The commentary by Bright and colleagues (Bright et al., 2012) on our article on the (un)sustainability of ambitious forestry bioenergy targets (Schulze et al., 2012) supports our argument by stressing an important point, namely that contributing to climate mitigation is only one among several objectives that need to be addressed in forest management. That is why we discussed changes in forest management intensity in terms of their effects on the human appropriation of net primary production (HANPP), on biodiversity, as well as on economic and other sustainability concerns.
Changes in albedo and other climate forcings related to altered management and area of forests are indeed important issues, and Bright et al. (2012) discuss some important recent references. Most of this literature, however, does not relate to our argument because it is focused on the effects of land-cover changes, e.g. deforestation, afforestation or re-afforestation (e.g., Anderson et al., 2011). In contrast, we discussed effects of increased forest management intensity without deforestation – that is, activities that would result in thinning or only transient forest-free patches of land, both of which are expected to have a smaller effect on albedo and other biophysical properties of the land than the complete removal of the forest (Amiro et al., 2006; O'Halloran et al., 2012). Moreover, albedo changes of deforestation are thought to be only significant in boreal forests with snow cover during a substantial part of the year, although they are minor in other climates (Bonan, 2008; O'Halloran et al., 2012). Hence, if climate change reduces snow cover, the benefits of increased albedo might also dwindle. Science on these complex interrelations between biogeochemical and physical climate forcings of forestry is in its early stages (Anderson-Teixeira et al., 2012), resulting in large knowledge gaps and uncertainties (Bonan, 2008; Anderson et al., 2011). Therefore, biophysical feedbacks have, to our knowledge, not been incorporated into forest policies, which was also the reason why we did not discuss them in our article. Once biophysical considerations such as albedo will enter the list of criteria to be considered in forest management – which promises to be a complex endeavour – they will need to be gauged against many other aspects of forests, including C sequestration, biodiversity, wood production, livelihoods, landscape amenities and many more. Such policies also need to consider that global warming is not the only effect of elevated atmospheric CO2; e.g., ocean acidification (Doney et al., 2009).
To avoid unnecessary confusion associated with the GHG balance issues addressed by Bright et al. (2012), it is essential to clarify that the global warming effect of CO2 in the atmosphere does not depend on its source. Biogenic CO2 emitted into the atmosphere has exactly the same global warming potential as CO2 from fossil-fuel combustion. Per unit of energy, the amount of CO2 released from biomass combustion is about as high as that of coal and substantially larger than that of oil and natural gas (Haberl et al., 2012). The decisive question is whether or not bioenergy production is causally linked with additional carbon sequestration compared to the situation without bioenergy use (Searchinger, 2010).
When forests are used for bioenergy, this is a complex issue because the carbon balance depends on stock/flow relations that change over time as a result of the forests’ changing age structure (Körner, 2006). The figure presented by Bright et al. (2012) refers to the GHG effects of a single harvest event (as do the articles by Cherubini et al., 2011a,b). In contrast, we have discussed the GHG effects of a large-scale, sustained increase of wood harvest in forests (Holtsmark, in press). On the basis of recent literature, (Hudiburg et al., 2011; Holtsmark, 2012; Hudiburg, 2012) we have argued that increased management intensity will result in a permanent, not transient, reduction of the carbon stock of forests at the landscape scale – in other words, it is a carbon debt (Fargione et al., 2008) that needs to be repaid by the avoided GHG emissions from fossil-fuel combustion. Measures aimed at speeding up tree growth, e.g. fertilization, may increase the amount of bioenergy produced at the landscape scale (and hence reduce payback times), but they will not increase the average carbon stock in the forest (Körner, 2006).
Moreover, one may also question a central assumption underlying all the expected GHG benefits of bioenergy. Specifically, it is assumed that one additional unit of bioenergy supplied reduces fossil energy use by the same amount. A recent empirical analysis (York, 2012) found substantially lower displacement effects. Consequently, payback times may be much longer.
Needless to say, we do not endorse continued combustion of fossil energy and the related GHG emissions – but we insist that we must make sure that the cure is not worse than the ailment, and that the full array of benefits from ecologically intact, biologically diverse forests need to be taken into account when making well-informed political decisions on the potential role of forests in climate-change mitigation.