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3. ARID ‘RAIN SHADOW’
INLAND PRAIRIE HABITAT
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This is the third of the major habitats to be discussed.
Orchids characteristic of this zone include the Cebolletae and Plurituberculata oncidiums, Laelia dayana, pumila
lundii and milleri, Leptotes bicolor, Cattleya walkeriana and nobilor, schomburgkia, Dendrobium speciosum and biggibum,
and Cymbidium suave, madidum and canalicculatum.
The habitat characteristically occurs in inland rain shadow areas, at an altitude of
500 to 1000 metres above sea level. Typical locations are coastal India, in Brazil including the central plateaus
of Minas Gerais, Goyas and Matto Grosso, and inland Australia, west of the Great Divide.
It is characterised by:-
- hot days followed by very cool nights
- Humidity is low during the day, but is high at night.
- Temperatures are warm, with significant diurnal variation.
- the amount of water is the major determinant of the number and type of orchids found
- many plants have developed water conservation adaptations to survive significant amounts
of aridity.
The majority of orchids found are terrestrials with epiphytes found bordering streams
where free water is available, with higher levels of humidity.
Growth and development, especially for the terrestrials, can be rapid, depending on the
length of the wet season.
Many of the epiphytes found in this zone have developed sophisticated physiological adaptations
to survive long periods of aridity. These include the following:-
- Leaves have become very thick and fleshy
- Leaf surfaces may be protected by a heavy wax coating. Under bright conditions, leaves
may develop a deep red colouration to protect sensitive photosynthetic tissues.
- Some leaves have become folded into a ‘V’ shape, decreasing the leaf surface exposed
to the dry atmosphere, in extreme cases the leaves becoming rolled into a round profile, only the lower surface
of the leaf exposed to the exterior.
- Stomata, through which the plant breathes, may be confined to the lower leaf surface,
or in pits or depressions, with large guard cells limiting water loss.
Some plants have developed a metabolic adaptation to limit water loss, as portrayed in
the chart which follows
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Normal plants respire and use sunlight for photosynthesis during the day, and rest at
night. Some orchids in arid habitats can split this food production process into two stages, breathing during the
night when there is less risk of water loss, and storing carbon dioxide for use during daylight hours when there
is plenty of sun for photosynthesis. During these daylight hours, when there could be significant water loss, the
stomata remain closed. Dendrobium speciosum is a typical plant showing this adaptation. The plants response is
shown in the chart. It can be seen that at dawn the air temperature falls to 15oC. rising to 32oC. during the day.
Solar radiation levels (sunshine) increase, and the water vapour differential increases indicating the plant stops
respiration and controls potential water loss. As photosynthesis is completed, the leaf acid content falls as food
is manufactured. In the evenings, temperatures fall and humidity increases, allowing respiration to start again
and more carbon dioxide stored for use the next day.
Not all plants show this metabolic adaptations, but if heavy leaves and other noted physical
adaptations are present, there is a good chance that the plant will have this adaptation. Plants known to have
this metabolic process include many laelias and cattleyas, Epidendrum radicans, Schomburgkia crispa, Sophronitis cernua, Dendrobium taruinum, speciosum,
Aerides odoratum, Angraecom sesquipedale, Arachnites flox-aeris and hookeriana, Ascocentrum ampullaceum, Phalaenopsis amabilis.
Some indicative climate information is available from site in Australia, which, because
it is maritime nature will not portray the extremes found in some habitats, but does show some of the basic climatic
characteristics.
Rainfall is minimal for most of the year, but the area is still often subjected to the
summer monsoons, when significant rain can be experienced for a period. Average humidity ranges from 68% just before
the summer monsoon conditions arrive, rising to 80% during that period. There is a marked variation in the day
and night humidity levels, which cannot be shown in this table.
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While in arid areas many fine days are expected, it is interesting that in may localities
portraying this habitat, there are many shady days, with most months having less than 5 sunny days. I am unsure
of the definition of ‘clear’ days used, but never the less the persistence of cloud cover is interesting. It is
not to be said that heavy shade is required for culture, but it is fair to day that some protection from the bright
midday sun experienced in this country appears necessary.
The habitat temperature levels and minimum and maximum and diurnal variation are also shown.
A feature is the consistency of the average temperatures, but the considerable variation in day and night temperatures
should be also noted, which cannot be overlooked
Characteristic environmental factors for this habitat zone suggest that a usual temperature
range of 15-35oC. can be expected, with a maximum range of 0-43oC. A diurnal day/night range of 35oC.
is widely experienced, the largest variation of all the habitats, with a maximum total temperature differential
of 43oC.
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Plants typical of this arid habitat require certain environmental conditions if growth under
artificial culture is to be maximised.
- Low daytime humidity is not detrimental to plant survival
- High night humidity si necessary to restore the plants water balance
- Relatively high light intensity is required to regulate this metabolism and to drive
the photosynthetic process
- Moderate night temperatures are needed to allow respiration and absorption of carbon
dioxide
- Moderately strong diurnal temperature fluctuations ensures the completion of this process.
This habitat usually contains the plants we classify as ‘warm’ or’ intermediate’ growing.
  
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