Read the following passage carefully and choose the most appropriate answer to the question out of the four alternatives. Riparian vegetation [the green band of vegetation along a watercourse] can help stabilize stream banks; filter sediment from surface runoff; and provide wildlife habitat, livestock forage,and scenic value. Well-developed vegetation also allows bank soils to absorb extra water during spring runoff, releasing it later during drier months, thus improving late-summer stream flows. In many parts of the arid West, trees and shrubs are found only in riparian areas. Woody plants are very important as winter cover for many wildlife species, including upland game birds such as pheasants and turkeys. Often this winter cover is the greatest single factor limiting game bird populations. Woody vegetation also provides hiding cover and browse for many other species of birds and mammals, both game and nongame. Dead trees ("snags") are an integral part of streamside habitats and should be left standing whenever possible. Woodpeckers, nuthatches, brown creepers, and other birds eat the insects that decompose the wood. These insects usually pose no threat to nearby living trees. Occasionally a disease organism or misuse of pesticides will weaken or kill a stand of trees. If several trees in a small area begin to die, contact your local extension agent immediately . What is discussed in the second paragraph of this passage?