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Site Limiting Factors

Once species and seedlot selection have been made, the selection of a stock type is, in most cases, reasonably simple. There are few stock type selection decisions where only one stock type is acceptable there is usually a choice. The stock type selection must be made partially on a cost basis the less expensive tree, in most years on most sites, may be the most cost-effective choice. In other years and on other sites, however, less expensive stock types may show lower survival and performance. Increasing planting densities under some circumstances is not recommended as stem distribution may become clumped. If survival is low, replanting may be required to compensate for losses. While the use of the less expensive and less reliable stock may save on initial costs, it may incur the risks and expenses associated with more variable performance.

It is at the edaphic, climatic, and logistic extremes that stock type selection becomes the most critical. Linking the stock type prescription with the species prescription at the SP stage helps to identify these thresholds. To deal with stock type selection under the environmental extremes, it is crucial to first identify the limiting factor(s) for the prescribed species on the intended site. A stock type choice can then be made with the goal of ameliorating specific limiting conditions for that species.

Although only a single factor may be considered limiting, a seedling weakened by one factor may become more prone to injury and poor growth from other factors. There are potentially many limiting factors but stock type selection is not capable of addressing the specifics of every limiting factor.


Frost and winter desiccation

No stock types can be used

to avoid frost damage.

If frost damage, frost heaving, or winter desiccation are expected, the initial damage/mortality is typically encountered in the first growing season. The principal ways of minimizing frost and winter desiccation damage and frost heaving are:

  • Species selection (e.g., Pl>Bl>Sx).
  • Site preparation.
  • Site avoidance.
  • Planting season.
  • Well-hardened seedlings.
  • Microsite selection.
  • Minimize exposure of mineral soil when screefing.
  • Select short, sturdy stock types.

Once an appropriate species has been chosen, planting should be timed to avoid early frost or late frost (see Table 4 for species relative frost tolerance). Avoid planting valley bottoms prone to cold air drainage and radiative frost until later in the season. Although these sites may be snow free early in the spring, depending upon the species, they should not be planted until after the risk of frost has passed. Some sites (e.g., high-elevation biogeoclimatic zones -- ESSF, MH, northern biogeoclimatic zones -- SBS, BWBS, and northern subzones of the ICH), may be prone to frost at any time of the year. The most conservative planting strategy is to plant these sites with summer-shipped stock that will not be as easily damaged by frost and to plant protected microsites.

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Species

Seedlot

Seedling Characteristics

Site Limiting Factors

Site Preparation Selection

Field Operational Considerations

Delivery Dates

Nursery Production Time

Cost

Nursery Treatments

What Have Other People Used?

Stock Type Development

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Table 4.   Relative tolerance to growing-season frost for provincial tree species


Relative tolerance to growing-season frost Tree species

Very low Cw, Dr, Fd, Hw, Mb
Low Bg, Bn, Lw, Ss
Moderate Ba, Bl, Pw, Py, Sx, Yc
High At, Acb, Act, Ep, Hm, Lt, Pl, Pj, Pa, Sb

Source: Province of British Columbia. 1995. Establishment to Free Growing guidebooks. B.C. Min. For., Victoria, B.C.

Frost heaving is a frequent occurrence on many sites with fine-textured soils, especially in northern British Columbia if the stock is planted late in the season. Small, shallow containers, such as the PCT211A and PCT/PSB 410, are not appropriate for these conditions as they are more prone to frost heaving than larger or deep-planted stock types such as PSB/PCT 313B/415B. For some species, PCT stock types may be more suitable than PSB stock types because of their better root egress in the upper soil layer.


Drought

Soil moisture can limit the length of the planting season and strongly influence seedling establishment. It is determined by five factors:

  • Season.
  • Soil characteristics (i.e., organic matter, texture, coarse fragments, density, depth).
  • Vegetation (particularly grasses).
  • Slope position, elevation, and aspect.
  • Evapotranspiration.

Droughty soil conditions are often accompanied by high temperatures and low relative humidity. Herbaceous and grass competition may further aggravate these droughty conditions by competing with seedlings for available soil moisture. Droughty site conditions are frequently associated with shallow, stony soil, and with south aspects, and low elevations in the southern interior.

The following is a list of the principal ways of dealing with drought conditions:

  • Select drought-tolerant species (e.g., Pl>Bl>Se).
  • Site prepare spots to minimize drought effects (e.g., disc trench).
  • Time planting to avoid drought period.
  • Select sheltered and/or shaded planting microsites.
  • Use only well-hardened seedlings.
  • Favour depressions as planting microsites.
  • Avoid planting in dense grass cover or other competing vegetation.

Stock types cannot be used as a

substitute for correct planting time.

Time planting to coincide with periods of acceptable soil moisture and low moisture demand during and following planting to assure adequate root growth prior to the onset of drought. Be aware of situations where soil moisture conditions are adequate for planting but the weather conditions are warm and dry; a high transpirational demand may be placed on seedlings.

Well-branched, woodier stem morphology, and hardened tissues are important seedling traits for minimizing transpirational water loss during establishment. For example, PSB 313B 1+0 seedlings may be more at risk to drought because they are not as woody or branched as the larger PSB 415B 1+0. Under shallow, stony soil conditions, the shallow-rooted stock types, such as PSB410 1+0 or PSB 412A, may be better suited. Similarly, a late-planted PSB 313B 1+0 Sp may be more sensitive to drought than a PSB 313B 1+0 Su because of the additional transpiration demands placed on the seedling by a succulent flush. Avoid stock with large shoots coupled with poorly developed or small root systems. A well balanced tree is critical for droughty sites.


Flooding

Flooding and saturated soil conditions with poor aeration are typically Sx, Ss, and Cw fluvial and alluvial sites as well as Plc bogs and Sb/Lt wetlands. Saturated soil conditions are accompanied by low soil temperatures and low oxygen. Correct species selection, raised microsite selection, drainage ditches, and mounding by mechanical site preparation (MSP) are ways to deal with high and fluctuating water tables.

Stock type selection cannot be used

to alleviate saturated soil conditions.


Physical damage

Physical damage may be due to a variety of factors depending on the site:

  • Snowpack effects (press, creep, abrasion, andglide).
  • Vegetation press (herbs, grasses).
  • Animals (browsing, trampling).
  • Falling debris (ravelling).

To a limited degree, physical damage can be moderated by stock type and species selection. A large, robust seedling will be the most resistant to these damaging factors. A large stem diameter will help prevent the seedling from being bent and trampled. A seedling with a well-branched stem is more likely to recover from physical damage by re-expression of apical dominance from a branch or subterminal buds. A well-branched stem also helps shed vegetation. For these reasons, seedlings that are sturdier or larger, such as the PSB 412A or PSB415D are preferred to a PSB 313B 1+0 for dealing with anticipated physical damage.

Seedlings can be protected from ungulate browsing by netting, sleeves, or plastic tubes but in some cases, particularly Cw, these devices may cause seedling deformities if not regularly maintained. Obstacle planting, locating the seedling near an obstacle such as a log or stump, is another way to protect them from ungulate browsing and trampling. For some species, such as Cw, the browsing may be so severe that it is better to select a species that is not as palatable.

There are no stock types that can

be used to avoid rodent browsing.

Large diameter woody stems are less palatable and better able to endure small mammal browsing than small diameter stems. Planting when rodents are not foraging, and at low periods in their population cycles, can be a successful avoidance strategy. Reduction of vegetation cover by the use of some site preparation techniques can reduce rodent habitat and improve seedling survival. Fertilization at time of planting may compound browsing problems.


Vegetation competition

Vegetation differs from most other site factors in that it can have both physical and physiological impacts on seedlings. Vegetation may affect seedlings physiologically by limiting moisture, nutrients, soil temperate, and light. Large, robust seedlings are better able to deal with both the physical and physiological effects of vegetation than smaller stock types. Sites with extreme competition from grasses, shrubs, or herbs, will require site preparation before any stock type will be successful.


Duff/Forest floor materials

On sites with a high level of debris, thick moss mats, or deep loose duff layers, site preparation, not stock type selection, may be the key to achieving reforestation objectives. If forest floor planting is prescribed under these conditions, larger stock types are recommended (e.g., 415B, 412A) as these can be planted more securely into the loose materials and there is less risk of burying the larger tops. BBR and PBR are not recommended for these conditions due to the difficulty in securing a proper planting microsite for these stock types.


Plantability

The three most common plantability problems are shallow soils, excessive rotten wood and debris, and high spring water table. Where shallow, stony soils or high water table prevail, select a shorter stock type, such as a PSB/PCT 313B/410/412A 1+0. For shallow, stony soils be sure to time the planting to take advantage of soil moisture. BBR and PBR stock types have special plantability considerations and should be restricted to sites where a planting depth of at least 30 cm can be assured.

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