TDUK SupplyingTimber Issue 8 DIGITAL - Magazine - Page 67
———— THE "FOREST PUMP" ————
When to fell a tree (from a
carbon storage perspective)
When a tree is felled, it can
no longer absorb carbon but
the wood continues to hold
the carbon it has previously
sequestered. However, it should
be noted that a growing tree
cannot go on absorbing carbon
forever. Older trees start to slow
down on absorption before
stopping altogether and dying,
whereupon the rotting trees
begin to release their carbon
back into the atmosphere as
methane (CH4), which is also a
greenhouse gas - one 80 times
more potent than CO2, albeit
with a shorter lifespan.
While it may seem a shame
to fell a tree in its prime, from
a climate perspective this is
the ideal time to do so – before
growth and hence carbon
absorption slows and stops.
(Farmers do the same as
foresters – they harvest their
crops at the optimum point in
their life cycle.) We can then
move the maximum amount
of carbon stored in the timber
into wooden products in the
built environment, for example,
house frames, 昀氀oor-boards,
beams and roof joists.
Given scientists across the
world are (rightly) working
hard to ascertain how we
can mechanically/chemically
remove carbon from the
atmosphere and then store it
safely underground (a process
known as carbon capture and
storage – CCS), why would
we not also want to maximize
the potential of the built
environment to safely store as
much carbon as possible? Every
existing building in Europe could
safely store between 2‒6 tonnes
of carbon in wood products,
including wood 昀椀bre insulation.
The "forest pump"
The often-cited paper Buildings
as a global carbon sink
(Churkina et al) has promoted
the idea of the 'forest pump'.
We grow trees in the forest and
they sequester carbon. We then
fell the trees. We turn the trees
into wooden building products.
These products store carbon in
the built environment for 50, 75,
perhaps 100 years. Meanwhile
we replant new trees, saplings,
and they start the carbon
sequestration process all over
again, and will in time produce
more timber for use in the built
environment, and so on.
There is no question that the
built environment’s ability to
store carbon is massive. For
instance, a 2021 report from
ASN Bank and Climate Cleanup
stated that there was the
potential in the Netherlands to
use the projected one million
new homes required before
2030 to store 50MtC. This is a
quarter of the country’s annual
emissions.
The USA is also waking up to
wood’s carbon-storing ability.
A 2023 report from the Rocky
Mountain Institute recognised
that the construction of new
SUPPLYING TIMBER
67
homes in the USA resulted in
50MtC of emissions annually.
However, it went on to argue
that the very same homes
could store signi昀椀cantly more
carbon than their construction
generated if carbon-storing
materials were deployed in their
construction, helping the US
reach its climate targets quickly
and e昀케ciently. The Institute
referred to this opportunity as
“low-hanging fruit”.
Timber as a substitute for
other materials
Timber’s ability to store carbon
safely in the built environment
is therefore a great asset in the
battle against climate change.
But the good wood news does
not end there, because wood
has another valuable attribute:
the ability to substitute for other
materials that are signi昀椀cantly
more carbon-intensive. This
SPRING 2025
↑
The Shambles:
a collection of
medieval timber
buildings still
standing in York.
TIMBER KNOWLEDGE
I
f we build with wood, we
safely store the carbon
in the built environment –
potentially for hundreds of
years. At the same time, back
in the forest, we replant the
felled trees with new saplings –
often three saplings per felled
tree – to ensure the process is
circular and hence sustainable.
In sustainably managed forests,
new trees are regenerated to
replace trees that are harvested
so that there is no net loss of
forest carbon.
