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Consequences of climate change: Palm trees thrive better than beeches in Bonn

Consequences of climate change : Palm trees thrive better than beeches in Bonn

Maximilian Weigend, the head of the Bonn Botanical Gardens, talks about the effects of climate change in the GA interview. According to the biologist, Bonn's plant world will change radically.

Professor Maximilian Weigend is convinced that nature in the Bonn region will change dramatically. In his view, the development of weather and climate can no longer be stopped. Bonn and the surrounding area will present itself very differently in 2050 than it does today, he says. Weigend expects increasingly dry and warmer summers and milder winters, which will lead to more and more plants from the Mediterranean area becoming native to the southern Rhineland. Holger Willcke spoke with the director of the Botanical Gardens at the University of Bonn.

During a panel discussion on the consequences of climate change in December 2019, you said that in Bonn it is now easier to plant and grow a palm tree than a beech tree. How do you explain this?

Maximilian Weigend: This statement is based on our experiences that we have been gathering for some time in the university's botanical gardens. Because of the hot and above all dry summer months, the chances of survival of many plants, some of which we have been calling native for centuries, are diminishing. One of these plants is beech. It does not tolerate long dry periods, especially not for several years in a row. Beech suffers heat damage, dries out and ultimately dies.

Beech is the showpiece tree in the Siebengebirge nature reserve. What does its prognosis mean for this natural area?

Weigend: The vegetation in the Siebengebirge will change significantly in the coming decades. The beech stands will decrease in size. Perhaps there will only be a few retreat areas at more humid and cooler places in the Siebengebirge. Other tree species will spread and shape the image of the observer.

Does this only apply to beech and only to the Siebengebirge?

Weigend: No, not at all. This prognosis applies to the entire Bonn basin and equally to oak, birch, ash and lime.

What is the reason?

Weigend: The Bonn basin is one of the warmest places in Germany. We register the mildest winters, the fewest frosty nights and below average precipitation. One can confidently call Bonn's city centre a dry island.

How should gardeners deal with your statement?

Weigend: They have to prepare for dramatic changes. You will have increasing problems with traditional garden plants. Conversely, plants that have not been classified as hardy in Germany will find a new home in outdoor gardens.

Which plants do you have in mind?

Weigend: For example pistachio trees, almond trees, cork oaks and strawberry trees. We will increasingly cultivate plants from the Mediterranean region. Hence my statement: palm trees instead of beeches.

What reactions to changes in climate and weather do you notice in the botanic gardens?

Weigend: A few years ago we replanted a Japanese beech. Since then it has been struggling to survive. In summer we have to water it. During the summer months it suffers from sunburn, which leads to drying out and consequently to the death of the leaves. How long this tree will survive I cannot predict. In contrast, our Chinese hemp palms grow faster than ever before. Whereas in the past, we assumed a growth of five centimeters per year, we had a growth of 90 centimeters in 2019.

What conclusions do you draw from this?

Weigend: We have decided to abandon the Alpine Garden, which we created in the 1980s. Unfortunately, it is no longer possible to cultivate Alpine plants in Bonn; instead, this year we will build a theme garden on Australia and New Zealand at the same location. All in all, we are increasingly rebuilding our outdoor departments in both the crop garden and the castle garden, with a focus on plant species that are more tolerant of heat and drought.

Are you worried that there might be no more cold winters in Bonn?

Weigend: There will certainly be cold winter phases from time to time. Climate change does not mean that it will become warmer and drier every year. Unfortunately, the public is mixing up the effects of weather and climate change.

Can you explain the difference?

Weigend: Weather and climate changes are largely independent of each other. Climate change represents the changes in average weather conditions. Weather makes itself felt in the short term. Accordingly, this does not mean that climate change will have slowed down if there is sufficient rainfall from June to August 2020. In other words: there will continue to be frosty nights in the future, which can lead to frost damage to the young shoots of fruit growers. The one has nothing to do with the other.

Forestry causes many problems because of the drought and the resulting pest infestation. What is your advice to foresters and forest workers?

Weigend: A rethink in forestry is absolutely necessary, but has already been initiated many times. The prerequisite, however, is that an agreement is reached on the difference between forest plantations and natural forest.

How is this to be understood?

Weigend: Spruce as the bread tree of the timber industry will not be replaced in the short term. But temperatures of more than 40 degrees Celsius and low rainfall mean that spruce trees cannot survive for long as flat-rooted trees. This tree species then suffers very quickly from so-called drought stress. Not enough tree resin is formed, which accelerates fungal and bark beetle infestation. This ultimately means the death of the spruce trees. If you want to keep this in mind, you can currently go to the Kottenforst. Barren, cleared areas look as if a war had taken place there.

But what is to be done?

Weigend: There is no definite answer. No one can predict exactly how temperatures and precipitation will develop in terms of quantity. The important thing is to rebuild the forest in such a way that it contains trees that are suitable for the location. A Douglas fir, for example, is not one of them; it has its habitat at much higher and more humid locations. Due to the unpredictable nature of climate change, it is also difficult to say whether what appears to be suitable for the location today is necessarily sustainable. But if, for example, oaks from Eastern Europe or sub-Mediterranean regions are planted, this increases their chances of survival in our region.

How?

Weigend: Mainly the environmental toxins are to blame. But the way in which agriculture has been practised for many years is also responsible for the development in insects. Field margins are sprayed dead by fertilising the land. Even the flowering strip subsidies of the federal and state governments do not change this. Agriculture in its present form leaves no room for biodiversity.

Are you criticising the state's nature conservation efforts?

Weigend: For certain topics and approaches my answer is yes. I am irritated, for example, by the approach of nature conservation, which has the unchangeability of nature as its goal. I'm not sure whether we should stick to this goal with regard to current climate-related developments. If it remains as dry for another three to four years as it has been for the past five years, then the change in nature will continue as dramatically as is already evident at present.

Let's be honest, does that worry you?

Weigend: No. Maybe in 50 to 100 years there won't be any more shipping on the Rhine. Maybe there won't be any native fir trees then either. And maybe in spring, instead of lilies of the valley, cacti will bloom in the meadows. Emotionally, the trend is regrettable, but not threatening. Climate change is particularly threatening for the human basis of life - food production and drinking water supply.

And which tree would you plant in your garden this spring?

Weigend: An olive tree. The hope for fruit is increasing from year to year.

(Original text: Holger Willcke; Translation: Mareike Graepel)