Seasonal isolation as a reproductive barrier among sympatric Salix species

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Abstract
Differences in flowering phenology may be an important premating reproductive barrier between willow species. The seven sympatric species studied fall into two flowering groups within which there is substantial seasonal overlap: an early flowering group that includes Salix bebbiana Sarg., Salix discolor Muhl., Salix eriocephala Michx., and Salix petiolaris Smith, and a later flowering group that includes Salix amygdaloides Anderss., Salix exigua Nutt., and Salix lucida Muhl. The species flowering sequence was reasonably consistent over the 3-year period from 1984 to 1986. Thermal control over flowering phenology was related to cumulative growing degree-days. The inheritance of phenological responses in artificially produced F1 interspecific hybrids was intermediate between that of the parental species.
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