Question

list all the causes forests in the last 200 years​

Answer 1

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.There are a few main drivers of forest degradation. One is climate change: higher temperatures and unpredictable weather patterns increase the risk and severity of forest fires, pest infestation, and disease. But the main cause of forest degradation is unsustainable and illegal logging.

Answer 2

Forest decline

In recent decades there have been observations of widespread declines in vigor and dieback of mature forests in many parts of the world. In many cases, pollution may be a factor contributing to forest decline, for example in regions where air quality is poor because of acidic deposition or contamination with ozone , sulfur dioxide , nitrogen compounds, or metals. However, forest decline also occurs in some places where the air is not polluted, and in these cases it has been suggested that the phenomenon is natural.

Forest decline is characterized by a progressive, often rapid deterioration in the vigor of trees of one or several species , sometimes resulting in mass mortality (or dieback) within stands over a large area. Decline often selectively affects mature individuals, and is thought to be triggered by a particular stress or a combination of stressors, such as severe weather, nutrient deficiency, toxic substances in soil , and air pollution . According to this scenario, excessively stressed trees suffer a large decline in vigor. In this weakened condition, trees are relatively vulnerable to lethal attack by insects and microbial pathogens . Such secondary agents may not be so harmful to vigorous individuals, but they can cause the death of severely stressed trees.

The preceding is only a hypothetical etiology of forest dieback. It is important to realize that although the occurrence and characteristics of forest decline can be well documented, the primary environmental variable(s) that triggers the decline disease are not usually known. As a result, the etiology of the decline syndrome is often attributed to a vague but unsubstantiated combination of biotic and abiotic factors.

The symptoms of decline differ among tree species. Frequently observed effects include: (1) decreased productivity; (2) chlorosis, abnormal size or shape, and premature abscission of foliage; (3) a progressive death of branches that begins at the extremities and often causes a "stag-headed" appearance; (4) root dieback; (5) an increased frequency of secondary attack by fungal pathogens and defoliating or wood-boring insects; and (6) ultimately mortality, often as a stand-level dieback.

One of the best-known cases of an apparently natural forest decline, unrelated to human activities, is the widespread dieback of birches that occurred throughout the northeastern United States and eastern Canada from the 1930s to the 1950s. The most susceptible species were yellow (Betula alleghaniensis ) and paper birch (B. papyrifera ), which were affected over a vast area, often with extensive mortality. For example, in 1951 at the time of peak dieback in Maine, an estimated 67% of the birch trees had been killed. In spite of considerable research effort, a single primary cause has not been determined for birch dieback. It is known that a heavy mortality of fine roots usually preceded deterioration of the above-ground tree, but the environmental cause(s) of this effect are unknown, although deeply frozen soils caused by a sparse winter snow cover are suspected as being important. No biological agent was identified as a primary predisposing factor, although fungal pathogens and insects were observed to secondarily attack weakened trees and cause their death.