The survival of the fattest: Problems and solutions, part 2

EXCESS OF ENERGY
 
In general, organisms can only survive if the quantities of water and energy entering are at least as big as those leaving them.
Succulent plants usually receive too much energy (solar radiation, wind) and too little water, which makes their balancing act even more complicated.

Radiation
In many arid areas the light intensity is usually high – often 2-2.5 times as much as in more temperate climates – and because there are few clouds, there are also more hours of sunshine.
When the radiation becomes too high it may damage the plants’ chlorophyll or overheat their tissues.
Succulents have developed a variety of means to reduce these dangers (upright stems or leaves, hairy or wax-covered skin, covers of spines etc.). Many columnar succulents bend in the direction of the sun and in leaf succulents the new and vulnerable leaves often grow upright at first, becoming more horizontal over time.

One of the many different forms of Cotyledon orbiculata. The whitish wax covering reduces absorption of light by about 10%.
More or less the same will apply to the greyish leaves of Crassula perfoliata ssp.minor. The fact that these leaves are upright also helps in diminishing the effects of high radiation.

 

The tiny upright columns of Anacampseros filamentosa.
In young plants of giant aloes like this Aloidendron pillansii,  the leaves are more or less upright.
In older plants of the same species, the leaves are about horizontal. The upright position of the young leaves most probably reduces the effect of solar radiation, which helps young plants survive.
The hairy leaves of this Pelargonium caroli-henrici do not only reflect the light, but also create a layer of still air (which diminishes transpiration) and are able to collect dew.

Another way to avoid the dangers of high radiation is hiding under shrubs, in rock crevices etc. This also helps against strong winds and browsing animals.

Haworthia scabra

 

Anacampseros telephiastrum
Crassula sericea

Temperatures
As a result of the large amounts of radiation, temperatures are often high during the day. In many cases there are great differences between day and night temperatures (often more than 50°C) as well as a great variation over the year.
The temperatures of soil surface may even reach 70°C.
Because the roots of succulents are usually near the surface, this is bound to cause problems. Under rocks it is much cooler, so many roots are found there. No wonder many succulents seek shelter near or in between rocks.

Cheiridopsis namaquensis
Dorstenia foetida

Whereas reduction in size is a good strategy for coping with lack of water, miniature succulents (by definition growing near the soil surface) will be  severely affected by these high temperatures. The same applies to young seedlings. Plants in general may get rid of excessive heat by means of transpiration, convection or long-wave emission, but often these options are not available to succulents:
– using transpiration to lower body temperature would lead to unaffordable water losses
– their low surface area-to-volume ratio reduces the boundary layer where convection can take place as well as the area from which long-wave radiation may be emitted.

Wind

Wind is usually present and often hot and strong. The continuous replacement of air around  the plants has a desiccating effect so that water loss can be extreme.
In many arid regions sandstorms are a regular occurrence, transporting not only dust and sand but often also small stones, damaging plants and remove hairs or wax cover through abrasion in the process. Seedlings are especially vulnerable in this respect.

Crassula columnaris ssp. prolifera with a protective cover of grains of sand.

PREDATORS

Predators: sometimes they eat you, sometimes they feed you. Othonna sedifolia surrounded by animal droppings.

 

In the case of this Gasteria disticha the advantages seem decidedly one-sided
Even a little browsing animal can do a lot of harm. (Tanquana prismatica)

 

Of the 10 spiny mesemb species Ruschia spinosa is probably the most widespread one. After the fruits have disappeared, the supporting bracts turn into sharp spines that protect the new leaves. (Other functions of the spines will be discussed later.)

Because of their juicy contents, succulents are attractive to herbivores, but many of them are unpalatable. This may be because they are very bitter (Aloes), contain a sour, salty sap (Augea capensis, Mesembryanthemum crystallinum, M. guerichianum) or a milky latex (Euphorbia ssp.), or even because they are poisonous (Cotyledon, Euphorbia, Sarcostemma, Tylecodon).

For other posts in this series click here.

 

 

 

 

 

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