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

==== 2.4.4.3 Observed Changes in Tree Mortality ====

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===== 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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===== 2.4.4.3.2 Observed tree mortality in tropical ecosystems =====

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In the Brazilian Amazon, deforestation to clear agricultural land comprises the principal cause of tree mortality, reducing forest cover by an average of 13,900 km 2 yr -1 from 1988 to 2020 ( [[#Assis--2019|Assis et al., 2019]] ). In addition, in a set of 310 Amazon field plots, an annual average temperature increase of 1.2°C from 1950 to 2018 ( [[#Marengo--2018|Marengo et al., 2018]] ) contributed to tree mortality of ~40% from 1983 to 2011 ( [[#Brienen--2015|Brienen et al., 2015]] ). In another set of plots, mortality among newly recruited trees of mesic genera increased and drought-tolerant genera became more abundant from 1985 to 2015 ( [[#Esquivel-Muelbert--2019|Esquivel-Muelbert et al., 2019]] ). In other plots, tree mortality did not show a statistically significant change from 1965 to 2016, but rose abruptly in severe drought years, mainly during warm phases of the ENSO ( [[#Aleixo--2019|Aleixo et al., 2019]] ). Nearly half the area of the Amazon has experienced extremely dry conditions during ENSO warm phases; this can cause extensive wildfire ( [[#2.4.4.2.3|Section 2.4.4.2.3]] ). Wildfires can increase tree mortality rates by &gt;600% above rates in non-burned areas, with the higher mortality persisting for up to a decade after a fire ( [[#Silva--2018|Silva et al., 2018]] ; [[#Berenguer--2021|Berenguer et al., 2021]] ). Climate change has contributed to tree mortality in the Amazon rainforest ( ''medium evidence'' , ''medium agreement'' ).

In the African Sahel, field research has continued to detect tree mortality, ranging from 20 to 90% in the period 1965–2018 ( [[#Kusserow--2017|Kusserow, 2017]] ; [[#Trichon--2018|Trichon et al., 2018]] ; [[#Dendoncker--2020|Dendoncker et al., 2020]] ), and declines in tree biodiversity, with up to 80% local losses of tree species in the period 1970–2014 ( [[#Hanke--2016|Hanke et al., 2016]] ; [[#Kusserow--2017|Kusserow, 2017]] ; [[#Ibrahim--2018|Ibrahim et al., 2018]] ; [[#Dendoncker--2020|Dendoncker et al., 2020]] ), consistent with, but not formally attributed to, climate change. In Algeria, mortality of the Atlas cedar ( ''Cedrus atlantica'' ) increased from 1980 to 2006, coinciding with a ~1°C spring temperature increase, but non-climate factors were not examined ( [[#Navarro-Cerrillo--2019|Navarro-Cerrillo et al., 2019]] ). Across southern Africa, nine of the 13 oldest known (1100–2500 years old) baobab trees ( ''Adansonia digitata'' ) have died since 2005, although the causes are unknown ( [[#Patrut--2018|Patrut et al., 2018]] ). In South Africa, savanna trees experienced an order of magnitude increase in mortality, related, but not formally attributed to, decreased rainfall ( [[#Case--2019|Case et al., 2019]] ). In Tunisia, insect infestations related, but not formally attributed to, hotter temperatures led to mortality of cork oaks ( ''Quercus suber'' ) ( [[#Bellahirech--2019|Bellahirech et al., 2019]] ).

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===== 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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===== 2.4.4.3.4 Tree mortality and fauna =====

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A global meta-analysis of 59 studies encompassing 631 cases of animal abundance changes in areas of tree mortality over the past 7–59 years, primarly in North America and Australia, with a few sites in other regions (e.g. Europe). Overall, in areas with documented high tree mortality, bird abundances increased (n=186 bird species), there was no significant trend for mammals (n=33 species), a slight trend towards declines in invertebrates (n=28 species), and insufficient information to categorize the responses of reptiles (n=20 species). However, within groups, significant differences appeared. Mammals that use trees as refugia showed declines with tree mortality ''(high confidence)'' , but flying mammals (e.g. bats) increased ''(medium confidence)'' . Ground-nesting, ground-foraging, tree-hole nesting and bark-foraging birds increased most, but nectar-feeding and foliage-gleaning birds declined ''(high confidence)'' . Within invertebrates, declines were strongest in ground-foraging predators and detritivores ''(medium confidence)'' ( [[#Fleming--2021|Fleming et al., 2021]] ).

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