Cold hardiness in genetically 'Improved' and wild stand coastal douglas-fir
Date
1998
Authors
Stevenson, Joshua Freedom
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Abstract
Breeding for increased growth in coastal Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb. Franco)) may affect the level of cold hardiness of seedlings used for reforestation. Cold hardiness was measured in seedlings grown from top-cross, first generation seed orchard, and unimproved wild stand seed throughout one growing season by visual assessment of artificial freeze tests. Some significant differences in freezing damage between genetically improved and wild stand trees were found in spring, fall, and mid-winter. Mitotic index of terminal buds of lateral branches was investigated as an indicator of dormancy, and a negative correlation between mitotic index and cold hardiness was found. During the spring period, there was a significant difference in mitotic index found between the genetic groups.
Using the rate of tracheid formation as an indicator of secondary growth, no xignificant correlations were made between secondary growth and cold hardiness. There were also no significant dilierences in rate of tracheid formation between the genetic groups. Date of bud burst and rates of shoot extension were related to levels of cold hardiness in the three groups of seedlings. Trees that burst bud earlier completed shoot extension earlier in the season and entered dormancy significantly earlier in the fall. Top-cross trees appeared to extend their growing season later into the fall, thereby gaining a height advantage over wild stand seedlings. These top-cross families do not appear to have an increased late fall frost damage risk, and may have a reduced risk of critical sp1ing frost damage due to delayed growth initiation.
A second study examined the effect of five nutrient treatments varying N:P ratio on cold tolerance of fast and slow growing Douglas-fir families. Trees receiving more N and P grew significantly taller and trees receiving higher levels of N had significantly higher mitotic indices in late fall. Trees with a high N:P ratio were significantly more hardy at all times of the year. Thus, a significant positive correlation existed between hardiness and mitotic index. Increased N and P nutrition may have prolonged the growing season, but did not decrease hardiness.