Woodland Expansions in the Platte River, Nebraska: Patterns and Causes

Abstract

This research was conducted to identify the factors that have permitted Populus—Salix woodland to expand into the formerly active channels of the Platte River and its two major tributaries, the South and North Platte rivers. The research included: pre—settlement vegetation reconstruction based on the General Land Office survey notes, a statistical comparison between historic rates of woodland expansion from aerial photographs and environmental variables, and a field study of seedling demography to isolate the factors controlling recruitment and survival in the modern river. Woodland expansion began in the South and North Platte rivers around 1900 and spread downstream into the Platte River. By the late 1930s, vegetation had occupied most of the former channel area of the South and North Platte rivers and was expanding into Platte River channels. Rates of channel loss in the Platte River have been as great as 10%/yr during droughts. By 1986, channel—to—woodland proportions were relatively uniform throughout the Platte River system. Statistical models indicated that sandbar succession to woodland was regulated by three environmental factors: June flows, summer drought, and ice. June flow regulated seedling recruitment and initial survival because it coincided with the main Populus—Salix seed germination period. Historic reductions in flow at this time for irrigation and to fill reservoirs exposed much of the riverbed and elevated recruitment and seedling survivorship. Late—summer seedling survival was regulated by factors that affect seedling water balance, including river stage, seedling elevation in the riverbed, and rainfall. Winter conditions exerted the largest effect on seedling survivorship. Dominant factors were air temperature, streamflow, and seedling elevation in the riverbed. Lowest survivorship occurred during cold, icy winters with relatively high flow and when most seedlings were growing on low sandbars. The dominant historic trend, of losses in channel area and gains in woodland area, has ceased in recent years. No significant declines in channel area have occurred since 1969; in several reaches channel area has significantly increased since 1969. Comparatively small changes in channel and woodland proportions are expected in the future as long as water use and climate do not change markedly. The steady state has developed because flows have come into balance with active channel area, thereby reducing recruitment and increasing the mortality of tree seedlings. Because of the importance of wide, unvegetated channels to certain avifauna, it may be desirable to manage future flows to ensure no further reduction in channel widths, even if narrowing is only temporary. Dominance by Populus and Salix on new sandbars can be explained by life history characteristics. These include large and dependable seed crops that are effectively dispersed by wind and water to optimal germination sites; rapid germination; rapid root and height growth to withstand flooding, drought, and sedimentation; tolerance of low soil fertility; and the ability of Salix to reproduce vegetatively. Pioneer vegetation and geomorphic processes (principally sedimentation) facilitate succession on floodplains by modifying the highly variable riverbed environment suitable for early successional species into relatively stable surfaces favorable for recruitment of later successional species. Much of the extensive Populus—Salix woodlands that now occupy the Platte River will be replaced by later successional tree and shrub species with lower associated faunal diversity. Maintenance of the current biotic diversity may require artificial regeneration, as is taking place along other river systems in western North America. The response of the Platte River to altered flow differed from that of other rivers. This divergent response despite similar disturbances points out the complex interrelationships among plants and hydrogeomorphic processes operating on floodplains and the difficulties associated with understanding, generalizing, and predicting the effects of human modification of streamflow on natural ecosystems.