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Trade-offs in using European forests to meet climate objectives

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The Paris Agreement advances forest management as one of the pathways to halt climate warming through carbon dioxide (CO2) emission reduction1. The climate benefits from carbon sequestration from forest management may, however, be reinforced, counteracted, or even offset by concurrent management-induced changes in surface albedo, surface roughness, biogenic volatile organic compound emissions, transpiration, and sensible heat flux2–4. Forest management could, thus, offset CO2 emissions without halting global temperature rise. It remains, therefore, to be confirmed that sustainable forest management portfolios for the end of the 21st-century for Europe would comply with the Paris Agreement, i.e., reduce the growth rate of atmospheric CO2, reduce the radiative imbalance at the top of the atmosphere, and neither increase the near-surface air temperature nor decrease precipitation. Here we show that a spatially-optimized portfolio that maximises the carbon sink through carbon sequestration, wood use and product and energy substitution, reduces the growth rate of atmospheric CO2 but does not meet any of the other criteria. The portfolios that maximise the carbon sink or forest albedo pass only one, albeit different, criterion. Managing the European forests with the objective to reduce near-surface air temperature, on the other hand, will also reduce the atmospheric CO2 growth rate, thus meeting two out of four criteria. Our results demonstrate that if present-day forest cover is sustained, the additional climate benefits through forest management would be modest and local rather than global. Based on these findings we argue that if adaptation would require large-scale changes in species composition and silvicultural systems over Europe5,6, these changes could be implemented with little unintended climate effects.
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LETTER https://doi.org/10.1038/s41586-018-0577-1
Trade-offs in using European forests to meet climate
objectives
Sebastiaan Luyssaert1,2*, Guillaume Marie1, Aude Valade3,5, Yi-Ying Chen2,6, Sylvestre Njakou Djomo4, James Ryder2,7,
Juliane Otto2,8, Kim Naudts2,9, Anne Sofie Lansø2, Josefine Ghattas3 & Matthew J. McGrath2
The Paris Agreement promotes forest management as a pathway
towards halting climate warming through the reduction of carbon
dioxide (CO2) emissions1. However, the climate benefits from
carbon sequestration through forest management may be reinforced,
counteracted or even offset by concurrent management-induced
changes in surface albedo, land-surface roughness, emissions of
biogenic volatile organic compounds, transpiration and sensible
heat flux2–4. Consequently, forest management could offset CO2
emissions without halting global temperature rise. It therefore
remains to be confirmed whether commonly proposed sustainable
European forest-management portfolios would comply with the
Paris Agreement—that is, whether they can reduce the growth rate
of atmospheric CO2, reduce the radiative imbalance at the top of the
atmosphere, and neither increase the near-surface air temperature
nor decrease precipitation by the end of the twenty-first century. Here
we show that theportfolio made up of management systemsthat
locallymaximize the carbon sink through carbon sequestration,
wood use and product and energy substitution reduces the growth
rate of atmospheric CO2, but does not meet any of the other criteria.
The portfolios that maximize the carbon sink or forest albedo pass
only one—different in each case—criterion. Managing the European
forests with the objective of reducing near-surface air temperature,
on the other hand, will also reduce the atmospheric CO
2
growth rate,
thus meeting two of the four criteria.Trade-off are thus unavoidable
when using European forests to meet climate objectives. Furthermore,
our results demonstrate that if present-day forest cover is sustained,
the additional climate benefits achieved through forest management
would be modest and local, rather than global. On the basis of these
findings, we argue that Europe shouldnot rely onforest management
to mitigate climate change.The modest climate effects fromchanges
in forest management imply, however, that if adaptation to future
climate were to require large-scale changes in species composition
and silvicultural systems over Europe
5,6
to climate change with neither positive nor negative climate effects.
Following the Paris Agreement, the European Union and its 28
member states have committed to a 40% domestic reduction in
greenhouse-gas emissions compared to 1990 levels by 2030. About 99%
of this reduction is expected to come from emission reductions and
the remaining 1% from land use, land-use change and forestry
7
. The
commitment to reduce domestic greenhouse-gas emissions through
forestry is in turn reflected in the national strategies of several European
countries for energy, climate change and forestry
8–10
. These strategies
typically focus on enhancing forestry-based sinks and reservoirs and
developing neutral- or negative-emission approaches based on woody
biomass. Furthermore, European forest owners who have reported to
have experienced climate change have indicated that this experience
influenced their management decisions11. Hence, climate change and
the Paris Agreement are already shaping forest-management decisions.
Despite being explicitly mentioned in both the Kyoto Protocol
12
and
the Paris Agreement
1
, little is known about the climate effects of forest
management, including the effects of human-induced changes in tree
species and silvicultural systems3,13,14.
This study searches for spatially explicit forest-management portfo-
lios for Europe that comply with the Paris Agreement up to the turn of
the twenty-first century. The agreement requires that forest manage-
ment jointly reduces the growth rate of atmospheric CO2 (Articles 4 and
5) and the radiative imbalance at the top of the atmosphere (Article 2).
Furthermore, forest management compliant with the Paris Agreement
should neither increase the near-surface air temperature (hereafter
referred to as ‘air temperature’) nor decrease precipitation, because
changing the climate of the terrestrial biosphere would make adaptation
to climate change (Article 7) even more difficult (seeSupplementary
Information, ‘Operationalizing the Paris Agreement’).
Simulation experiments that combine vegetation modelling, climate
modelling, vegetation–climate feedbacks and life-cycle analysis are used
to quantify the CO
2
emissions, radiative imbalance at the top of the
atmosphere, air temperature and precipitation of three spatially explicit
forest-management portfolios for Europe (Extended Data Fig.1). Each
portfolio has a distinct objective: maximize the forest carbon sink, max
-
imize forest albedo or reduce air temperature.
All portfolios start from the same 2010 species and age–class distri-
bution. Once an individual forest reaches maturity, six scenarios are
explored: (i) refrain from harvesting; (ii) har vest, replant the same species
and apply the same silvicultural system as before; (iii) harvest, replant
the same species and thin before the final felling; (iv) harvest, change to
the most common deciduous species in that region and thin before the
final felling; (v) harvest, change to the most common deciduous species
in that region and manage it as a coppice; and (vi) harvest, change to
the most common conifer species in that region and thin before the
final felling. Subsequently, portfolios are constructed by selecting the
best-performing management scenario for each of the three objectives
and for each 0.5° × 0.5° grid cell in the European domain.
In contrast to previous land-use simulation experiments, our portfo-
lios simulate a realistic rate of change for tree-species distributions and
silvicultural systems because changes are only implemented following
a harvest or stand-replacing mortality. Thus, management changes are
dictated by forest growth and human choices within natural constraints,
rather than through externally prescribed harvest volumes or through
strictly natural succession.
A management portfolio that maximizes the carbon sink
15,16
reflects
the widely held view that the net climate effect of forest management is
dominated by decreasing the growth rate of atmospheric CO2 through
forest-based carbon sequestration, carbon storage in wood products, and
material and energy substitution. Implementing the sink-maximizing
-
verting 475,000km
2
of deciduous forest in central and southern Europe
1Faculty of Science, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands. 2Laboratoire des Sciences du Climat et de l’Environnement (LSCE/IPSL), CEA-CNRS-UVSQ, Université Paris-Saclay,
Gif-sur-Yvette, France. 3Institut Pierre Simon Laplace (IPSL), Paris, France. 4Department of Agroecology, Aarhus University, Tjele, Denmark. 5Present address: Global Ecology Unit CREAF-UAB,
Cerdanyola del Vallès, Spain. 6Present address: Research Center for Environmental Changes (RCEC), Academia Sinica, Taipei, Taiwan. 7Present address: National Physical Laboratory, Teddington,
London, UK. 8Present address: Helmholtz-Zentrum Geesthacht (HZG), Climate Service Center Germany (GERICS), Hamburg, Germany. 9Present address: Max Planck Institute for Meteorology,
Hamburg, Germany. *e-mail: s.luyssaert@vu.nl
Corrected: Author Correction
11 OCTOBER 2018 | VOL 562 | NATURE | 259

Supplementary resource (1)

... Our global study particularly showed that the tree species composition of a forest, by determining the dominant plant traits, was the first driver of its capacity to store SOC, but that the role of forest composition was context-dependent 71 , meaning that probably no unique global mitigation strategy exists 72 . Indeed, our study suggests that functional traits of tree species do not play an important role for SOC storage in warm-wet regions, like tropical regions. ...
... Beyond their positive effect on SOC accumulation ( Supplementary Fig. S8), conservative tree species are adapted to maintain their growth in constrained and competitive environments 69,70 . They are more able to profit from the CO 2 enrichment of the atmosphere than acquisitive species 78 , and they only have a slight effect on air temperature through biophysical effects 72,77 . In boreal forests, where SOC constitutes the largest carbon pool of the ecosystem 2 and where conifers are welladapted to the widely-spread poor soils, forest composition is consequently a major force for enhancing SOC storage. ...
... Further, most land management practices are not included in the model used here and were absent in many other CMIP5 models. Recent modeling studies [Davin et al., 2014;Luyssaert et al., 2014a;Luyssaert et al., 2018;Naudts et al., 2016;Julia Pongratz et al., 2018] con rm the land management practices such as irrigation, no-tillage, grazing and forest management can considerably alter both biophysical and biogeochemical effects in large regions of the world. Therefore, our results stress the importance of improving Earth system model structures by implementing a consistent anthropogenic LCC reconstruction and including land management practices [Julia Pongratz et al., 2018; . ...
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