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

==== 2.4.3.5 Observed Changes in Savanna and Grasslands ====

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Savannas consist of co-existing trees and grasses in tropical and temperate regions ( [[#Archibald--2019|Archibald et al., 2019]] ). The global trend of woody encroachment reported in AR5 ( [[#Settele--2014|Settele et al., 2014]] ) is continuing ( ''robust evidence'' , ''high agreement'' , ''very high confidence'' ) (see Table SM2.1), with increases occurring in temperate savannas in North America (10–20% per decade) and tropical savannas in South America (8% per decade), Africa (2.4% per decade) and Australia (1% per decade) ( [[#O’Connor--2014|O’Connor et al., 2014]] ; [[#Espírito-Santo--2016|Espírito-Santo et al., 2016]] ; [[#Skowno--2017|Skowno et al., 2017]] ; [[#Stevens--2017|Stevens et al., 2017]] ; McNicol et al., 2018; [[#Venter--2018|Venter et al., 2018]] ; [[#Rosan--2019|Rosan et al., 2019]] ). Additionally, the forest expansion into mesic savannas reported in AR5 ( [[#Settele--2014|Settele et al., 2014]] ) is continuing in Africa, South America and Southeast Asia ( [[#Marimon--2014|Marimon et al., 2014]] ; [[#Keenan--2015|Keenan et al., 2015]] ; [[#Baccini--2017|Baccini et al., 2017]] ; [[#Ondei--2017|Ondei et al., 2017]] ; [[#Stevens--2017|Stevens et al., 2017]] ; Aleman et al., 2018; [[#Rosan--2019|Rosan et al., 2019]] ). Extreme high rainfall anomalies have also contributed to an increase in herbaceous and foliar production in the Sahel ( [[#Brandt--2019|Brandt et al., 2019]] ; [[#Zhang--2019a|Zhang et al., 2019a]] ).

New studies since AR5, using multiple study designs (experimental manipulations in lab and field, meta-analyses and modelling), attribute climate change increases in woody cover to elevated atmospheric CO 2 ( [[#Donohue--2013|Donohue et al., 2013]] ; [[#Nackley--2018|Nackley et al., 2018]] ; [[#Quirk--2019|Quirk et al., 2019]] ) and increased rainfall amount and intensity ( ''robust evidence'' , ''high agreement'' ) ( [[#Venter--2018|Venter et al., 2018]] ; [[#Xu--2018b|Xu et al., 2018b]] ; [[#Zhang--2019a|Zhang et al., 2019a]] ). Direct quantification of climate-change drivers is confounded with local LUC such as fire suppression ( [[#Archibald--2016|Archibald, 2016]] ; [[#Venter--2018|Venter et al., 2018]] ) '','' heavy grazing ( [[#du%20Toit--2014|du Toit and O’Connor, 2014]] ; [[#Archer--2017|Archer et al., 2017]] ), removal of native browsers and, specifically, loss of mega-herbivores in Africa ( ''medium evidence'' , ''medium agreement'' ) ( [[#Asner--2016b|Asner et al., 2016b]] ; [[#Daskin--2016|Daskin et al., 2016]] ; [[#Stevens--2016|Stevens et al., 2016]] ; [[#Davies--2018|Davies et al., 2018]] ). The relative importance of the climate- and non-climate-related causes of woody plants varies between regions, but there is general consensus that the impacts of climate change, specifically, increasing rainfall and rising CO 2 , are frequent and strong contributing factors of woody-cover increase ( ''robust evidence'' , ''high agreement'' ).

Extensive woody-cover increases in non-forested biomes is reducing grazing potential ( [[#Smit--2015|Smit and Prins, 2015]] ) as well as changing the carbon stored per unit of land area ( [[#González-Roglich--2014|González-Roglich et al., 2014]] ; [[#Puttock--2014|Puttock et al., 2014]] ; [[#Pellegrini--2016|Pellegrini et al., 2016]] ; [[#Mureva--2018|Mureva et al., 2018]] ) and the hydrological characteristics ( [[#Honda--2016|Honda and Durigan, 2016]] ; [[#Schreiner-McGraw--2020|Schreiner-McGraw et al., 2020]] ). Woody-cover encroachment also reduces biodiversity by threatening fauna and flora adapted to open ecosystems ( [[#Ratajczak--2012|Ratajczak et al., 2012]] ; [[#Smit--2015|Smit and Prins, 2015]] ; [[#Pellegrini--2016|Pellegrini et al., 2016]] ; [[#Andersen--2019|Andersen and Steidl, 2019]] ).

The global extent of grasslands is declining significantly because of climate change ( ''medium confidence'' ). In temperate and boreal zones, where about half of tree lines are shifting, they are overwhelmingly expanding poleward and upward, with an accompanying loss of montane and boreal grassland ( ''robust evidence'' , ''high agreement'' ) whereas tropical tree lines have been generally stable ( ''medium evidence'' , ''medium agreement'' ) ( [[#Harsch--2009|Harsch et al., 2009]] ; [[#Rehm--2015|Rehm and Feeley, 2015]] ; [[#Silva--2016|Silva et al., 2016]] ; [[#Andela--2017|Andela et al., 2017]] ; [[#Song--2018|Song et al., 2018]] ; [[#Aide--2019|Aide et al., 2019]] ; [[#Gibson--2019|Gibson and Newman, 2019]] ). The Eurasian steppes experienced a 1% increase in woody cover per decade since 2000 ( [[#Liu--2021|Liu et al., 2021]] ) and inner Mongolian grasslands in China experienced broad encroachment as well ( [[#Chen--2015|Chen et al., 2015]] ). Climatic drivers of woody expansion in temperature-limited grasslands, particularly alpine grasslands, are most frequently attributed to warming ( ''robust evidence'' , ''high agreement'' , ''high confidence'' ) ( [[#D’Odorico--2012|D’Odorico et al., 2012]] ; [[#Hagedorn--2014|Hagedorn et al., 2014]] ), an increase in water and nutrient availability from thawing permafrost ( ''medium evidence'' , ''high agreement'' ) ( [[#Zhou--2015b|Zhou et al., 2015b]] ; [[#Silva--2016|Silva et al., 2016]] ) and rising CO 2 ( ''medium evidence'' , ''medium agreement'' ) ( [[#Frank--2015|Frank et al., 2015]] ; [[#Aide--2019|Aide et al., 2019]] ). Interactions of LULCCs such as land abandonment, grazing management shifts and fire suppression with climate change are contributing factors ( [[#Liu--2021|Liu et al., 2021]] )

Remote sensing shows overall increasing trends in both the annual maximum Normalized Difference Vegetation Index (NDVI) and annual mean NDVI in global grassland ecosystems between 1982 and 2011 ( [[#Gao--2016|Gao et al., 2016]] ). Multiple lines of evidence indicate that changes in grassland productivity are positively correlated with increases in mean annual precipitation ( [[#Hoover--2014|Hoover et al., 2014]] ; [[#Brookshire--2015|Brookshire and Weaver, 2015]] ; [[#Gang--2015|Gang et al., 2015]] ; [[#Gao--2016|Gao et al., 2016]] ; [[#Wilcox--2017|Wilcox et al., 2017]] ; [[#Wan--2018|Wan et al., 2018]] ). Increasing temperatures positively impact grassland production and biomass, especially in temperature-limited regions ( [[#Piao--2014|Piao et al., 2014]] ; [[#Gao--2016|Gao et al., 2016]] ). However, it is expected that grasslands in hot areas will decrease production as temperatures increase ( ''limited evidence'' , ''low agreement'' ) ( [[#Gang--2015|Gang et al., 2015]] ) ''.'' Nevertheless, grassland responses to warming and drought are being ameliorated by increasing CO 2 and associated improved water-use efficiency ( [[#Roy--2016|Roy et al., 2016]] ). For example, in a cool temperate grassland experiment, warming led to a longer growing season and elevated CO 2 further extended growing by conserving water, which enabled most species to remain active longer ( ''medium evidence'' , ''medium agreement'' ) ( [[#Reyes-Fox--2014|Reyes-Fox et al., 2014]] ).

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