Climate change and forest health

Anthropogenic events

or human-caused, events are included in this Web site since the agent itself, i.e. fire or pollutants, is abiotic or non-living (photo ©Flickr/UN).

Air pollution has long been recognized as a detriment to the world’s flora. Pollutants such as nitrogen, sulphur dioxide, heavy metals and ozone can be conveyed in the atmosphere over great distances as gases or microscopic particles. Tree canopies are very efficient at capturing deposition of, or filtering, atmospheric pollutants of all kinds resulting in high inputs to forests. Deposition of pollutants can impact ecosystems directly or through soil acidification and eutrophication.
Ground-level ozone (O₃), the most important air pollutant affecting forests worldwide, is known to reduce photosynthesis, growth, and other plant functions. It also enhances susceptibility to pathogens and results in leaf chlorosis or senescence and forest decline.
The deposition of atmospheric nitrogenous pollutants emitted from industrial, urban and agricultural sources is also of great importance as they affect growth, biodiversity and biogeochemical cycles in forest ecosystems in many areas. Nitrogen oxides (NO_x) result in altered plant growth, enhanced sensitivity to secondary stresses, and eutrophication. At low levels and in nitrogen-limited ecosystems, such as boreal forests, nitrogen may enhance growth.
The importance of some industrial air pollutants, such as sulphur dioxide (SO₂) and heavy metals, has risen in recent years as a result of the rapid industrialization of some countries which often lack adequate environmental considerations and controls. The major source of SO₂ is from the combustion of fossil fuels containing sulphur. Impacts on vegetation include leaf chlorosis, reduced plant growth and vitality, and forest decline.

Fire has a major influence on the development and management of many of the world's forests. Some forest ecosystems have evolved in response to frequent fires from natural as well as human causes, but most others are negatively affected by wildfire.
Every year millions of hectares of the world's forests are consumed by fire, which results in significant loss of life and livelihoods and enormous economic losses from destroyed timber and infrastructure, the high costs of fire suppression, and loss of environmental, recreational and amenity values. FAO’s Global Forest Resources Assessment 2010 noted that, on average, one percent of all forests, or 19.8 million hectares, were reported to be significantly affected each year by forest fires. In addition to direct losses, the associated soil erosion, site deterioration and subsequent difficulties in re-establishing the forest due to the dry climate and poor soil conditions have major impacts on the forest sector.
Forest structure may be abruptly changed by intense canopy fires, which burn leaves and small branches and which are accompanied by surface fires that consume forest floor and understory vegetation. Notably, most of the larger tree boles are not burned, even if killed, and they often remain standing for 10–50 years after a fire. Such residual living and dead organic matter are important for wildlife habitat, nutrient dynamics, ecosystem function and forest recovery, though it may provide a breeding substrate for insect pests thus possibly leading to devastating outbreaks.
In recent decades a notable increase of large wildfires or mega-fires has been noted in all regions of the world. Mega-fires are the most costly, most destructive and most damaging of all wildfires. Not always a single wildfire, they are sometimes a group of multiple fires across a large geographic area. The risk of their occurrence likely increases as droughts deepen, fuels accumulate, and landscapes become more homogeneous. Mega-fires are often extraordinary for their size, but they are more accurately defined by their complex, deep and long-lasting social, economic and environmental impacts. They severely impact local communities and also have serious regional or global consequences. Environmental impacts include interrupting or adversely altering energy, water, nutrient and carbon cycles, declines in biodiversity, increased carbon emissions, and weed invasion.

Oil spills can have devastating impacts on coastal forests and mangroves. Mangroves are highly susceptible to oil exposure and can be affected in two main ways: from the physical effects of oiling and from the toxicological effects of the oil.
The physical effects of oiling (e.g. covering or blocking specialized tissues needed for respiration or salt management) may include either disruption or complete prevention of normal biological processes of exchange with the environment. When oil physically covers plants, animals and birds, they may die from suffocation, starvation, or other physical interference with normal physiological function.
Acute effects of oil (mortality) occur within six months of exposure and usually within a much shorter time frame (a few weeks). Visible signs of mangrove stress, including chlorosis, defoliation and death, often show within the first two weeks of a spill event. The tree may survive for a time only to succumb weeks or months later, or, depending on the severity of the damage, it may recover to produce new leaf growth. Seedlings and saplings, in particular, are susceptible to oil exposure. More subtle responses include branching of pneu­matophores (breathing roots), germination failure, decreased canopy cover, increased rate of mutation, and increased sensitivity to other stressors.
Chronic oil impacts on mangroves include altered growth rates and reproductive timing or strategy and can be measured over long time periods, potentially a decade or decades. They may also exhibit morphological adaptations in order to survive the oiling such as the development of branched secondary pneumatophores in mangroves as a response to impairment of normal respiration.

Accidents at nuclear power plants create obvious concerns about exposure of the human population to contamination in the immediate vicinity. There are potentially longer term problems due to the ecological impact from contamination with radionuclides. Radionuclides are radioactive atoms that are either man-made or naturally occurring but only a few are considered to present serious risks to human, wildlife and ecosystem health.|
Although many different kinds of radionuclides can be discharged following a major nuclear accident, some are very short-lived and others do not readily transfer into food and ecosystems. Caesium-137 is the primary radionuclide of concern regarding the long-term contamination of forests and forest products, owing to its 30 year half-life.
The transfer of radionuclides in the environment depends on the particular ecosystem. Forests with soils rich in organic matter and generally low in clay content leads to a higher transfer of radiocaesium to most forest products, such as berries or mushrooms. Over time, radioactivity can build up within food, as radionuclides are transferred through soil into crops or animals, or into rivers, lakes and the sea where fish and other seafood could take up the radionuclides. Food collected from the wild, such as mushrooms, berries and game meat, may continue to be a radiological problem for a long time.
After a nuclear accident, monitoring the agricultural, forestry and fisheries environment and restricting the movement and export of possibly contaminated products is an important factor. Preventing wildfires within contaminated areas also remains a high priority since they could release clouds of radioactive particles that still persist in the trees. Smoke from fires can spread thousands of kilometres which could substantially increase the area of impact to humans and the environment.

Climatological events

result from long-term changes in climatic variables such as temperature and precipitation (photo ©Flickr/WMO). 

Droughts are caused by a deficiency of precipitation over time and as such can develop slowly, sometimes over years. Often associated with the arid regions of Africa, particularly the Sahel, in recent years, droughts have also struck India and parts of China, the Near East, the Mediterranean, Australia, parts of North America, South America and Europe. Increases in the frequency, duration, and/or severity of drought and heat stress associated with climate change could fundamentally alter the composition, structure and biogeography of forests in many regions. Of particular concern are increases in tree mortality associated with climate-induced physiological stress and interactions with other disturbances such as pest outbreaks and fire.
Dozens of episodes of increased mortality due to drought and heat have been identified throughout a variety of forest types, from monsoonal savannas with mean precipitation 3000 mm/year, illustrating that drought-induced mortality is not restricted to forests normally considered water-limited.
A primary response of forests to future drought will be a reduction in net primary production (NPP) and stand water use. Forest drought usually results in reduced shoot growth, reduced nitrogen and water foliar concentrations, and increased allocation to secondary defensive compounds, such as tannins. Drought-induced reductions in decomposition rates may cause a buildup of organic material on the forest floor which could influence nutrient cycling and increase susceptibility to fire.
Young plants such as seedlings and saplings are particularly susceptible to drought whereas large trees with a more developed rooting system and greater stores of nutrients and carbohydrates tend to be less sensitive, though they are affected by more severe conditions. Shallow-rooted trees and plants as well as species growing in shallow soils are more susceptible to water deficits.
Strategies for forest managers to adapt to future drought events might include thinning stands to reduce competition or selecting appropriate genotypes, such as those with improved drought resistance.

Geophysical events

are natural phenomena capable of having devastating consequences for the world’s environment, economy and society (photo ©Flickr/Yisris).

In tectonically active regions of the world, large earthquakes disturb forests over extensive areas and as such are important determinants of forest structure and function. The intensity of damage to forests varies strongly with distance from the earthquake’s epicentre.
Earthquakes can trigger landslides while those occurring underwater can produce tsunamis. It is generally considered that the primary cause of tree mortality during earthquakes is as a result of the landslides they create; forests are often completely removed or submerged by such landslides. However much of the immediate impact of an earthquake is widespread, low-intensity tree mortality and injury.
Damage is caused by the shaking or shearing of tree roots, the uplift of the ground surface, or changes in the water table. The movement of soil or boulders downslope can also damage trees. Damaged trees may survive but they will exhibit signs of the disturbance such as fractures in the wood, growth suppression or the production of reaction wood. The diversity of earthquake impacts is a major source of heterogeneity in forest structure and regeneration

A tsunami is a series of enormous traveling ocean waves of extremely long length generated primarily by earthquakes occurring below or near the ocean floor. They may also be generated by underwater landslides and volcanic eruptions, or meteorites. Tsunami waves are distinguished from ordinary ocean waves by their great length between wave crests, often exceeding 100 kilometres or more in the deep ocean, and by the time between these crests, ranging from 10 minutes to an hour. In the deep ocean, tsunami waves travel at over 800 km/h, with a short wave height of only a few tens of centimetres or less. As they reach shallow coastal waters, the waves slow down and the water can pile up into a wall of destruction dozens of metres or more in height. Large tsunamis have been known to rise over 30 metres and even a tsunami 3-6 metres high can be very destructive and cause many deaths and injuries. Although 60 percent of all tsunamis occur in the Pacific Ocean, they can also threaten coastlines of countries in other regions, including the Indian Ocean, Mediterranean and Caribbean Seas, and the Atlantic Ocean.
Mangroves, coastal forests, home gardens, agroforestry systems and trees in coastal landscapes can be damaged by tsunamis. Trees may be snapped and uprooted by waves and strong currents. Observed changes in topography, soil salinity and freshwater input may also adversely affect mangroves, coastal forests and other trees in the longer term.

There are approximately 1 500 potentially active volcanoes worldwide, plus hundreds more on the ocean floor. About 500 of these have erupted in historical time. Many of these are located along the Pacific Rim in what is known as the "Ring of Fire".
The intense force of the blast, and the large amount of earth that is either moved or covered with various kinds of debris, makes volcanic eruptions more severe than the hottest fire or the most intense windstorm. Impacts diminish as distance from the volcano increases.
Volcano hazards that may impact forests include: gases, such as sulphur dioxide, carbon dioxide and hydrogen fluoride; lahars– volcanic flows composed of hot or cold water and rock fragments; landslides; lava flows; pyroclastic flows– fast-moving currents of hot rock and gas that travel downhill along slope depressions; tephra– fragments of volcanic rock and lava that become airborne through explosions or the rise of hot gases. The smallest fragments are volcanic ash.
Young forests are most at risk from ashfall; stands of trees less than two years old are likely to be destroyed by ash deposits thicker than 100 millimetres. Mature trees are unlikely to succumb from ashfall deposition alone, but the accumulated weight of ash can break large branches in cases of heavy ashfall (>500 mm). Defoliation of trees may also occur.

Hydrological events

are related to major changes in water levels on the earth’s surface. 
Winter storms with high winds and heavy snow or freezing rain can also contribute to avalanches on some mountain slopes and to high runoff or flooding later on in the melt season.  In temperate latitudes, severe summer weather can be accompanied by heavy hail and flash floods (photo ©Flickr/Freekstreetttt).

Avalanches are rapid, gravity driven mass flows of snow, air and debris. They can generally be classified into loose snow avalanches (starting at a single area or point) and slab avalanches (release of a cohesive snow layer initiated by a failure at depth in the snow cover). They can be highly destructive, moving at speeds in excess of 150 km/h.
Avalanches primarily affect subalpine forests (those forests closest to upper tree line). They can damage or kill individual trees over tens to hundreds of hectares in forests that are located in vulnerable areas.
At a stand scale, avalanches typically result in forest communities that are characterized by smaller and shorter trees, shade intolerant species, lower stem densities, and greater structural diversity. Such communities provide valued habitat for various animal and plant species and can contribute to overall higher biodiversity.
At a broader scale, avalanche tracks provide increased landscape heterogeneity and edge density and can serve as firebreaks.

Floods occur when the rate of water supply exceeds the capacity of stream-channel drainage such as during periods of heavy rains and rapidly melting snow and ice. They can be triggered by cyclones, severe thunderstorms, tornadoes and monsoons or can result from the building of dams (i.e. by beavers) or by dam breaks caused by general failure, ice jams, landslides, or by tectonic and other geological processes. Excessive rainfall on saturated soils in flat areas can also create floods. In coastal areas, storm surges caused by cyclones and tsunamis, or by a combination of high river flows and back-water effects as a result of high tides, can also cause flooding.
The ecological damage of regular flooding may be minor as floodplain plants and animals are well adapted to such conditions. In addition, the mechanical force of floodwater is not typically adequate to increase plant mortality rates, especially when flooding occurs in spring before bud break of deciduous trees. Oxbow formation, ice scouring, and bank erosion may cause the death of some trees and changes in the landscape mosaic, but impacts are generally affected only over limited areas. Floods caused by waterlogging of large, flat areas, however, can persist for several days and cause damage to trees and forests.
Flash floods can occur after heavy storms or after a period of drought when heavy rain falls onto very dry, hard ground that the water cannot penetrate. Such events may have much more impact on forests, especially in areas not accustomed to high waters. 

Landslides and mudslides occur when heavy rain, rapid snow or ice melt or an overflowing crater lake sends large amounts of earth, rock, sand or mud flowing swiftly down hill and mountain slopes. Earthquakes, volcanic eruptions, heavy rain storms, and cyclones can trigger landslides. Land-use intensification and climate change are increasing landsliding in mountainous regions.
Shallow landslides typically have little impact on trees. Deep landslides, triggered by major earthquakes or volcanic activity, however can denude hundreds or even thousands of square kilometres of land. In such major landslides, all of the soil down to bedrock is carried downslope, taking all of the trees and other vegetation with it.
Because no soil is left for new plants to grow on, the bare tracks of landslides can remain visible for hundreds of years. Shallow landslides can be prevented by tree cover whereas deep landslides can not be prevented, even with high forest cover.

Meteorological events

are severe weather phenomena that threaten life, property, and the environment (photo ©Flickr/Niccolo' Ubalducci).

Storms bringing wind, snow, ice or hail or a combination of these factors have always impacted the health of forests and thus are a regular consideration in forest management plans. They can occur as catastrophic events affecting entire landscapes, the quality of wildlife habitats, and forest stand structure, which can lead to major disruptions in management goals. Alternatively they may occur as small scale disturbances that affect individual trees or groups of trees within a stand increasing the amount of dead wood and diversifying stand structure, which can have positive benefits for biological diversity.
Snow most commonly impacts trees by breaking stems but trees can also be bent or uprooted. The severity of snow damage is related to tree characteristics; factors controlling the stability of trees such as stem taper and crown characteristics are the most important. Conifers are particularly damaged by heavy snowfall, while broadleaved trees are generally more resistant to storms and snow in the late autumn and winter due to better root systems and lack of foliage.
Damaging winds vary from short-lived gusts, to strong prevailing winds, to powerful hurricanes, to brief but intense downdrafts from thunderstorms.
Storm damage can include initial mechanical damage from the storm, subsequent damage from other biotic or abiotic factors (i.e. insects, fire, sun, snow, ice, etc.), and loss of production. The impact of wind on forests is determined by a complex of many biotic and abiotic factors and is similar to those experienced during cyclones.
In a matter of minutes an ice storm can deposit a layer of ice heavy enough to bring down power and telephone lines and snap branches from trees. Impacts of individual storms are highly patchy and variable, and depend on the nature of the storm, its severity, frequency, timing and extent. Ice accumulation on trees can cause minor branch breakage; major branch loss, up to total crown loss; temporarily or permanently bending over of crowns; root damage (when soil is not frozen); breakage of trunks within or below the crown; and for some hardwoods, split trunks. Softwoods seem to suffer less damage than hardwoods. Recently-thinned stands can be highly vulnerable, as crowns have spread into newly-opened space but branch strength may not be fully developed. Trees damaged by ice storms or windthrow can be more susceptible to other disturbances such as insect pests or fire.
Large hailstones that can reach diameters of over 10 centimetres and can fall at speeds of over 150 km/h can also cause considerable damage to forests.
Dust storms and sandstorms are natural events that occur throughout the world, especially in dryland areas which occur in Central and South Asia, the Australian Outback, South American Patagonia, the North American Great Plains, Sahara and sub-Saharan Africa and the Mediterranean region.
Dust storms and sandstorms are a result of wind erosion and are driven by poor land management and degradation of the dryland vegetation cover. Strong winds and favourable surface atmospheric conditions (i.e. turbulence level, stability, soil moisture) can allow for large amounts of sand and dust to be lifted from bare, dry soils into the atmosphere. Every year one and a half tonnes of sand and dust are emitted from drylands into the atmosphere where it can be transported downwind affecting regions hundreds to thousands of kilometres away depending on meteorological conditions. Dust from the Gobi Desert, for example, is carried to the Pacific coasts of North America and dust from the Sahara Desert is carried to the Caribbean islands and the Amazon basin.
Dust can have numerous impacts on human and veterinary health, the environment, agriculture, marine ecosystems, fisheries, transport, visibility, aviation, and weather and climate at larger scales. Crops can be destroyed, trees can be damaged or blown down and greenhouses and other nursery structures can be broken.
In some cases, however, the deposition of dust can produce positive results. For example, mineral-rich Saharan dust transported across the Atlantic Ocean to the Amazon rain forest in South America provides iron and phosphorus to the nutrient-poor rainforest soils acting as fertilizer.
Measures to combat the occurrence and impacts of sand and dust storms include the use of windbreaks or shelterbelts to reduce the impact of wind speeds and decrease soil erosion. 

Severe thunderstorms give rise to sudden electrical discharges in the form of lightning and thunder. They often bring heavy rain or hail, strong winds and occasionally snow and in some parts of the world they trigger tornadoes. In areas where lightning is not accompanied by rain, so-called dry lightning, it may also be a source of ignition for forest fires as noted in some remote areas of Canada and the Russian Federation. However it is generally recognized that the majority of forest fires are caused by humans and not by lightning. 
Tall trees tend to be the most vulnerable to lightning strikes, especially those growing singly in open areas such as on hills, in fields, near water or in urban environments. Lightning can impact a tree’s biological functions and structural integrity. Along the path of the strike, sap boils, steam is generated and cells explode in the wood, resulting in strips of wood and bark peeling or being blown off the tree. Trees may survive if only one side of the tree shows evidence of a lightning strike; however when the strike completely passes through the trunk, trees are usually killed. Many trees can suffer severe internal or below-ground injury despite the absence of visible, external symptoms when the lightning passes through the tree and dissipates in the ground. Major root damage may cause the tree to decline and die. 

Cyclones (a system of winds rotating inwards to an area of low barometric pressure) can and do occur at any latitude and in any climate. Those occurring within 30 degrees north or south of the equator are called tropical cyclones; those found above 60 degrees north or south of the equator are arctic or polar cyclones; and those between 30 and 60 degrees are called extratropical cyclones.
The terms 'hurricane' and 'typhoon' are regionally specific names for a strong tropical cyclone: western North Pacific Ocean and South China Sea - typhoon; Atlantic Ocean, Caribbean Sea, and in the eastern North and central Pacific Ocean – hurricane; Indian Ocean and South Pacific region – tropical cyclone.
Three primary features of cyclones that cause damage are rainfall, storm surges and winds.
Torrential rains accompanying tropical cyclones frequently cause extensive flooding, leading to tree mortality from anoxia (absence of oxygen). Flooding and rainfall saturates soil, which may increase susceptibility to windthrow in shallow soils.
Wind is the feature that is linked to a vast majority of a cyclone’s damage, both directly and indirectly, through waves and storm surge. The most common impacts of wind include defoliation, loosening and shredding of bark, and abrasion of stem surfaces. Trees can sway, twist and rock, and large branches may break off and cause damage to understory trees. Individual stems may bend, break or suffer some level of uprooting from leaning to complete blowdown of the tree.
A storm surge is a large dome of water, sometimes greater than five metres, that floods the coast at high speed and with immense force as the storm makes landfall. Storm surges can cause extensive damage to coastal vegetation by bending, breaking or uprooting trees. Scouring and erosion may expose root systems leading to desiccation, and deposition may lead to root suffocation. Salinity and inundation increased by the storm surge can cause plant and tree mortality.

Tornadoes are short-lived, relatively small, complex, violent and unpredictable storms that can cause severe damage though usually in limited areas. They are most common in spring in late afternoon and are concentrated in interior continental regions, particularly in Tornado Alley of the Great Plains of North America, but they can and do occur anywhere, especially in temperate latitudes. Tornado winds greatly surpass tropical cyclone winds in intensity, reaching an estimated maximum exceeding 400 km/h.
They develop under three meteorological conditions: long-lived supercell thunderstorms, which generate the largest and most damaging tornadoes; ordinary thunderstorms; and in cyclones after they make landfall. Damage to trees and forests can range from branch break and single tree gaps to extensive areas of complete blowdown.