How can flowers exist in the desert?
Life in the desert
The plants of the deserts
In the case of plants, the number of species in deserts is significantly reduced, as is the case with animals. In the Sahara, for example, only 1,400 plant species are known, a value that is reached in a few square kilometers in the tropical rainforest. Deserts are characterized by a lack of vegetation or even a lack of vegetation. In addition to climatic limit values, the occurrence or disappearance of certain plants is a reliable indicator for the delimitation of deserts. The northern border of the Sahara can be determined by the occurrence of the Halfa grass cushion and the southern border of the Sahara by the appearance of the cram cram grass.
The main enemies of desert life are water scarcity and extreme temperatures. They restrict life in the desert in general and that of the plants in particular. Since plants, unlike animals, do not have the opportunity to fly to a watering hole, to migrate to areas with more rain or to seek shade, they are more forced than animals to adapt to the extreme environmental conditions and to develop strategies for survival. The most important are described below. Furthermore, three typical desert plants - a more detailed description of the plant world of the deserts would go beyond the scope of this book.
First of all, a lack of water endangers the plants. It disrupts assimilation and photosynthesis and damages the protoplasm by causing deformations of the cell walls. If the plant does not get water again in time, the damage is irreversible. Plants dry out in deserts for two reasons: They do not receive sufficient water from precipitation or from the groundwater, and they are subject to the high evaporation typical of deserts.
In addition to a lack of water, high air and soil temperatures threaten their survival. Even though plants are somewhat more resistant than animals in this regard, temperatures of over 50ºC pose a threat to plants because the protoplasm coagulates. The high salt content of many desert soils also restricts plant growth, although some plants - the halophytes - have adapted to this.
Are there abiotic areas in the deserts of the earth in which neither animals nor plants live? Indeed, in the Sahara (Libyan desert, Tanezrouft) and in the Iranian Lut desert, hundreds of thousands of square kilometers are without any visible vegetation. However, unlike on the moon, there are traces of life there. Soil samples were examined near the southern Algerian border town in Guezzam, and it was found that one gram of soil contained 10,000 bacteria and 3,300 fungal spores. In some deserts, up to 10 million microorganisms per gram of soil have been detected.
Although it seems at first glance that the flora of the current deserts scattered all over the world is very different, there are similarities. This either indicates a common origin, perhaps from a time when the continents were still together, but it can also be due to the fact that under the selective pressure of the same factors, similar forms and strategies have emerged everywhere. As with the fauna, one speaks of convergence. The African drachee with a tuft of leaves at the top of the trunk is reminiscent of the American yuccas, for example.
If enough rain were to fall in the Sahara, it would not take long and it would resemble the savannahs of East Africa. However, water is only available to a very limited extent for plants in the desert - either in the form of precipitation or groundwater. Because the rain often only seeps a few centimeters into the ground, the plants absorb the rainwater through a widely ramified root system close to the surface. How much rainwater the plants receive depends on the type of subsoil. Sand is only apparently dry, but in reality it is a good water reservoir, since falling rain does not run off like a torrent, but is absorbed by the sand like a sponge. Although a thin layer of sand dries on the surface, the water absorbed is stored in the depths, because the large pores of the sand structure prevent capillary rise and thus also evaporation on the surface. So dunes are good water reservoirs. Even 50 mm of precipitation per year is sufficient for modest plant growth, while at least 400 mm must fall on dense clay soils. If the plant life in the dunes is sparse, it is due to the unstable terrain. Especially grasses hold their own here, thanks to the formation of extremely long roots.
The trees in the desert are less dependent on rainwater than on the groundwater, which they have to reach with their roots. A young tree can only grow if several climatically favorable years have followed one another and the seedlings have been supplied with water. If it has not fallen victim to cattle in the meantime, it can finally gain a foothold if it manages to get its roots to reach the water table. If this sinks after years with little rain or as a result of human intervention, the tree dies. When digging the well on the famous Arbre du Ténéré, an acacia tree, its roots were found 35 m below the ground. The view of the desert from the plane shows that trees are not randomly placed in the desert. They grow along the groundwater streams that often follow dry river valleys. The lack of water causes slow tree growth. Most of the thick-stemmed trees are over 100 years old.
The famous film "The Desert Lives" by Walt Disney leads one to believe that a single shower of rain can turn the desert into a sea of flowers. However, while germinating, many plants have developed strategies to prevent this from happening. Germ-inhibiting substances in the seed coat ensure that the seeds only come to life when sufficient rain has washed out these substances and at the same time moistened the soil so thoroughly that the plants not only begin their life cycle, but can also end their seed formation again. This requires 15 to 20 mm of precipitation per year. The phenomenon of the blooming desert only occurs when it rains for a long time. A sea of young plants then stretches for many square kilometers, and it becomes clear how many seeds actually lie in the ground.
Plants that have the ability to germinate quickly under favorable conditions and to complete their vegetation cycle in a short time are called ephemeral. Their productivity is therefore mainly used for the formation of new seeds, which remain in the sand until the next period of moisture. Some ephemeral seeds produce large numbers of seeds that become entangled and form a kind of ball that the wind disperses in the desert. Other plants produce seeds with hairs or barbs that get caught in the leg fur of grazing animals and are thus spread.
The onion plants known as geophytes pursue a different survival strategy. With their bulb, which is largely protected from temperature fluctuations in the soil at a depth of 20 to 30 cm, they have a storage organ for water and nutrients. As soon as moisture penetrates this depth, the onion sprouts and forms leaves and flowers on the surface of the earth. Here, too, the growing season only remains for a short period of time: the onion has to be filled with the nutrients used for sprouting, the seeds have to be formed, and when the drought sets in, nutrients from the withered leaves are returned to the onion.
The extreme climatic conditions in deserts have resulted in numerous adaptations in the construction of many plants. So they restrict z. B. the leaf surface or they keep the leaves small - sometimes so small that they cannot be recognized as such - to save water. Or the shoot and individual stem sections already contain the chlorophyll necessary for the assimilation and the conversion of carbon dioxide to sugar and thus take on the role of the leaves. If there are small leaflets, they often have a tough, waterproof coating that further restricts the loss of water from the leaf. Often the leaf margins are rolled up so that the evaporating leaf surface is reduced. Sometimes the tip of the leaf is transformed into a thorn.
THORNS AND POISON
Adaptation to the climate does not guarantee survival, because the risk of being eaten is considerable, especially in regions with sparse vegetation. It is beneficial to have thorns or to taste bad. Many desert plants therefore have thorns and thorns. But they offer only limited protection, because camels and goats still eat the plant. The fact that it tastes bad does not always deter, so that the plant is only protected if it turns out to be poisonous. If there are lush green bushes in the midst of eroded grass pads, they are certainly poisonous. In this way, poisonous plants can spread - the competitors disappear through cattle.
The parasitic plants are among the most beautiful desert plants. They have powerful yellow or lavish purple inflorescences up to one meter high, which burst straight out of the earth. They tap into the supply lines in the roots of other plants and extract what they need from them. Their reproduction is problematic. Since a parasitic plant does not develop chlorophyll itself, the seedling cannot feed itself either, so that it has to come into contact with a suitable host root in order to germinate.
Plants that can store water for a long time in particularly large-celled ground tissue are called succulents. Depending on the location of the water-storing tissue, a distinction is made between leaf, stem and root succulence. Succulents depend on regular rainfall, so that in hyperarid deserts like the Sahara there are largely no succulent plant species. The aloes and agaves with their fleshy, thickened leaves belong to the group of leaf succulents, whereas cacti have a pronounced stem succulence.
A trick used by succulents is to keep the stomata of the leaves tight during the hot time of the day so as not to lose water. During this time, the plants cannot absorb the carbon dioxide from the air, which is essential for growth. They do this at night when it gets cold in the desert and there is no risk of dehydration. Because the carbon dioxide cannot be processed further at night due to the lack of sunlight, it is stored in the tissue in the form of an organic acid. When the temperature rises in the morning, the plant closes the stomata again. However, by resorting to the carbon dioxide storage created at night, it can still carry out photosynthesis.
Hardly any other plant is more typical of a desert than the saguaro of the North American Sonora. It can live for several hundred years, the first arms branch off after 75 years, by the age of 150 to 200 years it is 15 meters high, weighs over ten tons and produces up to 50 arms.
The trunk contains up to several thousand liters of water and is held together by vertical ribs arranged in a circle. When it rains heavily, they expand like an accordion, storing as much water as possible. The large white flowers open in a single night in May or June to attract bats, the pollinators. A saguaro produces up to 40 million seeds in its lifetime, but only one of them may reach adulthood. Most of them are eaten by birds. Saguaro seedlings are often surrounded by nurse plants, without which a seedling would be defenseless against extreme soil temperatures of up to 70ºC. The nurse plants compete for water, but their fallen leaves donate nutrients again. The larger a cactus becomes, the more its water-storing volume increases compared to the surface, so that it can endure ever longer periods of drought. At the same time, the waxy, waterproof layer becomes more and more dense, which further reduces water loss. The saguaro is therefore less threatened by a lack of water than by frost and lightning strikes, livestock farming, air pollution and the destruction of the habitat.
In addition to the cactus, the date palm (Phoenix dactylifera) is the most famous plant in the desert. It requires an average annual temperature of 21ºC, is relatively salt-tolerant and depends on adequate irrigation and groundwater. These properties make them a typical useful plant in the oases of the Old World dry belt. In terms of their importance as a staple food for the local population, their fruits can be compared with the potato in Central Europe. The qualities range from the “Deglet Nour” variety, which is exported to Europe as confectionery, to fruits that are only suitable as food for camels and donkeys.
The date palm has male and female trees. The date fruits ripen on the latter. In palm groves, only one male out of 50 female palms is planted, which is completely sufficient to fertilize the female flowers with its pollen. The palm bears fruit three to seven years after germination. A palm can live up to 200 years, and its optimum yield is between 40 and 80 years of age. Up to 150 kg of dates can be harvested between August and December each year.
Welwitschia mirabilis is a botanical specialty. It occurs exclusively in the fog zone of the Namib Desert, so it is endemic. The mist adds moisture to the plant by precipitating on the long leaves and from there dripping into the sand. The fine roots then absorb the moisture from the soil. The Welwitschia is a typical desert plant, but in terms of vegetation geography it comes from the more humid savannah. In order to germinate and develop a root system that understands how to exploit the low humidity of the desert, the Welwitschia needs sufficient water for at least eight to ten years. Even if Welwitschiae do not form annual rings, it has been proven that some specimens are 500 years old. The Welwitschia survives so long in the desert, not least because its leaves are barely edible. Even undemanding desert animals such as the desert zebra or gemsbok only nibble on it in an emergency.
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