WOODLAND EXPANSION IN THE PLATTE RIVER, NEBRASKA - PATTERNS AND CAUSES

This research was conducted to identify the factors that have permitte d Populus-Salix woodland to expand into the formerly active channels o f the Platte River and its two major tributaries, the South and North Platte rivers. The research included: pre-settlement vegetation recons truction based on the General Land Office survey notes, a statistical comparison between historic rates of woodland expansion from aerial ph otographs and environmental variables, and a held study of seedling de mography to isolate the factors controlling recruitment and survival i n the modern river. Woodland expansion began in the South and North Pl atte rivers around 1900 and spread downstream into the Platte River. B y the late 1930s, vegetation had occupied most of the former channel a rea of the South and North Platte rivers and was expanding into Platte River channels. Rates of channel loss in the Platte River have been a s great as 10%/yr during droughts. By 1986, channel-to-woodland propor tions were relatively uniform throughout the Platte River system. Stat istical models indicated that sandbar succession to woodland was regul ated by three environmental factors: June hows, summer drought, and ic e. June flow regulated seedling recruitment and initial survival becau se it coincided with the main Populus-Salix seed germination period. H istoric reductions in flow at this time for irrigation and to fill res ervoirs exposed much of the riverbed and elevated recruitment and seed ling survivorship. Late-summer seedling survival was regulated by fact ors that affect seedling water balance, including river stage, seedlin g elevation in the riverbed, and rainfall. Winter conditions exerted t he largest effect on seedling survivorship. Dominant factors were air temperature, streamflow, and seedling elevation in the riverbed. Lowes t survivorship occurred during cold, icy winters with relatively high flow and when most seedlings were growing on low sandbars. The dominan t 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 signific antly 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 bec ause flows have come into balance with active channel area, thereby re ducing recruitment and increasing the mortality of tree seedlings. Bec ause of the importance of wide, unvegetated channels to certain avifau na, it may be desirable to manage future hows to ensure no further red uction in channel widths, even if narrowing is only temporary. Dominan ce by Populus and Salix on new sandbars can be explained by life histo ry characteristics. These include large and dependable seed crops that are effectively dispersed by wind and water to optimal germination si tes; rapid germination; rapid root and height growth to withstand floo ding, drought, and sedimentation; tolerance of low soil fertility; and the ability of Salix to reproduce vegetatively. Pioneer vegetation an d geomorphic processes (principally sedimentation) facilitate successi on on floodplains by modifying the highly variable riverbed environmen t suitable for early successional species into relatively stable surfa ces favorable for recruitment of later successional species.