Global warming and Berlin’s trees

A data based assessment of the evolution of Berlin’s tree stock

Introduction

Berlin, the capital of Germany, which is the city where I live, is above all a green-friendly city. According to the city’s Senate Department for the Environment, Transport and Climate Protection, there are, on average, 80 trees on every kilometer of the city’s streets. The city’s many trees are definitely a major relaxation factor and certainly have a role to play in attenuating urban pollution.

Berlin is so serious about its trees that every tree you see — in a public park or on the strees- comes with an identification plate, which looks like the image below. With the number and the address in hand, clicking here (for street trees) or here (for park trees), it is possible to easily find what type of tree it is, what is the tree’s age, among many other informations.

Berliner trees’ identification plate

While these days it is possible to easily identify what is a plant’s genus or species by using free apps such as PlantNet or iNaturalist, I think that the city of Berlin’s tool is pretty awesome too. The reason for this is that sometimes a specific tree may pretty much become one’s very close friend. If, for example, a ruthless politician decides to authorize some of your favorite tree’s felling, Berlin’s website for trees’ identification becomes a powerful tool that empowers democratic participation.

The fall of Berlin’s tree stock and its relation to global warming

So much so good. It happens that the stock of trees in Berlin is on the fall. According to a report by the BUND, Berlin has been loosing around 1,000 street trees in average in the 2012–2019 period. Indeed, as can be seen on the following graph for the evolution of Berlin’s street trees, a slight fall is observable, owing to a larger number of fellings in comparison to new plantings.

Berlin’s falling tree stock

According to BUND’s report:

”In 2017, 2018 and 2019 an above-average number of trees were felled. In 2017 Storm Xavier uprooted and damaged many trees in the city. As a result, 68% more trees were felled. But the persistent drought since 2018 has also had devastating effects on the Berlin tree population.”

To verify BUND’s hypothesis that droughts are a major factor behind Berlin’s dwindling tree stocks, I researched precipitation and temperature data from KNMI’s Climate Explorer, which can be found here. I gathered daily data for these two variables, which have been measured in the city of Berlin since 1878! I calculated the monthly average and plotted it in the Graph below. Indeed, as can be seen, the trendline suggests that there is a slight secular downward trend in the monthly mean of daily total precipitation, measured in mm/day.

While some recent years, such as 2017, had spectacular rainfalls, the last three unfortunately had very modest precipitation levels. All of this suggests that Berlin’s trees have been subjected to constant stress, particularly during the summer, which are getting hotter every year. From the same source, I also obtained data regarding average daily temperature (in °C) and plotted in the graph below. While the downward trend in daily precipitation is not that brisk, the rise of the average daily temperature in the city of Berlin has been nothing short of spectacular.

Now, I’m not a climate scientist, but there does seem to be a link between reduced precipitation and higher temperatures. According to the same report by BUND:

“If Berlin wants to keep pace with developments, then action must be taken now. And it starts before the trees are protected and cared for. Man-made climate change must be stopped and rising global warming limited to a minimum. The droughts of recent years correspond in a terrifying way to the projections of climate researchers and yet are only an early glimpse of what may come next.”

When it comes to global warming, some things of course are clearly beyond the reach of a municipality, even a large one, such as London, which controls large financial flows that could be potentially diverted to environmental protection. Hence, at this point, what can be clearly done is to take damage contention measures, for example at the municipal level.

Two counteractive measures to the trees crisis are to diversify the stock of trees and to plant more trees that are better adapted to harsher climatic conditions (in terms of precipitation and temperature). To understand what actions the city of Berlin is taking to protect its trees, I compiled data from 766,000 + of its streets’ and parks’ trees. As alluded above, the city of Berlin’s information system for its trees is very detailed, and contains informations ranging from the genus, the species, the size, address, to its year of plantation. In the graph below, I plot the evolution of the trees’ botanical genus and their year of plantation.

Evolution of Berlin’s trees by year of plantation and botanical genus

Of course, this statistic has its limitations, as the trees that have been felled over the years are not in the data. It only considers the trees still living by June 2021, which is when I extracted the data. Nonetheless, a clear trend is observable. The city has been taking steps to diversify its stock of trees. This strategy may have reflected purely aesthetical reasons, but in principle a diversification strategy does seem smart from the point of view of environmental protection, in the wake of climate change.

But this does not looks like a sufficient criteria to me, as it is also important to plant the right trees, which these days are the ones capable to resisting to more extreme weather. According to this source (unfortunately in German), some of the trees that are suitable for these harsher climate conditions, with the respective genus, are:

The table above suggests that Berlin’s tree diversification endeavour is not going in the right direction from the point of view of adaptation to climate change, for some of the tree types that are suitable for harsher weather conditions have had lower growth rates than Berlin’s overall trees stock. It should be stressed, however, that this is not a complete formal statistical study and that it is based on a limited understanding of trees desired properties.

Conclusions

In this article, building on BUND’s report on the evolution trend of Berlin’s tree stock, I analysed Berlin’s climatic data and related it to detailed data about the city’s trees.

1. Although Berlin is a very green city, it has lost around 1,000 trees on average per year in the 2012/2019 period, due to a number of fellings that is larger than newly planted trees.
2. While the evidence that Berlin’s average level of precipitation is falling is not that strong, there does seem to be strong evidence that the city’s average temperature is rising.
3. If in the last decades there has been a diversification of Berlin’s tree types, the stock of trees that are suggested to be more resilient to a harsher climate (drier and warmer) has grown below average in the 2000–2021 period.

To read more about this analysis, see the link to my Github available here.