Dehardening and budburst of Douglas-fir seedlings raised in three Pacific Northwest nurseries

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Abstract
This study was undertaken to test whether nursery location affected dehardening and budburst of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) seedlings from two seed sources. The seedlings, raised at three nurseries in Oregon and Washington, were measured with a whole-plant freezing test in January, February, and March, 1986. In general, seedlings raised at the nursery at highest elevation, and in a few cases, trees from the most northerly nursery, were more frost resistant than trees from a coastal nursery. From January to March, seedlings from the highest (975 m) seed source had less-hardy stem tissue than seedlings from the coastal source (450 m). A growth-chamber experiment confirmed the outdoor dehardening studies. A constant temperature of +5 °C with a 16-h photoperiod maintained cold hardiness, whereas +10 and +15 °C with a 16-h photoperiod promoted rapid dehardening after 20 days. The nursery environment influenced budburst; trees raised in the coastal nursery burst bud significantly earlier than trees from the other two nurseries. Trees of different provenances from the same nursery burst terminal buds only 2 days apart.