Editing IPCC:AR6/WGII/Cross-Chapter-Paper-3 (section)

==== CCP3.2.1.4 Woody Cover Increase ====

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Dryland ecosystems have shown mixed trends of decreases and increases in vegetation and biodiversity, depending on the time period, geographic region and vegetation type assessed (see Table CCP3.1 for examples of observed environmental changes and impacts in drylands and the role of climate change and non-climatic factors in causing these changes).

Increases in shrub cover in arid deserts and shrublands have been recorded in the North American drylands ( [[#Caracciolo--2016|Caracciolo et al., 2016]] ; [[#Archer--2017|Archer et al., 2017]] ; [[#Chambers--2019|Chambers et al., 2019]] ), the Namib desert ( [[#Rohde--2019|Rohde et al., 2019]] ), the Karoo ( [[#Ward--2014|Ward et al., 2014]] ; [[#Masubelele--2015b|Masubelele et al., 2015b]] ), north and central Mexico ( [[#Pérez-Sánchez--2011|Pérez-Sánchez et al., 2011]] ; [[#Báez--2013|Báez et al., 2013]] ; [[#Castillón--2015|Castillón et al., 2015]] ; [[#Sosa--2019|Sosa et al., 2019]] ), large parts of the West African Sahel with some local exceptions ( [[#Brandt--2016|Brandt et al., 2016]] ) and Central Asia ( [[#Jia--2015|Jia et al., 2015]] ; [[#Li--2015|Li et al., 2015]] ; [[#Deng--2016|Deng et al., 2016]] ; [[#Jiao--2016|Jiao et al., 2016]] ; [[#Wang--2016|Wang et al., 2016]] ). Increasing woodiness in the Namib is consistent with an increase in rainfall extremes and westward expansion of convective rainfall ( [[#Haensler--2010|Haensler et al., 2010]] ; [[#Rohde--2019|Rohde et al., 2019]] ). Increasing rainfall and rising CO 2 concentrations (which improves water use efficiency) benefits some shrubs ( [[#Polley--1997|Polley et al., 1997]] ; [[#Morgan--2004|Morgan et al., 2004]] ; [[#Donohue--2013|Donohue et al., 2013]] ). Together with changes in land use ( [[#Hoffman--2018|Hoffman et al., 2018]] ), improved land management ( [[#Reij--2005|Reij et al., 2005]] ) and improved irrigation ( [[#He--2019|He et al., 2019]] ), this contributes to woody cover increases. Extensive woody encroachment has been recorded in savannas (measured between 1920–2015, over the past century) in Africa (2.4% woody cover increase per decade), Australia (1% increase per decade), and South America (8% increase per decade) ( [[#O’Connor--2014|O’Connor et al., 2014]] ; [[#Stevens--2016|Stevens et al., 2016]] ; [[#Skowno--2017|Skowno et al., 2017]] ; [[#Venter--2018|Venter et al., 2018]] ; [[#Zhang--2019|Zhang et al., 2019]] ). Following drought in the Sahel (1968–1973 and 1982–1984), a rainfall increase since the mid-1990s has been linked to increases of woody cover between 1992–2011/2012 ( [[#Brandt--2016|Brandt et al., 2016]] ; [[#Brandt--2017|Brandt et al., 2017]] ; [[#Brandt--2019|Brandt et al., 2019]] ). See SRCCL [[IPCC:Wg2:Chapter:Chapter-3#3.2.1|Section 3.2.1.1]] for an evaluation of the normalized difference vegetation index (NDVI) and remote sensing approaches used in these studies. Tree regeneration by farmers has also increased woody cover, particularly next to villages ( ''high confidence'' ) ( [[#Reij--2005|Reij et al., 2005]] ; [[#Reij--2016|Reij and Garrity, 2016]] ; [[#Brandt--2018|Brandt et al., 2018]] ). Otherwise, savanna encroachment has been attributed to combinations of increased rainfall ( [[#Venter--2018|Venter et al., 2018]] ; [[#Zhang--2019|Zhang et al., 2019]] ), warming ( [[#Venter--2018|Venter et al., 2018]] ) and CO 2 fertilization ( [[#Kgope--2010|Kgope et al., 2010]] ; [[#Bond--2012|Bond and Midgley, 2012]] ; [[#Buitenwerf--2012|Buitenwerf et al., 2012]] ; [[#Stevens--2016|Stevens et al., 2016]] ; [[#Quirk--2019|Quirk et al., 2019]] ) interacting with changing land use ( [[#Archer--2017|Archer et al., 2017]] ; [[#Venter--2018|Venter et al., 2018]] ), where herbivory and fire regimes are altered ( [[#O’Connor--2014|O’Connor et al., 2014]] ; [[#Archer--2017|Archer et al., 2017]] ; see also discussion on fire and herbivory in [[IPCC:Wg2:Chapter:Chapter-2#2.4.3.1|Section 2.4.3.1]] ). In some cases, woody increase has been balanced locally by changes in runoff ( [[#Trichon--2018|Trichon et al., 2018]] ) or by land clearing and fuel wood harvesting, as seen in western Niger, northern Nigeria, and at the periphery of major towns (Montagné et al., 2016).

'''Table CCP3.1 |''' Observed ecological changes in drylands.

{| class="wikitable"
|-
! '''Region'''

! '''Observed change'''

! '''Climate change factors'''

! '''Attribution to climate change'''

! '''Non-climate change factors'''

! '''Confidence in observed change'''

! '''References'''

|-
| colspan="6"| ''Hyper-arid''

|
|-
| Asian hyper-arid regions (Gobi)

| Loss of shallow rooted desert plants

| Increase in extreme warm temperatures

|
| ''medium''

| [[#Li--2015|Li et al. (2015)]]

|-
| rowspan="3"| North America—Mojave Desert

| Loss of mesic bird species

| Decreased rainfall

| Yes. Analyses of causal factors find decreased rainfall more important than non-climate factors.

| Livestock, human-ignited fires

| ''medium''

| [[#Iknayan--2018|Iknayan and Beissinger (2018)]] ; [[#Riddell--2019|Riddell et al. (2019)]]

|-
| Decline of desert tortoise ( ''Gopherus agassizii'' ) population by 90% from 1993 to 2012 at one site in the Mojave

| Decreased rainfall

|
| [[#Lovich--2014|Lovich et al. (2014)]]

|-
| Reduced perennial vegetation cover, including trees and cacti, in the Mojave and Sonoran deserts of the southwestern USA

| Increased temperature, decreased rainfall, wildfire

|
| Land use change, invasive plant species

| ''high''

| [[#Defalco--2010|Defalco et al. (2010)]] ; [[#Munson--2016b|Munson et al. (2016b)]] ; [[#Conver--2017|Conver et al. (2017)]]

|-
| colspan="6"| ''Arid''

|
|-
| rowspan="2"| African Sahel

| Woody cover increase in parts of the Sahel

| Increase in rainfall since the mid-1990s (compared to 1968–1993)and increased CO 2

|
| Restoration planting, agroforestry

| ''high''

|
|-
| Increase in grass production across the Sahel

| Increases in rainfall since the mid-1990s (compared to 1968–1993) and increased CO 2

|
| ''medium''

| Hiernaux et al. (2009a; 2009b); [[#Dardel--2014|Dardel et al. (2014)]] ; Venter et al. (2018); [[#Zhang--2018|Zhang et al. (2018)]] ; [[#Brandt--2019|Brandt et al. (2019)]] ; [[#Bernardino--2020|Bernardino et al. (2020)]]

|-
| rowspan="2"|
| Decline of mesic tree species at field sites across the Sahel

| Decreased rainfall from 1901 to 2002 increased temperature

| Yes. Multivariate statistical analyses find climate factors more important than non-climate factors.

| Land clearing for cropland expansion,

increased pressure on wood resources (rural demography, urbanisation)

| ''high''

| [[#Gonzalez--2001|Gonzalez (2001)]] ; [[#Wezel--2006|Wezel and Lykke (2006)]] ; [[#Maranz--2009|Maranz (2009)]] ; Gonzalez et al. (2012); [[#Hänke--2016|Hänke et al. (2016)]] ; [[#Kusserow--2017|Kusserow (2017)]] ; Ibrahim et al. (2018); Zida et al. (2020b)

|-
| Increased tree mortality at field sites across the Sahel

| Decreased rainfall from 1901 to 2002, increased temperature

| Yes. Multivariate statistical analyses find climate factors more important than non-climate factors.

| Agricultural expansion, modified runoff on shallow soils

| ''high''

| [[#Helldén--1984|Helldén (1984)]] ; Gonzalez, (2001); [[#Wezel--2006|Wezel and Lykke (2006)]] ; [[#Maranz--2009|Maranz (2009)]] ; Vincke et al. (2010); [[#Hänke--2016|Hänke et al. (2016)]] ; [[#Trichon--2018|Trichon et al. (2018)]] ; Zwarts et al. (2018); [[#Wendling--2019|Wendling et al. (2019)]] ; [[#Bernardino--2020|Bernardino et al. (2020)]] ; [[#Zida--2020a|Zida et al. (2020a)]]

|-
|
| Latitudinal biome shift of the Sahel

| Decreased rainfall, increased temperature

| Yes. Multivariate statistical analyses find climate factors more important than non-climate factors.

|
| ''high''

| [[#Boudet--1977|Boudet (1977)]] ; [[#Tucker--1999|Tucker and Nicholson (1999)]] ; Gonzalez, (2001); [[#Hiernaux--2006|Hiernaux and Le Houérou (2006)]] ; [[#Hiernaux--2009a|Hiernaux et al. (2009a)]] ; [[#Maranz--2009|Maranz (2009)]] ; Gonzalez et al. (2012)

|-
| Namib desert

| Increase in woody plant cover and a shift of mesic species into more arid regions

| Increase in amount of fog from westward expansion of convective rainfall and increase in number of extreme rainfall events; elevated CO 2 and warming effects on the Benguela upwelling system

|
| ''medium''

| [[#Morgan--2004|Morgan et al. (2004)]] ; Haensler et al. (2010); [[#Donohue--2013|Donohue et al. (2013)]] ; [[#Rohde--2019|Rohde et al. (2019)]]

|-
| rowspan="3"| Southern Africa— Nama-Karoo

|
| Shifting rainfall seasonality (debate over whether it is cyclical or directional); elevated CO 2

|
| ''medium''

| [[#Du%20Toit--2014|Du Toit and O’Connor (2014)]] ; Du Toit et al. (2015); Masubelele et al. (2015a; 2015b)

|-
| Eastern Karoo has experienced a significant increase in the end of the growing season length

| Shift in rainfall seasonality and increase in Mean Annual Precipitation

|
| ''low''

| [[#Davis-Reddy--2018|Davis-Reddy (2018)]]

|-
| Woody encroachment observed throughout the Nama-Karoo in valley bottoms, ephemeral stream banks and the slopes of Karoo hills

| Rising concentration of CO 2

|
| Changing land use and herbivore management

| ''medium''

| [[#Polley--1997|Polley et al. (1997)]] ; [[#Morgan--2004|Morgan et al. (2004)]] ; [[#Donohue--2013|Donohue et al. (2013)]] ; Ward et al. (2014); Masubelele et al. (2015a); [[#Hoffman--2018|Hoffman et al. (2018)]]

|-
| Southern Africa— Succulent Karoo

| Range shift in tree aloe ''Aloidendron dichotomum'' with mortality in the warmer and drier range and increase in recruitment in the cooler southern range, populations have positive growth rates, possibly due to warming, although this finding has been challenged

| Warming and drying

|
| ''medium''

| [[#Foden--2007a|Foden et al. (2007a)]] ; [[#Jack--2016|Jack et al. (2016)]]

|-
| rowspan="3"| Northern Africa—Morocco

| Increased vulnerability of oases and reduced ecosystem service provision

| High temperature and reduced precipitation causing soil and water salinisation, drying up of surface water; hot winds and sandstorms

|
| Agricultural growth, high population growth and unregulated and indiscriminate land development

| ''medium''

| [[#Karmaoui--2014|Karmaoui et al. (2014)]]

|-
| Reduced surface water availability

| Increased temperature and reduced precipitation

|
| High demand (population growth) and land use change

| ''medium''

| [[#Rochdane--2012|Rochdane et al. (2012)]] ; [[#Choukri--2020|Choukri et al. (2020)]]

|-
| Reduction of resilience of ''Abies pinasapo–Cedrus atlantica'' forests to subsequent droughts

| Successive droughts

|
| ''medium''

| [[#Navarro-Cerrillo--2020|Navarro-Cerrillo et al. (2020)]]

|-
| rowspan="3"| North American drylands

| Drought adapted species are increasing in Chihuahuan deserts

| Increase in aridity and increased interannual variation in climate trends

|
| ''medium''

| [[#Collins--2015|Collins and Xia (2015)]] ; [[#Rudgers--2018|Rudgers et al. (2018)]]

|-
| Widespread woody plant encroachment; ''Prosopis'' sp. encroachment in arid desert regions (Chihuahuan and Sonoran Desert) at a rate of ~3% per decade

| Increasing temperature, elevated CO 2 and changing rainfall

|
| Fire suppression and altered grazing/browsing regimes

| ''high''

| [[#Caracciolo--2016|Caracciolo et al. (2016)]] ; [[#Archer--2017|Archer et al. (2017)]]

|-
| Plant desert community shift changes the albedo through the reduction in dark biocrusts

| Warming and drought

|
| ''medium''

| Rutherford et al. (2000)

|-
| rowspan="4"| South Chihuahuan Desert— North and Central Mexico

| Shrub encroachment of grassland ( ''Berberis trifoliolata'' , ''Ephedra aspera'' , ''Larrea tridentata'' ) changes dominant species in shrub areas; loss of less resistant shrubby species ( ''Leucophyllum laevigatum'' , ''Lindleya mespiloides'' , ''Setchellanthus caeruleu'' ); shrub encroachment of mesic and temperate areas

| Decreased rainfall, increase in temperature and

increase CO 2

|
| Urban growth, mechanised agriculture and changes in land use

| ''high''

| [[#Pérez-Sánchez--2011|Pérez-Sánchez et al. (2011)]] ; [[#Castillón--2015|Castillón et al. (2015)]] ; [[#Sosa--2019|Sosa et al. (2019)]]

|-
| Shifts in soil microbial community to being more abundant in fungi (Ascomycota and Pleosporales)

| Decreased rainfall and increase in temperature

|
| Changes in land use

| ''low''

| [[#Vargas-Gastélum--2015|Vargas-Gastélum et al. (2015)]]

|-
| Limited ecological connectivity of shrubby populations

| Decreased rainfall and increase in temperature

|
| ''medium''

| [[#Sosa--2019|Sosa et al. (2019)]]

|-
| Loss of cacti species ( ''Echinocactus platyacanthus'' , ''Pediocactus bradyi'' , ''Coryphantha werdermannii'' , ''Astrophytum'' ) due to decline in physiological performance, loss of seed banks and lower germination rates

| Decreased rainfall and increase in temperature

|
| Cattle grazing, looting

| ''high''

| [[#Aragón-Gastélum--2014|Aragón-Gastélum et al. (2014)]] ; Shryock et al. (2014); [[#Martorell--2015|Martorell et al. (2015)]] ; [[#Carrillo-Angeles--2016|Carrillo-Angeles et al. (2016)]] ; [[#Aragón-Gastélum--2018|Aragón-Gastélum et al. (2018)]]

|-
| Arid and semiarid territories in Argentina

| Decreases in vegetation indexes

| Decreased rainfall

|
| Human-induced land degradation

| ''low''

| Barbosa et al. (2015)

|-
| Argentina Chaco Region

| Dryland salinity

| Changes in rainfall

|
| Land use change, overexploitation of water resources

| ''medium''

| [[#Amdan--2013|Amdan et al. (2013)]] ; [[#Marchesini--2017|Marchesini et al. (2017)]]

|-
| South America Arid Diagonal

| Marked reduction in streamflow from the Andes mountain ‘water towers’ due to the persistent reduction in precipitation

| Decrease in precipitation in the upper Andes; the unprecedented 10-year extreme dry period has been called the ‘Mega- drought’

|
| ''high''

| [[#Bianchi--2017|Bianchi et al. (2017)]] ; [[#Rivera--2018|Rivera and Penalba (2018)]] ; [[#Masiokas--2019|Masiokas et al. (2019)]] ; [[#Rodríguez-Morales--2019|Rodríguez-Morales et al. (2019)]]

|-
| South American Andes

| Extensive glacier retreat across the Andes

| Increasing sub-continental temperature and regional reduction in snow precipitation

|
| ''high''

| [[#Dussaillant--2019|Dussaillant et al. (2019)]] ; [[#Falaschi--2019|Falaschi et al. (2019)]] ; [[#Masiokas--2019|Masiokas et al. (2019)]]

|-
| rowspan="2"| Patagonian Andes

| Widespread tree mortality of ''Austrocedrus'' and ''Nothofagus'' forests in the dry ecotone forest-steppe across Patagonia

| Increase in extreme drought events

|
| ''high''

| [[#Rodríguez-Catón--2019|Rodríguez-Catón et al. (2019)]]

|-
| Increase in elevation of the upper-forest ''Nothofagus'' treeline across Patagonia

| Increase in temperature and duration of the growing season at high elevation in the Patagonian Andes

|
| ''high''

| Srur et al. (2016; 2018)

|-
| Central Asian arid lands

| Shrub encroachment into arid grasslands within the past 10 years

| Temperature of central Asian arid regions experienced a sharp increase in 1997 and has been in a state of high variability since then

|
| ''medium''

| [[#Li--2015|Li et al. (2015)]]

|-
| Loess Plateau, China

| Widespread vegetation greening in the Loess Plateau region; soil moisture declining widely, and deficit in forests and orchards; Yellow River runoff declining

| Significant warming, slight increase in precipitation

|
| Land use and cover change, ecological restoration, mainly induced by Grain for Green Project

| ''high''

| [[#Jia--2015|Jia et al. (2015)]] ; [[#Wang--2015|Wang et al. (2015)]] ; [[#Deng--2016|Deng et al. (2016)]] ; [[#Jiao--2016|Jiao et al. (2016)]]

|-
| The Three-River Source Region of the Tibetan Plateau, China

| Runoff increases, total water storage and groundwater increasing, Net Primary Productivity increase

| Precipitation increasing and evapotranspiration slightly decreasing

|
| Grassland protection

| ''high''

| Xu et al. (2019)

|-
| colspan="6"| ''Semiarid''

|
|-
| Australian arid lands

| Widespread greening

| Elevated CO 2

|
| ''medium''

| [[#Donohue--2013|Donohue et al. (2013)]]

|-
| African savanna

| Doubling of tree cover from 1940–2010 in South Africa, changing land use and 20% increase in spread of woody areas into previously open areas in the last 20 years

| Warming, elevated CO 2 , altered rainfall regimes

|
| Removal of mega-herbivores, fire suppression, changed herbivore regime

| ''high''

| [[#Skowno--2017|Skowno et al. (2017)]] ; [[#Stevens--2017|Stevens et al. (2017)]] ; Venter et al. (2018); García Criado et al. 2020)

|-
| African savanna

| Widespread increase in tree cover across Africa with only three countries across the continent experiencing a net decline in tree cover

| Warming, changing rainfall, mention of CO 2

|
| Fire suppression

| ''high''

| Venter et al. (2018)

|-
| African savanna

| Biodiversity responses to changes in vegetation structure (woody encroachment) causing declines in functional groups that are open area specialists, records for birds, rodents, termites, mammals, insects

| Woody encroachment

|
| ''medium''

| [[#Blaum--2007|Blaum et al. (2007)]] ; [[#Blaum--2009|Blaum et al. (2009)]] ; [[#Sirami--2012|Sirami and Monadjem (2012)]] ; [[#Gray--2013|Gray and Bond (2013)]] ; [[#Péron--2015|Péron and Altwegg (2015)]] ; [[#Smit--2015|Smit and Prins (2015)]]

|-
| African semiarid regions (savanna)

| Reduced tourism experience due to woody encroachment

| Woody encroachment

|
| ''low''

| [[#Gray--2013|Gray and Bond (2013)]]

|-
| rowspan="3"| North American drylands – sagebrush steppes

| Sagebrush steppes are being invaded by non-native grasses

| Increase in temperature and favourable climates

|
| ''high''

| Bradley et al. (2016); [[#Hufft--2016|Hufft and Zelikova (2016)]] ; [[#Chambers--2018|Chambers (2018)]]

|-
| Shrub encroachment,( ''Prosopis glandulosa'' , ''Juniper ashei'' and ''Juniper pinchotti)'' occurring in the semiarid grasslands of the southern Great Plains at a rate of ~8% per decade

| Increasing temperature, elevated CO 2 and changing rainfall

|
| Fire suppression and altered grazing/browsing regimes

| ''high''

| [[#Caracciolo--2016|Caracciolo et al. (2016)]] ; [[#Archer--2017|Archer et al. (2017)]]

|-
| Woody encroachment in sagebrush steppes (cold deserts) ( ''Juniper occidentalis'' ) at a rate of 2% per decade

| Warming and associated decline in snowpack, less precipitation falling as snow and an increase in the rain fraction in winter

|
| ''high''

| [[#Chambers--2014|Chambers et al. (2014)]] ; [[#Mote--2018|Mote et al. (2018)]]

|-
| Central Mexico

| Desertification (as decreases in vegetation indexes)

| Decreased rainfall and increase in temperature

|
| Land use change and intensification

| ''medium''

| [[#Becerril-Pina--2015|Becerril-Pina et al. (2015)]] ; [[#Noyola-Medrano--2017|Noyola-Medrano and Martínez-Sías (2017)]]

|-
| Chinese drylands

| Widespread greening trend of vegetation in China over the last three decades; regional differences

| Warming, CO 2 increase

Rising atmospheric CO 2 concentration and nitrogen deposition are identified as the most likely causes of the greening trend in China, explaining 85% and 41% of the average growing season Leaf Area Index trend

Negative impacts of climate change in north China and Inner Mongolia and and positive impacts in the Qinghai-Xizang plateau

|
| Ecological protection

| ''medium''

| [[#Piao--2015|Piao et al. (2015)]]

|-
| colspan="6"| ''Dry subhumid''

|
|-
| African mesic savannas

| Forest expansion into mesic savannas

| Increased rainfall, elevated CO 2

|
| Fire suppression

| ''medium''

| [[#Baccini--2017|Baccini et al. (2017)]] ; Aleman et al. (2018)

|-
| South American cerrado

| 8% rate of woody cover increase

| Elevated CO 2

|
| Fire exclusion

| ''high''

| [[#Stevens--2017|Stevens et al. (2017)]] ; [[#Rosan--2019|Rosan et al. (2019)]]

|-
| South American cerrado

| Expansion of forest into cerrado

| Elevated CO 2

|
| Fire exclusion

| ''high''

| [[#Passos--2018|Passos et al. (2018)]] ; [[#Rosan--2019|Rosan et al. (2019)]]

|-
| Australian savannas

| 2% rate of woody cover increase and greening of drylands

|
| ''high''

| [[#Donohue--2013|Donohue et al. (2013)]] ; [[#Stevens--2017|Stevens et al. (2017)]] ; [[#Bernardino--2020|Bernardino et al. (2020)]]

|}

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