas in KwaZulu-Natal, Mpumalanga and Swaziland....
, 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
Anjajavy Forest
Anjajavy Forest is an element of the Madagascar dry deciduous forests situated on the Indian Ocean of northwest Madagascar. The Anjajavy Forest surrounds the village of Anjajavy and provides a habitat for many rare and endangered species. It covers roughly fifty square kilometres, and occupies a...
and other sites within the Madagascar dry deciduous forests
Madagascar dry deciduous forests
The Madagascar dry deciduous forests represent a tropical dry forest ecoregion generally situated in the western part of Madagascar. The area has high numbers of endemic plant and animal species but has suffered large-scale clearance for agriculture...
Pachypodium thrive growing above the limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
tsingy.
Substrate
Pachypodium grows in various types of substrateSubstrate (biology)
In biology a substrate is the surface a plant or animal lives upon and grows on. A substrate can include biotic or abiotic materials and animals. For example, encrusting algae that lives on a rock can be substrate for another animal that lives on top of the algae. See also substrate .-External...
s. In some cases, some species inhabit only one substrate. Pachypodium ambongense, P. decaryi, and P. windsorii grow exclusive on calcareous rocks
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
. P. brevicaule and P eburneum are contained to quartzite
Quartzite
Quartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...
. Other cases demonstrate that species can grow in multiple substrates. P. cactipes, P. densiflorum, and P. horombense dwell mainly on gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
and granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
. P. rutenbergianum grows in various substrates, but particularly on sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
and laterite
Laterite
Laterites are soil types rich in iron and aluminium, formed in hot and wet tropical areas. Nearly all laterites are rusty-red because of iron oxides. They develop by intensive and long-lasting weathering of the underlying parent rock...
. Sand provides a substrate for P. geayi and P. rosulatum. In African, P. namaquanum can grow on three substrates: granite, quartzite, and sand. The other species likewise can be found on myriad, various substrates: limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
, granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
, gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
, sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
, quartzite
Quartzite
Quartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...
, sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
, tert calcareous, sandy soils, basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
, and sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
y loam
Loam
Loam is soil composed of sand, silt, and clay in relatively even concentration . Loam soils generally contain more nutrients and humus than sandy soils, have better infiltration and drainage than silty soils, and are easier to till than clay soils...
in Madagascar. In Africa, the substrate ranges from: quartzite, sandstone, clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
, gravel, sandy soil, dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
, granite, schist, basalt, limestone, rhylite, and dolerite. The more adaptable the taxon is to substrate seems to be indication of how specialized the species is within its habitat in the landscape and climate.
Soil chemistry
Rapanarivo et al. sampled only half of the sites in Madagascar for pHPH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
level reading. The results show no significant difference in the soil type pH Level recorded in literature.
Pachypodium have a pH range from strictly acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
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
Simulacrum
Simulacrum , from the Latin simulacrum which means "likeness, similarity", was first recorded in the English language in the late 16th century, used to describe a representation, such as a statue or a painting, especially of a god...
" 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 significant factor in defining habitat type.
Soil pH
Rapanarivo et al. measured the pHPH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
levels for the soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
in half the sites of Pachypodium endemic to Madagascar. The pH ranges approximately from 3.5 to 7. Strictly acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
s that had a pH level of 3.5 to 5 were preferred by Pachypodium brevicaule, P. cactiples, P. densiflorum, P.eburneum, and P. rosulatum. The next level, acid to basic soils, with a pH between 4.5 and 7 were suitable for P. lamerei and P. rutenbergianum. On soils with a neutral pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
level of 7, P. meridionale inhabit. P. sofiense grows on calcareous soils but also in acid soils with a pH level of 4. P. lealii grows on outcrops of granite in fairly fertile
Fertile
The term fertile describes a condition whereby organisms are able to produce physically healthy offspring.Fertile may also refer to:...
, acid soil.
Altitude
In the mainland African continent, PachypodiumPachypodium
Pachypodium is a genus of succulent spine-bearing trees and shrubs, native to Africa. It belongs to the dogbane family, Apocynaceae. Pachypodium comes from a Latin form from Greek pachus and podion , hence meaning thick-footed.-Genus characteristics:All Pachypodium are succulent plants that...
are known to inhabit areas from sea level for the species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
P. bispinosum to 1600 meters for P. lealii. In Madagascar, the range is roughly the same with P. cactiples, P. geayi, and P. rutenbergainum inhabiting at sea level to P. brevicaule reaching an altitude of 1900 meters, virtually the uppermost limit for the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
.
Temperature
Average annual temperature regimes vary approximately 13°C (55°F) for Pachypodium brevicaule, P. densiflorum, and P. eburneum to 26.7°C (81°F) for the species P. decaryi, P. rutenbergainum, and P. windsorii. In continental southern Africa, the extreme temperatures range from -10°C (14°F) for P. succulentum locations to as much as 45°C (113°F) for P. bispinosum, P. lealii, and P. namaquanum. In winter time, snow remains a possibility for Africa's south-eastern Mountain Grassland. Importantly the African species of Pachypodium live in habitatsHabitat (ecology)
A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...
with a heat regime resembling greater amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
s than those of Madagascar. There, in the central part where most species come from, frost occasionally occurs. One station there has recorded a temperature of -2.6°C (27°F) whereas another has recorded a low of -6.3°C (21°F). In the sub–arid regions of Madagascar, the maximum temperature recorded can climb up to 40°C (104°).
Precipitation
Annually in the southern part of the African continent the rainfall varies from 75 mm (2.95 inches) for the Pachypodium namaquanum to 800 mm (31.50 inches) for P. saundersii. The west coast of South Africa and NamibiaNamibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
receives annually less than 100 mm (3.94 inches) but the fog coming off the Atlantic Ocean plays a significant role in maintaining plant diversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
. Madagascar, on the other hand, can receive precipitation from 344 mm (13.54 inches) annually for the regional habitats of P. geayi, P. lamerei, and P. meridionale to 1985 mm (78.15 inches) for P. baronii, P rosulatum, and P. rutenbergianum. These lower values of precipitation apply to the spiny desert and Madagascar dry deciduous forests
Madagascar dry deciduous forests
The Madagascar dry deciduous forests represent a tropical dry forest ecoregion generally situated in the western part of Madagascar. The area has high numbers of endemic plant and animal species but has suffered large-scale clearance for agriculture...
while the higher rainfall regimes apply to the (eastern) lowland rainforests. There is little commonality in precipitation regimes for Pahypodium. Having said that, a precipitation regime for a species of Pachypodium depends upon a habitat's location relative to the influences of the Atlantic and India Oceans and the various mountain ranges and open expanses of southern continental Africa and the island of Madagascar.
One common thread holds true for the genus: all Pachypodium are succulent plants making use of their trunks and their spines to inhabit dry, xeric climates.
Number of dry months
Pachypodium represent the dryDry season
The dry season is a term commonly used when describing the weather in the tropics. The weather in the tropics is dominated by the tropical rain belt, which oscillates from the northern to the southern tropics over the course of the year...
flora of continental Africa and Madagascar. The genus grows in areas where there are significant periods of dry months. This dry period can vary from 5 months for Pachypodium brevicaule to 10 or more months for P. cactipes, P. geayi, P. lamerei, and P. meridionale. It would seem likely that the Atlantic and India Oceans pay a big role in the creation of weather conducive to rainfall. The effect of mountains might also affect the localized conditions of the climate for Pachypodium.
Evolution of Pachypodium
There are no fossilFossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
records of Pachypodium, a fact that does exclude analysis to determine common ancestry and current relationships between taxa. Yet certain conclusions can be drawn from the geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
of the landscape itself to the past natural history
Natural history
Natural history is the scientific research of plants or animals, leaning more towards observational rather than experimental methods of study, and encompasses more research published in magazines than in academic journals. Grouped among the natural sciences, natural history is the systematic study...
of Pachypodium. Geological history demonstrates that Pachypodium and other genera like Aloe
Aloe
Aloe , also Aloë, is a genus containing about 500 species of flowering succulent plants. The most common and well known of these is Aloe vera, or "true aloe"....
, Euphorbia, Cissus
Cissus
Cissus is a genus of approximately 350 species of woody vines in the grape family . They have a cosmopolitan distribution, though the majority are to be found in the tropics. In Tamil they are known as pirandai .-Medicinal:...
, Sesamothamnus
Sesamothamnus
Sesamothamnus is a genus of plant in family Pedaliaceae. It contains the following species :* Sesamothamnus benguellensis, Welw.* Sesamothamnus busseanus, Engl.* Sesamothamnus guerichii, E.A. Bruce....
, Kalanchoe
Kalanchoe
Kalanchoe , also written Kalanchöe or Kalanchoë, is a genus of about 125 species of tropical, succulent flowering plants in the Family Crassulaceae, mainly native to the Old World but with a few species now growing wild in the New World following introduction of the species.Most are shrubs or...
, and Adansonia
Adansonia
Adansonia is a genus of eight species of tree, six native to Madagascar, one native to mainland Africa and the Arabian Peninsula and one to Australia. The mainland African species also occurs on Madagascar, but it is not a native of that island....
existed before the separation of Madagascar from continental Southern Africa
Southern Africa
Southern Africa is the southernmost region of the African continent, variably defined by geography or geopolitics. Within the region are numerous territories, including the Republic of South Africa ; nowadays, the simpler term South Africa is generally reserved for the country in English.-UN...
. Pachypodium and these other genera, for instance, are represented on both Madagascar and the mainland, suggesting that their populations were once continuous within the landscape before the Gondwanaland contental separation about 65 million years ago in the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
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 contribute 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 an 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 economic development of the Malagasy people. Conservation may become 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.