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

==== 2.4.3.6 Observed Changes in Tropical Forest ====

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Overall declines of tropical forest cover ( [[#Kohl--2015|Kohl et al., 2015]] ; [[#Liu--2015|Liu et al., 2015]] ; [[#Baccini--2017|Baccini et al., 2017]] ; [[#Harris--2021|Harris et al., 2021]] ), with declines more than triple the gains ( [[#Harris--2021|Harris et al., 2021]] ) have been driven primarily by deforestation and land conversion ( ''robust evidence'' , ''high agreement'' ) ( [[#Lewis--2015|Lewis et al., 2015]] ; [[#Curtis--2018|Curtis et al., 2018]] ; [[#Assis--2019|Assis et al., 2019]] ). In opposition to this general trend, expansion of tropical forest cover into savannas and grasslands has occurred in Africa, South America and Australia ( [[#Marimon--2014|Marimon et al., 2014]] ; [[#Baccini--2017|Baccini et al., 2017]] ; [[#Ondei--2017|Ondei et al., 2017]] ; [[#Stevens--2017|Stevens et al., 2017]] ; Aleman et al., 2018; [[#Staver--2018|Staver, 2018]] ; [[#Rosan--2019|Rosan et al., 2019]] ).

Specific examples of climate change-driven range shifts of tropical deciduous forests upslope into alpine grasslands have been documented in the Americas ( [[#Chacón-Moreno--2021|Chacón-Moreno et al., 2021]] ; [[#Jiménez-García--2021|Jiménez-García et al., 2021]] ) and Asia ( [[#Sigdel--2018|Sigdel et al., 2018]] ). However, tree line behaviours are diverse. A study in Nepal recorded that the tree line fomed by ''Abies spectabilis'' had been stable for more than a century, while the upper limit of large shrubs ( ''Rhododendron campanulatum'' ) had been advancing ( [[#Mainali--2020|Mainali et al., 2020]] ). In both the Andes ( [[#Harsch--2009|Harsch et al., 2009]] ) and Himalayas ( [[#Singh--2021|Singh et al., 2021]] ), most tree lines have been stable, leading ( [[#Rehm--2015|Rehm and Feeley, 2015]] ) to postulate a ‘grass ceiling’ that has been difficult for trees to penetrate. The tree line shifts that have occurred are probably driven by interactions between changing land use (e.g., fire suppression) and climate changes such as increased rainfall, warming and elevated CO 2 (via CO 2 fertilisation or increases in water-use efficiency) ( ''medium evidence'' , ''medium agreement'' ) ( [[#Cernusak--2013|Cernusak et al., 2013]] ; [[#Huang--2013|Huang et al., 2013]] ; [[#Van%20Der%20Sleen--2015|Van Der Sleen et al., 2015]] ; [[#Yang--2016|Yang et al., 2016]] ).

Increases in productivity of tropical forests ( [[#Gatti--2014|Gatti et al., 2014]] ; [[#Brienen--2015|Brienen et al., 2015]] ; [[#Baccini--2017|Baccini et al., 2017]] ), Africa and southeast Asia ( [[#Qie--2017|Qie et al., 2017]] ) have been attributed to elevated CO 2 ( ''robust evidence'' , ''medium agreement'' ) ( [[#Ballantyne--2012|Ballantyne et al., 2012]] ; [[#Brienen--2015|Brienen et al., 2015]] ; [[#Sitch--2015|Sitch et al., 2015]] ; [[#Yang--2016|Yang et al., 2016]] ; [[#Mitchard--2018|Mitchard, 2018]] ). The rates of these increases have been slowing down in the central Amazon ( [[#Brienen--2015|Brienen et al., 2015]] ; [[#de%20Meira%20Junior--2020|de Meira Junior et al., 2020]] ) and Southeast Asia ( [[#Qie--2017|Qie et al., 2017]] ). In contrast, the carbon sink (and hence the rate of biomass gain) in intact African forests was stable until 2010 and has only recently started to decline, indicating asynchronous carbon sink saturation in Amazonia and Africa, the difference being driven by rates of tree mortality ( [[#Hubau--2020|Hubau et al., 2020]] ). At the global level, [[#Hubau--2020|Hubau et al. (2020)]] argue that the carbon sink associated with intact tropical forests peaked in the 1990s and is now in decline.

Declines in productivity are most strongly associated with warming ( [[#Sullivan--2020|Sullivan et al., 2020]] ), reduced growth rates during droughts ( [[#Bennett--2015|Bennett et al., 2015]] ; [[#Bonai--2016|Bonai et al., 2016]] ; [[#Corlett--2016|Corlett, 2016]] ), drought-related mortality ( [[#Brando--2014|Brando et al., 2014]] ; [[#Zhou--2014|Zhou et al., 2014]] ; [[#Brienen--2015|Brienen et al., 2015]] ; [[#Corlett--2016|Corlett, 2016]] ; [[#McDowell--2018|McDowell et al., 2018]] ), fire ( [[#Liu--2017|Liu et al., 2017]] ) and cloud-induced radiation limitation ( ''robust evidence'' , ''high agreement'' ) ( [[#Deb%20Burman--2020|Deb Burman et al., 2020]] ) ''.'' Increases in the frequency and severity of droughts and shorter tree residence times due to increases in growth rates caused by elevated CO 2 may be additional interactive factors increasing tree mortality ( [[#Malhi--2014|Malhi et al., 2014]] ; [[#Brienen--2015|Brienen et al., 2015]] ). Vulnerability to drought varies between tree species and sizes, with large, older trees at the highest risk of mortality ( [[#McDowell--2018|McDowell et al., 2018]] ; [[#Meakem--2018|Meakem et al., 2018]] ). Mortality risk also varies between forest types, with seasonal rainforests appearing to be the most vulnerable to drought ( [[#Corlett--2016|Corlett, 2016]] ).

Lianas (long-stemmed woody vines) generally negatively impact trees, significantly reducing the growth of heavily infested trees ( [[#Reis--2020|Reis et al., 2020]] ). Lianas would benefit from climate change and disturbance ( [[#LingZi--2014|LingZi et al., 2014]] ; [[#Hodgkins--2018|Hodgkins et al., 2018]] ). The extent of their suitable niche can increase ( [[#Taylor--2016|Taylor and Kumar, 2016]] ), thereby decreasing forest biomass accumulation ( ''robust evidence'' , ''high agreement'' ) ( [[#van%20der%20Heijden--2013|van der Heijden et al., 2013]] ; [[#Fauset--2015|Fauset et al., 2015]] ; [[#Estrada-Villegas--2020|Estrada-Villegas et al., 2020]] ).

Climate change continues to degrade forests by reducing resilience to pests and diseases, increasing species invasion, facilitating pathogen spread ( [[#Malhi--2014|Malhi et al., 2014]] ; [[#Deb--2018|Deb et al., 2018]] ) and intensifying fire risk and potential dieback ( [[#Lapola--2018|Lapola et al., 2018]] ; [[#Marengo--2018|Marengo et al., 2018]] ). Drought, temperature increases and forest fragmentation interact to increase the prevalence of fires in tropical forests ( ''robust evidence'' , ''high agreement'' ). Warming increases water stress in trees ( [[#Corlett--2016|Corlett, 2016]] ) and, together with forest fragmentation, dramatically increases the desiccation of forest canopies—resulting in deforestation that then leads to even hotter and drier regional climates ( [[#Malhi--2014|Malhi et al., 2014]] ; [[#Lewis--2015|Lewis et al., 2015]] ). Warming and drought increase the invasion of grasses into forest edges and increase fire risk ( ''robust evidence'' , ''high agreement'' ) ( [[#Brando--2014|Brando et al., 2014]] ; [[#Balch--2015|Balch et al., 2015]] ; [[#Lewis--2015|Lewis et al., 2015]] ). Droughts and fires additively increase mortality and, consequently, reduce canopy cover and above-ground biomass (Cross-Chapter Paper 7) ( [[#Brando--2014|Brando et al., 2014]] , 2020; [[#Balch--2015|Balch et al., 2015]] ; [[#Lewis--2015|Lewis et al., 2015]] ).

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