When air pollution, including acid rain, is combined with other environmental stresses, such as low winter temperatures, prolonged droughts, insects, and bacterial, fungal, and viral diseases, it can cause plants to decline and die. More than half of the red spruce trees in the mountains of the northern United States have died since the mid-1970s. Other tree species, such as sugar maples, for example, are also dying. Many still-living trees are exhibiting symptoms of forest decline, characterized by a gradual deterioration and often eventual death. The general symptoms of forest decline are reduced vigour and growth, but some plants exhibit specific symptoms, such as yellowing of needles in conifers. Air pollutants may or may not be the primary stress that results in forest decline, but the presence of air pollution lowers plant resistance to other stress factors. When one or more stresses weaken a tree, then an additional stress may be enough to cause death.