Seed germination of willow species from a desert riparian ecosystem

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The restoration of riverine riparian areas following mechanical, herbicidal, or biological control of the invasive species tamarisk (Tamarix ramosissima Ledeb.) is a major issue in the western United States. Recruitment of seedlings of native woody species is necessary in these restoration efforts. Species of willow (Salix) are often considered essential in these efforts. We studied the germination of seeds of tree willow (Salix lutea Nutt.) and coyote willow (S. exigua Nutt.) at a wide range of constant or alternating incubation temperatures. Seeds were collected from native stands in the delta of the Walker River in western Nevada over a 3 year period. Seed germination was very similar for both species. On 2 of the 3 years of testing the seeds had 100% germination at some incubation temperatures and some germination over almost all of the 55 temperature regimes used in the experiments. A late frost in May of 2000 markedly reduced total germination of both species, but did not greatly restrict the temperature regimes where some germination occurred. Optimum germination, defined as that not lower than the maximum observed minus one half the confidence interval at the 0.01 level of probability, occurred over a very wide range of temperatures, but for tree willow only the temperature regimes 15/25 (15 degrees C for 12 hours and 25 degrees C for 8 hours in each 24 hour period) and 15/30 degrees C always supported optimum germination. No temperature regime always supported optimum germination of coyote willow seeds, but the most frequent optima tended to be at lower temperatures than for tree willow. Because of the similarity in germination responses and overlapping phenology, seeds of these 2 species probably compete for germination safesites.