Timber iQ February - March 2019 // Issue: 42 | Page 11

NEWS
CLIMATE CHANGE IS MAKING THE
WOOD LIGHTER
With the combination of wood samples from the 1870s to
the present day coupled with the latest measurement
technology, the team at the School of Life Sciences
Weihenstephan demonstrated that the annually growing
wood has gradually become lighter since observations
began: by up to 8 to 12% since 1900. Within the same
period, the volume growth of the trees in central Europe has
accelerated by 29 to 100%.
In other words, even though a greater volume of wood is
being produced today, it now contains less material than
just a few decades ago. However, the explanation which
immediately comes to mind does not apply.
"Some people may surmise that the more rapid growth
could itself be the cause for our observations," says Dr
Peter Biber, co-author of the study. "In some tree species, it
is in fact the case that wider annual rings also tend to have
lighter wood. But we have taken this effect into account. The
decrease in wood density we are talking about is due to
other factors."
Instead, Pretzsch and his team see the causes as being
the long-term increase in temperature due to climate
change and the resulting lengthening of the vegetation
period. But the nitrogen input from agriculture, traffic and
industry also play a part. A number of details lead experts
to surmise this, such as the decrease in the density of late
wood and the increase in the percentage of early wood in
the annual rings.
LIGHTER WOOD – WHAT'S THE PROBLEM?
Lighter wood is less solid and it has a lower calorific value,
which is crucial for numerous application scenarios ranging
from wood construction to energy production. Less solid
wood in living trees also increases the risk of damage
events such as breakage due to wind and snow in forests.
But the most important finding for practical and political
aspects is that the current climate-relevant carbon
sequestration of the forests is being overestimated as long
as it is calculated with established but outdated wood
densities. "The accelerated growth still results in surplus
carbon sequestration," says Pretzsch. "But scaling up for the
forests of central Europe, the traditional estimate would be
too high by about 10 million metric tons of carbon a year."
The research group at the Chair for Forest Growth and
Yield Science at the TUM School of Life Sciences
Weihenstephan, led by Hans Pretzsch, investigates the
effect of climate change on the growth, stability and vitality
of trees. An important basis for this research are the
experimental plots of the Chair, on which the dynamics of
forests have been measured since 1879 to answer
ecological and economic questions. In the study reported
here, they contribute to measuring the human footprint in
forest ecosystems.