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Threats and Conservation
Environmental Threats.
Humans are affecting natural and secondary environments in North America in ways that impinge on the conservation of trees. We can identify six major human influences on the environment: 1) the degradation and destruction of natural habitats through residential development, industrialization, poor forestry practices, agricultural expansion, mining, and road building( broadly categorized as land-use change); 2) the exploitation of species through commercial collecting in the wild of medicinal plants, timber trees, ornamentals, and the like; 3) pollution of the air, land, and waters with substances other than greenhouse gases, including industrial chemicals, plastics, and pesticides; 4) urbanization, through expansion of major cities and the further isolation of humans from nature; 5) the introduction of invasive species that outcompete native species; and 6) the spread of plant and animal diseases and pests between formerly isolated continents and land masses.
Vulnerability to Climate Change and Global Warming.
Perhaps the least predictable but possibly most devastating environmental factor affecting tree species is the rise in global temperatures associated with the accumulation of greenhouse gases in the atmosphere. There is little doubt that the geographic ranges of species will shift as temperatures rise and trees migrate through fruit dispersal to track equitable climates. If temperatures rise too quickly for trees to adapt and migrate, then populations will decline. Equally threatening are the events associated with rises in atmospheric and surface temperatures, such as the amplified dispersal of exotic and native pathogens; the decline of native pollinators and fruit dispersers; and changes in patterns of precipitation, storms, and fires. Taken together, global warming may have massive effects on the native trees of North America.
In order to evaluate the vulnerability of our trees to climate change, the U. S. Forest Service recently developed a global-warming assessment covering over 300 species. 4 Each
species was categorized according to risk factors reflecting its exposure to, sensitivity to, and capacity to adapt to climate change. Eight species of the common trees are in the highest risk category. For example, Chinkapin( Castanea pumila) has a high exposure to climate change because of where it grows and only a moderate ability to adapt to a changing climate. A second species in the same genus, the American Chestnut( Castanea dentata), has a somewhat lower exposure but an even poorer capacity to adapt.
Conservation Assessment.
Even though 11 – 16 percent of tree species in North America are threatened with extinction, most of the recognized common trees are not currently under threat, due to their abundance. 5 However, such assessments can change rapidly. For example, the Emerald Ash Borer, an invasive beetle imported from Asia and first discovered near Detroit in 2002, has now spread to thirty-six states, primarily east of the Rocky Mountains. This pest has had a major detrimental impact on populations of ash( Fraxinus spp.) across North America and was recently detected in Oregon. 6 The conservation status of the once-common ash trees has been significantly and rapidly altered as a result.
The two most widely used threat-assessment protocols for North American trees are those of the International Union for Conservation of Nature( IUCN) Red List and NatureServe Explorer. 7 The tree extinction risks provided below follow the main IUCN categories of conservation assessment: 8 Critically Endangered( CR)— at extremely high risk of extinction in the wild Endangered( EN)— at very high risk of extinction in the wild Vulnerable( VU)— at high risk of unnatural( human-caused) extinction without further human intervention Near Threatened( NT)— close to being at high risk of extinction in the near future Least Concern( LC)— unlikely to become extinct in the near future Data Deficient( DD)— insufficient data to assess status Not Evaluated( NE)— yet to be assessed for status