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[[Image:Acid rain woods1.JPG|right|thumb|[[Jizera Mountains]] in Central Europe in 2006]]
[[Image:Acid rain woods1.JPG|right|thumb|[[Jizera Mountains]] in Central Europe in 2006]]
[[Image:Baumsterben. tree dieback near Ruderitz - saxony.jpg|thumb|Tree dieback because of persistent drought in the [[Saxony|Saxonian]] [[Vogtland]] in 2020]]
[[Image:Baumsterben. tree dieback near Ruderitz - saxony.jpg|thumb|Tree dieback because of persistent drought in the [[Saxony|Saxonian]] [[Vogtland]] in 2020]]
'''Forest dieback''' (also "{{lang|de|Waldsterben}}", a German [[German loan words|loan word]]) is a condition in [[Tree|trees]] or [[Woody plant|woody plants]] in which peripheral parts are killed, either by [[Pathogen|pathogens]], [[Parasite|parasites]] or conditions like [[acid rain]], [[drought]]<ref name="FAO20092">{{cite web|year=2009|title=Climate-induced forest dieback: an escalating global phenomenon?|url=http://www.fao.org/docrep/011/i0670e/i0670e10.htm|access-date=March 16, 2010|publisher=Food and Agricultural Organization (FAO)}}</ref>, and more. These episodes can have disastrous consequences such as reduced resiliency of the ecosystem<ref name=":0">{{Cite journal|last=Sangüesa-Barreda|first=Gabriel|last2=Linares|first2=Juan Carlos|last3=Camarero|first3=J. Julio|date=2015-12|title=Reduced growth sensitivity to climate in bark-beetle infested Aleppo pines: Connecting climatic and biotic drivers of forest dieback|url=http://dx.doi.org/10.1016/j.foreco.2015.08.017|journal=Forest Ecology and Management|volume=357|pages=126–137|doi=10.1016/j.foreco.2015.08.017|issn=0378-1127}}</ref>, disappearing important symbiotic relationships<ref name=":1">{{Cite journal|last=Štursová|first=Martina|last2=Šnajdr|first2=Jaroslav|last3=Cajthaml|first3=Tomáš|last4=Bárta|first4=Jiří|last5=Šantrůčková|first5=Hana|last6=Baldrian|first6=Petr|date=2014-03-27|title=When the forest dies: the response of forest soil fungi to a bark beetle-induced tree dieback|url=http://dx.doi.org/10.1038/ismej.2014.37|journal=The ISME Journal|volume=8|issue=9|pages=1920–1931|doi=10.1038/ismej.2014.37|issn=1751-7362}}</ref>and thresholds<ref name=":2">{{Cite journal|last=Evans|first=P. M.|last2=Newton|first2=A. C.|last3=Cantarello|first3=E.|last4=Martin|first4=P.|last5=Sanderson|first5=N.|last6=Jones|first6=D. L.|last7=Barsoum|first7=N.|last8=Cottrell|first8=J. E.|last9=A’Hara|first9=S. W.|last10=Fuller|first10=L.|date=2017-07-28|title=Thresholds of biodiversity and ecosystem function in a forest ecosystem undergoing dieback|url=http://dx.doi.org/10.1038/s41598-017-06082-6|journal=Scientific Reports|volume=7|issue=1|doi=10.1038/s41598-017-06082-6|issn=2045-2322}}</ref>. Two of the nine [[Tipping point (climatology)|tipping points]] for major [[Climate change|climate changes]] forecast for the next century, are directly related to [[forest]] diebacks<ref name=":3">{{Cite journal|last=Lenton|first=Timothy M.|last2=Held|first2=Hermann|last3=Kriegler|first3=Elmar|last4=Hall|first4=Jim W.|last5=Lucht|first5=Wolfgang|last6=Rahmstorf|first6=Stefan|last7=Schellnhuber|first7=Hans Joachim|date=2008-02-12|title=Tipping elements in the Earth's climate system|url=https://www.pnas.org/content/105/6/1786|journal=Proceedings of the National Academy of Sciences|language=en|volume=105|issue=6|pages=1786–1793|doi=10.1073/pnas.0705414105|issn=0027-8424|pmc=PMC2538841|pmid=18258748}}</ref>.
'''Forest dieback''' (also "{{lang|de|Waldsterben}}", a German [[German loan words|loan word]]) is a condition in [[tree]]s or [[woody plant]]s in which peripheral parts are killed, either by [[pathogen]]s, [[parasite]]s or due to conditions like [[acid rain]] and [[drought]].<ref name="FAO2009">{{cite web|url=http://www.fao.org/docrep/011/i0670e/i0670e10.htm |title=Climate-induced forest dieback: an escalating global phenomenon? |publisher=Food and Agricultural Organization (FAO) |year=2009 |access-date=March 16, 2010}}</ref> Two of the nine [[Tipping point (climatology)|tipping points]] for major [[climate change]]s forecast for the next century, are directly related to [[forest]] diebacks.<ref name="Tipping2008">{{cite web|url=http://www.uea.ac.uk/mac/comm/media/press/2008/feb/%27Tipping+points%27+could+come+this+century |title='Tipping points' could come this century |publisher=University of East Anglia |date=February 5, 2008 |access-date=March 16, 2010}}</ref>


==Definition==
==Definition==
Forest dieback refers to the phenomenon of a stand of [[tree]]s losing health and dying without an obvious cause. This condition is also known as forest decline, forest damage, canopy level dieback, and stand level dieback (also Waldsterben and Waldschäden, German loan words).<ref name="Ciesla1994">Ciesla, William M., Donaubauer, Edwin. (1994) ''Decline and dieback of trees and forests: A global overview.'' Rome, Italy: Food and Agriculture Organization of the United Nations.</ref> This usually affects individual [[species]] of trees, but can also affect multiple species. Trees and woody plants are killed by a complex group of factors that include parasites, such as fungi and beetles, pollution, such as acid rain and organic compounds, and [[drought]].<ref name="FAO2009" /> Diseases and pests can kill large groups of trees very easily, but the premature and progressive loss of clusters of trees without an obvious or specific cause is known as forest dieback. Dieback is an episodic event<ref name="Ciesla1994" /> and may take on many locations and shapes. It can be along the perimeter, at specific elevations, or dispersed throughout the forest ecosystem.<ref name="Krahl1988">Krahl-Urban, B., Papke, H.E., Peters, K. (1988) ''Forest Decline: Cause-Effect Research in the United States of North America and Federal Republic of Germany.'' Germany: Assessment Group for Biology, Ecology and Energy of the Julich Nuclear Research Center.</ref>
Forest dieback refers to the phenomenon of a stand of [[Tree|trees]] losing health and dying without an obvious cause. This condition is also known as forest decline, forest damage, canopy level dieback, and stand level dieback.<ref name="Ciesla19942">Ciesla, William M., Donaubauer, Edwin. (1994) ''Decline and dieback of trees and forests: A global overview.'' Rome, Italy: Food and Agriculture Organization of the United Nations.</ref> This usually affects individual [[species]] of trees, but can also affect multiple species. Dieback is an episodic event<ref name="Ciesla19942" /> and may take on many locations and shapes. It can be along the perimeter, at specific elevations, or dispersed throughout the forest ecosystem.<ref name="Krahl19882">Krahl-Urban, B., Papke, H.E., Peters, K. (1988) ''Forest Decline: Cause-Effect Research in the United States of North America and Federal Republic of Germany.'' Germany: Assessment Group for Biology, Ecology and Energy of the Julich Nuclear Research Center.</ref>


Forest dieback presents itself in many ways: falling off of leaves and needles, discolouration of leaves and needles, thinning of the crowns of trees, dead stands of trees of a certain age, and changes in the roots of the trees. It also has many dynamic forms. A stand of trees can exhibit mild symptoms, extreme symptoms, or even death. Forest decline can be viewed as the result of continued, widespread, and severe dieback of multiple species in a forest.<ref name="Ciesla1994" /> Current forest decline can be defined by: rapid development on individual trees, occurrence in different forest types, occurrence over a long duration (over 10 years), and occurrence throughout the natural range of affected species.<ref name="Krahl1988" />
Forest dieback presents itself in many ways: falling off of leaves and needles, discolouration of leaves and needles, thinning of the crowns of trees, dead stands of trees of a certain age, and changes in the roots of the trees. It also has many dynamic forms. A stand of trees can exhibit mild symptoms, extreme symptoms, or even death. Forest decline can be viewed as the result of continued, widespread, and severe dieback of multiple species in a forest.<ref name="Ciesla19942" /> Current forest decline can be defined by: rapid development on individual trees, occurrence in different forest types, occurrence over a long duration (over 10 years), and occurrence throughout the natural range of affected species.<ref name="Krahl19882" />


==History==
==History==
Much [[research]] was done in the 1980s when a severe dieback happened in Germany and the Northeast United States. Previous diebacks were regionally limited, but starting at the end of the 1970s a decline took over the forests of Central Europe and parts of North America. The forest damage in Germany was different because the decline was severe, up to 50% of the trees, prolonged, over 5 years, and widespread across tree species. Affected trees went from 8% in 1982 to 50% in 1984 and stayed at 50% through 1987.<ref name="Krahl1988" /> Many hypotheses have been proposed for this dieback, ''see below.''
A lot [[research]] was done in the 1980s when a severe dieback occurred in Germany and the Northeast United States. Previous diebacks were regionally limited, however, starting at the end of the 1970s, a decline took over the forests in Central Europe and parts of North America. The forest damage in Germany, specifically, was different as the decline was severe: the damage was widespread across various tree species. The percentage of affected trees increased from 8% in 1982 to 50% in 1984 and stayed at 50% through 1987.<ref name="Krahl198823">Krahl-Urban, B., Papke, H.E., Peters, K. (1988) ''Forest Decline: Cause-Effect Research in the United States of North America and Federal Republic of Germany.'' Germany: Assessment Group for Biology, Ecology and Energy of the Julich Nuclear Research Center.</ref> Many hypotheses have been proposed for this dieback, ''see below.''


In the 20th century, [[North America]] was hit with five notable [[hardwood]] diebacks. They occurred following the maturation of the forest and each episode had lasted about eleven years. The most severe temperate forest dieback targeted [[white birch]] and [[yellow birch]] trees. They experienced an episode that started between 1934 and 1937 and ended between 1953 and 1954. This followed a wave pattern that first appeared in Southern regions and moved to Northern regions, where a second wave was evident between 1957 and 1965 in Northern Quebec<ref name="Auclair19973">Auclair, A.N.D., Eglinton, P.D., Minnemeyer, S.L. (1997) ''Principle Forest Dieback Episodes in Northern Hardwoods: Development of Numeric Indices of Aereal Extent and Severity.'' Netherlands: Kluwer Academic Publishers.</ref>.
[[Drought]] and temperature ([[heat stress]]) have been causal factors in many diebacks. This condition is common in [[semi-arid]] regions and the trees are already stressed by [[dehydration]]. There have been reported diebacks in the 50% mortality range across multiple species in Africa, Asia, Europe, North America, South and Central America, and Australia. Notable drought related diebacks have occurred globally from the 1970s through the 2000s. A few drought related diebacks have occurred before this. For instance, a 23% mortality rate was reported between 1945 and 1993 in [[Senegal]] Africa.<ref name="Drought2009">Allen, C.D., et al. (2009) ''A Global Overview of Drought and Heat-induced Tree Mortality Reveals Emerging Climate Change Risks For Forests.'' Forest Ecology and Management, 259, 660–684. {{doi|10.1016/j.foreco.2009.09.001}}</ref> Trees stressed by drought and temperature are more susceptible to parasites.<ref name="Ciesla1994" />


In [[North America]] there were five notable [[hardwood]] diebacks in the 20th century. These have occurred with the maturation of the forest and each episode has lasted about eleven years. The most severe temperate forest dieback was on [[white birch]] and [[yellow birch]] trees. The others are [[ash (tree)|ash]], [[oak]], and [[maple]] species. White and yellow birches experienced an episode starting between 1934 and 1937 and ending between 1953 and 1954. This followed a wave pattern that first appeared in southern regions and moved to northern regions. A second wave is evident between 1957 and 1965 in northern Quebec. [[Acer saccharum|Sugar maple]] experienced a wave of dieback in parts of the United States during the 1960s. A second wave occurred primarily in Canada during the 1980s, but also reached the United States. These diebacks were numerically analyzed to exclude natural tree [[Death|mortality]]. It is hypothesized that a mature forest is more susceptible to extreme [[environmental stresses]].<ref name="Auclair1997">Auclair, A.N.D., Eglinton, P.D., Minnemeyer, S.L. (1997) ''Principle Forest Dieback Episodes in Northern Hardwoods: Development of Numeric Indices of Aereal Extent and Severity.'' Netherlands: Kluwer Academic Publishers.</ref>
Dieback can also affect other species such as a[[Ash (tree)|sh]], [[oak]], and [[maple]]. [[Acer saccharum|Sugar maple]], particularly, experienced a wave of dieback in parts of the United States during the 1960s. A second wave occurred primarily in Canada in the 1980s, but also managed to reach the United States. These diebacks were numerically analyzed to exclude natural tree [[Death|mortality]]. It is hypothesized that a mature forest is more susceptible to extreme [[environmental stresses]]<ref name="Auclair19973" />.

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[[Pathogen]]s are responsible for many diebacks. It is difficult to isolate and identify exactly which pathogens are responsible and how they interact with the trees. For instance ''[[Phomopsis azadirachtae]]'' is a fungus of the genus ''[[Phomopsis]]'' that has been identified as responsible for the dieback in ''[[Azadirachta indica]]'' (Neem) in the regions of India.<ref name="Girsh2008">Girsh, K., Shankara Bhat, S. (2008) ''Phomopsis azadirachtae – The Die-Back of Neem Pathogen.'' Electronic Journal of Biology, 4(3), 112-119.</ref> Neem is a very hardy and drought tolerant tree, has antifungal and antibacterial properties, but it still has many diseases. Different pathogens affect every part of the tree: the twigs, shoots, leaves, roots, and bark. Scientists have isolated both the fungus and the toxin. They have identified the pathogen in the tissue of all affected trees. Proper knowledge of toxin chemistry and its role in pathogenesis requires further investigations.<ref name="Girsh2008" /> Some experts consider dieback as a group of diseases with incompletely understood origins influenced by factors which predispose trees under stress to invasion.<ref name="Ciesla1994" />

[[Top dying disease]] is a disease of ''[[Heritiera fomes]]'', the dominant species of mangrove tree growing in the [[Sundarbans]]. This disease has become more prevalent since about 1970 and may be linked to an increase in the [[Heavy metal (chemistry)|heavy metal]] concentration of the sediment deposited in the [[Ganges Delta]].<ref>{{cite journal |author1=Awal, M.A. |author2=Hale, W.H.G. |author3=Stern, B. |year=2009 |title=Trace element concentrations in mangrove sediments in the Sundarbans, Bangladesh |journal=Marine Pollution Bulletin |volume=58 |issue=12 |pages=1944–1948 |doi=10.1016/j.marpolbul.2009.08.016 |arxiv=1506.05421 }}</ref>


==Hypothesis==
==Hypothesis==
The components of a [[Forest ecology|forest ecosystem]] are complex and identifying specific cause–effect relationships between dieback and the environment is a difficult process. Etiology is the science of identifying the causes of death. Because there is no one single and clear cause of dieback, there are multiple hypotheses that could explain its causes and effects. The following hypotheses were agreed upon from the scientific exchanges of Germany and the United States in 1988:<ref name="Krahl1988" />
The components of a [[Forest ecology|forest ecosystem]] are complex and identifying specific cause–effect relationships between dieback and the environment is a difficult process. Etiology is the science of identifying the causes of death. Over the years, a lot of research has been conducted and some hypotheses have been agreed upon such as:


* Bark beetle: Bark beetles use the soft tissues of a tree for shelter, subsistence and nest. Their arrival usually also includes other organisms such as fungi and bacteria. Together, they form symbiotic relationships where the condition of the tree gets exacerbated<ref name=":4">{{Cite web|last=Allen|first=C|last2=Ayres|first2=M|last3=Berg|first3=E|last4=Carroll|first4=A|last5=et al.|date=2005|title=Bark Beetle Outbreaks in Western North America: Causes and Consequences.|url=https://www.fs.fed.us/rm/pubs_other/rmrs_2009_bentz_b001.pdf|url-status=live|access-date=17 March 2021|website=US Forestry Service}}</ref>. Their life cycle is dependent on the presence of a tree as they lay their eggs in them. Once hatched, the larva can form a parasitic relationship with the tree, where it lives off it and cuts the circulation of water and nutrients from the roots to the shoots<ref name=":4" />.
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* ''[[Soil acidification]]/[[aluminum toxicity]]'': As a soil becomes more acidic, aluminum gets released, damaging the tree’s roots. Observed effects are: a reduction of uptake and transport of some cations, reduction in [[root respiration]], damage to fine feeder roots and root morphology, and reduction in elasticity of the [[cell wall]]s. This was proposed by Professor Bernhard Ulrich in 1979.<ref name="Krahl1988" />
* Groundwater conditions: A study conducted in Australia found that conditions such as depth and salinity could potentially help predict diebacks before they occur. In one bioregion, when both depth and salinity concentrations increased, standing of forests increased. However, in another bioregion in the same study area, when depth increased but the water had lower concentrations of salts (i.e. freshwater), diebacks increased<ref>{{Cite journal|last=CUNNINGHAM|first=SHAUN C.|last2=THOMSON|first2=JAMES R.|last3=MAC NALLY|first3=RALPH|last4=READ|first4=JENNIFER|last5=BAKER|first5=PATRICK J.|date=2011-02-21|title=Groundwater change forecasts widespread forest dieback across an extensive floodplain system|url=http://dx.doi.org/10.1111/j.1365-2427.2011.02585.x|journal=Freshwater Biology|volume=56|issue=8|pages=1494–1508|doi=10.1111/j.1365-2427.2011.02585.x|issn=0046-5070}}</ref>.
* Drought and heat stress: Drought and heat stress are hypothesized to cause dieback. Their apparent reason comes from two mechanisms<ref name=":02">{{Cite journal|last=Sangüesa-Barreda|first=Gabriel|last2=Linares|first2=Juan Carlos|last3=Camarero|first3=J. Julio|date=2015-12|title=Reduced growth sensitivity to climate in bark-beetle infested Aleppo pines: Connecting climatic and biotic drivers of forest dieback|url=http://dx.doi.org/10.1016/j.foreco.2015.08.017|journal=Forest Ecology and Management|volume=357|pages=126–137|doi=10.1016/j.foreco.2015.08.017|issn=0378-1127}}</ref>. The first one, hydraulic failure<ref name=":02" />, results in transportation failure of water from the roots to the shoots of a tree. This can cause dehydration and possibly death<ref name=":5">{{Cite journal|last=Adams|first=Henry D.|last2=Zeppel|first2=Melanie J. B.|last3=Anderegg|first3=William R. L.|last4=Hartmann|first4=Henrik|last5=Landhäusser|first5=Simon M.|last6=Tissue|first6=David T.|last7=Huxman|first7=Travis E.|last8=Hudson|first8=Patrick J.|last9=Franz|first9=Trenton E.|last10=Allen|first10=Craig D.|last11=Anderegg|first11=Leander D. L.|date=2017-09|title=A multi-species synthesis of physiological mechanisms in drought-induced tree mortality|url=http://www.nature.com/articles/s41559-017-0248-x|journal=Nature Ecology & Evolution|language=en|volume=1|issue=9|pages=1285–1291|doi=10.1038/s41559-017-0248-x|issn=2397-334X}}</ref>. The second, carbon starvation<ref name=":02" />, occurs as a plant’s response to heat is to close its stomata. This phenomenon cuts off entry of carbon dioxide, thereby making the plant rely on stored compounds like sugar. If the heat event is long and if the plant runs out of sugar, it will starve and die<ref name=":5" />.
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* [[Pathogen|Pathogens]] are responsible for many diebacks. It is difficult to isolate and identify exactly which pathogens are responsible and how they interact with the trees. For instance ''[[Phomopsis azadirachtae]]'' is a fungus of the genus ''[[Phomopsis]]'' that has been identified as responsible for the dieback in ''[[Azadirachta indica]]'' (Neem) in the regions of India.<ref name="Girsh20082">Girsh, K., Shankara Bhat, S. (2008) ''Phomopsis azadirachtae – The Die-Back of Neem Pathogen.'' Electronic Journal of Biology, 4(3), 112-119.</ref> Neem is a very hardy and drought tolerant tree, has antifungal and antibacterial properties, but it still has many diseases. Different pathogens affect every part of the tree: the twigs, shoots, leaves, roots, and bark. Scientists have isolated both the fungus and the toxin. They have identified the pathogen in the tissue of all affected trees. Proper knowledge of toxin chemistry and its role in pathogenesis requires further investigations.<ref name="Girsh20082" /> Some experts consider dieback as a group of diseases with incompletely understood origins influenced by factors which predispose trees under stress to invasion.<ref name="Ciesla199422">Ciesla, William M., Donaubauer, Edwin. (1994) ''Decline and dieback of trees and forests: A global overview.'' Rome, Italy: Food and Agriculture Organization of the United Nations.</ref>


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* ''Complex High-Elevation Disease'': The combination of high ozone levels, acid deposition and nutrient deficiencies at high elevations kills trees. High ozone concentrations damage the leaves and needles of trees and nutrients get leached from the foliage. The chain of events gets magnified over time. This was proposed by a group of professors: Bernhard Prinz, Karl Rehfuess, [[:de:Heinz W. Zöttl|Heinz Zöttl]].<ref name="Krahl1988" />


Some other hypotheses could explain the causes and effects of dieback. As agreed upon between the scientific exchanges of Germany and the United States in 1988<ref name="Krahl19884">Krahl-Urban, B., Papke, H.E., Peters, K. (1988) ''Forest Decline: Cause-Effect Research in the United States of North America and Federal Republic of Germany.'' Germany: Assessment Group for Biology, Ecology and Energy of the Julich Nuclear Research Center.</ref>:
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* ''[[Red-needle disease]] of [[spruce]]'': This disease causes needle drop and [[crown thinning]]. Needles turn a rust colour and fall off. This is caused by foliar fungi, which are secondary parasites attacking already weakened trees. This was proposed by Professor Karl Rehfuess.<ref name="Krahl1988" />


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* ''[[Soil acidification]]/[[aluminum toxicity]]'': As a soil becomes more acidic, aluminum gets released, damaging the tree’s roots. Some of the observed effects are: a reduction of uptake and transport of some cations, reduction in [[root respiration]], damage to fine feeder roots and root morphology, and reduction in elasticity of the [[Cell wall|cell walls]]. This was is proposed by Professor Bernhard Ulrich in 1979.<ref name="Krahl19884" />
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* ''General Stress'': The increased concentration of [[atmospheric pollutants]] hurts the root system and leads to the accumulation of toxins in new leaves. Pollutants can alter growth, reduce photosynthetic activity, and reduce the formation of [[secondary metabolites]]. It is believed that even low concentrations levels can be considered toxic. This was proposed by a group of professors led by Peter Schütt.<ref name="Krahl1988" />
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* Complex High-Elevation Disease: The combination of high ozone levels, acid deposition and nutrient deficiencies at high elevations kills trees. High ozone concentrations damage the leaves and needles of trees and nutrients get leached from the foliage. The chain of events gets magnified over time. This was proposed by a group of professors: Bernhard Prinz, Karl Rehfuess, and [[Heinz W. Zöttl|Heinz Zöttl]] <ref name="Krahl19884" />.
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* Red-needle disease of [[spruce]]: This disease causes needle drop and crown thinning. Needles turn a rust color and fall off. This is caused by foliar fungi, which are secondary parasites attacking already weakened trees. This was proposed by Professor Karl Rehfuess <ref name="Krahl19884" />.
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* Pollution General Stress: The increased concentration level of atmospheric pollutants hurts the root system and leads to the accumulation of toxins in new leaves. Pollutants can alter the growth, reduce the photosynthetic activity, and reduce the formation of [[secondary metabolites]]. It is believed that low concentrations levels can be considered are toxic. This was proposed by a group of professors led by Peter Schütt<ref name="Krahl19884" />.
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** Organic Air Pollutants: this subsection focuses on organic compounds. The three compounds seriously discussed are [[ethylene]], [[aniline]], and [[dinitrophenol]]. Even at low levels, these organic chemical compounds have caused: abnormal dropping of foliage, twisted foliage, and killing of seedlings. This was proposed by Fritz Führ<ref name="Krahl19884" />.
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* Excess Nitrogen Deposition: The increased level of nitrogen and [[ammonium]], both commonly found in [[fertilizer]], could have the following possible effects: it could inhibit beneficial fungi, delay chemical reactions, disturb normal balances between shoot growth and root growth, and increase [[soil leaching]]. However, there is no experimental proof. This was proposed by Carl Olaf Tamm<ref name="Krahl19884" />. See also: [[Nutrient pollution]]


==Consequences==
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* ''Excess Nitrogen Deposition'': The increased level of nitrogen and [[ammonium]], both commonly found in [[Fertilizers|fertilizer]], could have the following possible effects: it could inhibit beneficial fungi, delay chemical reactions, disturb normal balances between shoot growth and root growth, and increase [[soil leaching]]. However, there is no experimental proof. This was proposed by Carl Olaf Tamm.<ref name="Krahl1988" /> {{see also|Nutrient pollution}}
Forest dieback can be caused by a multitude of factors, however, once they occur, they can have certain consequences.


* Fungal community: Ectomycorrhizal fungi form a symbiotic relationship with trees. Following a bark beetle outbreak, dieback can occur. This process can decrease photosynthesis, nutrient availability and decomposition rates and processes. Once this occurs, the symbiotic relationship, previously mentioned, gets negatively affected: the ectomycorrhizal fungi community decreased and then the relationship disappeared altogether<ref name=":12">{{Cite journal|last=Štursová|first=Martina|last2=Šnajdr|first2=Jaroslav|last3=Cajthaml|first3=Tomáš|last4=Bárta|first4=Jiří|last5=Šantrůčková|first5=Hana|last6=Baldrian|first6=Petr|date=2014-03-27|title=When the forest dies: the response of forest soil fungi to a bark beetle-induced tree dieback|url=http://dx.doi.org/10.1038/ismej.2014.37|journal=The ISME Journal|volume=8|issue=9|pages=1920–1931|doi=10.1038/ismej.2014.37|issn=1751-7362}}</ref>. This is problematic as certain plants depend on their presence for survival<ref name=":6">{{Cite journal|last=Policelli|first=Nahuel|last2=Horton|first2=Thomas R.|last3=Hudon|first3=Aimée T.|last4=Patterson|first4=Taylor R.|last5=Bhatnagar|first5=Jennifer M.|date=2020-08-06|title=Back to Roots: The Role of Ectomycorrhizal Fungi in Boreal and Temperate Forest Restoration|url=https://www.frontiersin.org/article/10.3389/ffgc.2020.00097/full|journal=Frontiers in Forests and Global Change|volume=3|pages=97|doi=10.3389/ffgc.2020.00097|issn=2624-893X}}</ref>.
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* ''Organic Air Pollutants'': This is similar to the general stress hypothesis, but focuses on organic compounds. The three compounds seriously discussed are [[ethylene]], [[aniline]], and [[dinitrophenol]]. Even at low levels, these organic chemical compounds have caused: abnormal dropping of foliage, twisted foliage, and killing of seedlings. This was proposed by Fritz Führ.<ref name="Krahl1988" />
* Soil chemistry: Soil chemistry changed following a dieback episode. It resulted in the increase of base saturation as biomass left behind set free certain ions such as calcium, magnesium and potassium<ref name=":7">{{Cite journal|last=Kaňa|first=Jiří|last2=Kopáček|first2=Jiří|last3=Tahovská|first3=Karolina|last4=Šantrůčková|first4=Hana|date=2019-02|title=Tree dieback and related changes in nitrogen dynamics modify the concentrations and proportions of cations on soil sorption complex|url=http://dx.doi.org/10.1016/j.ecolind.2018.10.032|journal=Ecological Indicators|volume=97|pages=319–328|doi=10.1016/j.ecolind.2018.10.032|issn=1470-160X}}</ref>. This can be considered a positive consequence as base saturation is essential for plant growth and soil fertility<ref>{{Cite web|title=Cation Exchange Capacity and Base Saturation {{!}} UGA Cooperative Extension|url=https://extension.uga.edu/publications/detail.html?number=C1040&title=Cation%20Exchange%20Capacity%20and%20Base%20Saturation|access-date=2021-03-29|website=extension.uga.edu}}</ref>. Therefore, this signifies that soil chemistry following a dieback even could aid in recovering acidic soils<ref name=":7" />.


==Global climate change==
=== Climate change ===
Changes in mean annual temperature and drought are major contributing factors to forest dieback. As more carbon is released from dead trees, especially in the [[Amazon rainforest|Amazon]] and [[Boreal forest|Boreal]] forests, more [[greenhouse gas]]es are released into the atmosphere. Increased levels of greenhouse gases increase the temperature of the atmosphere. The feedback loop is reinforced and the biological adaptations of the species determine its survival. Projections for dieback vary, but the threat of [[Global warming|global climate change]] only stands to increase the rate of dieback.<ref name="Allen2009">Allen, C.D. (2009) ''Climate-induced forest dieback: An escalating global phenomenon?'' Unasylva 231/232, 60, 43-48.</ref>
Changes in mean annual temperature and drought are major contributing factors to forest dieback. As more carbon is released from dead trees, especially in the [[Amazon rainforest|Amazon]] and [[Boreal forest|Boreal]] forests, more [[Greenhouse gas|greenhouse gases]] are released into the atmosphere. Increased levels of greenhouse gases increase the temperature of the atmosphere. Projections for dieback vary, but the threat of [[Global warming|global climate change]] only stands to increase the rate of dieback<ref name=":42">{{Cite web|last=Allen|first=C|last2=Ayres|first2=M|last3=Berg|first3=E|last4=Carroll|first4=A|last5=et al.|date=2005|title=Bark Beetle Outbreaks in Western North America: Causes and Consequences.|url=https://www.fs.fed.us/rm/pubs_other/rmrs_2009_bentz_b001.pdf|url-status=live|access-date=17 March 2021|website=US Forestry Service}}</ref>.


* Reduced resiliency: Trees can be resilient. However, that can be changed when the ecosystem is hit with a drought episode. This results in trees becoming more susceptible to insect infestations, thereby triggering a dieback event<ref name=":03">{{Cite journal|last=Sangüesa-Barreda|first=Gabriel|last2=Linares|first2=Juan Carlos|last3=Camarero|first3=J. Julio|date=2015-12|title=Reduced growth sensitivity to climate in bark-beetle infested Aleppo pines: Connecting climatic and biotic drivers of forest dieback|url=http://dx.doi.org/10.1016/j.foreco.2015.08.017|journal=Forest Ecology and Management|volume=357|pages=126–137|doi=10.1016/j.foreco.2015.08.017|issn=0378-1127}}</ref>. This is a problem as climate change is predicted to increase drought in certain regions of the world.
⚫
Scientists do not know the exact tipping points of climate change and can only estimate the timescales. When a [[Tipping points in the climate system|tipping point]] - the critical threshold - is reached, a small change in human activity can have long-term consequences on the [[Natural environment|environment]]. Two of the nine tipping points for major climate changes forecast for the next century are directly related to forest diebacks. Scientists are worried that forest dieback in the [[Amazon rain forest]]<ref name="Blaustien2011">Blaustein, R.J. (2011). ''Amazon Dieback and the 21st Century.'' Bioscience, 61(3), 176-182. {{doi|10.1525/bio.2011.61.3.3}}</ref> and the [[Boreal evergreen forest]]<ref name="Krankina1997">Krankina, O.N., et al. (1997) ''Global Climate Change Adaption: Examples From Russian Boreal Forests.'' Climatic Change, 36, 197–216.</ref> will trigger a tipping point in the next 50 years.<ref name="Tipping2008" />
* Thresholds: A number of thresholds exist in relation to forest dieback such as “biodiversity [...], ecological condition [...] and ecosystem function” <ref name=":22">{{Cite journal|last=Evans|first=P. M.|last2=Newton|first2=A. C.|last3=Cantarello|first3=E.|last4=Martin|first4=P.|last5=Sanderson|first5=N.|last6=Jones|first6=D. L.|last7=Barsoum|first7=N.|last8=Cottrell|first8=J. E.|last9=A’Hara|first9=S. W.|last10=Fuller|first10=L.|date=2017-07-28|title=Thresholds of biodiversity and ecosystem function in a forest ecosystem undergoing dieback|url=http://dx.doi.org/10.1038/s41598-017-06082-6|journal=Scientific Reports|volume=7|issue=1|doi=10.1038/s41598-017-06082-6|issn=2045-2322}}</ref>. As climate change has the power to cause diebacks through multiple processes (discussed earlier), these thresholds are becoming more and more achievable where, in some cases, they have the ability to induce a positive feedback process<ref name=":22" />: when the basal area in an ecosystem decreases by 50%, species richness of ectomycorrhizal fungi follows. As mentioned earlier, ectomycorrhizal fungi are important for the survival of certain plants<ref name=":62">{{Cite journal|last=Policelli|first=Nahuel|last2=Horton|first2=Thomas R.|last3=Hudon|first3=Aimée T.|last4=Patterson|first4=Taylor R.|last5=Bhatnagar|first5=Jennifer M.|date=2020-08-06|title=Back to Roots: The Role of Ectomycorrhizal Fungi in Boreal and Temperate Forest Restoration|url=https://www.frontiersin.org/article/10.3389/ffgc.2020.00097/full|journal=Frontiers in Forests and Global Change|volume=3|pages=97|doi=10.3389/ffgc.2020.00097|issn=2624-893X}}</ref>, turning dieback into a positive feedback mechanism.
⚫
* Tipping points: Scientists do not know the exact tipping points of climate change and can only estimate the timescales. When a [[Tipping points in the climate system|tipping point]] is reached, a small change in human activity can have long-term consequences on the [[Natural environment|environment]]. Two of the nine tipping points for major climate changes forecast for the next century are directly related to forest diebacks<ref name=":32">{{Cite journal|last=Lenton|first=Timothy M.|last2=Held|first2=Hermann|last3=Kriegler|first3=Elmar|last4=Hall|first4=Jim W.|last5=Lucht|first5=Wolfgang|last6=Rahmstorf|first6=Stefan|last7=Schellnhuber|first7=Hans Joachim|date=2008-02-12|title=Tipping elements in the Earth's climate system|url=https://www.pnas.org/content/105/6/1786|journal=Proceedings of the National Academy of Sciences|language=en|volume=105|issue=6|pages=1786–1793|doi=10.1073/pnas.0705414105|issn=0027-8424|pmc=PMC2538841|pmid=18258748}}</ref>. Scientists are worried that forest dieback in the [[Amazon rain forest]]<ref name="Blaustien20112">Blaustein, R.J. (2011). ''Amazon Dieback and the 21st Century.'' Bioscience, 61(3), 176-182. {{doi|10.1525/bio.2011.61.3.3}}</ref> and the [[Boreal evergreen forest]]<ref name="Krankina19972">Krankina, O.N., et al. (1997) ''Global Climate Change Adaption: Examples From Russian Boreal Forests.'' Climatic Change, 36, 197–216.</ref>will trigger a tipping point in the next 50 years<ref>{{Cite web|title='Tipping points' could come this century|url=http://www.eurekalert.org/pub_releases/2008-02/uoea-pc020408.php|access-date=2021-03-29|website=EurekAlert!|language=en}}</ref>.


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Revision as of 17:04, 29 March 2021

Jizera Mountains in Central Europe in 2006
Tree dieback because of persistent drought in the Saxonian Vogtland in 2020

Forest dieback (also "Waldsterben", a German loan word) is a condition in trees or woody plants in which peripheral parts are killed, either by pathogens, parasites or conditions like acid rain, drought[1], and more. These episodes can have disastrous consequences such as reduced resiliency of the ecosystem[2], disappearing important symbiotic relationships[3]and thresholds[4]. Two of the nine tipping points for major climate changes forecast for the next century, are directly related to forest diebacks[5].

Definition

Forest dieback refers to the phenomenon of a stand of trees losing health and dying without an obvious cause. This condition is also known as forest decline, forest damage, canopy level dieback, and stand level dieback.[6] This usually affects individual species of trees, but can also affect multiple species. Dieback is an episodic event[6] and may take on many locations and shapes. It can be along the perimeter, at specific elevations, or dispersed throughout the forest ecosystem.[7]

Forest dieback presents itself in many ways: falling off of leaves and needles, discolouration of leaves and needles, thinning of the crowns of trees, dead stands of trees of a certain age, and changes in the roots of the trees. It also has many dynamic forms. A stand of trees can exhibit mild symptoms, extreme symptoms, or even death. Forest decline can be viewed as the result of continued, widespread, and severe dieback of multiple species in a forest.[6] Current forest decline can be defined by: rapid development on individual trees, occurrence in different forest types, occurrence over a long duration (over 10 years), and occurrence throughout the natural range of affected species.[7]

History

A lot research was done in the 1980s when a severe dieback occurred in Germany and the Northeast United States. Previous diebacks were regionally limited, however, starting at the end of the 1970s, a decline took over the forests in Central Europe and parts of North America. The forest damage in Germany, specifically, was different as the decline was severe: the damage was widespread across various tree species. The percentage of affected trees increased from 8% in 1982 to 50% in 1984 and stayed at 50% through 1987.[8] Many hypotheses have been proposed for this dieback, see below.

In the 20th century, North America was hit with five notable hardwood diebacks. They occurred following the maturation of the forest and each episode had lasted about eleven years. The most severe temperate forest dieback targeted white birch and yellow birch trees. They experienced an episode that started between 1934 and 1937 and ended between 1953 and 1954. This followed a wave pattern that first appeared in Southern regions and moved to Northern regions, where a second wave was evident between 1957 and 1965 in Northern Quebec[9].

Dieback can also affect other species such as ash, oak, and maple. Sugar maple, particularly, experienced a wave of dieback in parts of the United States during the 1960s. A second wave occurred primarily in Canada in the 1980s, but also managed to reach the United States. These diebacks were numerically analyzed to exclude natural tree mortality. It is hypothesized that a mature forest is more susceptible to extreme environmental stresses[9].

Hypothesis

The components of a forest ecosystem are complex and identifying specific cause–effect relationships between dieback and the environment is a difficult process. Etiology is the science of identifying the causes of death. Over the years, a lot of research has been conducted and some hypotheses have been agreed upon such as:

  • Bark beetle: Bark beetles use the soft tissues of a tree for shelter, subsistence and nest. Their arrival usually also includes other organisms such as fungi and bacteria. Together, they form symbiotic relationships where the condition of the tree gets exacerbated[10]. Their life cycle is dependent on the presence of a tree as they lay their eggs in them. Once hatched, the larva can form a parasitic relationship with the tree, where it lives off it and cuts the circulation of water and nutrients from the roots to the shoots[10].
  • Groundwater conditions: A study conducted in Australia found that conditions such as depth and salinity could potentially help predict diebacks before they occur. In one bioregion, when both depth and salinity concentrations increased, standing of forests increased. However, in another bioregion in the same study area, when depth increased but the water had lower concentrations of salts (i.e. freshwater), diebacks increased[11].
  • Drought and heat stress: Drought and heat stress are hypothesized to cause dieback. Their apparent reason comes from two mechanisms[12]. The first one, hydraulic failure[12], results in transportation failure of water from the roots to the shoots of a tree. This can cause dehydration and possibly death[13]. The second, carbon starvation[12], occurs as a plant’s response to heat is to close its stomata. This phenomenon cuts off entry of carbon dioxide, thereby making the plant rely on stored compounds like sugar. If the heat event is long and if the plant runs out of sugar, it will starve and die[13].
  • Pathogens are responsible for many diebacks. It is difficult to isolate and identify exactly which pathogens are responsible and how they interact with the trees. For instance Phomopsis azadirachtae is a fungus of the genus Phomopsis that has been identified as responsible for the dieback in Azadirachta indica (Neem) in the regions of India.[14] Neem is a very hardy and drought tolerant tree, has antifungal and antibacterial properties, but it still has many diseases. Different pathogens affect every part of the tree: the twigs, shoots, leaves, roots, and bark. Scientists have isolated both the fungus and the toxin. They have identified the pathogen in the tissue of all affected trees. Proper knowledge of toxin chemistry and its role in pathogenesis requires further investigations.[14] Some experts consider dieback as a group of diseases with incompletely understood origins influenced by factors which predispose trees under stress to invasion.[15]


Some other hypotheses could explain the causes and effects of dieback. As agreed upon between the scientific exchanges of Germany and the United States in 1988[16]:

  • Soil acidification/aluminum toxicity: As a soil becomes more acidic, aluminum gets released, damaging the tree’s roots. Some of the observed effects are: a reduction of uptake and transport of some cations, reduction in root respiration, damage to fine feeder roots and root morphology, and reduction in elasticity of the cell walls. This was is proposed by Professor Bernhard Ulrich in 1979.[16]
  • Complex High-Elevation Disease: The combination of high ozone levels, acid deposition and nutrient deficiencies at high elevations kills trees. High ozone concentrations damage the leaves and needles of trees and nutrients get leached from the foliage. The chain of events gets magnified over time. This was proposed by a group of professors: Bernhard Prinz, Karl Rehfuess, and Heinz Zöttl [16].
  • Red-needle disease of spruce: This disease causes needle drop and crown thinning. Needles turn a rust color and fall off. This is caused by foliar fungi, which are secondary parasites attacking already weakened trees. This was proposed by Professor Karl Rehfuess [16].
  • Pollution General Stress: The increased concentration level of atmospheric pollutants hurts the root system and leads to the accumulation of toxins in new leaves. Pollutants can alter the growth, reduce the photosynthetic activity, and reduce the formation of secondary metabolites. It is believed that low concentrations levels can be considered are toxic. This was proposed by a group of professors led by Peter Schütt[16].
    • Organic Air Pollutants: this subsection focuses on organic compounds. The three compounds seriously discussed are ethylene, aniline, and dinitrophenol. Even at low levels, these organic chemical compounds have caused: abnormal dropping of foliage, twisted foliage, and killing of seedlings. This was proposed by Fritz Führ[16].
  • Excess Nitrogen Deposition: The increased level of nitrogen and ammonium, both commonly found in fertilizer, could have the following possible effects: it could inhibit beneficial fungi, delay chemical reactions, disturb normal balances between shoot growth and root growth, and increase soil leaching. However, there is no experimental proof. This was proposed by Carl Olaf Tamm[16]. See also: Nutrient pollution

Consequences

Forest dieback can be caused by a multitude of factors, however, once they occur, they can have certain consequences.

  • Fungal community: Ectomycorrhizal fungi form a symbiotic relationship with trees. Following a bark beetle outbreak, dieback can occur. This process can decrease photosynthesis, nutrient availability and decomposition rates and processes. Once this occurs, the symbiotic relationship, previously mentioned, gets negatively affected: the ectomycorrhizal fungi community decreased and then the relationship disappeared altogether[17]. This is problematic as certain plants depend on their presence for survival[18].
  • Soil chemistry: Soil chemistry changed following a dieback episode. It resulted in the increase of base saturation as biomass left behind set free certain ions such as calcium, magnesium and potassium[19]. This can be considered a positive consequence as base saturation is essential for plant growth and soil fertility[20]. Therefore, this signifies that soil chemistry following a dieback even could aid in recovering acidic soils[19].

Climate change

Changes in mean annual temperature and drought are major contributing factors to forest dieback. As more carbon is released from dead trees, especially in the Amazon and Boreal forests, more greenhouse gases are released into the atmosphere. Increased levels of greenhouse gases increase the temperature of the atmosphere. Projections for dieback vary, but the threat of global climate change only stands to increase the rate of dieback[21].

  • Reduced resiliency: Trees can be resilient. However, that can be changed when the ecosystem is hit with a drought episode. This results in trees becoming more susceptible to insect infestations, thereby triggering a dieback event[22]. This is a problem as climate change is predicted to increase drought in certain regions of the world.
  • Thresholds: A number of thresholds exist in relation to forest dieback such as “biodiversity [...], ecological condition [...] and ecosystem function” [23]. As climate change has the power to cause diebacks through multiple processes (discussed earlier), these thresholds are becoming more and more achievable where, in some cases, they have the ability to induce a positive feedback process[23]: when the basal area in an ecosystem decreases by 50%, species richness of ectomycorrhizal fungi follows. As mentioned earlier, ectomycorrhizal fungi are important for the survival of certain plants[24], turning dieback into a positive feedback mechanism.
  • Tipping points: Scientists do not know the exact tipping points of climate change and can only estimate the timescales. When a tipping point is reached, a small change in human activity can have long-term consequences on the environment. Two of the nine tipping points for major climate changes forecast for the next century are directly related to forest diebacks[25]. Scientists are worried that forest dieback in the Amazon rain forest[26] and the Boreal evergreen forest[27]will trigger a tipping point in the next 50 years[28].

See also

References

  1. ^ "Climate-induced forest dieback: an escalating global phenomenon?". Food and Agricultural Organization (FAO). 2009. Retrieved March 16, 2010.
  2. ^ Sangüesa-Barreda, Gabriel; Linares, Juan Carlos; Camarero, J. Julio (2015-12). "Reduced growth sensitivity to climate in bark-beetle infested Aleppo pines: Connecting climatic and biotic drivers of forest dieback". Forest Ecology and Management. 357: 126–137. doi:10.1016/j.foreco.2015.08.017. ISSN 0378-1127. {{cite journal}}: Check date values in: |date= (help)
  3. ^ Štursová, Martina; Šnajdr, Jaroslav; Cajthaml, Tomáš; Bárta, Jiří; Šantrůčková, Hana; Baldrian, Petr (2014-03-27). "When the forest dies: the response of forest soil fungi to a bark beetle-induced tree dieback". The ISME Journal. 8 (9): 1920–1931. doi:10.1038/ismej.2014.37. ISSN 1751-7362.
  4. ^ Evans, P. M.; Newton, A. C.; Cantarello, E.; Martin, P.; Sanderson, N.; Jones, D. L.; Barsoum, N.; Cottrell, J. E.; A’Hara, S. W.; Fuller, L. (2017-07-28). "Thresholds of biodiversity and ecosystem function in a forest ecosystem undergoing dieback". Scientific Reports. 7 (1). doi:10.1038/s41598-017-06082-6. ISSN 2045-2322.
  5. ^ Lenton, Timothy M.; Held, Hermann; Kriegler, Elmar; Hall, Jim W.; Lucht, Wolfgang; Rahmstorf, Stefan; Schellnhuber, Hans Joachim (2008-02-12). "Tipping elements in the Earth's climate system". Proceedings of the National Academy of Sciences. 105 (6): 1786–1793. doi:10.1073/pnas.0705414105. ISSN 0027-8424. PMC 2538841. PMID 18258748.{{cite journal}}: CS1 maint: PMC format (link)
  6. ^ a b c Ciesla, William M., Donaubauer, Edwin. (1994) Decline and dieback of trees and forests: A global overview. Rome, Italy: Food and Agriculture Organization of the United Nations.
  7. ^ a b Krahl-Urban, B., Papke, H.E., Peters, K. (1988) Forest Decline: Cause-Effect Research in the United States of North America and Federal Republic of Germany. Germany: Assessment Group for Biology, Ecology and Energy of the Julich Nuclear Research Center.
  8. ^ Krahl-Urban, B., Papke, H.E., Peters, K. (1988) Forest Decline: Cause-Effect Research in the United States of North America and Federal Republic of Germany. Germany: Assessment Group for Biology, Ecology and Energy of the Julich Nuclear Research Center.
  9. ^ a b Auclair, A.N.D., Eglinton, P.D., Minnemeyer, S.L. (1997) Principle Forest Dieback Episodes in Northern Hardwoods: Development of Numeric Indices of Aereal Extent and Severity. Netherlands: Kluwer Academic Publishers.
  10. ^ a b Allen, C; Ayres, M; Berg, E; Carroll, A; et al. (2005). "Bark Beetle Outbreaks in Western North America: Causes and Consequences" (PDF). US Forestry Service. Retrieved 17 March 2021. {{cite web}}: Explicit use of et al. in: |last5= (help)CS1 maint: url-status (link)
  11. ^ CUNNINGHAM, SHAUN C.; THOMSON, JAMES R.; MAC NALLY, RALPH; READ, JENNIFER; BAKER, PATRICK J. (2011-02-21). "Groundwater change forecasts widespread forest dieback across an extensive floodplain system". Freshwater Biology. 56 (8): 1494–1508. doi:10.1111/j.1365-2427.2011.02585.x. ISSN 0046-5070.
  12. ^ a b c Sangüesa-Barreda, Gabriel; Linares, Juan Carlos; Camarero, J. Julio (2015-12). "Reduced growth sensitivity to climate in bark-beetle infested Aleppo pines: Connecting climatic and biotic drivers of forest dieback". Forest Ecology and Management. 357: 126–137. doi:10.1016/j.foreco.2015.08.017. ISSN 0378-1127. {{cite journal}}: Check date values in: |date= (help)
  13. ^ a b Adams, Henry D.; Zeppel, Melanie J. B.; Anderegg, William R. L.; Hartmann, Henrik; Landhäusser, Simon M.; Tissue, David T.; Huxman, Travis E.; Hudson, Patrick J.; Franz, Trenton E.; Allen, Craig D.; Anderegg, Leander D. L. (2017-09). "A multi-species synthesis of physiological mechanisms in drought-induced tree mortality". Nature Ecology & Evolution. 1 (9): 1285–1291. doi:10.1038/s41559-017-0248-x. ISSN 2397-334X. {{cite journal}}: Check date values in: |date= (help)
  14. ^ a b Girsh, K., Shankara Bhat, S. (2008) Phomopsis azadirachtae – The Die-Back of Neem Pathogen. Electronic Journal of Biology, 4(3), 112-119.
  15. ^ Ciesla, William M., Donaubauer, Edwin. (1994) Decline and dieback of trees and forests: A global overview. Rome, Italy: Food and Agriculture Organization of the United Nations.
  16. ^ a b c d e f g Krahl-Urban, B., Papke, H.E., Peters, K. (1988) Forest Decline: Cause-Effect Research in the United States of North America and Federal Republic of Germany. Germany: Assessment Group for Biology, Ecology and Energy of the Julich Nuclear Research Center.
  17. ^ Štursová, Martina; Šnajdr, Jaroslav; Cajthaml, Tomáš; Bárta, Jiří; Šantrůčková, Hana; Baldrian, Petr (2014-03-27). "When the forest dies: the response of forest soil fungi to a bark beetle-induced tree dieback". The ISME Journal. 8 (9): 1920–1931. doi:10.1038/ismej.2014.37. ISSN 1751-7362.
  18. ^ Policelli, Nahuel; Horton, Thomas R.; Hudon, Aimée T.; Patterson, Taylor R.; Bhatnagar, Jennifer M. (2020-08-06). "Back to Roots: The Role of Ectomycorrhizal Fungi in Boreal and Temperate Forest Restoration". Frontiers in Forests and Global Change. 3: 97. doi:10.3389/ffgc.2020.00097. ISSN 2624-893X.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  19. ^ a b Kaňa, Jiří; Kopáček, Jiří; Tahovská, Karolina; Šantrůčková, Hana (2019-02). "Tree dieback and related changes in nitrogen dynamics modify the concentrations and proportions of cations on soil sorption complex". Ecological Indicators. 97: 319–328. doi:10.1016/j.ecolind.2018.10.032. ISSN 1470-160X. {{cite journal}}: Check date values in: |date= (help)
  20. ^ "Cation Exchange Capacity and Base Saturation | UGA Cooperative Extension". extension.uga.edu. Retrieved 2021-03-29.
  21. ^ Allen, C; Ayres, M; Berg, E; Carroll, A; et al. (2005). "Bark Beetle Outbreaks in Western North America: Causes and Consequences" (PDF). US Forestry Service. Retrieved 17 March 2021. {{cite web}}: Explicit use of et al. in: |last5= (help)CS1 maint: url-status (link)
  22. ^ Sangüesa-Barreda, Gabriel; Linares, Juan Carlos; Camarero, J. Julio (2015-12). "Reduced growth sensitivity to climate in bark-beetle infested Aleppo pines: Connecting climatic and biotic drivers of forest dieback". Forest Ecology and Management. 357: 126–137. doi:10.1016/j.foreco.2015.08.017. ISSN 0378-1127. {{cite journal}}: Check date values in: |date= (help)
  23. ^ a b Evans, P. M.; Newton, A. C.; Cantarello, E.; Martin, P.; Sanderson, N.; Jones, D. L.; Barsoum, N.; Cottrell, J. E.; A’Hara, S. W.; Fuller, L. (2017-07-28). "Thresholds of biodiversity and ecosystem function in a forest ecosystem undergoing dieback". Scientific Reports. 7 (1). doi:10.1038/s41598-017-06082-6. ISSN 2045-2322.
  24. ^ Policelli, Nahuel; Horton, Thomas R.; Hudon, Aimée T.; Patterson, Taylor R.; Bhatnagar, Jennifer M. (2020-08-06). "Back to Roots: The Role of Ectomycorrhizal Fungi in Boreal and Temperate Forest Restoration". Frontiers in Forests and Global Change. 3: 97. doi:10.3389/ffgc.2020.00097. ISSN 2624-893X.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  25. ^ Lenton, Timothy M.; Held, Hermann; Kriegler, Elmar; Hall, Jim W.; Lucht, Wolfgang; Rahmstorf, Stefan; Schellnhuber, Hans Joachim (2008-02-12). "Tipping elements in the Earth's climate system". Proceedings of the National Academy of Sciences. 105 (6): 1786–1793. doi:10.1073/pnas.0705414105. ISSN 0027-8424. PMC 2538841. PMID 18258748.{{cite journal}}: CS1 maint: PMC format (link)
  26. ^ Blaustein, R.J. (2011). Amazon Dieback and the 21st Century. Bioscience, 61(3), 176-182. doi:10.1525/bio.2011.61.3.3
  27. ^ Krankina, O.N., et al. (1997) Global Climate Change Adaption: Examples From Russian Boreal Forests. Climatic Change, 36, 197–216.
  28. ^ "'Tipping points' could come this century". EurekAlert!. Retrieved 2021-03-29.
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