Wildfires are a natural phenomenon in many ecosystems, including forests along the U.S. Pacific West Coast, as natural fires serve to rejuvenate ecological communities. However, from mid-August through mid-September 2020, forests in California, Oregon, and Washington State suffered through unprecedented wildfires caused by extreme weather events that can be attributed to climate change. By September 15, 2020, in Washington state, 700,000 acres (280,000 hectares) had burned; in Oregon, about 1 million acres (405,000 hectares) had burned; and in California, the worst-hit state, more than 7700 fires had burned 3 million acres (1.2 million hectares), damaged or destroyed more than 5000 structures, and killed at least 35 people. The single largest fire to date was the California Complex fire in Northern California, which was still raging on September 15 after it had already burned about 800,000 acres (320,000 hectares). These epic blazes have created some of the worst air pollution in the world from Seattle, Washington, to Southern California, with haze reaching as far east as New York City. See also: Air pollution; Extreme weather events; Forest fire; Global climate change; Wildfire impact on air quality

The Joshua tree (Yucca brevifolia) is a remarkable evergreen, treelike plant found only in the southwestern United States. It was named by Mormon settlers in the early 1800s. Upon seeing the limbs of this plant branching upwards to heaven, the Mormon travelers were reminded of the biblical prophet Joshua, who raised his arms in prayer for guidance to the Promised Land. The Joshua tree is the largest member of the Yucca genus of monocotyledonous trees and shrubs, which is placed in the agave family (Agavaceae; order Asparagales) or the lily family (Liliaceae; order Liliales), depending on the taxonomic system being used for classification. The height of the Joshua tree can reach 15 to 40 feet (4.6 to 12.2 meters) and its diameter ranges from 1 to 3 feet (0.3 to 0.9 meters). Life spans of 150 to 200 years have been documented for this resilient species, with some unverified ages of 500 to 1000 years being estimated. See also: Asparagales; Liliales; Monocotyledons; Tree

Wildfires, also termed bushfires or forest fires, are currently devastating large swaths of land in Australia. Detrimental ecologic consequences of these uncontrolled combustions of forest fuels and vegetation are numerous. One of the most serious issues pertains to the survival of the koala, which is a small marsupial (pouch-bearing mammal) found only on that continent. Conservationists and scientists are concerned that the hundreds of fires presently raging in Australia, which were brought about by hotter and drier conditions exacerbated by anthropogenic climate change, may cause significant enough reductions in the population of koalas to threaten the continued existence of this species. See also: Australia; Forest fire; Global climate change; Global warming; Koala; Marsupialia

The American beech (Fagus grandifolia) is native to forest regions east of the Mississippi River and is a key tree species in the United States. In particular, it is very abundant from Ohio to the New England states. The tree is also found in the southernmost portions of eastern Canada, especially southern Ontario. Since 2012, though, American beech trees have been distressed by a lethal leaf disease—termed beech leaf disease—that has spread rapidly from Ohio into the other aforementioned geographical areas. Most cases of the disease have been detected in forest regions, but landscaped locations also have been affected. The external symptoms of the disease include striped banding and shriveling of tree leaves, aborted budding, reduced production of leaves, and premature leaf drop. More importantly, beech trees with beech leaf disease often die within a relatively short span of time after being inflicted by the disease—within 2 to 3 years for saplings and within 6 years for larger and older trees. See also: Beech; Forest; Forestry; Leaf; Tree; Tree diseases

Many trees enjoy extremely long lives in comparison to humans and other animals. Yet one group of trees, the bristlecone pines, stands out as the most ancient of all, with individual trees that commonly live for thousands of years, including one that has survived more than 5000 years. See also: Tree

Global climate change affects all areas of the Earth, with some regions experiencing greater impacts than others. In particular, the Arctic treeline—that is, the southern limit of the Arctic Bioclimate Zone—is being affected to various degrees by the present-day shifts in global climate patterns. Bioclimate (or bioclimatic) zones are broad areas of vegetation that correspond to mean annual temperatures at various latitudes and altitudes. Located predominantly between 60° and 70° north latitude, and stretching across North America from northern Alaska to the coast of Labrador, and across Eurasia from Scandinavia to eastern Siberia, the Arctic treeline forms the biogeographic boundary between the boreal forests to the south (dominated by coniferous trees) and tundra vegetation to the north (dominated by small shrubs, mosses, lichens, and bare ground). The Arctic treeline is not a sharply delineated boundary; instead, it is a transitional zone in which continuous forest cover gives way to increasingly scattered fragments of forest and increasingly smaller individual trees northward. See also: Arctic Circle; Arctic Ocean; Climate modification; Forest; Global climate change; Global warming; Plant geography; Tree; Tree growth; Tundra; Vegetation and ecosystem mapping

Forests absorb and store carbon. If this ability is hampered, then levels of atmospheric carbon will increase and global temperatures will rise, leading to potentially drastic climatic changes. In particular, deforestation (the permanent elimination of forests in order to utilize the land for other uses) in the tropics has contributed to soaring carbon dioxide (CO₂) emissions, creating an acknowledged troubling trend in modern times. However, investigators analyzing the conditions of tropical forests have revealed another source—forest degradation and disturbance—that is contributing significantly to elevations in CO₂ emission levels. See also: Carbon dioxide; Climate modification; Deforestation; Forest; Forest ecosystem; Forestry; Global climate change; Global warming; Greenhouse effect; Tropical forest

Forest fires are becoming more problematic across the globe. For example, climate change has extended fire season in the United States by about 80 days since 1970 because of extreme weather events, such as prolonged drought, according to the U.S. Forest Service. In more fire-prone regions, such as the western United States, fire season now lasts almost year-round. While it cannot extinguish fires, fire retardant is an important tool for slowing the spread of forest fires. Such retardants typically incorporate ammonium phosphate [active ingredient (NH₄)₂HPO_4], a thickener that helps adhere the retardant to vegetation, a red colorant for visual tracking, and water. Upon heating, the retardant’s ammonium phosphate decomposes to ammonia (NH₃) and phosphoric acid (H₃PO₄). Phosphoric acid then reacts with cellulose on the surface of burning plants to produce a carbon layer (char) that shields the underlying organic material from heat and oxygen to prevent further combustion. See also: Ammonia; Combustion; Drought; Extreme weather events; Fire; Flame; Flameproofing; Forest fire; Global climate change; Phosphorus

Concrete and steel have been the materials of choice for constructing high-rise buildings for over 100 years. Definitions of high-rise buildings vary, but according to the U.S. National Fire Protection Association high-rise buildings are 23 meters (75 feet) or more in height; that is, about seven stories and up. An innovation that seems like nothing new, called mass timber construction, is showing promise for constructing wood buildings 7 to 15 stories tall, and potentially up to 40 stories tall. See also: Architectural engineering; Buildings; Concrete; Steel construction; Structural materials; Structural steel

Of all the air pollutants, one of the most dangerous to human health is fine-particulate matter (PM_2.5)—solid or liquid particles less than 2.5 micrometers in diameter. The health risk for PM_2.5 is that these particles can be inhaled deep into the lungs, where they can be trapped and can impair lung function. The smallest of these particles, with diameters of less than 0.1 micrometer, can pass from the lungs into the bloodstream and can affect organs such as the heart and brain or cause cancer. The most common source of PM_2.5 in urban areas is fuel combustion by power plants, diesel-powered vehicles, and oil-burning furnaces; the average diameter of diesel exhaust particles is in the range of 0.05 to 0.08 micrometers. Two recent studies found that urban tree leaves can reduce PM_2.5 concentrations. See also: Air pollution; Combustion; Environmental toxicology; Particulates; Reduction of diesel engine particulate emissions