REGULATION OF SEED‐GERMINATION TIMING BY MOIST CHILLING IN WESTERN HEMLOCK

Abstract

SUMMARYExistence of a natural system is proposed, in which duration of winter chilling of seeds of western hemlock, Tsuga heterophylla (Raf.) Sarg., regulates timing of germination in spring. Seeds from 12 populations (from Oregon and Washington, U.S.A.) were germinated in 12 combinations of three chilling durations and four germination temperatures. Embryo development rates, calculated as 100/Y where Y= days to 50% germination, increased directly with temperature, and the relation could be described by an empirical logistic equation. Chilling increased embryo developmental rates at all germination termperatures, altering coefficients in the equation and the shape of the resulting temperature‐response curve. Each population, deviated slightly from the average curve for each chilling duration. Population deviations were described by a complex cline based on latitude, elevation, and distance from the ocean of the parent tree. Results are discussed as being consistent with Levins' dormancy‐strategy hypothesis.