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

==== 2.5.3.3 Risks to Ecosystems and Services from Tree Mortality ====

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Under continued climate change, increased temperature, aridity, drought, wildfire ( [[#2.5.3.2|Section 2.5.3.2]] ) and insect infestations ( [[#2.4.4.3.3|Section 2.4.4.3.3]] ) will tend to increase tree mortality across many parts of the world ( [[#McDowell--2020|McDowell et al., 2020]] ). Loss of boreal and temperate forest to fire, wind and bark beetles could cause more negative than positive effects for most ecosystem services, including carbon storage to regulate climate change (Sections 2.4.4.3, 2.5.2.6, 2.5.2.7, 2.5.3.4), water supply for people ( [[#2.5.3.6.1|Section 2.5.3.6.1]] ), timber production and other forest products (Chapter 5) and protection from hazards ( [[#Thom--2016|Thom and Seidl, 2016]] ). In addition, deforestation in tropical and temperate forests can increase local temperatures by 0.3°C–2°C ( [[#Hesslerová--2018|Hesslerová et al., 2018]] ; [[#Lejeune--2018|Lejeune et al., 2018]] ; [[#Zeppetello--2020|Zeppetello et al., 2020]] ) and this effect can extend up to 50 km ( [[#Cohn--2019|Cohn et al., 2019]] ).

In Amazon rainforests, the relatively lower buffering capacity for plant moisture during drought increases the risk of tree mortality and, combined with increased heat from climate change and fire from deforestation, the possibility of a tipping point of extensive forest dieback and a biome shift to grassland ( [[#Oyama--2003|Oyama and Nobre, 2003]] ; [[#Sampaio--2007|Sampaio et al., 2007]] ; [[#Lenton--2008|Lenton et al., 2008]] ; [[#Nepstad--2008|Nepstad et al., 2008]] ; [[#Malhi--2009|Malhi et al., 2009]] ; [[#Salazar--2010|Salazar and Nobre, 2010]] ; [[#Settele--2014|Settele et al., 2014]] ; [[#Lyra--2016|Lyra et al., 2016]] ; [[#Zemp--2017b|Zemp et al., 2017b]] ; [[#Brando--2020|Brando et al., 2020]] ). This could occur at a 4°C–5°C temperature increase above that of the pre-industrial period ( [[#Salazar--2010|Salazar and Nobre, 2010]] ). Under RCP8.5, half the Amazon tropical evergreen forest could turn into grassland through drought-induced tree mortality and wildfire, but lower emissions (RCP4.5) could limit this loss to ~5% ( [[#Lyra--2016|Lyra et al., 2016]] ). The decline in precipitation due to reduced evapotranspiration inputs after forest loss could cause additional Amazon forest loss of one-quarter to one-third ( [[#Zemp--2017a|Zemp et al., 2017a]] ). Similarly, in Guinean tropical deciduous forest in Africa, climate change under RCP8.5 could increase mortality 700% by 2100 or 400% under lower emissions (RCP4.5; ( [[#Claeys--2019|Claeys et al., 2019]] ). These projections indicate risks of climate change-induced tree mortality reducing tropical forest areas in Africa and South America by up to half under a 4°C increase above the pre-industrial period, but a lower projection of a 2°C increase could limit the projected increases in tree mortality ( ''robust evidence'' , ''high agreement'' ).

Temperate and boreal forests possess greater diversity of physiological traits related to plant hydraulics, so they are more buffered against drought than tropical forests ( [[#Anderegg--2018|Anderegg et al., 2018]] ). Nevertheless, in temperate forests, drought-induced tree mortality under RCP8.5 could cause the loss of half the Northern Hemisphere conifer forest area by 2100 ( [[#McDowell--2016|McDowell et al., 2016]] ). In the western USA, under RCP8.5, one-tenth of forest area is highly vulnerable to drought-induced mortality by 2050 ( [[#Buotte--2019|Buotte et al., 2019]] ). In California, increased evapotranspiration in Sierra Nevada conifer forests increases the potential fraction of the area at risk of tree mortality by 15–20% per degree Celsius ( [[#Goulden--2019|Goulden and Bales, 2019]] ). In Alaska, fire-induced tree mortality from climate change under RCP8.5 could reduce the extent of spruce forest ( ''Picea'' sp.) by 8–44% by 2100 ( [[#Pastick--2017|Pastick et al., 2017]] ). Under RCP8.5, tree mortality from drought, wildfire and bark beetles could reduce the timber productivity of boreal forests in Canada by 2100 below the current levels ( [[#Boucher--2018|Boucher et al., 2018]] ; [[#Chaste--2019|Chaste et al., 2019]] ; [[#Brecka--2020|Brecka et al., 2020]] ). In Tasmania, projected increases in wildfire ( [[#Fox-Hughes--2014|Fox-Hughes et al., 2014]] ) increase the risk of mortality of mesic vegetation ( [[#Harris--2018b|Harris et al., 2018b]] ) and threaten the disappearance of the long-lived endemic pencil pine ( ''Athrotaxis cupressoides'' ) ( [[#Holz--2015|Holz et al., 2015]] ; [[#Worth--2016|Worth et al., 2016]] ) and temperate montane rainforest ( [[#Mariani--2019|Mariani et al., 2019]] ). These projections indicate risks of climate change-induced tree mortality reducing some temperate forest areas by half under emissions scenarios of 2.5°C–4°C above the pre-industrial period ( ''medium evidence'' , ''high agreement'' ).

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