Streamflow requirements for cottonwood seedling recruitment - An integrative model

This paper describes the 'recruitment box,' an integrative model that defines the stream stage patterns that enable successful establishment of riparian cottonwood seedlings. In western North America, cottonwood reed dispersal generally occurs after annual peak river flows. The receding stream exposes moist sites upon which seeds land after transport by wind anti water. Germination is rapid, and initial seedling establishment is often prolific. However, the vast majority of seedlings die, primarily due to drought stress, as root growth is insufficient to maintain contact with the receding zone of moisture, Cottonwood roots grow about 0.5 to I cm per day or 60 to 100 cm in the first year. Along the 'losing' streams in semi-arid regions, the riparian water table is an almost horizontal extension from the stream stage. A capillary fringe exists above: the water table and is often 30 to 100 cm in elevation, but can range from about 5 to 130 cm depending on substrate texture, The combination of root growth MJ capillary fringe define the successful recruitment band. which is; usually from about 0.6 to 2 m in elevation above the late summer stream stage. Within this range, higher elevation establishment occurs (i) for the Aigeiros cottonwoods, Populus deltoides, and P. fremontii, which grow more rapidly than Tacamahaca species and occur in warmer areas with longer growing seasons, (ii) along larger rivers that are characterized by more gradual stage fluctuations; and (iii) alone streams with finer substrate. The rate cif stream stage decline is also critical for seedling survival and should not exceed 2.5 cm per dal;. The recruitment box model is consistent with dendrochronological interpretations that moderate flood events are naturally required for cottonwood recruitment. Flood events with recurrences of about 1 in 5 to 1 in 10 years often satisfy the model and provide stream stage patterns with a gradual decline through the recruitment bos. The model will facilitate analyses of the reproductive ecology of riparian cottonwoods and also permit the prescription of stream stage patterns for cottonwood seedling recruitment along dammed rivers.