Seasonal changes in the cold tolerance of pitch pine

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Abstract
Primary needles, secondary needles, and stems of 1-year-old pitch pine (Pinusrigida Mill.) seedlings were evaluated for cold tolerance on eight dates from October to April. Male and female strobili and elongating shoots of mature trees were evaluated on four dates in May and June. Stems acclimated more rapidly than secondary needles, but were less cold tolerant in midwinter. Secondary needles and primary needles had similar levels of tolerance on most dates. Strobili and emerging shoots were essentially identical in tolerance except on May 17, about 10 days before pollination, when strobili were 5.8–7.7 °C more tolerant than elongating shoots.