> Fertilization
Stand Selection
Guidelines
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)
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Age
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Priority
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40 - 79
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1
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15 - 39
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2
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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 supplement
The 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:
- The live crown of the remaining crop trees is of insufficient
size (e.g., <30%) to utilize the added nutrients.
- 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.
- 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.
- 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.
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.
