Forests in a changing world

The Earth is facing increasing anthropogenic pressure. At the beginning of the 21^st century it has become clear that human activities strongly and durably affect the atmosphere and the biosphere, putting at risk ecosystems and thereby human popu-lations (MA, 2005; IPCC, 2014). Since the 19^th century, continuously rising anthro-pogenic greenhouse gases emissions have led to an increase in atmospheric CO2

concentrations from 280 ppm to 415 ppm as of 2019, an unprecedented value since the early Miocene, 24M years ago (Pearson & Palmer, 2000). During the last dec-ade, CO2 emissions to the atmosphere reached on average 10 PgC year^-1, over-whelmingly as a result of fossil energy consumption and, to a lower extent, land use changes (Le Quéré et al., 2017). Anthropogenic greenhouse gas emissions are unequivocally the origin of ongoing climate change (Marotzke et al., 2017), trigger-ing, among others, a global average warming of 1 ºC compared to the preindustrial era (Sévellec & Drijfhout, 2018), which is projected to reach 4 ºC by the end of the century in a business as usual scenario (IPCC, 2014). Concomitantly, anthropogeni-zation, i.e. the direct modification of ecosystems by human populations, affects an increasing proportion of land surface (Ellis, 2015). Taken together, these global changes characterize a distinctive geologic epoch, the Anthropocene, where human societies have become a significant geological force (Crutzen, 2006). In the An-thropocene, human activities profoundly alter ecological patterns, processes and ecosystem services, which human populations depend upon.

Forests cover about 30% of land surface, and provide crucial ecosystem services to human societies. The most evident services of forest ecosystems are the provision-ing of woody (i.e. timber, firewood and fiber) and non-woody products (e.g. wild game, mushrooms, compounds with medicinal or industrial applications). Moreo-ver, forests support life as primary producers and by being key agents of soil for-mation. Forests are also involved in climate regulation by exchanging CO2, water and energy with the atmosphere, improve water quality and supply and have a high cultural value (MA, 2005). Regarding the global carbon (C) economy, forest eco-systems contain 45% of the terrestrial ecosystem C reserves (1600 PgC, twice the atmospheric C mass) and account for 50% of terrestrial Net Primary Production (NPP) (Bonan, 2008), i.e. the proportion C assimilated minus that respired by autotrophic organisms. Moreover, the balance between forest NPP and C losses to the atmosphere by heterotrophic respiration – i.e. the Net Ecosystem Production – NEP) is positive, resulting in forests accounting for 50% of the global C sink (Pan et al., 2011) and mitigating atmospheric C growth by 25% (Le Quéré et al., 2017).

Forests nevertheless are sensitive to global change impacts, which may compromise the delivery of associated ecosystem services (Mooney et al., 2009; Anderegg et al., 2013b; Trumbore et al., 2015; Seidl et al., 2016).

Until recently, global changes appear to have led to an overall increase in land surface greenness, at least in some regions (Myneni et al., 1997; Zhu et al., 2016;

Piao et al., 2020). Models point toward a large fertilization effect of elevated at-mospheric [CO2] as well as land cover changes (Chen et al., 2019; Piao et al., 2020). In Europe and in the Mediterranean, land abandonment seems to be the main driver of observed greening (Buitenwerf et al., 2018; Pausas & Millán, 2019). In addition to increased photosynthetic area, global warming and increased [CO2] resulted in global NPP increase during the 20^th century (Smith et al., 2016;

Campbell et al., 2017). However, a saturation of this trend is observed from the beginning of the 21^st century on, suggesting a shift in the effects of global changes on vegetation functioning, and particularly forests (Zhao & Running, 2010;

Peñuelas et al., 2017; Zhang et al., 2019). Amongst global threats, forest condition is heavily sensitive to climate variations and the fate of forests is highly uncertain in the context of climate change (Franklin et al., 2016). Importantly, climate change may affect forest functioning both positively and negatively, depending on the considered process, tree physiology and environmental conditions. Whereas atmos-pheric [CO2] increase potentially has a positive fertilizing effect on forest growth, through improved photosynthetic rates (McMurtrie & Wang, 1993; Ainsworth &

Rogersl, 2007), its realized effect depends on canopy temperature and water stress and on the actual limitation of forest growth by C assimilation (Van Der Sleen et al., 2015; Körner, 2015). Similarly, a hotter climate can enhance forest growth under mild conditions through an extension of the growing season and an increase of growth rates (Moser et al., 2009; Cuny et al., 2015). However, higher tempera-tures can also have a direct negative effect if optimum temperatempera-tures are overrun (Parent et al., 2010) and indirect negative effects through increased vapor pressure deficit, evapotranspiration and edaphic drought (Williams et al., 2012). Historical global patterns arising from these complex interactions lead to the expectation that climate change should improve forest growth in cold and humid environments and be deleterious in hot and dry environments (Harsch et al., 2009; Carnicer et al., 2011; Sánchez-Salguero et al., 2012; Franklin et al., 2016; Klesse et al., 2018).

However, recent heat waves and droughts have been shown to cause negative impacts on vegetation even in cold and humid areas (Williams et al., 2010; Allen et al., 2010; Babst et al., 2019), as temperature-related increases of atmospheric water

demand may offset the beneficial effects of warming (Barber et al., 2000; Allen et al., 2015). These effects tend to be underestimated in many dynamic global vegeta-tion models, which predict an overall positive response of vegetavegeta-tion to climate and atmospheric changes (Notaro et al., 2007; Smith et al., 2016; Pugh et al., 2018).

This discrepancy might arise from model overestimation of the direct fertilization effect resulting from increasing atmospheric [CO2] (Smith et al., 2016), which might be more limited than expected (Gedalof & Berg, 2010; Peñuelas et al., 2011;

Silva & Anand, 2012; Van Der Sleen et al., 2015). Overall, recent widespread increases in drought and heat-induced forest die-off events (Allen et al., 2010, 2015) highlight the vulnerability of forests in the face of climate change and poten-tially large societal consequences. Reducing uncertainties regarding simulated forest response to past and future climate is critical for anticipating future societies–

atmosphere–forest interactions (Friedlingstein et al., 2014; Huntzinger et al., 2017) and enabling potential mitigation and adaptation (Millar et al., 2007; Lindner et al., 2014; Keenan, 2015).