Growth and survival of seedlings planted in the field are influenced by seedling quality, which varies appreciably with seed quality, nursery practices, and handling of nursery stock during lifting from the nursery and in outplanting.
The quality of seedlings reflects integration of many physiological and morphological characteristics. Ritchie (1984) divided components of seedling quality into "material attributes" (e.g., dormancy status, water relations, nutrition, and morphology) and "performance attributes" (e.g., root growth potential, hardiness to frost, and resistance to stress). Material attri butes are more easily measured than performance attributes but, considered individually, they often have relatively low predictive value for seedling success unless they fall outside some normal range. Nevertheless, seedling size often has been a useful index of seedling success. For example, survival of grade 1 (root collar diameter >4.7 mm) loblolly pine seedlings was higher than that of grade 2 seedlings (root collar diameter 3.2 to 4.7 mm). Volume production of grade 1 seedlings was 17.5% higher than that of grade 2 seedlings (South et al., 1985). On some sites, however, short seedlings may grow better than tall seedlings. For example, on very dry sites short seedlings, with small transpiring leaf areas, sometimes outperform tall seedlings.
It is important that nursery practices are favorable for maintaining physiological processes of seedlings that will result in high capacity for growth and survival after outplanting. The loss of many of the small absorbing roots during lifting and handling of nursery stock often leads to dehydration of transplanted trees (Kozlowski, 1975, 1976a; Kozlowski and Davies, 1975a,b). Hence, important requirements for survival of transplanted trees are a high root-shoot ratio and rapid growth of roots into a large volume of soil in order to maintain high rates of absorption of water and mineral nutrients. Reserve foods also are essential because photosynthesis of outplanted seedlings may not return to normal for several weeks.
Production of high-quality seedlings requires close attention to all phases of nursery management. These include preparation of nursery beds, soil management, planting procedures, control of seedling density, use of fertilizers, irrigation, and pest control. Sometimes they also may include root pruning and inoculation with mycorrhizal fungi (Chapter 7). For discussion of nursery practices, see Chapter 7 and books by Duryea and Brown (1984), Duryea and Landis (1984), Duryea and Dougherty (1991), and van den Driessche (1991a,b).
It often is necessary to place seedlings in cold storage because readiness of planting sites does not coincide with the time of lifting seedlings at the nursery. Furthermore, if the planting season must be extended, planting stock in cold storage can be kept dormant for a longer time than if left in nursery beds. If lifted, handled, and stored properly, seedlings from cold storage may grow better and survive as long or longer than recently lifted seedlings (Hocking and Nyland, 1971). Seedlings should be physiologically dormant when lifted for storage. If seedlings are to be stored over winter, their lifting from the nursery should be delayed as long as possible.
Storage Conditions The quality of planting stock can be greatly lowered by improper storage. Seedlings should not be allowed to dehydrate either before or during cold storage. Dehydration of seedlings during storage may greatly lower the root growth potential (RGP) (Colombo, 1990; Deans et al., 1990). To prevent dehydration of planting stock most storage areas are maintained at a relative humidity of 85% or higher. Often seedlings are wrapped in plastic films. While slowing dehydration, this practice increases molding, which can be counteracted by low temperature.
Temperatures of most seedling storages are maintained at either 1 to 2°C above freezing or 2 to 4°C below freezing (Camm et al., 1994). However, satisfactory storage temperatures range from 3°C to — 6°C. In northwestern parts of the United States and in Canada conifer seedlings are routinely stored at temperatures below freezing to reduce losses of reserve carbohydrates by respiration and to inhibit growth of molds (Simpson, 1990; Omi et al., 1991). In British Columbia most conifers can be satisfactorily stored at — 2°C (Van Eerden and Gates, 1990). Unfortunately, however, frozen seedlings usually thaw slowly before they are planted, often resulting in appreciable respiratory depletion of carbohydrates. Furthermore, molds sometimes develop during the periods of thawing.
Although many cold storages are maintained in darkness, a daily photo-period during storage may improve growth and increase survival of outplanted seedlings that had been lifted in the autumn. Cold hardiness also may be increased by a daily photoperiod in storage. Camm et al. (1994) suggested that it might be useful to combine early lifting of seedlings with cold storage in the light.
Lifting Date The date of lifting of seedlings for immediate planting or for cold storage is important because it influences the RGP of outplanted seedlings. Usually the RGP increases from a low value in the autumn to a high value in the early spring and then decreases.
Seedlings of Douglas fir, western hemlock, black spruce, and jack pine that were lifted from the nursery in September or October had less vigor than those lifted after mid-November (Camm et al, 1994). Ponderosa pine seedlings lifted early in autumn also had lower RGP than those lifted in November. Stone et al. (1962) showed that mortality of transplanted conifers and RGP were inversely related. Hocking and Nyland (1971) suggested that to minimize adverse effects on seedling vigor lifting from the nursery should be delayed as long as possible.
In some regions (e.g., interior Washington State, British Columbia) the ground freezes and seedlings cannot be lifted in midwinter when their RGP is highest. Hence, seedlings have been lifted in the autumn and planted after overwinter storage, or they have been lifted in the spring and planted immediately or after a short period of storage. Ritchie et al. (1985)
showed that autumn lifting of lodgepole pine and interior spruce (Picea glauca X P. engelmannii) in British Columbia, beginning November 1 and with overwinter storage, was preferable to spring lifting and planting. Seedlings that were lifted in the spring had low RGP, low resistance to stress, low frost hardiness, and poor storage performance.
Duration of Storage Seedlings have been cold-stored from a few days in the southern United States to as long as 8 months in northern states and Canada. A moderate period of cold storage does not significantly influence mechanisms of seedling development. However, under certain conditions prolonged storage may be harmful because of depletion of reserve carbohydrates by respiration and a decrease in RGP. In a forest nursery in Scotland, the amounts of reserve carbohydrates in bulked needles, stems, and roots of unlifted Sitka spruce and Douglas fir seedlings were relatively constant (100 to 150 mg g"1) from September to April. In comparison, carbohydrates were depleted by respiration in intact seedlings in cold storage at a rate of 0.4 to 0.6 mg g_1 day-1, until only 40 to 50 mg g_1 of carbohydrate remained (Cannell et al, 1990). Ritchie (1982) noted a decrease in RGP after storage of Douglas fir seedlings for 6 months. Dry weight losses of seedlings in cold storage often are associated with reduced survival of outplanted seedlings. The harmful effects of prolonged storage are more serious for seedlings lifted early than for late-lifted seedlings. Seedlings lifted in December and planted the next May had the benefit of a short storage time and development of high resistance to environmental stresses (Camm et al., 1994).
All of the skill devoted to maintaining appropriate nursery regimes can be negated by improper handling of nursery stock during lifting from the nursery and outplanting after storage. Hocking and Nyland (1971) recommended that stock should be lifted only when the leaves are dry. Roots should be kept moist, and the air temperature should be cool in the sorting and packing sheds. Injured, weak, diseased, and undersized plants should be culled.
Exposure of bare-rooted trees for even short periods during the out-planting process may severely inhibit growth and decrease survival. Even after seedlings are planted in the field, there often is excessive loss of water at a time when the roots grow too slowly to keep up with transpirational losses (Kozlowski et al, 1991). Trees that survive transplanting may show growth inhibition for a long time. For example, height growth of white spruce transplants was reduced by about half in the first year after outplanting. In some cases, growth was inhibited for at least 10 years (Mullin, 1964).
70 2. Seed Germination and Seedling Growth
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