Effects of flooding on native and exotic plant seedlings: Implications for restoring south-western riparian forests by manipulating water and sediment flows

Unregulated streams of the U.S. South-west typically carry large sediment loads during high flow. Seedlings can be injured or killed by floods that deposit sediment, but as seedlings grow larger they develop greater resistance to damage from flood flows. On dam-regulated streams, natural sedimentation processes are interrupted, possibly altering competitive dynamics among seedlings. In this suite of experiments, seeds of three native woody species, Baccharis salicifolia, Populus fremontii, Salix gooddingii, and an exotic, Tamarix ramosissima, were sown in greenhouse pots. Seedlings were experimentally buried by sediment at progressive age intervals up to ninety days. Sediment treatments varied in terms of depth (1 and 2 cm) and method of application (undisturbed sifting or forceful burial). Our studies indicated that at an early age, P. fremontii had significantly greater ability than T. ramosissima, S. gooddingii and B. salicifolia to survive 1 cm sifted sediment deposition. However, as the seedlings became taller, survivorship of S. gooddingii and T. ramosissima approached or equalled that of P. fremontii. Tamarix ramosissima had greater survivorship than native species only when it also was taller; in some cases, natives had greater survivorship when they were shorter than T. ramosissima. Populus fremontii survived 1 cm of sediment deposition in large numbers by the time it reached 2 cm tall; a size that it reached at a very young age (<2 weeks). Salix gooddingii showed high survivorship at 3-4 cm, but required five weeks to achieve this threshold height. Tamarix ramosissima survived in large numbers when it reached a height range of 4-6 cm, a value it did not attain until it also was nearly 5 weeks of age. These results suggest that P. fremontii may be more competitive than T. ramosissima under natural sediment flow regimes. One approach to managing for native species involves restoring the physical conditions under which they are more competitive. Based on the results of our experiments and those of others, we suggest ways in which discharges of water and sediment in degraded reaches could be managed to favor the establishment of desired native species.