Stand selection guide for forest fertilization - 2006
Consider a stand's site conditions, health, biodiversity and potential for integrated resource management in the selection process. Stand level activities should be consistent with forest level objectives. Evaluate candidate stands according to biological factors. Those stands that are biologically acceptable should then be checked for operational feasibility to ensure they can indeed be treated and are suitable for treatment.
Species preference: Douglas-fir and spruce. (avoid Pl until MPB population crashes)
Age preference: (avoid stands that are not free growing, stands age 80 and older should only be treated on a trial basis)
Age |
Priority |
40 - 79 |
1 |
15 - 39 |
2 |
Site index: Douglas-fir, avoid dry-belt D-f, otherwise D-f responds on all sites. Spruce, select sites with SI in the range from 15 to 24. Avoid sites poorer than SI =15 or SI greater than 24.
The live crown of the crop trees is greater than 30%, to utilize the added nutrients. This may be dominant and co-dominant trees or a spaced or thinned stand.
There should be room for crowns to expand, and the stand should be fully stocked.
The height/diameter breast height (dbh) ratio for Douglas-fir should be less than 85. Avoid fertilizing conifer stands with a height/dbh ratio greater than 100.
The following four operational factors should be considered during the evaluation of candidate stands.
Location: Choose sites closest to communities as distance to haul the fertilizer affects transportation costs. Also, costs of future harvests are partly determined by hauling distances to manufacturing plants and markets.
Access: Conditions of access also affect costs of transporting material and personnel in fertilizer operations, in addition to later expenses of hauling timber to manufacturing plants. Avoid areas that require long ferry flights, choose areas less than 2km from roads.
Slope: Costs of future management and harvesting usually increase as terrain becomes steeper. Furthermore, flying over steep or irregular, contoured land may not be conducive to efficient and uniform aerial distribution of fertilizer.
Project and Block Size: Project and block sizes effect efficiency and cost of operation. Large-scale projects (e.g., >300 ha) are generally more cost effective than small-scale.
Stand selection supplementThe following discusses broadcast fertilizer applications used to increase tree growth of interior spruce and interior Douglas-fir. This information does not deal with fertilization at the time of planting. This information is primarily excerpts from the Forest Fertilization Guidebook.
Response to fertilization
Fertilization accelerates the rate of stand development. Fertilization is a silvicultural treatment that can be effectively used to increase the merchantable yield and value of established forests. By adding nutrients that are limited on a site, fertilizers can improve the growth of individual stands. However, with knowledge of the timber supply profile and the timing and magnitude of wood supply needs, fertilization can also be strategically used to accelerate the development of specific age classes and timber types. This is done to facilitate an even supply of wood at the forest level.
Forest level planning
Forest level plans may require an increase in forest production in order to meet timber supply objectives. A large-scale forest fertilization program may help meet these objectives.
Stand level planning
Stand selection should be consistent with forest level objectives. The stand level objectives for a fertilization treatment are chosen considering a stand's site conditions, health, biodiversity and potential for integrated resource management. The costs incurred to achieve the necessary stand level objectives should be kept at a minimum.
With the decision to fertilize comes the need to evaluate candidate stands for suitability and priority.
Evaluate candidate stands according to biological factors. Those stands that are biologically acceptable should then be checked for operational feasibility to ensure they can indeed be treated and are suitable for treatment.
The following section explains the major factors to be considered.
Biological principles
Trees respond to added nutrients by increasing the rate of photosynthesis per unit of foliage area (i.e., photosynthetic efficiency) and by increasing photosynthetic surface area through the production of more foliage and expansion of live crowns. These response mechanisms will apply regardless of the treatment objectives.
Increased bole wood production during the first year after fertilization is primarily due to increased photosynthetic efficiency caused by higher foliar nitrogen concentration. However, foliar nitrogen levels of fertilized trees generally return to pre-fertilization levels after about three years. It is the increased foliage mass caused by increased needle size, number of needles per shoot, and number of shoots that results in enhanced bole wood production over the majority of the response period (five or more years). This is why it is critical that crop trees have room for crown expansion following fertilization. If not, the growth response to fertilization will be limited to the short-lived increase in photosynthetic efficiency, rather than the prolonged response due to increased foliage mass.
The growth response to fertilization is largely dependent on the amount of the added nitrogen that is taken up by trees during the short period following treatment. In most forest soils, urea fertilizer is quickly converted to ammonium (NH4+) nitrogen, which is readily taken up by trees and other vegetation. However, the recovery of added nitrogen in crop trees is generally quite low, ranging from less than 10% to approximately 30%. Most of the added nitrogen is rapidly immobilized in soil microbial biomass and organic matter. The immobilized nitrogen is largely unavailable for tree uptake and is generally mineralized too slowly to have much practical value in improving the growth of crop trees.
Under certain conditions, significant losses of added nitrogen can occur from gaseous losses of ammonia (NH3). Volatilization losses will increase with high air temperature, wind speed, and soil pH. Volatilization can be minimized by timing nitrogen applications to coincide with cool (<10°C), calm weather with a high probability of rain in the next 24 hours.
Biological factors
Species
Douglas-fir and spruce respond positively to fertilization with nitrogen in the B.C. interior.
Other species, including dry-belt Douglas-fir, black spruce, western redcedar, western hemlock, and western larch, are not recommended for operations at this time.
Age and size
Preference should be given to fertilizing older stands, provided that live crowns are of favorable size and vigor, and there is room for crown expansion (i.e., naturally occurring lower density or a suitable number of well-spaced dominants i.e., healthy, vigorous crowns), and that other forest level objectives do not take priority.
Because of the unfavorable structure of many older stands in the B.C. interior, the highest fertilization priority is generally assigned to free growing plantations. Stands 15-20 years old should not be fertilized unless trees are at least 2 m taller than competing vegetation. Plantations less than 15 years may exhibit a large relative response to fertilizer additions but a small absolute stem volume response due to their small stem diameter. The site occupancy of such stands may also be too low to efficiently utilize the applied fertilizer.
Stand density
The magnitude of fertilizer response is related to the space available for crown expansion. Stands selected for fertilization should have well-spaced dominant and codominant trees. This means that some stands need to be spaced before fertilization.
Fertilizer treatment of recently spaced stands can reduce spacing shock. In most cases the best time to fertilize is at the time of spacing. However, fertilization should be delayed if one or more of the following factors exist:
Soil moisture and nutrient regimes
Moisture and nutrient regimes of forest soils are two of the main factors that effect tree responses to fertilizing and must therefore be carefully considered. The plant community reflects soil moisture and nutrient regimes. Use regional guides to relate plant associations to soil moisture and nutrient conditions.
Soil moisture regime
Sites that are slightly dry to fresh should receive the highest priority (e.g., submesic and mesic), with lesser priority assigned to drier (subxeric) and wetter (subhygric and hygric) sites. Because soil moisture, either by deficiency or excess, may exert the primary limitation on tree growth, fertilization of stands with very dry (xeric and very xeric) or very wet (subhydric) conditions should be avoided. Avoid sites in dry-belt Douglas-fir.
Nutrient regime
Although benefits from fertilizing occur most consistently on infertile sites, very infertile sites (site quality is low) are unsuitable for treatment because natural growth rates are too slow. Avoid very rich sites also, since the soil probably contains adequate nutrients. Stands on poor and medium regimes should respond best, and therefore will be given the highest priority. The most reliable information on nutrient availability is obtained from foliar analysis. While evidence of chlorotic tree foliage can indicate soil infertility, caution should be used in making interpretations based on these indications (see Crown condition).
Site quality
Preference should be given to fertilizing medium sites (e.g., submesic to mesic soil moisture regimes and poor to medium soil nutrient regimes) in the B.C. interior. A lower priority should be assigned to extremely rich sites until such time that additional research information is available. Fertilization on poor sites will have low priority since site productivity is probably influenced strongly by inadequate soil moisture or extreme climate. Even where relative growth responses are favorable, the absolute volume gains on these lower productivity sites may be too small to make fertilization profitable. Fertilizer should not be applied on low sites. Fertilize spruce from SI=15 to 24 inclusive.
Crown condition
The size and condition of live crowns provide an indication of the nutrient status and productive potential of stands. Considerations related to crown size and foliar characteristics are described below.
Size
Evidence of many trees with short, narrow crowns suggests competitive stress is, or has been, strong. Application of fertilizer to stands in this condition will enhance crown expansion by stimulating growth of branches and foliage. Assign a priority to the treatment of these stands provided they meet the criteria described in the guideline in the section on stand density.
Foliage color
Small, yellowish, and sparse foliage throughout the stand may indicate that one or more soil nutrients are deficient. In this situation fertilization may achieve a substantial growth response. However a chlorotic appearance may also be caused by drought or pathological conditions. In the absence of chemical analysis of soils or foliage, the interpretation of visual symptoms requires expertise and local knowledge. Look for other symptoms that may identify insect, disease or animal damage. Also consider soil conditions, ground vegetation, and rainfall patterns to infer drought.
If foliar appearance in a stand can be reliably interpreted to indicate nutrient deficiency and sufficient response is anticipated to make the treatment economical, the site can be assigned a high priority for fertilizing. However, an absence of visual symptoms does not preclude the possibility of growth limiting nutrient deficiencies.
Nutrient diagnosis
Foliar analysis
When used properly and efficiently, foliar analysis can be an effective tool for planning and monitoring operational fertilization projects. Foliar analysis information can be used to: 1) diagnose possible nutritional reasons for poor quality or rate of tree growth; 2) identify stands that will likely respond well to nutrient additions; 3) prescribe fertilizer formulations to correct inferred nutrient deficiencies and stimulate tree growth; and 4) assess post-fertilization uptake of applied nutrients and foliar nutrient balance. However, stand nutrient status is only one of several factors to be considered when assessing the suitability and priority of candidate stands for aerial fertilizer operations. Foliar sampling should only be undertaken on sites that satisfy other forest- and stand-level selection criteria. For example, foliar sampling is a wasted expense if stand structure or health indicate poor fertilization response potential or if there are serious non-nutritional constraints on site productivity. Also, extensive foliar nutrition and fertilization research has been undertaken by the B.C. Ministry of Forests and Range to determine the nutrient status and fertilization response potential of interior forests. Foliar sampling may be unnecessary if foliar nutrient data or fertilizer growth response information is available from nearby stands of similar age, site, and stand conditions. Foliar sampling should be undertaken if local knowledge indicates that unusual nutritional problems are likely (e.g., S or B deficiencies).
It is recommended that a forest nutrition specialist be consulted to determine whether foliar sampling is necessary. Rob Brockley (Research Silviculturist, Ministry of Forests and Range) is qualified and willing to provide this service. Where sampling is recommended, Rob can also assist with the interpretation of foliar analytical results. He can be contacted by phone (250 260-4768) or email (Rob.Brockley@gov.bc.ca).
Foliar sampling
Procedures for sampling foliage are in Extension Note 52, by Rob Brockley at http://www.for.gov.bc.ca/hfd/pubs/Docs/En/En52.htm
Screening trials
Screening trials are not normally necessary. The advice of a specialist is recommended.
Priority for fertilization: The best financial return is with a short duration following fertilizer application. Applying ten years prior to harvest allows time for the maximum volume response with a short financial time frame.
Forest Health
The susceptibility of a stand to certain damaging agents may, or may not, be increased by fertilization. The degree of damage that can be accepted will vary by forest health factor and severity of impact.
Little is known about the effects of fertilization on the spread of root diseases in interior forests. However, N-fertilized Douglas-fir in the Inland Northwest (northern Idaho, Montana, and eastern Washington appears to be more susceptible to Armillaria (Armillaria ostoyae) root disease than unfertilized stands. There is some published evidence linking susceptible stands with low foliar potassium (K) status (< 0.60% foliar K). Foliar N/K imbalance and subsequent mortality may be related to fertilizer-induced changes in root biochemistry (i.e., reduced phenol/sugar ratio) that favours the spread of Armillaria. In the B.C. interior, Douglas-fir candidate stands within the Interior Cedar-Hemlock (ICH) biogeoclimatic zone should be carefully assessed for the presence of root disease.
The white pine weevil (Pissodes strobe) is the principal insect that may affect priorities for fertilizing interior spruce stands. Results from a recent study indicate that fertilization of young spruce plantations in the SBSwk and SBSmk biogeoclimatic subzones will exacerbate weevil leader damage. However, despite the increased weevil damage to fertilized trees, the height losses due to weevil attack were not as great as the height gains due to fertilization. When combined with large increases in stem radial increment, the beneficial effects of fertilization on the growth of young interior spruce plantations in the SBSwk and SBSmk subzones likely outweigh the negative effects associated with increased incidence and severity of leader damage from the white pine weevil. Study results also indicated that large-scale fertilization of spruce plantations in the SBSmc biogeoclimatic subzone probably entail a low risk of increasing damage by the white pine weevil, because the climate is likely too cold to sustain large weevil populations. The summarized results from this study can be downloaded at http://www.for.gov.bc.ca/hfd/pubs/Docs/En/En75.pdf.
Small wildlife
In the interior, sharp increases of red squirrel feeding damage on lodgepole pine have been observed after fertilization. If any pre-fertilization animal damage is noted in the general area of the stand, caution might be indicated.