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GCB Bioenergy. 2020;12:1034–1035.
Received: 12 May 2020
Accepted: 13 May 2020
LETTER TO THE EDITOR
Recognizing the importance of unmanaged forests to mitigate
The most effective means for keeping carbon out of the at-
mosphere to meet climate goals is to protect primary forests
(Mackey etal.,2020) and continue growing secondary forests
to accumulate additional carbon (proforestation; Moomaw,
Masino, & Faison,2019) while reducing emissions from all
sources including bioenergy. We find several assumptions
and conclusions in the Opinion piece by Schulze etal.(2020)
inaccurate or questionable.
The Opinion compares differences in carbon storage be-
tween ‘unmanaged’ and ‘managed’ European forests, and
states, ‘(f)or unmanaged forests, the contribution to climate
change mitigation through storage is very small or close to
nil’. However, no evidence is cited to support this conten-
tion, which is inconsistent with several published empirical
studies and theoretical analyses (Erb et al., 2017; Houghton
& Nassikas,2018; Lutz etal.,2018). In order to support ef-
fective climate policy, we are responding to identify some of
the errors of omission in the Opinion.
Based on UNFCCC accounting rules, climate mitigation
depends on the relative carbon stocks in the biosphere and
atmosphere, not on sequestration rates as annual flows. In
order to minimize the amount of carbon in the atmosphere,
the cumulative carbon in trees and soils must be maximized
(Mackey etal., 2020). The maximum carbon stored in for-
ests occurs when forests are allowed to continue growing,
a management practice called ‘proforestation’ (Moomaw
etal.,2019). As the harvest rotation period is shortened, less
carbon is stored in trees averaged over the harvest period in-
tervals, leaving more in the atmosphere (Sterman, Siegel, &
Rooney-Varga, 2018). Each harvest also releases additional
biogenic and fossil fuel carbon emissions from the harvest
process (Harris etal.,2016).
Schulze et al. cite two studies from outside Europe, which
found higher area-averaged stand volumes in unmanaged
compared to managed forests. However, they state that this
difference does not exist in the temperate zone of Central
Europe without offering any data to support the claim. A
number of studies confirm higher stand volumes in older
forests in Central Europe. The results from Jacob etal.(2012)
show a higher standing biomass carbon pool in old-growth
forest than younger developmental stages. Horváth et al.
(2012) also found that old-growth forests in reserves had the
highest proportion of large trees in diameter and height, and
the largest volume of dead wood, meaning that total biomass
carbon stock is higher compared with secondary forests that
were being actively managed or recently abandoned.
Schulze et al. do not acknowledge or investigate the ca-
pacity for unmanaged forest to store more carbon than
currently. The Opinion does not provide the age class dis-
tribution of the forests that were analysed, or reflect the fact
that even presently unmanaged forests in Europe have likely
been harvested at some time in the past. Forests in Europe do
not differ from North America or Australia in terms of their
ecological processes of carbon accumulation, but rather, have
not yet reached their ultimate cumulative carbon capacity.
Considering Fagus and Picea as the dominant tree spe-
cies in Central Europe is an over-simplification, as the
European Environment Agency maps five biogeographical
regions in this region: Continental, Pannonian, Alpine and to
a smaller extent Atlantic and Mediterranean. While Sabatini
etal.(2018) found an uneven distribution of primary forests
in Europe, with concentration often in mountainous areas
with Fagus and Picea, excluding the importance of oak forest
habitats and soft-wood riverine forests is a major oversight.
In their conclusions about mitigation benefits of forest
management, Schulze et al. use annual rates of carbon seques-
tration and emissions (flows) that are the incorrect metric to
assess forest carbon stocks or alternative forest management
on those stocks. It is also an inappropriate metric for meeting
the Paris Agreement goals. The authors also greatly underes-
timate the carbon stock in biomass, because only stemwood is
counted, and branches, bark, canopies and roots are excluded
along with soil carbon that is greatest in older forests.
The importance of primary (unlogged) forests lies in the
magnitude and longevity of their carbon stock. In order to re-
verse the decreasing forest carbon stocks in Europe (European
Environmental Agency, 2019), the largest forest carbon stores
must be protected and additional forests must be allowed to
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© 2020 The Authors. GCB Bioenergy Published by John Wiley & Sons Ltd
This is a letter regarding Schulze et al. 12, 186–197.
LETTER TO THE EDITOR
continue accumulating carbon (proforestation). Harvesting
for bioenergy increases atmospheric carbon, and slows the
accumulation of forest carbon (Sterman etal.,2018).
Based on the above, we disagree with the conclusions
by Schulze et al. The climate change mitigation potential of
unmanaged forests is significantly greater because of their
greater cumulative carbon storage than for forests managed
for lumber and bioenergy. Scientific findings are critically
important to decision makers addressing climate change.
We urge managers to disregard the erroneous carbon metrics
for forest carbon accounting in the Opinion and utilize more
We thank Griffith University that financially supports the re-
search project on ‘Boreal and Temperate Primary Forests and
1Frankfurt Zoological Society, Frankfurt am Main,
2GEOS Institute, Ashland, KY, USA
3Griffith University, Brisbane, Qld, Australia
4Wild Heritage, Berkeley, CA, USA
5University of Cambridge, Cambridge, UK
6Tufts University, Medford, MA, USA
7Faculty of Ecology and Environmental Science, TU
in Zvolen, Slovakia/European Parliament, Brussels,
Zoltán Kun https://orcid.org/0000-0002-4688-8936
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