The outer leaves of this specimen of Aloe peglerae, growing in a protected pocket between some rocks, were scorched by a fire, but the damage did not prevent the plant from flowering.
The dried leaves of the miniature Aloe petrophila remain attached to the stem, even in a habitat that is well protected against fires. In the dry winter season, the leaves turn a deep purple, contrasting with their white leaf spots.
The corky bark that covers Aloe plicatilis stems provides protection against the heat of the fires that regularly ravage the southwestern Cape fynbos.
Aloe marlothii’s ‘skirt’ of dried leaves around its stem may play a role in insulating the stem during fires.
Every natural habitat in which sufficient combustible material is produced has its own characteristic fire regime. These habitats range from being highly susceptible to fire to essentially fire-free.
In southern Africa many species of Aloe occur in habitats that can be regarded as fire-free. This includes parts of the Great Karoo, where the availability of fuel is insufficient to sustain fire because the distance between adjacent plants is too great and the production of material that can burn is too low. Aloe species also occur as a component of succulent thicket vegetation, where many plants with fat, moisture-filled leaves make it difficult for fire to take hold. This is often the case in the hot, arid valleys of east-flowing rivers in the Eastern Cape and KwaZulu-Natal.
However, some habitats in which aloes occur are prone to regular, almost predictable fires. The Cape Floristic Region, which coincides closely with the Fynbos Biome, is the best known of these, but the grassland and savanna regions are also fire-prone.
Under normal conditions, fynbos vegetation burns at a frequency of between five and 20 years. However, as a result of recent human impacts, such as the introduction and subsequent uncontrolled spread of alien vegetation, the frequency and intensity of fires have increased dramatically, with disastrous effects on property and the natural environment.
Fynbos vegetation can be divided into various subtypes, depending on altitude and a number of climatic criteria. On a broad level, however, we can distinguish mountain from lowland fynbos. Comparatively few aloes occur naturally in mountain fynbos, notable exceptions being the tree-like Aloe plicatilis and the low-growing A. haemanthifolia. Both of these, rather uncommonly for species of Aloe, have tongue- or strap-shaped leaves arranged in a fan-like manner, but they differ considerably from one another in terms of growth form and height. A. plicatilis, commonly known as the fan aloe or waaier-aalwyn in Afrikaans, is a sparsely to much-branched tree, carrying its rosettes at the tips of the branches. One adaptation to surviving in a fire-prone habitat is seen in the stems and branches of this species, which are protected by a thick, corky bark, a very uncommon character in the Aloe family. In addition, A. plicatilis favours rocky and scree habitats where hot fires generally cannot reach the plants.
Aloe haemanthifolia, on the other hand, is a low-growing, more or less stemless herb. It has an underground rootstock with a strong root system from which new stems and leaves develop (sprout) if the above-ground parts are ravaged by fire. Although the above-ground growth may be completely destroyed by fires, plants are rarely killed. The plant is therefore a resprouter, one of the more effective strategies (a common alternative strategy, though not occurring among aloes, is displayed by so-called reseeders: plants are killed by fire and must regenerate anew from seed) evolved by plants in the fynbos (and elsewhere) to survive regular fires. The shrubby A. commixta also has a strongly developed underground rootstock and roots, and is a resprouter. So too is A. micracantha, the only species of grass aloe that occurs in grassy fynbos in the Eastern Cape province. It has a rootstock with exceptionally thick and robust roots, so much so that the below-ground biomass (mass of living matter) of this species by far outstrips the above-ground biomass (as you discover when trying to remove a plant from the ground without damaging its spindle-shaped roots!). In this species, new rosettes develop from the rootstock if the plants are burned or, as often happens in urban open spaces, are cut off at ground level with industrial-strength lawn-mowers.
Aloes found in lowland fynbos, including the creeping Aloe perfoliata and the miniature A. brevifolia from the chalk fields near Bredasdrop, have developed different strategies for survival. They evade fire through a preference for rocky habitats that are often, at best, only sparsely covered with vegetation that can act as fuel during a fire in the dry summer season. This strategy is also found in the summer-rainfall region, where a number of grass aloes, such as A. verecunda, are inclined to grow in protected rock crevices.
But fynbos is not the only habitat prone to regular fires. Grassland and savanna (bushveld) are too, and here the grass aloes survive fire through strongly developed rootstocks from which new plants grow in the ensuing rainy season. A miniature relative of Aloe, Chortolirion angolense, which has rather insignificant haworthioid flowers, survives fire by resprouting leaves from a typical underground bulb-like structure. The renewal (perennating) structure of this species is as close to a conventional bulb as one will get in the Aloe family.
Among the possible reasons for the retention of a skirt of dry leaves around the stems of single-stemmed, tree-like aloes is that these act as insulators against the heat of fires. Aloe ferox and A. marlothii are a case in point, as both species grow naturally in areas that carry sufficient fuel to ensure the occasional occurrence of warm to hot fires.
In spite of having developed various strategies for surviving fire, all aloes, especially the medium-sized ones, can be very detrimentally affected by fire. If a fire that is too hot occurs in the flowering season, the inflorescences will be seared and no fruit will set. Furthermore, many winter-flowering aloes occur in winter-dry savanna and grassland habitats, precisely the time of year when fires are most likely to occur. And of course, in some cases, aloes – and their relatives – may simply be killed outright by fire.
But then there are also some aloes that benefit from fire. Seemingly, some grass aloes will not flower unless they are exposed to fire. The most notable of these is Aloe chortolirioides which may not flower at all during years when the grassland remains unburnt. In addition, the subsequent release of nutrients into the soil through ash production enriches the soil, thereby enhancing seed germination, among other things. A grassland fire, followed by good early summer rains, is sure to stimulate extensive flowering in many of the grass aloe species. An indication that grass aloes are dependent on fire is evident in the condition of their dead winter foliage, which burns with ease (as does the grass in which they occur). The ideal timing of fires for the grass aloe species is late July and early August, just before the beginning of the growth cycle.
This undescribed Aloe species, growing in Limpopo Province, survived after being exposed to a grass fire, and still flowered shortly thereafter.
A Mediterranean climate is one in which the summers are hot and dry and the winters wet and mild, such as is experienced in parts of southern Europe (macchia/maquis vegetation), southern California (chaparral), the Western Cape (fynbos), southern Australia (kwongan) and Chile (matorral). Within these relatively high-rainfall climates, aloes occur naturally only in the southwestern Cape, although many aloes flourish in cultivation in the other areas mentioned. In contrast to other ‘Mediterranean-type’ regions, the Cape experiences additional precipitation, albeit in limited form, from rain and mist throughout the warm, dry summer months, especially at higher altitudes on the Cape Fold Belt mountains. Furthermore, water released from higher up the surrounding Cape mountains creates a steady run-off, even in the driest months.
The designation ‘Mediterranean climate’ is not applied to all regions with predominantly winter rainfall. Regions that experience very low winter rainfall are referred to as winter-rainfall deserts. Such arid regions include the Succulent Karoo (incorporating Namaqualand) in southern Africa and the desert of Baja California in North America. Winter-rainfall deserts are very rich in succulent plants, but the Succulent Karoo is the only one that harbours aloes (about 20 species, including the tall and sparingly branched tree aloe, Aloe pillansii).
In Lesotho, where winter temperatures regularly drop below freezing, Aloe striatula is often planted as a hedge. Here, Agave americana subsp. americana, a native of Mexico, grows behind the hedge.
Not a common sight: Aloe microstigma, growing here in its natural habitat in Verlatekloof, Northern Cape, had to endure a fall of snow during the winter of 2006.
In general, succulents are surprisingly cold tolerant. This certainly also applies to species of Aloe. Exceptions to this rule are those species whose natural distribution is restricted to subtropical and near-tropical areas. Aloe thraskii, which occurs naturally along the humid subtropical coastline of KwaZulu-Natal, is such an example. It invariably requires protection against the mild to severe frosts that are prevalent on the Highveld during the winter months. In contrast, species that occur in high-lying parts of the subcontinent, for example, in the Drakensberg of the Eastern Cape, Lesotho and Mpumalanga, as well as in the Great Karoo, can tolerate very low temperatures. Some species have a distribution that straddles areas that are both frost-free and subjected to either light, mild or severe frost. In such instances the species may well have local forms differentiating into distinct ecotypes (see page 107).
Aloe polyphylla, which can grow to the size of half a wine barrel, is indigenous to Lesotho, one of the coldest areas in southern Africa. It can easily withstand subzero temperatures.
The flowers of this specimen of Aloe variegata were destroyed by frost during a cold snap.
Unable to retreat into a burrow or migrate to warmer climates, plants seem especially vulnerable to cold. Unlike warm-blooded animals, plants are also not able to maintain (thermoregulate) their tissues at a constant temperature on their own (except in a few highly specialized flowers). Yet, many survive freezing temperatures for months on end. Damage to plant tissues by extreme cold is not due to the low temperatures per se, but is usually caused by sharp ice crystals that form inside cells with resultant damage to cell membranes. To increase cold-tolerance, cell membranes in some chilling-resistant (or cold-acclimated) plants are chemically different from the membranes in chilling-sensitive plants. Membranes in such cold-adapted plants are able to maintain membrane flexibility to much lower temperatures, thus protecting the cell and its contents from damage by ice crystal formation.
In another strategy, some plants, including members of the aloe family, produce antifreeze compounds that protect cells against intracellular ice formation. Some of these compounds (sugars, amino acids and other solutes) induce supercooling in the plant’s tissues by inhibiting the growth of ice crystals. Supercooling refers to water in cells that is maintained in a liquid state below 0°C.
Contrary to what we would expect, the freezing of water may, under certain conditions, help to protect cells from frost-damage. When water changes from a liquid to a solid (ice) during freezing, energy in the form of so-called latent heat is released. As the temperature drops below freezing, living plant cells lose heat, but solutes prevent the watery contents of the cells from forming ice crystals and the liquid supercools. However, as the temperature drops further, ice starts to form in the spaces filled with moist air between the cells. As the ice crystals grow, water (but not any solutes) moves out of the cells through the cell membrane and porous cell wall and adds to the growing ice crystals outside the cell. This slow drying out (dehydration) concentrates solutes within the living cell contents (protoplast), resulting in a lowering of the freezing point further by 2–3°C. Ice formed outside a cell (extracellular ice) does not kill plant cells. As ice forms in the spaces between the cells, heat energy is released and the temperature of the liquid inside the cells rises a few degrees and stays at the higher level until all the extracellular spaces are frozen. When that point is reached, latent heat release stops and the temperature begins to fall again. When the ice outside cells melts in frost-hardy plants, the water goes back into the cells and they resume their normal metabolism. The release of heat energy during ice formation is the basis for the common practice of spraying plants with water during frost. As long as the water continues to freeze outside cells, it releases heat that prevents freezing inside cells.
Because of their high water content, most succulents are particularly prone to damage by subzero temperatures. Yet, many aloes are surprisingly cold-tolerant. Worldwide, succulents hardly ever occur naturally in areas experiencing severe frost, especially during the season of active growth. On high mountains, succulents tend to decrease in abundance with altitude and are virtually absent above the mean cloud line where severe night frosts regularly occur throughout the year. Aloe polyphylla (spiral aloe), the national floral emblem of Lesotho, is unusual among succulents in general and aloes in particular for being adapted to flourish in the extreme cold of its mountain habitat above 2 400 m in Lesotho. Plants are often covered by snow in winter. The specific mechanisms employed by this remarkable aloe to avoid lethal frost damage are not known.
After the sun has set, the land radiates heat that it has stored during daylight hours, and soil and plants eventually cool to a point at which they are colder than the surrounding air. At this point (dew point), any moisture in the air condenses in the form of water droplets (dew) on plants (and, of course, on other structures, such as outdoor garden furniture, vehicles etc).
If the air humidity is very low, i.e. if the air is dry, the dew point will be reached at a much lower temperature, closer to freezing. This typically occurs in inland, continental areas away from the coast. If the air temperature drops below freezing, the moisture in the air is deposited in solid form as frost. Thus, frost is a solid deposition of water vapour from saturated air, and NOT, as many people assume, dew that has subsequently frozen. Since a small amount of heat is released as the water vapour converts to frost, the temperature of the plant sap usually stops dropping for a short while and the visible frost on the plant surface provides some ‘cold protection’, especially if the air is reasonably moist.
Black frost occurs on very cold nights when the air humidity is so low that ‘protective’ visible white frost does not actually occur. Under such conditions the temperature of the leaves of plants, including aloes, drops to that of the air and the plants lose moisture to the air through evaporation, which further lowers the temperature of the leaves. This results in the freezing of sap inside the cells of susceptible plants. Moisture loss and evaporation is even greater when a strong, cold wind is blowing at the time. These conditions result in severe freezing damage. As the frozen sap of plants melts when temperatures rise again, damage becomes visible as a blackening of dead plant parts (hence the term ‘black frost’).
To prevent frost damage to flowers and inflorescences, some forms of species (ecotypes) that are generally not frost resistant are genetically programmed to postpone their flowering events to fall outside the cold winter months. This is particularly evident in summer-flowering forms of Aloe ferox from parts of the Northern and Eastern Cape and southern Free State, and spring-flowering forms of Aloe marlothii from the Winterton and Bergville areas in KwaZulu-Natal.
Sometimes, in the flowering season, you may encounter an aloe population with not a single flower in evidence. There are several potential reasons for this: the flowers may have been removed by baboons, who destructively break off inflorescences in search of nectar; the preceding growing season may not have been favourable for plant growth, resulting in a shortage of water and food reserves to invest in flowering and fruiting; or the plants sensed an oncoming event, such as a drought or severe cold snap, that could destroy any flowers produced. Plants may also ‘deliberately’ deprive destructive predators of a season’s worth of food, in the form of flowers and seeds, so as to minimize the impact of such pests on subsequent seed crops (see ‘masting’ below).
So, do plants have built-in ‘intelligence’ that can anticipate environmental conditions that could be conducive to the destruction of the plant and/or its flowers, fruits and seeds?
A well-established phenomenon in many perennial plant populations, especially trees, is a reproductive pattern wherein an entire population of one species reproduces heavily at once, but at long intervals.
Such episodic productions of large seed crops result from mass flowering. Referred to as ‘masting’ or ‘mast-fruiting’ by biologists, such events are generally followed by one or more years of comparatively poor flowering and/or seed set.
It is believed that masting plants put so much energy into producing a good seed crop that they are forced to forgo reproduction in at least one subsequent year. Although masting has never been described in aloes, it is possible that the non-flowering behaviour observed in groups of aloes may be related to comparable periodic cycles of an exceptionally good seed crop in one year, followed by one or more years of relatively poor crops.
One of the most popular theories to explain masting is that in ‘mast’ years, the bumper crops satiate seed eaters, providing them with so much food that some seeds escape being eaten. Furthermore, the small crops produced in subsequent seasons serve to reduce populations of seed predators, resulting in fewer animals to eat the seeds produced during mast years. Thus, a higher overall proportion of seeds ultimately escapes predation. Another suggestion is that masting might enhance pollination efficiency, especially in wind-pollinated trees. It has also been proposed that the year after a period of water stress (or other suboptimal conditions for growth), forest trees may respond with high seed production. Although the cues that trigger masting are poorly understood, past and present weather conditions, especially fluctuations in temperature, seem, in some instances, to correlate with this still largely mysterious ecological phenomenon. But, what about the possibility that such synchronized reproductive patterns in plants may also be in anticipation of future climatic conditions?
Traditional methods of predicting weather often make use of animal behavioural patterns, and in recent years even scientists have been studying animal behaviour in the context of predicting natural disasters. Furthermore, it has long been observed that weather change over the short term is associated in humans with increased complaints, especially of chronic pain. Stormy weather causes the air pressure to drop and then rise as the storm moves past. Experiments based on animal behaviour have shown that low barometric pressure and low ambient temperature augment pain intensity. These observations support reports from humans with certain pathological conditions that pain is aggravated by an approaching low-pressure system or exposure to a mildly cold environment.
If some humans can predict approaching inclement weather in this way, then could plants do the same? Currently, there is no scientific evidence to support this. It is nevertheless widely claimed by gardeners (at least in South Africa) that the so-called rain or storm lilies (Zephyranthes spp.) of South America display mass blooming a few days before such stormy weather. On the other hand, reports in the popular literature state that such flowering only happens after good rains. It would be informative to observe a stand of these lilies and to record weather patterns over one or more seasons to establish the facts and whether they can predict certain weather conditions.
Plant behaviour is still very poorly understood. In folklore, influences of the phases of the Moon on plants have long been claimed. Although still treated as myth by many scientists, some of these assertions may well have some scientific foundation. Using sensitive measuring equipment, scientists have shown that the Moon does influence the flow of water between different parts of a tree. Moreover, plant neurobiology, a newly emerging field of plant sciences, depicts plants as social organisms with surprisingly complex forms of behaviour. In their own special way, plants can see, smell, taste, touch and, perhaps, even hear. They have the power to compute, they show foresight and they remember what happens to them – all qualities associated with intelligence in animals!