Quantitative data concerning the magnitude of losses caused by low temperatures are generally lacking, although one large land management concern in the Pacific Northwest estimates that 10% of planting stock are adversely affected. The reason for the lack of truly definitive data may be the difficulty of correctly assessing damage caused by cold. We can generalize, however, to the extent that young trees are most vulnerable and, for this reason, nurseries and plantations are likely to be damaged more frequently than older stands. Development and effectiveness of preventive measures depend on a tolerance or avoidance of low-temperature stress.
This section will review the current data describing the induction and development of cold hardiness in plants. Methodology appropriate for reducing the incidence of frost damage in nurseries and plantations will be described. You should read this section through once, and then re-read with references cited at the end of the section.
At the end of this section, you should be able to answer the following questions:
1. What is frost hardiness? How does it differ from dormancy? What parts of seedlings become frost hardy? Describe the annual frost hardiness cycle. What environmental conditions induce frost hardiness? How do these conditions differ from those which induce dormancy? What one factor in the nursery should the nursery manager control if he/she is to produce the most frost hardy stock?
2. What physiographic features are likely to increase the incidence of frost damage. What will decrease such damage? How many concentrations of slash affect seedling losses to frost? Why may shelterwood harvest systems reduce frost damage?
Annual Cycle of Frost Hardiness
Figure 13 outlines the sequence of events which have either been observed or hypothesized to occur during the annual growth cycle of a plant. It is
FIGURE 13 A hypothesis to explain acclimation in hardy woody plants. Numbered arrows indicate the hypothetical sequence events resulting in the most efficient and complete acclimation. Arrows without numbers identify sequential relationships ... alternate acclimation pathways. Parentheses denote events which can be observed experimentally. After Weiser, 1970. (Take a moment and really look at this.)
important to note that the development of the frost hardiness in plants is associated with the same sequence of environmental changes, i.e., long warm days, warm nights--short, mild days, cool nights--short, cool days, cold nights, as that associated with the development of the first two stages of dormancy. One major difference, however, between the development of dormancy and that of frost resistance is that the latter may require exposure to low temperatures, generally about -3° to -5°C, whereas a plant may become fully dormant without exposure to low temperatures. You should also note the reference to photosynthesis. Plants cannot become fully frost hardy without accumulation of significant carbohydrate reserves. Therefore, if everything else is equal, seedlings grown in an overcrowded seed bed are less likely to be frost hardy in the fall than are plants grown at proper spacing.
Figure 14 illustrates a second important aspect of frost hardiness, i.e., frost hardy races or ecotypes do not necessarily develop a greater resistance to cold than less hardy members of a given species, but this hardiness is developed earlier in the fall. This phenomenon is particularly important in the Pacific Northwest where coastal seed sources of Douglas-fir are raised in Willamette Valley nurseries, or where westside seedlings are grown in eastside nurseries. Recent years have seen such stock severely impacted by early season frosts, whereas seedlings raised from seed collected near the nurseries were relatively unscathed.
FIGURE 14 Typical seasonal patterns of cold resistance in the living bark of three climatic races of Cornus stolonifers. The acclimation curves shown are for clones from North Dakota, Minnesota, and Washington grown in the field in Minnesota. Races from regions with mild climates and long growing seasons acclimate later and more slowly than clones from regions with severe climates and short growing seasons. Over 25 clones which have been collected from widespread locations in North America became resistant to -196°C by midwinter in Minnesota. After Weiser, 1970.
Despite substantial research in the area, we are still not truly conversant with the mechanism of low temperature damage. The most tenable current hypotheses are that plants can tolerate ice crystal formation in extracellular locations, but when ice crystal form with the cells, the ice ruptures membranes and leads to general disorganization of cell contents or the formation of ice within the plant leads eventually to extreme desiccation of cells, in effect, physiological drought. Macroscopic evidence of frost damage commonly takes the form of brown or necrotic tissue. However, if the damaging frost is followed by a period of low temperatures, such symptoms may require weeks to develop.
As we noted in an earlier unit, only buds develop dormancy. In contrast, the whole plant develops some degree of cold-hardiness, generally the shoots more so than the roots and all parts of a seedling may be damaged by frost or low temperatures. The symptoms of such damage and their significance are outlined in Table XVIII.
If more than about 25% of the seedlings in a given seed bed in the nursery are shown to have significant damage to the stem, it would be well to leave them in the nursery so that possible survivors may be transplanted in late spring after the full extent of the damage is known.
| Tissue | Symptom | |||
|---|---|---|---|---|
| Buds | primordia blackened instead of green | Damage to terminal bud in some species can adversely impact ability of seedling to compete with weeds | | |
| Needles | Discolored often purplish or grey-purple changing to brown | partially killed foliage is common | Not important unless the damage occurs by late fall. | Needles sense environmental changes which trigger development of dormancy |
| Stem | Bark loose,cambial region brown | If less than one half of the circumference of the stem is affected, seedlings may survive | more seedlings have a low survival potential | |
| Roots | Bark loose, cambial region discolored | seedling survival potential very low |
At least some of the loss to frost damage may be prevented by one or more of the following:
1. Plant properly conditioned seedlings whose resistance and dormancy were induced by proper nursery irrigation schedules.
2. Use a shelterwood system of harvest in areas of extreme radiation frosts. The overwood will modify the temperatures sufficiently to prevent frost damage in many areas. If possible, cover seedlings in frost pockets with light branches or similar debris to protect against radiation cooling. (In New Zealand they practice absolutely clean ground forestry to maximize heat transfer from soil to air on nights when radiation cooling occurs.)
3. Plant frost pockets with the most frost resistant stock available. Avoid creating frost pockets with clear-cut design or with method of disposing of slash.
4. Avoid the use of container stock in areas where frost heaving is probable.
5. Avoid the use of exotic species native to areas which do not have severe winter frosts. Seedlings may survive for many years, but eventually are likely to be killed by a severe frost in British Columbia or the Pacific Northwest.
6. Use seedlings with early bud break characteristics only on southern slopes where danger of late frost is slight.
7. When container grown seedlings are overwintered out-of-doors, the root systems should be protected against temperatures below -5°C in fall; -10°C in winter.
1. How may seedling frost resistance be affected by nursery management?
2. Are cold-hardy plants resistant to lower temperatures in January than other plants of the same species which are not considered so cold hardy? If not, what is the difference between cold hardy and less cold hardy plants of the same species?
Readings
1. Burr,K.E. 1990. The Target Seedling Concepts: Bud Dormancy and Cold Hardiness. in The Target Seedling Symposium. pp. 79-90.
2. Alden, John and R.K. Hermann. 1971. Aspects of the Cold-hardiness Mechanism in Plants. Botanical Review 37(1):37-142. General reference, scan for material which may interest you.
Additional Information:
1. Hermann, R.K. 1976. Frost damage in the nursery - effect on seedlings. Northwest Lookout.
2. Weiser, C.J. 1970. Cold resistance and injury in woody plants. Science 169:1269-1277.
