Editing IPCC:AR6/SRCCL/Chapter-4 (section)

==== 4.4.1.2 Climate-induced vegetation changes, implications for land degradation ====

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The spatial mosaic of vegetation is determined by three factors: the ability of species to reach a particular location, how species tolerate the environmental conditions at that location (e.g., temperature, precipitation, wind, the topographic and soil conditions), and the interaction between species (including above/below ground species (Settele et al. 2015 <sup>[[#fn:r562|562]]</sup> ). Climate change is projected to alter the conditions and hence impact on the spatial mosaic of vegetation, which can be considered a form of land degradation. Warren et al. (2018) <sup>[[#fn:r563|563]]</sup> estimated that only about 33% of globally important biodiversity conservation areas will remain intact if global mean temperature increases to 4.5°C, while twice that area (67%) will remain intact if warming is restricted to 2°C. According to AR5, the clearest link between climate change and ecosystem change is when temperature is the primary driver, with changes of Arctic tundra as a response to significant warming as the best example (Settele et al. 2015 <sup>[[#fn:r564|564]]</sup> ). Even though distinguishing climate-induced changes from land-use changes is challenging, Boit et al. (2016) <sup>[[#fn:r565|565]]</sup> suggest that 5–6% of biomes in South America will undergo biome shifts until 2100, regardless of scenario, attributed to climate change. The projected biome shifts are primarily forests shifting to shrubland and dry forests becoming fragmented and isolated from more humid forests (Boit et al. 2016 <sup>[[#fn:r566|566]]</sup> ). Boreal forests are subject to unprecedented warming in terms of speed and amplitude (IPCC 2013b <sup>[[#fn:r567|567]]</sup> ), with significant impacts on their regional distribution (Juday et al. 2015 <sup>[[#fn:r568|568]]</sup> ). Globally, tree lines are generally expanding northward and to higher elevations, or remaining stable, while a reduction in tree lines was rarely observed, and only where disturbances occurred (Harsch et al. 2009 <sup>[[#fn:r569|569]]</sup> ). There is limited evidence of a slow northward migration of the boreal forest in eastern North America (Gamache and Payette 2005 <sup>[[#fn:r570|570]]</sup> ). The thawing of permafrost may increase drought-induced tree mortality throughout the circumboreal zone (Gauthier et al. 2015 <sup>[[#fn:r571|571]]</sup> ).

Forests are a prime regulator of hydrological cycling, both fluxes of atmospheric moisture and precipitation, hence climate and forests are inextricably linked (Ellison et al. 2017 <sup>[[#fn:r572|572]]</sup> ; Keys et al. 2017 <sup>[[#fn:r573|573]]</sup> ). Forest management influences the storage and flow of water in forested

watersheds. In particular, harvesting, forest thinning and the construction of roads increase the likelihood of floods as an outcome of extreme climate events (Eisenbies et al. 2007 <sup>[[#fn:r574|574]]</sup> ). Water balance of at least partly forested landscapes is, to a large extent, controlled by forest ecosystems (Sheil and Murdiyarso 2009 <sup>[[#fn:r575|575]]</sup> ; Pokam et al. 2014 <sup>[[#fn:r576|576]]</sup> ). This includes surface runoff, as determined by evaporation and transpiration and soil conditions, and water flow routing (Eisenbies et al. 2007 <sup>[[#fn:r577|577]]</sup> ). Water-use efficiency (i.e., the ratio of water loss to biomass gain) is increasing with increased CO <sub>2</sub> levels (Keenan et al. 2013 <sup>[[#fn:r578|578]]</sup> ), hence transpiration is predicted to decrease which, in turn, will increase surface runoff (Schlesinger and Jasechko 2014 <sup>[[#fn:r579|579]]</sup> ). However, the interaction of several processes makes predictions challenging (Frank et al. 2015 <sup>[[#fn:r580|580]]</sup> ; Trahan and Schubert 2016 <sup>[[#fn:r581|581]]</sup> ). Surface runoff is an important agent in soil erosion.

Generally, removal of trees through harvesting or forest death (Anderegg et al. 2012 <sup>[[#fn:r582|582]]</sup> ) will reduce transpiration and hence increase the runoff during the growing season. Management-induced soil disturbance (such as skid trails and roads) will affect water flow routing to rivers and streams (Zhang et al. 2017 <sup>[[#fn:r583|583]]</sup> ; Luo et al. 2018 <sup>[[#fn:r584|584]]</sup> ; Eisenbies et al. 2007 <sup>[[#fn:r585|585]]</sup> ).

Climate change affects forests in both positive and negative ways (Trumbore et al. 2015 <sup>[[#fn:r586|586]]</sup> ; Price et al. 2013 <sup>[[#fn:r587|587]]</sup> ) and there will be regional and temporal differences in vegetation responses (Hember et al. 2017 <sup>[[#fn:r1650|1650]]</sup> ; Midgley and Bond 2015 <sup>[[#fn:r589|589]]</sup> ). Several climate-change-related drivers interact in complex ways, such as warming, changes in precipitation and water balance, CO <sub>2</sub> fertilisation, and nutrient cycling, which makes projections of future net impacts challenging (Kurz et al. 2013 <sup>[[#fn:r590|590]]</sup> ; Price et al. 2013 <sup>[[#fn:r591|591]]</sup> ) (Section 2.3.1.2). In high latitudes, a warmer climate will extend the growing seasons. However, this could be constrained by summer drought (Holmberg et al. 2019 <sup>[[#fn:r592|592]]</sup> ), while increasing levels of atmospheric CO <sub>2</sub> will increase water-use efficiency but not necessarily tree growth (Giguère-Croteau et al. 2019 <sup>[[#fn:r593|593]]</sup> ). Improving one growth-limiting factor will only enhance tree growth if other factors are not limiting (Norby et al. 2010 <sup>[[#fn:r594|594]]</sup> ; Trahan and Schubert 2016 <sup>[[#fn:r595|595]]</sup> ; Xie et al. 2016 <sup>[[#fn:r596|596]]</sup> ; Frank et al. 2015 <sup>[[#fn:r597|597]]</sup> ). Increasing forest productivity has been observed in most of Fennoscandia (Kauppi et al. 2014 <sup>[[#fn:r598|598]]</sup> ; Henttonen et al. 2017 <sup>[[#fn:r599|599]]</sup> ), Siberia and the northern reaches of North America as a response to a warming trend (Gauthier et al. 2015 <sup>[[#fn:r600|600]]</sup> ) but increased warming may also decrease forest productivity and increase risk of tree mortality and natural disturbances (Price et al. 2013 <sup>[[#fn:r601|601]]</sup> ; Girardin et al. 2016 <sup>[[#fn:r602|602]]</sup> ; Beck et al. 2011 <sup>[[#fn:r603|603]]</sup> ; Hember et al. 2016 <sup>[[#fn:r604|604]]</sup> ; Allen et al. 2011 <sup>[[#fn:r605|605]]</sup> ). The climatic conditions in high latitudes are changing at a magnitude faster than the ability of forests to adapt with detrimental, yet unpredictable, consequences (Gauthier et al. 2015 <sup>[[#fn:r606|606]]</sup> ).

Negative impacts dominate, however, and have already been documented (Lewis et al. 2004 <sup>[[#fn:r607|607]]</sup> ; Bonan et al. 2008 <sup>[[#fn:r608|608]]</sup> ; Beck et al. 2011 <sup>[[#fn:r609|609]]</sup> ) and are predicted to increase (Miles et al. 2004 <sup>[[#fn:r610|610]]</sup> ; Allen et al. 2010 <sup>[[#fn:r611|611]]</sup> ; Gauthier et al. 2015 <sup>[[#fn:r612|612]]</sup> ; Girardin et al. 2016 <sup>[[#fn:r613|613]]</sup> ; Trumbore et al. 2015 <sup>[[#fn:r614|614]]</sup> ). Several authors have emphasised a concern that tree mortality (forest dieback) will increase due to climate-induced physiological stress as well as interactions between physiological stress and other stressors, such as insect pests, diseases, and wildfires (Anderegg et al. 2012 <sup>[[#fn:r615|615]]</sup> ; Sturrock et al. 2011 <sup>[[#fn:r616|616]]</sup> ; Bentz et al. 2010 <sup>[[#fn:r617|617]]</sup> ; McDowell et al. 2011 <sup>[[#fn:r618|618]]</sup> ). Extreme events such as extreme heat and drought, storms, and floods also pose increased threats to forests in both high – and low-latitude forests (Lindner et al. 2010 <sup>[[#fn:r619|619]]</sup> ; Mokria et al. 2015 <sup>[[#fn:r620|620]]</sup> ). However, comparing observed forest dieback with modelled climate-induced damages did not show a general link between climate change and forest dieback (Steinkamp and Hickler 2015 <sup>[[#fn:r621|621]]</sup> ). Forests are subject to increasing frequency and intensity of wildfires which is projected to increase substantially with continued climate change (Price et al. 2013 <sup>[[#fn:r622|622]]</sup> ) (Cross-Chapter Box 3 in Chapter 2, and Chapter 2). In the tropics, interaction between climate change, CO <sub>2</sub> and fire could lead to abrupt shifts between woodland – and grassland-dominated states in the future (Shanahan et al. 2016 <sup>[[#fn:r623|623]]</sup> ).

Within the tropics, much research has been devoted to understanding how climate change may alter regional suitability of various crops. For example, coffee is expected to be highly sensitive to both temperature and precipitation changes, both in terms of growth and yield, and in terms of increasing problems of pests (Ovalle-Rivera et al. 2015 <sup>[[#fn:r624|624]]</sup> ). Some studies conclude that the global area of coffee production will decrease by 50% (Bunn et al. 2015 <sup>[[#fn:r625|625]]</sup> ). Due to increased heat stress, the suitability of Arabica coffee is expected to deteriorate in Mesoamerica, while it can improve in high-altitude areas in South America. The general pattern is that the climatic suitability for Arabica coffee will deteriorate at low altitudes of the tropics as well as at the higher latitudes (Ovalle-Rivera et al. 2015 <sup>[[#fn:r626|626]]</sup> ). This means that climate change in and of itself can render unsustainable previously sustainable land-use and land management practices, and vice versa (Laderach et al. 2011 <sup>[[#fn:r627|627]]</sup> ).

Rangelands are projected to change in complex ways due to climate change. Increasing levels of atmospheric CO <sub>2</sub> directly stimulate plant growth and can potentially compensate for negative effects from drying by increasing rain-use efficiency. But the positive effect of increasing CO <sub>2</sub> will be mediated by other environmental conditions, primarily water availability, but also nutrient cycling, fire regimes and invasive species. Studies over the North American rangelands suggest, for example, that warmer and dryer climatic conditions will reduce NPP in the southern Great Plains, the Southwest, and northern Mexico, but warmer and wetter conditions will increase NPP in the northern Plains and southern Canada (Polley et al. 2013 <sup>[[#fn:r628|628]]</sup> ).

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