Heat stress: The climate is putting European forests under sustained
pressure

Date:
July 16, 2020
Source:
University of Basel
Summary:
No year since weather records began was as hot and dry as 2018. A
first comprehensive analysis of the consequences of this drought
and heat event shows that central European forests sustained
long-term damage. Even tree species considered drought-resistant,
such as beech, pine and silver fir, suffered.



FULL STORY ==========================================================================
No year since weather records began was as hot and dry as 2018. A first comprehensive analysis of the consequences of this drought and heat event
shows that central European forests sustained long-term damage. Even tree species considered drought-resistant, such as beech, pine and silver fir, suffered. The international study was directed by the University of Basel, which is conducting a forest experiment unique in Europe.


========================================================================== Until now, 2003 has been the driest and hottest year since regular
weather records began. That record has now been broken. A comparison of
climate data from Germany, Austria and Switzerland shows that 2018 was significantly warmer.

The average temperature during the vegetation period was 1.2DEGC above
the 2003 value and as high as 3.3DEGC above the average of the years
from 1961 to 1990.

Part of the analysis, which has now been published, includes measurements
taken at the Swiss Canopy Crane II research site in Basel, where extensive physiological investigations were carried out in tree canopies. The
goal of these investigations is to better understand how and when trees
are affected by a lack of water in order to counter the consequences of
climate change through targeted management measures.

When trees die of thirst Trees lose a lot of water through their
surfaces. If the soil also dries out, the tree cannot replace this water,
which is shown by the negative suction tension in the wood's vascular
tissue. It's true that trees can reduce their water consumption, but if
the soil water reservoir is used up, it's ultimately only a matter of
time until cell dehydration causes the death of a tree.

Physiological measurements at the Basel research site have shown the researchers that the negative suction tension and water shortage in
trees occurred earlier than usual. In particular, this shortage was
more severe throughout all of Germany, Austria and Switzerland than ever measured before.

Over the course of the summer, severe drought-related stress symptoms
therefore appeared in many tree species important to forestry. Leaves
wilted, aged and were shed prematurely.

Spruce, pine and beech most heavily affected The true extent of the summer heatwave became evident in 2019: many trees no longer formed new shoots
-- they were partially or wholly dead. Others had survived the stress
of the drought and heat of the previous year, but were increasingly
vulnerable to bark beetle infestation or fungus. Trees with partially
dead canopies, which reduced the ability to recover from the damage,
were particularly affected.

"Spruce was most heavily affected. But it was a surprise for us that
beech, silver fir and pine were also damaged to this extent," says
lead researcher Professor Ansgar Kahmen. Beech in particular had until
then been classified as the "tree of the future," although its supposed
drought resistance has been subject to contentious discussion since the
2003 heatwave.

Future scenarios to combat heat and drought According to the latest projections, precipitation in Europe will decline by up to a fifth by
2085, and drought and heat events will become more frequent.

Redesigning forests is therefore essential. "Mixed woodland is often propagated," explains plant ecologist Kahmen, "and it certainly has
many ecological and economic advantages. But whether mixed woodland
is also more drought-resistant has not yet been clearly proven. We
still need to study which tree species are good in which combinations, including from a forestry perspective. That will take a long time."
Another finding of the study is that it is only possible to record
the impacts of extreme climate events on European forests to a limited
extent using conventional methods, and thus new analytical approaches are needed. "The damage is obvious. More difficult is precisely quantifying
it and drawing the right conclusions for the future," says Kahmen. Earth observation data from satellites could help track tree mortality on a
smaller scale. Spatial patterns that contain important ecological and forestry-related information can be derived from such data: which tree
species were heavily impacted, when and at which locations, and which
survived without damage? "A system like this already exists in some
regions in the US, but central Europe still lacks one."

========================================================================== Story Source: Materials provided by University_of_Basel. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Bernhard Schuldt, Allan Buras, Matthias Arend, Yann Vitasse, Carl
Beierkuhnlein, Alexander Damm, Mana Gharun, Thorsten E.E. Grams,
Markus Hauck, Peter Hajek, Henrik Hartmann, Erika Hiltbrunner,
Gu"nter Hoch, Meisha Holloway-Phillips, Christian Ko"rner, Elena
Larysch, Torben Lu"bbe, Daniel B. Nelson, Anja Rammig, Andreas
Rigling, Laura Rose, Nadine K. Ruehr, Katja Schumann, Frank Weiser,
Christiane Werner, Thomas Wohlgemuth, Christian S. Zang, Ansgar
Kahmen. A first assessment of the impact of the extreme 2018 summer
drought on Central European forests.

Basic and Applied Ecology, 2020; 45: 86 DOI:
10.1016/j.baae.2020.04.003 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200716123000.htm

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