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

===== 2.4.4.3.1 Observed tree mortality globally =====

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Anthropogenic climate change can cause tree mortality directly via increased aridity or drought ( [[#2.4.4.3.3|Section 2.4.4.3.3]] ) or indirectly through wildfire ( [[#2.4.4.2.1|Section 2.4.4.2.1]] ) and insect pests ( [[#2.4.4.3.3|Section 2.4.4.3.3]] ). Catastrophic failure of the plant hydraulic system, in which a lack of water causes the xylem to lose hydraulic conductance, is the principal mechanism of drought-induced tree death ( [[#Anderegg--2016|Anderegg et al., 2016]] ; [[#Adams--2017|Adams et al., 2017]] ; [[#Anderegg--2018|Anderegg et al., 2018]] ; [[#Choat--2018|Choat et al., 2018]] ; [[#Menezes-Silva--2019|Menezes-Silva et al., 2019]] ; [[#Brodribb--2020|Brodribb et al., 2020]] ).

Up through the AR5 ( [[#Settele--2014|Settele et al., 2014]] ), detection and attribution analyses had found that anthropogenic climate change, with global temperature increases of 0.3°C–0.9°C above the pre-industrial period and the increases in aridity exceeding the effects of local non-climate change factors, caused three cases of drought-induced tree mortality of up to 20% in the period 1945–2007 in western North America ( [[#van%20Mantgem--2009|van Mantgem et al., 2009]] ), the African Sahel ( [[#Gonzalez--2012|Gonzalez et al., 2012]] ) and North Africa ( [[#le%20Polain%20de%20Waroux--2012|le Polain de Waroux and Lambin, 2012]] ). Increased wildfire and pest infestations, driven by climate change, also contributed to North American tree mortality ( [[#van%20Mantgem--2009|van Mantgem et al., 2009]] ). In addition, a meta-analysis of published cases found that drought consistent with, but not formally attributed to, climate change had caused tree mortality at 88 sites in boreal, temperate and tropical ecosystems ( [[#Allen--2010|Allen et al., 2010]] ), with 49 additional cases found by the AR5 ( [[#Settele--2014|Settele et al., 2014]] ).

Since the AR5 ( [[#Settele--2014|Settele et al., 2014]] ), global meta-analyses found at least 15 ( [[#Allen--2015|Allen et al., 2015]] ) and 25 ( [[#Hartmann--2018|Hartmann et al., 2018]] ) additional sites, respectively, of drought-induced tree mortality around the world. These and other global analyses found more rapid mortality than previously ( [[#Allen--2015|Allen et al., 2015]] ), rising background mortality ( [[#Allen--2015|Allen et al., 2015]] ), mortality increasing with drought severity ( [[#Greenwood--2017|Greenwood et al., 2017]] ), mortality of tropical trees increasing with temperature ( [[#Locosselli--2020|Locosselli et al., 2020]] ), mortality increasing with tree size for many species ( [[#Bennett--2015|Bennett et al., 2015]] ), mortality predominantly at the dry edge of species ranges ( [[#Anderegg--2019|Anderegg et al., 2019]] ) and three-quarters of drought-induced mortality cases leading to a change in the dominant species ( [[#Batllori--2020|Batllori et al., 2020]] ). Multiple non-climate factors contribute to tree mortality, including timber cutting, livestock grazing and air pollution ( [[#Martinez-Vilalta--2016|Martinez-Vilalta and Lloret, 2016]] ). Globally, tropical dry forests lost, from all causes, 95,000 km 2 , 8% of their total area, from 1982 to 2016, the most extensive area of mortality of any biome ( [[#Song--2018|Song et al., 2018]] ).

In summary, anthropogenic climate change caused drought-induced tree mortality of up to 20% in the period 1945–2007 in western North America, the African Sahel and North Africa, via global temperature increases of 0.3°C–0.9°C above the pre-industrial period and increases in aridity, and it contributed to over 100 other cases of drought-induced tree mortality in Africa, Asia, Australia, Europe and North and South America ( ''high confidence'' ). Field observations document accelerating mortality rates, rising background mortality and post-mortality vegetation shifts ( ''high confidence'' ). Water stress, leading to plant hydraulic failure, is the principal mechanism of drought-induced tree mortality. Timber cutting, agricultural expansion, air pollution and other non-climate factors also contribute to tree death.

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