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TREE CROWNS AND CLIMATE CHANGE – SCIENTISTS FROM UWR IN ‘SCIENCE’

Tree crowns protect forests from global warming – but only for a time

Coolness that is generated by dense tree crowns protects forest organisms from extreme temperatures and facilitates their adjustment to global warming. That is the result of a study done by scientists engaged in the Swiss Federal Institute for Forest, Snow and Landscape Research project (WSL) that was published in ‘Science.’ In the international team, there is Dr Kamila Reczyńska and Dr hab. Krzysztof Świerkosz from the Faculty of Biological Sciences at the University of Wrocław, as well as scholars from other Polish institutions, including the University of Agriculture in Cracow, the University of Rzeszów, and the University of Warsaw.

Forest as a ‘climatic time capsule’

Weather stations are usually situated in the open space, and the temperature is being measured 1.5-2m above the ground (not in a forest area and far from the earth’s surface where most of the animals and plants live). Therefore, data gathered from these stations are of less relevance to the forest communities and they do not say much about the potential risks.

An international group of researchers managed by Florian Zellweger from WSL and the University of Cambridge, have received the first detailed data regarding warming under the tree canopy, which showed differences between this process taking place inside and outside the forest. It was possible thanks to the measurements taken in 100 different locations inside the forest, which then were compared with the data that have been gathered for past 80 years from 3 000 locations in Europe.

W tym miejscu 120 lat temu znajdowało się pastwisko z pojedynczymi starymi drzewami. Wystarczyło 120 lat bez ingerencji człowieka by odtworzył się w pełni wykształcony ekosystem leśny. Góry Złote
Photo Krzysztof Świerkosz

“We all know that the world around us is changing, and many scientists want to learn more about it. For us, the primary interest is how forests are changing, both under the influence of natural factors, climate change, as well as people,” Dr Świerkosz says.“ For this purpose, we search for places where the measurement was taken some time ago (we go as far back in time as to 1950s), and then we make it again in the same place.”

“In these locations, we take so called phytosociological photos, which include a list of species from all the forest layers: trees, shrubs and undergrowth in the area of 200-400m²,” Dr Reczyńska explains. “While comparing archival photos with the modern ones, we check how the flora has changed, which species have disappeared and which new ones have appeared. With the use of special statistical software, we try to investigate why these changes have happened. This is the standard procedure in such publications.”

Las grądowy. Mszana i Obłoga, Pogórze Kaczawskie
Photo Katarzyna Reczyńska

Many people associate field research in the forest with marveling at the beauty of nature or listening to singing birds. However, the reality is not that romantic. Majority of scientists has suffered from Lyme disease, and some have had meningitis.

“We are bitten by mosquitos or flies, and increasingly often we meet hornets. Also, weather is now more and more unpredictable, bringing thunderstorms and tree-breaking gales. This is not a job for everyone,” Świerkosz says.

In ‘Science,’ the scientists write that the climate change measurements that were taken in the open space do not reflect the real change of temperature under the tree canopy. If tree stands are too dense, they buffer climate change and create a sort of ‘time capsule’ in which the changes do not happen that fast. However, if the stand becomes  less dense, the temperature of the lower layers will drastically increase.  As Florian Zellweger says, that knowledge is crucial for understanding the influence of climate change on the biodiversity of forests.

Masowe kwitnienie śnieżycy wiosennej Leucoium vernum to jeden z najpiękniejszych widoków kwietniowego lasu, okolice Żukowa, Wzgórza Dalkowskie
Photo Krzysztof Świerkosz

The fact that nature reacts to climate change is not a surprise to anybody. The scale and speed of this process, as well as mutual dependence of different factors and living organisms, are a subject of constant research.

“The scale of buffering climate change through keeping microclimate stable in a forest environment has baffled us all,” Dr Reczyńska says.“ Implications of these findings have a dark side, too. Until recently, we had thought that exposing forest floor through cutting trees might result in quick recovery thanks to growing new generations of trees and, thus, returning plants that are typical of a forest floor, as it was in the past. However, we have learnt that today this process might look differently. The typical forest plants might have to give place to other species that have higher temperature adaptability or to nitrophiles, as we found out in our recently published study. Therefore, our forests are increasingly more impoverished and endangered.”

Delay in adaptation to climate change

All organisms have an optimal range of temperature in which they grow. When the climate becomes warmer, many of them are replaced by thermophiles, which benefit from the warmer temperature. Because forest organisms have lower optimal temperature range, they lag behind when it comes to adaptation to global warming.

As Zellweger says, “In the context of climate change, many species live in an increasingly less optimal range of temperature.”

Consequently, when the canopy layer protecting forest from the heat becomes more open, either naturally or by humans, the plants that grow below it will experience drastic increase of temperature – something that they are not prepared for. Many species are not able to adapt quickly enough and, therefore, they might become obsolete on a global scale.

Considering the forecasted heatwave in Europe, it will probably change the biodiversity in the forests and may cause problems for plants and animals that are used to cooler and more humid environment. For that reason, it is important that forest managers take into consideration the impact of work being done in the forest on the biodiversity. Too extensive tree cutting in natural deciduous forests will lead to significant and irreversible changes.


Research described in ‘Science’ is being done in many different institutions all over the world but it yields different results locally. Only after having synthetized data from different stations in different countries or continents, a bigger and clearer picture emerges. Therefore, scientists join forces and create databases on which they can work together. Scholars from the University of Wrocław are engaged in work on ‘forestREplot’ database, which was created seven years ago by the Forest & Nature Lab at the Ghent University in Belgium. Until now, it has covered 4646 km² in 87 stations in Europe and North America.

Published by: Dariusz Tomaszczyk

25 May 2020

last modification: 23 Jun 2020