How have trees spread over the earth

Plants: Interview: Do plants rule the world?

Why trees, bushes and flowers are more important for the earth's ecosystems than animals, why an inconspicuous bluegrass is the most successful plant in the world - and more: the botanist Norbert Jürgens on the global importance of plants

GEOkompakt: Professor Jürgens, plants can neither think nor do they have the opportunity to walk around and explore the world. Have they not - compared to animals - been disadvantaged by evolution?

Professor Norbert Jürgens: Not at all. Your question reflects the anthropocentric view of many people. We tend to define ourselves primarily through our brain and experience ourselves as the crown of creation. But even in the animal kingdom, brains are not absolutely necessary. Sedentary animals without a thinking organ - corals - are able to build gigantic reefs. And the supposedly disadvantaged plants rule the earth after all.

Their dominance ...

... is evident in their productivity alone: ​​Plants make up more than 99.9 percent of global biomass, animals make up less than a thousandth. Plants are also pioneers. It is they who are the first to conquer new realms, for example when they germinate on volcanic ash or green a newly created island in the ocean. This is how plants create living space: because only they give other organisms the chance to develop in previously completely dead places. So plants ensure that biodiversity develops. It is amazing how ingenious plants are when it comes to revitalizing inhospitable regions.

Can you give an example?

In South Africa there are areas where the wind blows so forcefully over the ground that even rocks are sandblasted away like a sandblast. Actually nothing can survive there. And yet, certain types of ice plant have found a way to withstand these extremely harsh conditions. The plants excrete glue on their entire surface. Grains of sand stick to it and over time form a protective crust. The plants use the sand in a clever way to shield themselves from sand.

Are these plants the only organisms that live there?

No, but just because these plant species are there, other living things can also populate these regions. For example some insects that feed on the plants. So one amazing innovation made it possible for a species to conquer new ecosystems in which there was previously no life. We call this "key matching".

Does such an adaptation characterize many plant species?

Not just many, but all of them. Behind every species there is a key adaptation, a very specific solution to a very specific problem, which enables survival in the ecosystem. It's an evolutionary principle. The innovation in question may not always be particularly obvious, and in many cases it is not known at all. But I'm sure: the 350,000 plant species worldwide stand for at least 350,000 inventions, each of which has changed the world. For this reason alone, not a single species should become extinct.

Which environmental factor do plants have to adapt to in particular?

An important parameter is the soil. Is the subsurface acidic or calcareous? Is it damp, sandy, or loamy? Depending on how complex the structure of the soil is, plant species can grow in close proximity to one another. For example, some species have specialized in granite that a few meters away - say, on rock containing gypsum - would have no chance of surviving. Other species thrive there, and a little further on it may be boggy, and there, too, one encounters adapted species. Geodiversity is an engine for biodiversity: Wherever there are many different rocks and slopes in close proximity - and where conditions such as temperature, solar radiation, humidity alternate - evolution develops a particularly large number of plant species. Each one is adapted to a sub-ecosystem; In a mountain range, for example, one species specializes in living on a sunlit southern slope, while another can make the most of the cool shade of a northern slope with months of snow cover. Therefore, in such regions there is a particularly high multiplication of the key adaptations and thus of the species: radiation.

Mountains have the highest biodiversity?

Exactly. Even in the tropics, which have the highest biodiversity, these so-called hotspots are in mountainous regions and not in other landscapes. In Latin America, for example, the Andean chains have the richest flora. In the tropics of East Asia, too, most species grow on mountains such as the Kinabalu. There are other remarkable centers of biodiversity, for example the Cape Flora in South Africa, which is astonishingly rich in species in a very small area. It was created by a special climate history, but also by mountain formations. It is similar in the European Mediterranean area with its mountainous countries and island worlds.

Are there species that can grow in very different regions? Who is the global player in the plant kingdom?

There are opportunistic species whose strategy is to defy different conditions. Ferns and lichens, which spread through particularly wide-flying spores, are often found almost worldwide. The flowering plant that does this best is the annual bluegrass. It grows in the lowlands as well as in the mountains, on nutrient-rich and sandy soils. It even grows where car tires roll, as long as only a few cracks are free. This survival specialist even occurs in Antarctica. The grasses are already extremely successful plants, they represent one of the most species-rich plant families of all - there are around 10,000 species worldwide. For example, many have adapted to dry regions.

Which strategy will help you with this?

The lower part of their leaves nestles tightly around the stem and protects it from drying out. In addition, the grasses of the hot and dry regions have a special enzyme that attracts the carbon dioxide required for photosynthesis and binds it to itself more quickly than in normal plants.

Why is this beneficial?

All plants have small, adjustable pores, the stomata, in their leaves to absorb carbon dioxide. They are constantly losing water through these holes. It is therefore a problem to open the crevice, especially in arid regions. The plants run the risk of dying of thirst if they open the crevice for too long, or of starvation if they close the crevice for too long and then do not absorb carbon dioxide. Thanks to the optimized enzyme, the grasses can absorb more carbon dioxide without losing water. In addition, grasses have adapted particularly well to a threat to which the vast majority of plants are exposed: hungry animals attack them. While many plants die when they are eaten bare, the grass usually does not matter. As long as their roots and a growth zone just above the ground remain intact, they can grow back quickly and form new leaves. And finally, they resist being exterminated too much. They then form pointed silicate grains which, over time, sand down the teeth of the herbivores. Some grasses may even change their palatability.

As the?

Some researchers suspect that the grasses register when a herd plucks at them. They then produce substances that give them an inedible taste or are even slightly toxic. They may also send out special signaling substances that other grass plants can perceive and thus warn them. In any case, similar relationships have already been proven several times. For example, poplars produce toxic substances when insects gnaw on neighboring conspecifics. There are certainly many other interactions between plants that are so far unknown to us. Unfortunately, much is still unclear on this question - as is the case with the highly complex molecular control processes that take place inside the plants. There is an enormous need for research here.

As a botanist, what else would you like an answer to?

A crucial question for me is to what extent it can be controlled whether certain plants in an ecosystem grow better or become fewer. In Namibia, Botswana, Zambia and Angola, for example, 100 years ago there were still fantastic pastures that were dominated by grasses and in which there were only a few individual savannah trees. Flightless ratites such as ostriches lived in these open landscapes, and farmers could raise thousands of cattle. Today there is a scrub landscape of thorn bushes, which are often so dense that neither humans nor cattle can get through and no ostrich can live there anymore.

What happened?

It's an example of the consequences of overuse. The grass was so eaten up by the high number of livestock that it could no longer recover. Then thorn acacias spread over the damaged areas, because they were spurned by the cattle. Finally, only thorn bushes grew on the previous pastures. This development was favored by another factor: Due to the enormous burning of fossil raw materials, the carbon dioxide content rises all over the world and acts like a fertilizer on the plants. Woody plants such as thorn acacias, which now thrive more luxuriantly than other plants, benefit from this - forests and wooded landscapes are increasing all over the world. At the same time, however, the risk of forest fires increases, with dramatic consequences, for example in the Mediterranean region.

Can you reverse this process?

Yes, but we still lack precise knowledge about this. Farmers in Namibia are now converting their farms to kudu farms. The kudu do not eat grass, but the branches of woody trees, and can contain the undergrowth again. But now there are already farms with overuse by too many kudu. As you can see, the management of ecosystems and land use is extremely difficult because too little is known about the properties of plants. We don't even understand how immigrated exotic species fit into existing communities - how they manage to become at home in the foreign environment.

Do such human-introduced plant species cause damage?

In some countries they are a huge problem. Because we are reversing evolution - millions of years in which groups of plants and animals on different continents have specialized. We bring them into contact with each other and thereby create completely new competitive structures. This inevitably leads to some species becoming extinct because stronger ones displace them. The Cape flora in South Africa, for example, is almost on the verge of complete destruction because acacia species have been introduced there from Australia several times over the past 100 years. It was partly about protecting coastal dunes. It was believed that the sand could be strengthened by planting acacia trees, as was the case before on the fifth continent. But the Australian acacias have proven to be much more competitive than the native Cape flora - and have destroyed them across the board.

What exactly happened there?

The acacias have overgrown the native shrubs because they have adapted better to fire in Australia than the plants in Africa. Fires also occur regularly in South Africa during the summer months; on average, every hectare burns down once every 15 years; that is quite natural. But the Australian acacia burns much hotter than the South African "fine bush". It only has very thin axes and does not involve high fire temperatures. The hellishly hot fires of the acacias, on the other hand, not only kill all the native plant species, but also turtles, which were previously able to survive ten centimeters deep in the ground and now burn with them.

What are the other consequences of the change?

Water is getting scarce around Cape Town. Scientists have calculated that the Australian acacias consume an enormous amount because they absorb it with their deep roots and evaporate with their large leaf areas. In contrast, the native species use water much more sparingly. Based on the recommendations of scientists, the local politicians have therefore decided to completely remove the Australian acacias. They started a program in which tens of thousands of people chop off the acacia trees and still pull out germinating seeds by hand decades later. In the meantime, huge areas are again free of the foreign plants. It is amazing to what extent landscapes have been destroyed and how little you can tell from them at times. Many of these completely degraded areas that have been reshaped by human hands are even perceived by many as extremely beautiful, unspoiled.

Which ecosystem are you thinking of?

To the north German heathland. As I understand it, this is the world's worst case of ecosystem destruction. Because there humans first completely destroyed the natural primeval forests, partly by chopping, partly by burning and probably very much by grazing the entire area. The grasslands that were created in this way were later transformed into heather landscapes through permanent overgrazing. As a result, willow weeds finally spread. For example the completely inedible, super-pointed juniper.

What poets sing about as beautiful heather is actually an ecological catastrophe?

It is for sure. In their distress, people have often even started to cut out the heather, including the roots and part of the soil, in order to use the pests as litter for the animals in the barn and thus bring their cattle through the wet winter. It was a maximum destruction of the soil. Not even the burned down tropical forests in Brazil have been so radically destroyed.

What is the greatest threat to our plants today?

In addition to the conversion of natural ecosystems into agricultural or settlement areas, I would clearly mention overfertilization. I don't mean the deliberate fertilization of agricultural land. But the fact that internal combustion engines and industry are constantly blowing masses of nitrogen into the air. And that gets through the rain on all sorts of soils - in fields as well as nature reserves, cities as well as forests or mud flats. You have to be clear about the extent: A farmer in the Middle Ages could not, with great difficulty - with three-field farming and the dung from cows - bring as much nitrogen onto his fields as it rains on such an area today.

Why is this additional fertilization so bad?

This is due to a fundamental phenomenon related to biodiversity - and the special adaptation of each species to a specific area of ​​each environmental factor. Regardless of which factor you change in an ecosystem, be it the amount of light or the temperature, the humidity or the nutrient content: There are always species that benefit and others that cannot cope with the change and therefore perish. Here in Germany there are plants that need a lot of nitrogen - such as nettles, elderberries, ground elder - as well as those that avoid nitrogen - such as sundew and peat moss - and others that prefer a medium concentration. Due to the increasing over-fertilization, there are now no more nutrient-poor locations, which means that sundew, peat moss and bog lily are wiped out. Another danger is the rising carbon dioxide content in the atmosphere.

You already mentioned that this increases woody plants in many ecosystems.

Yes, but there is another aspect, a danger that affects us humans directly. The more carbon dioxide there is in the atmosphere, the less far plants have to open their crevices to absorb the gas. As a result, they lose less water. And that in turn means: The roots of the plants soak up less rainwater than before. As a result, the soil is filled with water much earlier than in the past over the course of a year, which in some places leads to flooding. The risk of flooding will therefore increase in the future. Here, too, it can be seen that plants are much more important to the earth's ecosystems than animals.

Do we actually have the possibility, through modern genetic engineering, of manipulating some plants in such a way that they develop certain properties and become resistant to drought or salt water, for example?

I have not yet heard of any scientific breakthroughs that have actually led to application on this issue. And I also think the much easier way would be to exploit the potential of plants that have already evolved to adapt to salt or drought.For example, there is an extreme biodiversity of salt-tolerant plants on the fringes of all oceans. Everyone knows the mangroves that populate the mud flats of tropical coastal areas. They grow there in salt water and are highly productive. These plants create supplies of wood that can be used as fuel. They bear fruits that are edible. They also protect the coasts from tidal waves such as cyclones or tsunamis.

How could the mangroves be better used?

For example, technicians and scientists are considering pumping salt water into desert areas and growing mangroves there - but I don't know of any project in which this is actually tried out on a large scale. Sometimes I am surprised that the idea that one can create something completely new through breeding or genetic manipulation is more attractive and more likely to be pursued than to research, understand, protect and use the natural flora that has grown over millions of years. In my opinion that would make a lot more sense. Plants still harbor many unknown innovations. There we have the chance to raise a whole treasure trove of knowledge.

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