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

===== 2.4.4.3.3 Observed tree mortality in boreal and temperate ecosystems =====

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The most extensive research into tree mortality since the AR5 has been in the western USA, where anthropogenic climate change accounted for half the magnitude of a drought in the period 2000–2020 that has been the most severe since the 1500s, ( [[#Williams--2020|Williams et al., 2020]] ) and for one-tenth to one-quarter of the magnitude of the 2012–2014 period of th e severe drought in California that lasted from 2012 to 2016 ( [[#Williams--2015a|Williams et al., 2015a]] ). Across the western USA, anthropogenic climate change doubled tree mortality between 1955 and 2007 ( [[#van%20Mantgem--2009|van Mantgem et al., 2009]] ). Lodgepole pine ( ''Pinus contorta'' ) mortality increased 700% from 2000 to 2013 ( [[#Anderegg--2015|Anderegg et al., 2015]] ) and piñon pine ( ''P. edulis'' ) experienced &gt;50% mortality from 2002 to 2014 ( [[#Redmond--2018|Redmond et al., 2018]] ). In montane conifer forest in California, anthropogenic climate change has increased tree mortality by one-quarter ( [[#Goulden--2019|Goulden and Bales, 2019]] ). One-quarter of the trees died in some areas, with mortality rates of ponderosa pine ( ''P. ponderosa'' ) and sugar pine ( ''P. lambertiana'' ) increasing to up to 700% of pre-drought rates ( [[#Stephenson--2019|Stephenson et al., 2019]] ; [[#Stovall--2019|Stovall et al., 2019]] ). Substantial field evidence shows that anthropogenic climate change has caused extensive tree mortality in North America ( ''robust evidence'' , ''high agreement'' ).

In western North America, increased infestations of bark beetles and other tree-feeding insects that benefit from higher winter temperatures (section 3.3.1.1 in ( [[#IPCC--2021a|IPCC, 2021a]] )) and longer growing seasons (section 2.3.4.3.1 in ( [[#IPCC--2021a|IPCC, 2021a]] )) have killed drought-stressed trees ( [[#2.4.2.1|Section 2.4.2.1]] ) ( [[#Anderegg--2015|Anderegg et al., 2015]] ; [[#Kolb--2016|Kolb et al., 2016]] ; [[#Lloret--2018|Lloret and Kitzberger, 2018]] ; [[#Redmond--2018|Redmond et al., 2018]] ; [[#Stephens--2018|Stephens et al., 2018]] ; [[#Fettig--2019|Fettig et al., 2019]] ; [[#Restaino--2019|Restaino et al., 2019]] ; [[#Stephenson--2019|Stephenson et al., 2019]] ). Increasing temperatures have allowed bark beetles to move further north and to higher elevations, survive through the winter at sites where they would previously have died and reproduce more often ( [[#Raffa--2008|Raffa et al., 2008]] ; [[#Bentz--2010|Bentz et al., 2010]] ; [[#Jewett--2011|Jewett et al., 2011]] ; [[#Macfarlane--2013|Macfarlane et al., 2013]] ; [[#Raffa--2013|Raffa et al., 2013]] ; [[#Hart--2017|Hart et al., 2017]] ; [[#Stephenson--2019|Stephenson et al., 2019]] ; [[#Teshome--2020|Teshome et al., 2020]] ; [[#Koontz--2021|Koontz et al., 2021]] ). Under warmer conditions, some insects that were previously innocuous have become important agents of tree mortality ( [[#Stephenson--2019|Stephenson et al., 2019]] ; [[#Trugman--2021|Trugman et al., 2021]] ). Field observations show mixed effects of bark beetle-induced tree mortality on subsequent fire-caused tree mortality ( [[#Andrus--2016|Andrus et al., 2016]] ; [[#Meigs--2016|Meigs et al., 2016]] ; [[#Candau--2018|Candau et al., 2018]] ; [[#Lucash--2018|Lucash et al., 2018]] ; [[#Talucci--2019|Talucci and Krawchuk, 2019]] ; [[#Wayman--2021|Wayman and Safford, 2021]] ). From 1997 to 2018, ~5% of the forest area in the western USA died from bark beetle infestations ( [[#Hicke--2020|Hicke et al., 2020]] ). Under most circumstances, trees that have been weakened by drought are more vulnerable to being killed by bark beetles ( [[#Anderegg--2015|Anderegg et al., 2015]] ; [[#Kolb--2016|Kolb et al., 2016]] ; [[#Lloret--2018|Lloret and Kitzberger, 2018]] ; [[#Redmond--2018|Redmond et al., 2018]] ; [[#Stephens--2018|Stephens et al., 2018]] ; [[#Fettig--2019|Fettig et al., 2019]] ; [[#Restaino--2019|Restaino et al., 2019]] ; [[#Stephenson--2019|Stephenson et al., 2019]] ; [[#Koontz--2021|Koontz et al., 2021]] ). In summary, climate change has contributed to bark beetle infestations that have caused much of the tree mortality in North America ( ''robust evidence'' , ''high agreement'' ) (see also [[#2.4.2.1|Section 2.4.2.1]] ).

Across Europe, rates of tree mortality in field inventories from 2000 to 2012 were highest in Spain, Bulgaria, Sweden and Finland, positively correlated to maximum winter temperature and inversely correlated to spring precipitation ( [[#Neumann--2017|Neumann et al., 2017]] ). Tree mortality in Austria, the Czech Republic, Germany, Poland, Slovakia and Switzerland doubled from 1984 to 2016, correlated with intensified logging and increased temperatures ( [[#Senf--2018|Senf et al., 2018]] ). Drought-related tree mortality rates from 1987 to 2016 were highest in the Ukraine, Moldova, southern France and Spain ( [[#Senf--2020|Senf et al., 2020]] ). Climate contributed to tree mortality across Europe from 1958 to 2001 ( [[#Seidl--2011|Seidl et al., 2011]] ). In addition, insect infestations related to higher temperatures ( [[#Okland--2019|Okland et al., 2019]] ) have caused the extensive mortality of Norway spruce ( ''Picea abies'' ) across nine European countries ( [[#Marini--2017|Marini et al., 2017]] ; [[#Mezei--2017|Mezei et al., 2017]] ). Across the Mediterranean Basin, a combination of drought, wildfire, pest infestations and livestock grazing ( [[#Peñuelas--2021|Peñuelas and Sardans, 2021]] ) has driven tree mortality. In summary, climate change has contributed to tree mortality in Europe ( ''high confidence'' ) (see also [[#2.4.2.1|Section 2.4.2.1]] ).

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