Editing IPCC:AR6/WGII/Cross-Chapter-Paper-3 (section)

==== CCP3.2.1.2 Ecosystem Processes ====

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Semiarid ecosystems have a disproportionately large role in the global carbon cycle, driving trends and interannual variability of the global carbon sink (Alstrom et al., 2015). These systems are highly sensitive to annual precipitation and temperature variations ( ''high confidence'' ) (Alstrom et al., 2015, Poulter et al., 2014). The positive trend in semiarid regions is consistent with widespread woody encroachment and increased vegetation greenness ( [[#Andela--2013|Andela et al., 2013]] ; [[#Piao--2019|Piao et al., 2019]] ; [[#Piao--2020|Piao et al., 2020]] ) driven by CO 2 fertilization and rainfall increases ( [[#Sitch--2015|Sitch et al., 2015]] ; [[#Piao--2020|Piao et al., 2020]] ), although some trends are complicated by irrigation practices ( [[#He--2019|He et al., 2019]] ). Increases in temperature and drought diminish this trend through reduced vegetation productivity and increased vegetation mortality ( [[#Brandt--2016|Brandt et al., 2016]] ; [[#Ma--2016|Ma et al., 2016]] ; [[#Fernández-Martínez--2019|Fernández-Martínez et al., 2019]] ; [[#Maurer--2020|Maurer et al., 2020]] ) with indications that this trend is declining or reversing in some locations ( [[#Yuan--2019|Yuan et al., 2019]] ; [[#Wang--2020|Wang et al., 2020]] ).

Changed climates have increased water constraints of vegetation growth most notably in the Mediterranean (Sections [[#CCP1.2.3.2|CCP1.2.3.2]] ; [[#CCP4.2.1|CCP4.2.1]] ) and West and Central Asia ( [[#Jiao--2021|Jiao et al., 2021]] ) ''.'' Climate change and elevated CO 2 have both increased and decreased vegetation sensitivity to rainfall throughout drylands, with the degree of variation shaped by region, land use and vegetation traits ( [[#Haverd--2017|Haverd et al., 2017]] ; [[#Abel--2021|Abel et al., 2021]] ). Mineral nitrogen production in drylands may become increasingly decoupled from consumption by plants over prolonged dry periods, and more extreme hydrological events can drive multiple changes to nutrient cycling ( [[#Manzoni--2019|Manzoni et al., 2019]] ). Soil biocrusts (composed of lichens, bryophytes and soil microorganisms), which contribute to dryland ecosystem function, including carbon uptake and soil stabilisation ( [[#Reed--2019|Reed et al., 2019]] ), are sensitive to warming and altered rainfall in a shift in biocrust communities of mosses and lichens in favour of early successional cyanobacteria-dominated biocrusts ( [[#Escolar--2012|Escolar et al., 2012]] ; [[#Reed--2012|Reed et al., 2012]] ), which can increase surface albedo ( [[#Rutherford--2017|Rutherford et al., 2017]] ).

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