Selection of a stock type is a biologically and economically crucial phase in the regeneration process. Silviculturists can exert control over the morphology of seedlings through stock type selection. The diversity of available stock types provides silviculturists with a set of tools for dealing with a wide range of reforestation situations (Plate 3).
Many factors must be understood and considered in order to select the most effective stock type for a given situation. This section provides detailed information on seedling biology, site factors, field logistics, and nursery culture that will influence your final stock type selection.
It is important to remember that correct species selection is necessary before a stock type selection can be made. In many cases, stock type selection may be directly influenced by species choices.
Species selection should be based on a variety offactors:
Seedlot selection should be based on the following four factors:
An appropriate stock type selection can optimize the match of a species/seedlot selection to specific site conditions.
Plate 3. Selection of common regeneration species and stock types based on 1997 sowing requests. Notice the difference in root collar diameter between the Sx 1+0 and Sx 2+0 stock types.
The physiological condition and carbohydrate reserves of the seedling determines its resistance to stress and its ability to establish quickly. An acceptable stock type selection represents a blueprint that, when grown and handled properly, will reliably produce seedlings of optimum size and satisfactory physiological quality.
Seedling size is the most important morphological characteristic of a stock type. Select the size of seedling that overcomes the site limiting factors -- then select the stock type that matches those requirements. Stock type morphological specifications are the basis for selecting stock by field staff and grading stock in the nursery. Table 3 gives an example of the 199798 morphological specifications for some Sx stock types.
Table 3. Acceptable Sx stock type morphological specifications determined by the Ministry of Forests for 199798 ministry-funded crops. Morphological specifications vary by species. Specifications for other species are available from Ministry of Forests Nursery and Seed Operations Branch. These specifications are reviewed and revised annually. These specifications are guidelines for licensees.
|Sx Interior Spruce||Height (cm)||Root collar diameter (mm)|
|PSB 313B 1+0||11||17||25||2.2/2.4*||2.8/3.0*|
|PSB 410 1+0||12||18||27||2.4/2.6*||3.0/3.2*|
|PSB 415B 1+0||13||22||28||2.6/2.8*||3.3/3.5*|
|PSB 412A 1+0||14||24||35||2.8/3.0*||3.6/3.8*|
|PSB 415D 1+0||14||25||40||3.0/3.2*||3.8/4.0*|
|PSB 512A 1+0||15||27||40||3.3||4.0/4.2|
|PSB 615A 1+0||20||40||50||4.0||5.0|
|PSB 415B 2+0||14||27||40||3.6/4.0*||4.8/5.0*|
|PSB 415D 2+0||16||30||44||4.0/4.4*||5.3/5.7*|
|PSB 615A 2+0||20||35||50||4.5/5.0*||6.0/6.5*|
|*||The smaller number represents the specifications on stock lifted prior to September 15. The larger number applies after this date.|
Three basic principles are used in choosing a container size and seedling age:
As nursery culture has developed, size expectations have increased. Whereas in the past large stock could only be provided in 2+0 seedlings or transplants, now tall, robust seedlings can be grown as 1+0 stock. These seedlings also achieve large root masses quickly. However, if growth cycles are not matched to appropriate planting windows (refer to "Nursery Production Time," Figure 6), seedlings can become root-bound and prone to root diseases. This change in culture, morphology, and root health is the reason for preferring any 1+0 over a 2+0 Sp culture. As a seedling becomes larger and older, there are fewer cultural techniques that a nursery can use to control growth and prevent disease without reducing the vigour of the seedling. Seedlings cannot be left in the nursery forever!
In addition to root disease, 2+0 seedlings at the nursery are subject to risk of over-winter damage. The 2+0 crops are typically over-wintered in open compounds, hence risk being damaged or killed by low temperatures.
The size of seedlings can be expressed in terms other than height and root collar diameter. Shoot/root ratio is a comparison of shoot dry weight to root dry weight. It is manipulated primarily by seedling spacing in the nursery. Higher nursery densities tend to produce trees with higher shoot/root ratios and increased incidence of foliage disease. Seedlings with high shoot/root ratios are more prone to water loss after out-planting from transpiration because a higher proportion of foliage is shade adapted, lacking sufficient cuticular waxes. They are also less sturdy hence more prone to being damaged by vegetation or snowpress. These issues have been integral in the continued shift in preference for larger stock types.
Sturdiness ratio is another means for expressing the shoot to root relationship. It is the ratio of seedling height to the root collar diameter. In the nursery, seedling height and stem diameter are measured as illustrated in Figure 3. The lower the value, the more sturdy the seedling and the less likely it will suffer physical damage from vegetation or snow.
Both shoot/root and sturdiness ratios change with the age of the seedling in the nursery. More importantly, these ratios change after out-planting. Do not look for a stock type in the nursery with exactly the same ratios as a seedling after a few years in the field. Instead, select seedling stock types that grow into the target ratios established as field performance measures.
Figure 3. Spruce seedling with a height of 18 cm and root collar diameter of 3.0 mm showing where measurements are taken in the nursery. This seedling would have a sturdiness ration of 18:3 = 6.