14 | New Scientist | 20 November 2021
News
A EUROPE-wide study has found
that the drying out of soils across
the continent as the planet heats
up is leading to the deaths of
more and more trees.
“European forests are suffering,”
says Jan-Peter George at the
University of Tartu in Estonia.
“It needs to be made clear to
everyone in Europe, regardless
of whether you are in the
north or the south, that this
will become a huge problem.”
There is already growing
evidence that severer droughts
due to climate change are killing
off more trees in Europe. But most
previous studies have focused
on particular areas or been based
on satellite surveys, where it
is hard to know whether a tree
has died or been felled.
George and his colleagues have
instead analysed about 3 million
on-the-ground observations
made as part of an initiative
called ICP Forests, which was
set up in the 1980s.
Using this data set meant the
team could exclude tree deaths
that were down to felling. The
analysis also excluded ash trees,
which are being wiped out by
the double whammy of a fungal
disease and an invasive beetle.
The researchers found that
annual mortality rates are rising
for all tree species. Norway
spruce has been hardest hit,
with mortality rates 60 per cent
higher on average between 2010
and 2020 than between 1995 and- Rates are up 40 per cent
for Scots pine, 36 per cent for
European beech and 3.5 per cent
for oak (bioRxiv, doi.org/g5xd).
What’s more, for all species and
regions, annual mortality rates
have been positive since 2012. This
means that more trees have been
dying than usual, compared with
the long-term average. “This could
mean that European forests
reached a critical point in 2012,”
says George.
The researchers also looked
at soil moisture, as estimated
by a model fed with rainfall and
run-off data. They found that
abnormally low soil moisture
was the main driver of tree
deaths over the following year.
In northern regions such as
Scandinavia, many people still
think local forests won’t be
affected by warming-driven
droughts, says George, but
all forests across Europe
are already being affected,
including northern boreal ones.
“It is a very unsettling study, but
also not surprising,” says Hannah
Mowat at Fern, an organisation in
Brussels, Belgium, that campaigns
to protect forests. “It’s not just the
Amazon forest that is drying up,
our own forests are parched, and
it is causing them to die back too.”The researchers cannot be sure
why trees often die a year or so
after droughts, but there are many
possible reasons, says George.
Severe water stress starves trees of
resources and can also damage the
water-conducting tissues, making
them vulnerable to other stresses
such as pests and diseases.
For instance, drought-hit
conifers produce much less resin
than they would normally, andresin is what protects them from
bark beetles. “Bark beetles love
to attack drought-stressed trees,”
says George.
Rising tree deaths will have
many consequences. For starters,
forests will soak up less carbon
as older trees are replaced by
younger ones.
“There is no question that
old-growth forests are able tosequester more carbon than
young forests,” says George.
However, he thinks it is
unlikely that Europe’s forests
will become a carbon source
in the long term.
Europe is also already suffering
from more frequent and severe
wildfires, and more dying trees
will increase the danger even
further. “As more and more dry and
dead material accumulates, forests
will be much more vulnerable
to wildfires,” says George. “This
is the dynamics we usually know
from the US and Australia.”
The timber industry could also
be affected. One of the reasons
why Norway spruce is the species
hardest hit is that it has been
planted in areas outside its natural
range for timber production,
typically in monoculture
plantations. Forest owners need
to start growing a mix of species
to make plantations more
resilient, says George.
There could be economic
consequences, too. Timber prices
in Europe are already high because
of demand from the US and China,
says George. Rising tree losses
could push prices even higher
and have knock-on effects on the
building industry and beyond.
“This is also yet again evidence
that to climate-proof our forests,
to make them more resilient, we
need to make them more diverse,”
says Mowat. “This study is a
rude awakening, but it could
mean that the forests of tomorrow
are actually more biodiverse,
if we heed the scientists making
this wake-up call.”
A lack of water due to global
warming is far from the only
threat to Europe’s trees. Exotic
pests and diseases have decimated
elms and now ashes, and threaten
other trees including oaks. Wood
harvesting has also increased
by 50 per cent since 2016. ❚“This is evidence that to
climate-proof our forests,
we need to make them
more diverse”EcologyMichael Le PageMATTHIAS^ SCHRA
DER/AP/SHUTT
ER
ST
OCKDry soils mean Europe’s trees
are dying at a faster rate
A forest on Brocken
mountain near
Schierke, Germany