Considering that the current carbon sink in the EU LULUCF sector is about 300 Mt CO2/year (about 400 MtCO2e/year for forests alone)—offsetting about 7% of total EU GHG emissions, with rather stable values in the last 25 years , reaching a 25% contribution would require (i) that the current LULUCF sink nearly doubles by 2030, something not supported by any peer-reviewed publication, and (ii) that this sink is entirely counted as a mitigation effort towards the EU 2030 target. This second point has never been seriously considered in the political debate, because it has long been recognized that the existing forest sink is not entirely a result of direct and recent mitigation actions, but instead largely due to historic management activities and the effects of environmental change [6, 7] (see Additional file 1: Section S2.1). Accordingly, in the recent EU LULUCF legislation  not all of the forest management sink will count toward the mitigation target. Instead, only the portion that will exceed a predefined science-based “forest reference level” benchmark will count [7, 9], reflecting the atmospheric impact of additional actions. In the event that the forest sink is smaller than this benchmark, then the corresponding accounted “debit” from forest management will need to be compensated for, through extra emission reductions in other land uses or in other GHG sectors, such as energy.
Furthermore, the EU climate legislation  has capped both the amount of possible “credits” from managed forest land (equal to 3.5% of 1990 emissions) and the maximum contribution from the LULUCF sector toward the EU target at 280 MtCO2e for the period 2021–2030. If this latter value is annualized (i.e., 28 MtCO2e/year), it corresponds to about 1% of the EU 2030 emission reduction target. Therefore, contrary to the assumption of Luyssaert et al., almost all of the EU mitigation effort in 2030 is expected to come from emission reductions from non-LULUCF sectors and only a very small part directly from LULUCF.
Forests may contribute to mitigation also indirectly, especially through the utilization of wood as an energy source in place of fossil fuels. When the harvesting of forest biomass for energy purposes is increased, a decrease in carbon stock is reported in the LULUCF sector whilst GHG emission savings appear in the energy sector. For the EU, these savings are currently estimated to be about 130 MtCO2e/year , relative to about 90 MtCO2e/year in 1990 (see Additional file 1: Section S2.2). Several studies suggest a larger future potential associated with additional sustainable harvest of EU wood for bioenergy . However, since this additional harvest would temporarily lower the forest sink, the relevant question is which portion of this bioenergy potential can be realized without generating accounted debits in the forestry and consequently in the LULUCF sector by 2030. Based on various sources [7, 11, 13], and assuming no LULUCF debits, we estimate that EU forest-based bioenergy derived from additional harvest could save about 150 MtCO2e/year in 2030. Relative to the EU’s climate target under the Paris Agreement (reduction of about 2250 Mt CO2e/year from 1990 to 2030), the indirect contribution of EU forest-based bioenergy to the EU 2030 emission reduction target would realistically add another 3% ((150 − 90)/2250).
We fully share with Luyssaert et al.  the view that forest management strategies aiming at climate change mitigation should not focus solely on GHG emissions, but should consider also any robust evidence on the potential impact of biophysical effects. However, we think that the uncertainty of the findings by Luyssaert et al.  has not been adequately emphasized. Although the local and seasonal climate biophysical effects can be retrieved with some confidence—e.g. afforestation warms winter surface temperature (decreased albedo) and cools summer surface temperature (increased evapotranspiration) —the net annual impact of combined local and non-local effects in temperate zones, such as most of the EU, is highly uncertain. This is because in temperate regions radiative and non-radiative effects have similar magnitude but opposite impacts on the mean annual temperature. As a result, observation-based assessments and models don’t agree on the magnitude, and often not even on the sign, of the net annual biophysical climate effects of forestry in temperate zones [15, 16]. Despite the good advancements in Luyssaert et al.’s model (e.g., in representing differences between tree species and stand structures), the net annual biophysical climate impact of forest management in the EU remains more uncertain than the net CO2 impact. Rather than emphasizing these crucial caveats, Luyssaert et al.  used their results on the combined biophysical and biogeochemical effects to challenge their perception of EU policy towards forestry and climate change.
If the aim is to encourage countries to start considering biophysical effects in their policies, more emphasis should be put on seasonal and local impact of biophysical effects of forest cover change, including synergies and trade-offs with a carbon-oriented management, rather than on the net annual biophysical climate impact at EU level. These seasonal and local impacts are less uncertain and more relevant in the context of changes in diurnal temperature excursions  and heat extremes , and therefore for our perception of climate change. Concrete and accessible tools should be developed to allow countries to assess themselves the biophysical effects of different forest management scenarios . Furthermore, model projections should be complemented with observational evidences, and a comprehensive communication of the uncertainty and range of applicability of the scientific findings is required to gain credibility in the policy domain.
Irrespective of the high uncertainty of biophysical effects on climate, the argument by Luyssaert et al. , that efforts for enhancing the CO2 sink from forest management are counterbalanced by negative biophysical climate effects—resulting in a “zero-sum” climate outcome, could be interpreted as forest management not being important to fight climate change. We think that would be a wrong conclusion. In fact, the recent inclusion of forests into the EU 2030 economy-wide climate targets  represents a key incentive for identifying the country-specific optimal mix, in terms of overall GHG balance, between strategies focused on conserving and/or enhancing the sink, as explicitly requested by the Paris Agreement (Art. 5), and strategies focused on using more wood to reduce emissions in other GHG sectors (including both energy and material substitution ). Without political “sticks and carrots” on GHG emissions, i.e. if forests were excluded from climate change mitigation strategies, there would be no incentive for conserving the current forest CO2 sink, and no disincentive for a possible over-use of forest resources (e.g., for bioenergy purposes), which could drastically reduce the current CO2 sink.