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

===== 2.4.3.2.2 Observed biome shifts from combined land use change and climate change =====

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Research has detected biome shifts in areas where agriculture, fire use or suppression, livestock grazing, harvesting of timber and wood for fuel and other local land use substantially altered vegetation, in addition to changes in climatic factors and CO 2 fertilisation. These studies were not designed or conducted in a manner to make climate change attribution possible, although many vegetation changes are consistent with climate change. For example, a global review of observed changes in tree lines found that, globally, two-thirds of tree lines have shifted upslope in elevation over the past 50 years or more, (( [[#Hansson--2021|Hansson et al., 2021]] ).

Upslope and poleward forest shifts have occurred where timber harvesting or livestock grazing has been abandoned, allowing the regeneration of trees at sites in Canada ( [[#Brice--2019|Brice et al., 2019]] ; [[#Wang--2020b|Wang et al., 2020b]] ), France ( [[#Feuillet--2020|Feuillet et al., 2020]] ), Italy ( [[#Vitali--2017|Vitali et al., 2017]] ), Spain ( [[#Ameztegui--2016|Ameztegui et al., 2016]] ) and the USA ( [[#Wang--2020b|Wang et al., 2020b]] ) as well as in mountainous areas across Europe ( [[#Cudlin--2017|Cudlin et al., 2017]] ). Intentional use of fire drove an upslope forest shift in Peru ( [[#Bush--2015|Bush et al., 2015]] ) while mainly human-ignited fires drove the conversion of shrubland to grassland in a drought-affected area of the USA ( [[#Syphard--2019b|Syphard et al., 2019b]] ). In eastern Canada, timber harvesting and wildfire drove the conversion of mixed conifer–broadleaf forests to broadleaf-dominated forests ( [[#Brice--2020|Brice et al., 2020]] ; [[#Wang--2020b|Wang et al., 2020b]] ).

Shrub encroachment onto savanna has occurred at numerous sites, particularly across the Southern Hemisphere, mainly between 1992 and 2010 ( [[#Criado--2020|Criado et al., 2020]] ). Globally, overgrazing initiates shrub encroachment by reducing grasses more than woody plants, while fire exclusion maintains the shrub cover ( [[#D’Odorico--2012|D’Odorico et al., 2012]] ; [[#Caracciolo--2016|Caracciolo et al., 2016]] ; [[#Bestelmeyer--2018|Bestelmeyer et al., 2018]] ). The magnitude of woody cover change in savannas is not correlated with mean annual temperature change ( [[#Criado--2020|Criado et al., 2020]] ); however, higher atmospheric CO 2 increases shrub growth in savannas ( [[#Nackley--2018|Nackley et al., 2018]] ; [[#Manea--2019|Manea and Leishman, 2019]] ). A global remote-sensing analysis of biome changes from all causes, including agricultural and grazing expansion and deforestation, estimated that 14% of pixels changed between 1981 and 2012, although this approach can overestimate global changes, since it uses a new biome classification system which doubles the conventional biome classifications ( [[#Higgins--2016|Higgins et al., 2016]] ). In addition to climate change, LULCC causes vegetation changes at the biome level ( ''robust evidence'' , ''high agreement'' ).

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