Forest dieback

Forest dieback (also "Waldsterben", a German loan word, pronounced [ˈvaltˌʃtɛʁbn̩] ) is a condition in trees or woody plants in which peripheral parts are killed, either by pathogens, parasites or conditions like acid rain, drought, and more.
These episodes can have disastrous consequences such as reduced resiliency of the ecosystem, disappearing important symbiotic relationships and thresholds. Some tipping points for major climate change forecast in the next century are directly related to forest diebacks.
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. This usually affects individual species of trees, but can also affect multiple species. Dieback is an episodic event and may take on many locations and shapes. It can be along the perimeter, at specific elevations, or dispersed throughout the forest ecosystem.
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. 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.
A lot of 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. 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.
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.
The components of a forest ecosystem are complex and identifying specific cause–effect relationships between dieback and the environment is a difficult process. Over the years, a lot of research has been conducted and some hypotheses have been agreed upon such as:
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:
Forest dieback can be caused by a multitude of factors, however, once they occur, they can have certain consequences.
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.