Editing IPCC:AR6/WGII/Chapter-2 (section)

==== 2.5.2.5 Risk to Grasslands and Savannas ====

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Worldwide, woody cover is increasing in savannas ( [[#Buitenwerf--2012|Buitenwerf et al., 2012]] ; [[#Donohue--2013|Donohue et al., 2013]] ; [[#Stevens--2017|Stevens et al., 2017]] ), as a result of interactions of elevated CO 2 and altered fire and herbivory impacts, some of which stems from LULCC ''(high confidence)'' (see [[#2.4.3.5|Section 2.4.3.5]] ; Cross-Chapter Paper 3.2) ( [[#Venter--2018|Venter et al., 2018]] ; [[#Wu--2021|Wu et al., 2021]] ). In some regions, altered climate may also contribute (Cross-Chapter Paper 3.2). Elevated CO 2 benefits plants with C3 photosynthesis (often woody plants), more than C4 species ( [[#Moncrieff--2014a|Moncrieff et al., 2014a]] ; [[#Scheiter--2015|Scheiter et al., 2015]] ; [[#Knorr--2016a|Knorr et al., 2016a]] ). Increases in woody vegetation in grassy ecosystems could provide some carbon increase ( ''medium confidence'' ) ( [[#Zhou--2017|Zhou et al., 2017]] ; [[#Mureva--2018|Mureva et al., 2018]] ), but is expected to decrease biodiversity ( [[#Smit--2015|Smit and Prins, 2015]] ; [[#Abreu--2017|Abreu et al., 2017]] ; [[#Andersen--2019|Andersen and Steidl, 2019]] ) and water availability ( [[#Honda--2016|Honda and Durigan, 2016]] ; [[#Stafford--2017|Stafford et al., 2017]] ) and alter ecosystem services like grazing and wood provision ( ''high confidence'' ) ( [[#Anadón--2014b|Anadón et al., 2014b]] ).

The relative importance of climate, disturbance (e.g., fire/herbivory) and plant feedbacks in shaping present and future savanna distribution varies between continents ( [[#Lehmann--2014|Lehmann et al., 2014]] ), which makes projections of changing the biome extent challenging ( [[#Moncrieff--2016|Moncrieff et al., 2016]] ). It has been shown that simulation studies that do not account for CO 2 interactions but only consider climate change impacts do not realistically capture the future distribution of savannas ( ''high confidence'' ) ( [[#Higgins--2012|Higgins and Scheiter, 2012]] ; [[#Moncrieff--2016|Moncrieff et al., 2016]] ; [[#Scheiter--2020|Scheiter et al., 2020]] ). Due to the continued strong effect of CO 2 on tree and shrub-to-grass ratios in future, models suggest a loss of savanna extent and conversion into closed canopy forest/thicket and an expansion of savanna-type vegetation into arid grasslands ( [[#Wårlind--2014|Wårlind et al., 2014]] ; [[#Moncrieff--2016|Moncrieff et al., 2016]] ). In arid savannas and their interface to grasslands, survival of woody vegetation (which may be stimulated to grow by increasing CO 2 ) will depend on their capacity to survive potentially more severe and frequent droughts ( [[#Sankaran--2019|Sankaran and Staver, 2019]] ). Across a range of models, for RCP4.5 future climate change and CO 2 concentrations, savanna expanse declines by around 50% (converting to closed canopy systems) by 2070 in Africa and South America, 25% in Asia and with small changes in Australia ( [[#Moncrieff--2016|Moncrieff et al., 2016]] ; [[#Kumar--2021|Kumar et al., 2021]] ). Future fire-spread is expected to be reduced with increased woody dominance ( [[#Scheiter--2015|Scheiter et al., 2015]] ; [[#Knorr--2016b|Knorr et al., 2016b]] ; [[#Scheiter--2020|Scheiter et al., 2020]] ), feeding back to further increase tree-to-grass ratios ( ''high confidence'' ).

Like the tropical forest biome, savannas are at a high risk, given the projected climate changes in combination with LULCC (see Cross-Chapter Paper 3). About 50% of the Brazilian Cerrado has been converted to agricultural land and pastures ( [[#Lehman--2016|Lehman and Parr, 2016]] ), and African savannas have been proposed to follow a similar tropical agricultural revolution pathway to enhance agronomic prosperity ( [[#Ryan--2016|Ryan et al., 2016]] ). In fact, indirect climate change impacts arising from mitigation efforts may be particularly perilous to savannas; extensive tree-planting to restore ecosystems and remove CO 2 from the atmosphere, as pledged, for example, under the African Forest Restoration Initiative, could lead to carbon losses and the loss of biodiversity as well as damage the water balance if trees are planted on what was naturally grassland or savanna (Box 2.2; FAQ 2.6) ( [[#Bond--2019|Bond et al., 2019]] ).

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