March 2018 - enjoysucculents

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.

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.

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

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.

The survival of the fattest: Problems and solutions, part 1

The harsh environment where succulents usually grow causes many problems and the interactions between problems and solutions are often quite complicated.
I realise that studying a diagram like the one below is not everyone’s cup of tea, but if you are interested in the way succulents survive in nature, spending a little time on it may be well worth it.

Legend
abrasion: damage caused by strong winds transporting sand and small stones.
evapotranspiration: the process by which water is transferred from the earth to the atmosphere by evaporation from soil and water surfaces and by transpiration from plants.
evasion: avoiding an undesirable situation by hiding under bushes, in rock crevices etc.
life forms the shape and structure of a plant as a result from adaptation to its environment.
predator: an organism that obtains food by eating other animals or plants.

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The survival of the fattest: Where do succulents grow?

Succulents are mainly found in areas where dry periods are regularly followed by a rainy season, so that the plants can fill their storage organs for the next dry season. This means they are relatively rare in regions where precipitation is very unpredictable or the dry periods are often longer than a year.
In other words, although we tend to think of succulents as typical desert-dwellers, in fact they rarely occur in extremely arid areas. (An area is usually called arid when it receives 70–150 mm precipitation per year and semi-arid when the figures are 150–400 mm per year).
In some coastal deserts with extremely low rainfall, such as the Atacama in South America and the Namib in southern Africa, succulents can survive and even thrive because of the runoff from nightly fog. To give you some idea of the importance of this phenomenon: for Swakopmund in Namibia, fog is recorded on 150 days per year.
The presence of succulents is limited by low temperatures, especially when these occur in the growing period. Several species though can stand temperatures as low as -10°C for a short while.
Factors such as soil type, drainage, wind speed, aspect, relief etc. play a role too. As a result of all this, succulent plants are not only found in more or less extended dry zones, but also in smaller pockets of dryness in environments that are generally much wetter (think of places like dunes, inselbergs, rocky outcrops etc.)

There are 2 main distribution centres for succulents:
Firstly we have the dry areas of the new world: Arizona, Mexico, central and South America. Here we mainly find cacti, but there also many other succulents.
The second centre is formed by the dry regions of Africa, including the Canary islands, Madagascar and Socotra, as well as the southern part of the Arabian peninsula. Far richer than any other region here is the Succulent Karoo, the winter-rainfall area of southern Africa, which includes one of the mist deserts mentioned above.
Europe, Asia and Australia are home to relatively very few succulents.

In conjunction with this post a new photo gallery is published with pictures of
— habitats where succulents are known to occur and of
— succulents showing much more of their natural environment than usually in my pictures.
Go to gallery.

For other posts in this series click here.

The survival of the fattest: What are succulents, actually?

The one thing that sets succulents apart from all other plants is the ability to store water to allow the plants to remain active even in periods when no water is available.
Although this does not make them immune to extreme lack of water, it of course helps to alleviate problems caused by temporary droughts.
Succulence is often described as a structural phenomenon, something you can see and feel, so that plants are called ‘‘succulent’’ when the leaves, stems and/or roots have a swollen appearance.
We can also look at succulence from a functional perspective, wondering how it affects the way a plant functions and survives in its habitat.
As these posts are not intended for professional botanists but for amateur plant aficionados, they are mainly concerned with the visible adaptations succulents have developed.
When looking at a collection of succulent plants it is easy to see that “some of them are more succulent than others”.
It is sometimes less easy, or even nearly impossible, to say whether a certain plant should be called a succulent or not. In other words, succulence is a continuum and there is no clear cut-off point between succulent and non-succulent plants.

So when we talk about ‘‘succulents’’, we should bear in mind that this is in fact shorthand for more correct but also slightly complicated phrases, such as ‘‘plants with notable succulence’’ or ‘‘very succulent plants’’.
There are many different kinds of succulent features, which may be combined in several ways and water may be stored in more than one organ. For these reasons it is very difficult to give a comprehensive (as well as comprehensible) definition of what succulents are.

The following definition is easy to understand:
“A succulent is a plant that stores water in its tissues as a mechanism to survive periods of drought in the growing phase.”
List of southern African succulent plants, G. F. Smith e.a., 1997.

The two definitions below are more comprehensive:
“A succulent is a plant possessing at least one succulent tissue. A succulent tissue is a living tissue that, besides possible other tasks, serves and guarantees an at least temporary storage of utilizable water, which makes the plant temporarily independent from external water supply, when soil water conditions have deteriorated such that the root is no longer able to provide the necessary water from the soil”
Life Stategies of succulents in deserts, D. J. Von Willert e.a., 1992.

“Succulence can be defined as follows: = storage of utilizable water in living tissues in one or several plant parts in such a way to allow the plant to be temporarily independent from external water supply but to retain at least some physiological activity.”
Living under temporarily arid conditions succulence as an adaptive strategy, Eggli U. and R.Nyffeler, Bradleya 27/2009, pages 13-36.

Succulence is a tried and tested solution for plants living in dry environments, but it is not the only possible strategy for surviving there. In the wild, succulents are often growing together with plants that use very different adaptations to stay alive.
The ability to store water has developed independently in around 50 different plant families. For many plants it is apparently important to have some degree of succulence, as it occurs in 4-5 % of higher plants. Estimates vary from about 8000 to plus or minus 13.000 -out of approximately 260.000 species (depending on what you call a succulent and how you define a species).

As it is of little use to store something that you cannot keep safe, succulents have also developed a variety of means to conserve the stored water.
It is this combination of storing and conserving water that causes the peculiar appearances of these plants.

When you follow this link, you will be taken to a gallery giving you some idea of the enormous diversity in size, shape and structure of (mainly South African) succulents.

For other posts in this series click here.