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 in detail the major factors to be considered. These factors, which should be considered in the development of stand mangement prescriptions and stand selection, are summarized in Appendix 1. When assigning a priority to a particular stand, consider as many factors as possible. These ranking criteria were developed for stands when the main objective of the fertilization is an increase in net merchantable volume growth, or earlier harvest. This ranking criteria must be modified if the objectives of the treatment are different.
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.
For coastal areas, Douglas-fir has shown consistent and significant growth response to nitrogen fertilization. Reliable methods exist for predicting response of Douglas-fir to fertilization. Stands with >80% Douglas-fir component should be assigned the highest priority. Fertilizing Douglas-fir outside of its normal ecological range (see Establishment to Free Growing Guidebook) is not recommended, as those stands may have greater susceptibility to snow and frost damage.
Western hemlock has shown erratic response to fertilization. More consistent response to N + P applications has been shown in young stands growing on substrates rich in decaying wood in the CWHvh subzone and salal-prone sites at low elevations in the CWHvm subzone. On other sites, hemlock stands should only be considered for research or operational trials.
Limited local experience indicates that Sitka spruce stands, particularly those in chlorotic condition, respond to fertilizing. Fertilization of Sitka spruce may increase the incidence of leader attack by the spruce weevil (Pinus strobi). Until research or operational trial results better document positive results, extensive fertilizing should be delayed and a low priority assigned to this species. Screening trials are recommended prior to treatment.
For commercial tree species other than Douglas-fir, Sitka spruce, and western hemlock, as discussed above, fertilizer application should be restricted to research installations and operational trials.
For coast–interior transition areas, little information is available about growth responses of trees to fertilizer. Only consider Douglas-fir stands for inclusion in large-scale fertilizer programs. Also note that this recommendation is based entirely on extrapolation of coastal experience. To ensure soil moisture does not seriously limit growth responses, do not fertilize on sites with more than three months of growing season water deficit (e.g., avoid very dry and moderately dry soil moisture regimes).
Table 1. Species-specific site characteristics that may be useful indicators of nutrient deficiencies. (From: Soo TSA Forest Fertilization 10 Year Plan, R.E. Carter, E.R.G. McWilliams.)
Research throughout the B.C. interior has shown that lodgepole pine is consistently deficient in nitrogen. Nitrogen additions often have a substantial positive effect on tree and stand growth. On sites with marginal sulphur status (<60 ppm foliar sulphate – S), fertilizer nitrogen response may be improved when sulphur is applied with the nitrogen. Induced boron deficiencies following nitrogen fertilization have also been documented. Boron deficiency causes top dieback which can have a large negative impact on stem value. The addition of a small amount of B in the operational fertilizer mix is generally effective in preventing damage. However, it is probably most cost effective to avoid fertilizing stands where pre-fertilization foliar analysis indicates a potential B problem (i.e., pre-fertilization foliar B <10 ppm).
There is currently limited fertilization response information for Douglas-fir in the B.C. interior. However, results from fertilizer trials in the Intermountain region of the United States indicate that N fertilization has produced consistent growth response over a broad range of site and stand conditions. Douglas-fir stands in northern Idaho have responded very well to N additions. In many respects, these stands are similar to Douglas-fir growing in the Interior Cedar-Hemlock (ICH) Biogeoclimatic Zone in south-central B.C. However, extensive fertilization should not be undertaken until research or operational trial results document positive results for wet-belt Douglas-fir in southern B.C.
Other species, including dry-belt Douglas-fir, black spruce, white spruce, western redcedar, western hemlock, and western larch, are recommended for trials only until more response information is available.
For Douglas-fir there is no clear relationship between stand age and the percentage response in periodic increment following fertilization. Factors such as crown vigor and room for crown expansion are apparently more important than age in predicting the response. Since the volume increment of a stand tends to decline with age, the absolute increases in volume from fertilization will also decline with age. However, from a financial viewpoint, older immature stands might be the most desirable investment. The costs of fertilizing will be compounded for only a few years before harvest. Therefore, provided that live crowns are of favorable size and vigor, and there is room for crown expansion, preference should be given to fertilizing older stands rather than younger ones. Stands recently commercially thinned, with favorable stand structure are good candidates for fertilizing in these older age classes. Given these conditions and the absence of priority ratings based on the need to mitigate specific age class distribution gaps, the highest priority should be assigned to stands 10–15 years from expected rotation end, followed in order of preference, by stands 16–30 years, and 31 or more years from rotation end.
Generally, very young Douglas-fir (15–20 years) should not be fertilized until trees are at least 2 m taller than the ground vegetation. This height advantage is necessary so that competing vegetation will not overtake the crop trees. An exception is when early fertilization is necessary to meet free growing or green-up requirements.
As with coastal species, preference should be given to fertilizing older stands in the interior, provided that live crowns are of favorable size and vigor, there is room for crown expansion, and that other forest level objectives do not take priority. Older, spaced stands and unspaced stands shown to have appropriate structures for fertilization (i.e., naturally occurring lower density or a suitable number of well-spaced dominants) or good potential for spacing or commercial thinning (i.e., healthy, vigorous crowns) may be assigned a high priority. However, the yield implications should be carefully considered before these stands are spaced or commercially thinned to provide fertilization opportunities.
Unfortunately, many older, unmanaged interior stands (e.g., age class 3 or 4 lodgepole pine) exhibit poor fertilization response potential. These stands often have unfavorable stand structure and low potential for response to spacing or commercial thinning. As such, combined fertilization and thinning treatments in older, unmanaged stands with questionable structure is recommended for operational trials only until their responsiveness to treatment is documented. Fertilization should probably be delayed 3–5 years after spacing or commercial thinning in these stands in order to allow the remaining crop trees to adjust to the new environment and develop sufficient new foliage to utilize the added nutrients.
Because of the unfavorable structure of many older stands in the B.C. interior, the highest fertilization priority is generally assigned to 15- to 40-year-old stands. As with coastal Douglas-fir, stands 15–20 years old should not be fertilized unless trees are at least 2 m taller than competing vegetation. Very young spaced stands or plantations (under 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.
Priority for 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. Fertilization response data for lodgepole pine indicate that relative and absolute volume responses are generally larger for stands fertilized at the time of spacing compared to fertilization 2–3 years after spacing. However, fertilization should be delayed if one or more of the following factors exist:
a. The live crown of the remaining crop trees is of insufficient size (e.g., <30%) to utilize the added nutrients.
b. The height/diameter breast height (dbh) ratio of remaining crop trees is large enough to put trees at risk of toppling and breakage. Fertilized trees produce substantially more foliage the year following treatment, and the larger crowns increase the susceptibility of snow and wind damage. For Douglas-fir, it is recommended that the height/dbh ratio should be less than 85. No conifer stands with a height/dbh ratio greater than 100 should be fertilized. The same criteria apply for spaced lodgepole pine. Lodgepole pine stands with pre-spacing densities >10 000 have been shown to be especially susceptible to snow press where spacing and fertilization have been undertaken simultaneously. In these stands, risks can be substantially reduced by delaying fertilization for 2–3 years after spacing.
c. Fertilization should be delayed 1–3 years in situations where there is a heavy cover of fine thinning slash. Decomposition of slash can act as a “green manure,” thereby providing a short-term increase in nutrient availability. A heavy cover of slash can also prevent fertilizer prills from reaching the soil, thus increasing risks of N volatilization losses.
d. In older age class 3 and 4 stands, fertilization should be delayed for 3–5 years after spacing or commercial thinning unless pre-treatment densities were below 4000 stems per hectare and there is no reason to anticipate any wind throw losses.
Fertilizing too soon after spacing may increase the risks of wind or snow damage. Delay fertilizing for two or more years after spacing when: