Editing IPCC:AR6/WGII/Chapter-5 (section)

=== 5.6.2 Projected Impacts ===

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AR5 stated that other stressors such as human-driven land use change and pollution will continue to be the main causes of forest cover change in the next three decades ( [[#Settele--2014|Settele et al., 2014]] ). In the second half of this century, it was projected that climate change will be a strong stressor of change in forest ecosystems. Many forest species may not be able to move fast enough to adjust to new climate conditions. In some cases, a warmer climate could lead to extinction of species.

The SR15 concluded that limiting warming to 1.5°C will be more favourable to terrestrial ecosystems, including forests, relative to a 2°C warming ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ). In general, a 2°C warming could lead to two times more area of biome shifts compared with a 1.5°C warming. As a result, keeping a cooler average global temperature will lead to lower extinction risks. The special report supports the AR5 conclusion that a warmer planet will impact wide swaths of forests adversely. For example, higher temperatures will promote fire, drought and insect disturbances. Consistent with AR5, SRCCL projected that tree mortality will increase with climate change ( [[#Barbosa--2019|Barbosa et al., 2019]] ). In addition, forests will be more exposed to extreme events such as extreme heat, droughts and storms. The incidence of forest fires will likewise increase.

Additional evidence since the above reports were published supports their overall conclusions. For example, at the global scale, modelling the vulnerability of 387 forest ecoregions under future climate change (to 2080 using the average of five Global Circulation Models (GCMs) and RCP4.5 and 8.5) across different biomes, biogeographical realms and conservation statuses showed that 8.8% of global forest ecoregions are highly vulnerable in a low-greenhouse-gas-concentration scenario, and 32.6% of the global forest ecoregions are highly vulnerable in a high-greenhouse-gas-concentration scenario ( [[#Wang--2019a|Wang et al., 2019a]] ). Furthermore, a recent synthesis of the literature suggests that climate change will result in younger and shorter forests globally ( [[#McDowell--2020|McDowell et al., 2020]] ). In Asia, a systematic review of climate change impacts on tropical forests revealed that future climate may lead to changes in species distribution and forest structure and composition as well as phenology ( [[#Deb--2018|Deb et al., 2018]] ).

Overall, studies indicate both negative and positive climate change impacts on forest production systems. Some forests in the USA could benefit slightly from CO 2 fertilisation (using IGSM-CAM and MIROC3.2 till 2100) resulting in increased productivity especially for hardwoods ( [[#Beach--2015|Beach et al., 2015]] ). A study across Europe showed that both productivity gains (mostly in Northern and Central Europe, up to +33%) and losses (predominately in Southern Europe, up to −37%) are possible until the end of the 21st century ( [[#Reyer--2017|Reyer et al., 2017]] ). The study further indicated that disturbances would reduce gains and exacerbate losses of productivity throughout Europe under climate change ( [[#Reyer--2017|Reyer et al., 2017]] ). For Central and Eastern Canada, decreasing biomass production is projected as a result of increasing disturbance from wildfire and drought ( [[#Brecka--2020|Brecka et al., 2020]] ). Climate-induced disturbances could also reduce the temporal stability of ecosystem service supply ( [[#Albrich--2018|Albrich et al., 2018]] ), increasing the volatility of timber markets ( ''medium confidence'' ). More broadly, climate change could lead to abrupt changes and the crossing of tipping points, resulting in profoundly altered future forest development trajectories ( [[#Turner--2020|Turner et al., 2020]] ). Some studies suggest that such threshold could already be crossed at relatively low warming levels of +2°C ( [[#Elkin--2013|Elkin et al., 2013]] ; [[#Albrich--2020|Albrich et al., 2020]] ), with substantial implications for ecosystem service supply ( ''limited evidence'' , ''high agreement'' ).

Regional studies on the potential future effects of climate change on forest production systems indicate diverse impacts. In Germany, drier conditions in 2070 (RCP8.5; GCMs INM-CM4, ECHAM6 and ACCESS1.0) are expected to benefit the mean annual increment at biological rotation age of Scots pine and oak, while beech might suffer losses of up to 3 m 3 ha –1 yr –1 depending on climate scenario and region ( [[#Albert--2018|Albert et al., 2018]] ). In India, 46% of the forest grid points were found to have high, very high or extremely high vulnerability under future climate in the short term (2030s) under both RCP4.5 and 8.5, increasing to 49% and 54%, respectively, in the long term (2080s) ( [[#Sharma--2017|Sharma et al., 2017]] ). In addition, forests in the higher rainfall zones show lower vulnerability as compared with drier forests under future climate, which is in contrast to dry forests in Central and South America cited above. Warming and drying trends are projected to reduce timber production in the neotropics in some cases ( [[#Hiltner--2021|Hiltner et al., 2021]] ). Also in India, a study using CMIP5 (RCP4.5 and 8.5 with two time slices 2021–2050 and 2070–2099) shows how forests in five districts in Himachal Pradesh in Western Himalayan region are vulnerable to global warming ( [[#Upgupta--2015|Upgupta et al., 2015]] ). In the Guiana Shield, climate projections under RCP2.5 and 8.5 led to decreasing the basal area, above-ground fresh biomass, quadratic diameter, tree growth and mortality rates of tropical forests ( [[#Aubry-Kientz--2019|Aubry-Kientz et al., 2019]] ). In Central Africa, projections under RCP4.5 and 8.5 showed a general increase in growth, mortality and recruitment leading to a strong natural thinning effect, with different magnitudes across species ( [[#Claeys--2019|Claeys et al., 2019]] ).

On a global and regional scale, there is ''limited evidence'' and ''high agreement'' ( ''medium confidence'' ) that climate change will increase global and regional supply of timber and other forest products. To date, there are eight studies assessing the total economic impacts of climate change on the forestry sector at the global level. Some of them have assumed only flow effects of climate change by using the projected changes in yields of forest types from integrated economic models ( [[#Perez-Garcia--1997|Perez-Garcia et al., 1997]] ; [[#Perez-Garcia--2002|Perez-Garcia et al., 2002]] ; [[#Buongiorno--2015|Buongiorno, 2015]] ), while other studies have assumed both flow and stock effects by accounting for changes in forest yields, die back effects and biome migration ( [[#Sohngen--2001|Sohngen et al., 2001]] ; [[#Lee--2004|Lee and Lyon, 2004]] ; [[#Tian--2016|Tian et al., 2016]] ; [[#Favero--2018|Favero et al., 2018]] ; [[#Favero--2021|Favero et al., 2021]] ).

According to these studies, global timber supply will increase as the result of an increase in global forest growth under climate change scenarios ( ''medium confidence'' ). Some studies indicate that timber supply is projected to increase more in tropical and subtropical areas because of the assumed availability of short-rotation species which might could adaptation easier for forest owners in these regions relative to others ( [[#Sohngen--2001|Sohngen et al., 2001]] ; [[#Perez-Garcia--2002|Perez-Garcia et al., 2002]] ; [[#Tian--2016|Tian et al., 2016]] ), while others indicate that temperate areas will experience the largest increase in supply ( [[#Favero--2018|Favero et al., 2018]] ; [[#Favero--2021|Favero et al., 2021]] ). The results are very sensitive to the climate change scenarios tested, the climate and vegetation models used and the climate drivers that are considered. For example, [[#Tian--2016|Tian et al. (2016)]] and Favero et al. (2018; 2021) used the same economic model (the global timber model) but different climate scenarios and vegetation models, obtaining different results.

The increasing supply induces lower global timber prices ( ''medium confidence'' ). Studies estimate that the prices will decline between 1% and 38% in 2100 with respect to a no climate change scenario depending on the model and the climate change scenario assumed (climate change is represented as a change in GHG concentration, global average temperature or radiative forcing) ( [[#Favero--2018|Favero et al., 2018]] ; [[#Favero--2021|Favero et al., 2021]] ). Clearly, further studies are needed considering a wider set of vegetation and climate models and incorporating the impacts of extreme events (such as droughts and wildfires).

There are a number of national and regional scale studies exploring the impact of climate change on yields and markets of wood products, with mixed results. In Finland, it is projected that timber yield in the north will increase in Scots pine and birch stands by 33–145% and 42–123%, compared with the current climate, depending on the GCM and thinning regime using a 90-year rotation (10 individual GCM projections under the RCP4.5 and RCP8.5 forcing scenarios) ( [[#ALRahahleh--2018|ALRahahleh et al., 2018]] ). However, in Norway spruce stands, yield could decline by up to 35%, under GFDL-CM3 RCP8.5 and increase by up to 39%, under CNRM-CM5 RCP8.5, compared with the current climate.

In Germany, timber harvest was projected to increase slightly (&lt;10%) in 2045 using the process-based forestry model (4C) driven by three management strategies (nature protection, biomass production and a baseline management) and an ensemble of regional climate scenarios (RCP2.6, RCP4.5, RCP8.5) ( [[#Gutsch--2018|Gutsch et al., 2018]] ). Similarly, average production of pulpwood in slash pine stands in the southeastern USA are projected to increase by 7.5 m 3 ha −1 for all climatic scenarios using the Physiological Processes Predicting Growth (3-PG) forest growth model by 2100 (RCP4.5 and RCP8.5; CanESM2) ( [[#Susaeta--2018|Susaeta and Lal, 2018]] ).

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'''Box 5.6: Contributions of Indigenous and Local Knowledge: An Example'''
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Indigenous and local people have long histories of adaptation to climate hazards in forests (see [[#Eriksen--2014|Eriksen and Hankins, 2014]] ; [[#Neale--2019|Neale et al., 2019]] ; [[#Bourke--2020|Bourke et al., 2020]] ; [[#Long--2020|Long et al., 2020]] ; [[#Williamson--2021|Williamson, 2021]] for notable examples in Australia and North America). In this section, we present a North American example of an indigenous adaptation practice developed by the Karuk Tribe in northern California. The Karuk Climate Adaptation Plan focuses on the use of cultural fire as climate adaptation, places a central importance on restoring human ecological caretaking responsibilities, and emphasises the need for collaboration, public education and policy advocacy to achieve these outcomes.

The Karuk Climate Adaptation Plan utilises a combination of Western science and Karuk traditional ecological knowledge. The plan centres on 22 focal species as '''cultural indicators''' as cues for human responsibilities and the particular techniques of fire application across seven habitat management zones (e.g., multiple forest types as well as riverine, riparian and montane systems). These adaptations range from specific prescriptions for the use of fire to lower river temperatures in acute scenarios ( [[#David--2018|David et al., 2018]] ), to protocols for treatment of grasslands and the use of high elevation meadows as fuel breaks. The plan also includes chapters on adaptations for tribal sovereignty, the mental and physical health effects of the changing climate and the protection of critical tribal infrastructure.

One aspect of Indigenous fire knowledge featured in the Karuk Climate Adaptation Plan is the culture-centric perspective on vegetation zones which are organised in relation to the elevation band in which smoke inversions occur (Figure Box 5.6.1). Within this system, burn timing follows a gradient that tracks the reproductive life cycles of season and elevational migrant species, the calving of elk and the nesting of birds. Within this system, elevational migrants are indicators of when to stop burning at one location and move upslope, following receding snows.

The plan also calls for the restoration of Indigenous fire science in emergency scenarios such as when rivers become too hot for salmon. With such fires localised, smoke inversions cool water temperatures through a variety of mechanisms, including shading river systems and reducing evapo-transpiration, thereby increasing stream flow ( [[#David--2018|David et al., 2018]] ).

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'''Figure Box 5.6.1 |''' '''Seasonality and elevation dynamics of cultural indicators in Karuk Cultural Management Zones based in Karuk traditional ecological knowledge.'''

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