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25
NUTRITION OF DOUGLAS-FIR
M. A. Radwan and H. Brix
Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) is a major
timber species in the Pacific coastal forests of Oregon, Wash­
ington, and British Columbia. Success in the intensive man­
agement of this timber type requires thorough knowledge of
the species ecology and physiology, including tree nutrition.
This paper summarizes the basic nutritional information
now available for Douglas-fir, with emphasis on recent find­
ings and gaps in present knowledge. The paper is not an all­
inclusive literature review on the subject; rather it includes se­
lected references to document the concepts presented. Some
information on species other than Douglas-fir is also included
whenever it is helpful in understanding Douglas-fir nutrition.
Additional aspects related to nutrition are discussed by others
elsewhere in this volume.
THE ESSENTIAL NUTRIENT ELEMENTS
Higher plants, including Douglas-fir, contain many chemi­
cal elements in their tissues; these elements include arsenic,
mercury, and silver. The mere presence of an element in the
plant, however, does not constitute an essential requirement of
the element. It is now generally agreed that only certain ele­
ments are essential for the normal growth and development of
the higher green plants. Some of these elements are required in
relatively large amounts. These macronutrients include hydro­
gen, oxygen, and carbon, obtained by plants from water and
atmospheric gases, as well as nitrogen, phosphorus, po­
tassium, calcium, magnesium, and sulfur, which enter the
plant primarily from the soil. Other essential elements are re­
quired only in trace amounts; these micronutrients include
iron, manganese, zinc, copper, boron, molybdenum, and chlo­
rine.
NUTRIENT REQUIREMENTS
OF DOUGLAS-FIR
Proof of the essentiality of nutrient elements comes mainly
from work with herbaceous test plants. As for many economi­
cally important crop plants, work on the nutritional require­
ments of Douglas-fir and other northwestern conifers has been
very limited (Gessel et al. 1950, Walker et al. 1955). It has
been generally assumed that Douglas-fir requires the same es­
sential elements as other higher green plants for normal growth
and development. This may not be true. Gerloff (1963) stated
that "the mineral nutrient requirements of all plants are not the
same. This is true both qualitatively and quantitatively." The
forest ecosystem where Douglas-fir grows is vastly different
from other environments. Also, under some conditions, certain
higher plants are known to require elements such as sodium
(Brownell 1965) or cobalt (Ahmed and Evans 1961) in addi­
tion to the essential nutrients. Under other conditions, some
higher plants may not require one or more of the essential ele­
ments, or may have the ability to substitute other elements for
some of the essential nutrients (Epstein 1972).
Like other higher plants (Gerloff 1963), the rriinimum
amounts of the different elements required for Douglas-fir are
not absolute, but depend on the relative amounts of the other
elements available. And the nutrient requirements of the spe­
cies probably change with age and during flower and seed pro­
duction. Cole et al. (1967) estimated the total requirements per
year of some macronutrients for 36-year-old Douglas-fir as fol­
lows: nitrogen, 38.8 kg/ha; phosphorus, 7.2; potassium, 29.4;
and calcium, 24.4. Part of such requirements is satisfied by
internal redistribution (Sollins et al. 1980).
Analysis of leaves (or other plant parts) of many crops has
long been used to establish nutrient concentrations associated
with optimum growth. Work with Douglas-fir in this regard
has been very limited. Van den Driessche (1979) reviewed the
literature and summarized "adequate" levels of nutrients in
current year's needles of Douglas-fii: as follows: nitrogen,
1.8%; phosphorus, 0.22%; potassium, 0.80%; calcium,
0.18%. By compar­
0.20%; magnesium, 0.12%, and sulfur
ison, Epstein (1972) considered the following nutrient concen­
trations in dry matter adequate for plant growth in general: ni­
trogen, 1.5%; phosphorus, 0.2%; potassium, 1.0%; calcium,
0.5%; magnesium, 0.2%; and sulfur, 0.1%. Furthermore, op­
timum foliar concentrations of nutrients may change with stand
age (Miller et al. 1981). Ratios of mineral element concentra­
tions may also be useful in interpreting nutritional status and
expected response to fertilization as discussed by van den Dri­
=
177
essche (1979). For instance, the nitrogen to sulfur (N :S) ratio
may indicate whether or not nitrogen fertilization is likely to
induce sulfur deficiency or produce a growth response to nitro­
gen (Turner et al. 1977). A similar but more refined approach.
has been taken by Ingestad (1979). Using sand cultures, he es­
tablished optimum macronutrient weight proportions for seed­
lings of several tree species, and thereby provided a basis for
evaluating the nutritional status and requirements of seedlings.
SOURCES AND FORMS OF NUTRIENTS
Like other terrestrial plants, Douglas-fir obtains its nutrients
from the soil and the atmosphere. The soil system, with its
mineral and forest floor components, is the main source of
most nutrients. Native soil nutrients are sometimes supple­
mented with other introduced nutrients, such as the nitrogen
added in the form of synthetic fertilizer, or through biological
Nrfixation.
Release of nutrients by weathering of rocks and minerals
provides the main input of cations. As expected, this varies
greatly by site. Absolute rates are difficult to establish, and es­
timates are by indirect methods, as in nutrient cycling studies
(Clayton 1979). Some rates per year, summarized from vari­
ous studies by Zasoski (1979), are: calcium, 8 to 24 kg/ha; po­
tassium, 4 to 15; and magnesium, 8.
Fallen litter in Douglas-fir stands returns to the soil nutrients
previously absorbed by the trees. The composition and amount
of litterfall varies considerably by such factors as thinning in­
tensity, time of the year, and climatic conditions (Reukema
1964). Similarly, varying amounts of nutrients are released an­
nually from litterfall in different stands. In one Douglas-fir
stand, Gessel and Turner (1974) estimated annual nutrient re­
turns by litter as follows: nitrogen, 19.2 kg/ha; phosphorus,
0.05; potassium, 12.0; calcium, 47.4; and magnesium, 5.4.
Nitrogen input from biological fixation is symbiotic by Rhi­
zobium- and actinomycete-nodulated plants or nonsymbiotic
by blue-green algae and free-living bacteria. In the Pacific
Northwest, the main actinorhizal species are alder (Alnus spp.)
and snowbrush (Ceanothus velutinus Doug!.). Stands of red
alder (Alnus rubra Bong.) have been estimated to fix from 27
kg N/ha per year (DeBell et a!. 1983) to more than 300 kg N/ha
(Newton et al. 1968). Similarly, nitrogen accretion by
snowbrush varied from 20 kg/ha per year (Zavitkovski and
Newton 1968) to 100 kg/ha or more (Youngberg and Wollum
1976, Binkley et al. 1982).
Amounts of nitrogen fixed by free-living soil microorga­
nisms are not well known. They are considered to be low--O. l
to 0.2 kg N/ha per year (Miller et a!. 1976)-although rates up
to 5 kg N/ha year have been reported (Davey and Wollum
1979). The high biomass of lichens in some forest stands may
also add a significant amount of nitrogen (Pike et al. 1970,
178
Radwan and Brix
1972). Furthermore, N2-fixing bacteria found on the leaf sur­
faces of Douglas-fir (Jones 1970) could provide 5 to 12 kg NI
ha per year (Davey and Wollum 1979).
The atmosphere provides Douglas-fir with the carbon and
oxygen required for photosynthesis and respiration. Water in
the surrounding air also contributes to the water balance of the
trees, especially during dry periods. In contrast, atmospheric
inputs of mineral nutrients are much more limited in the Pacific
Northwest, where the air is mostly clean of industrial contami­
nants. Nutrients from the atmosphere may include particulate,
gaseous, and dissolved chemicals. For a Douglas-fir forest in
western Oregon, Fredriksen (1972) estimated that nutrient in­
puts per year via precipitation were: nitrogen, 0.90 to 1.08 kgl
ha; phosphorus, 0.27; potassium, 0.11 to 0.27; calcium, 2.33
to 7.65; and magnesium, 0.72 to 1.32.
The form of an element in an introduced chemical may af­
fect nutrition, chemical composition, and growth of plants. For
example, plant species have long been known to differ in their
ability to absorb and assimilate different forms of nitrogen
(Nightingale 1937). Work with Douglas-fir shows conflicting
results. Some studies indicated that ammonium produced more
growth (van den Driessche 1971), whereas others concluded
that p.itrate was better for growth (Radwan et al. 1971, Krajina
et al. 1973) and for seed cone production (Ebell and McMullan
1970), and still others showed no difference between the two
sources (van den Driessche and Webber 1975). Douglas-fir,
like other forest tree species (e.g. Radwan et al. 1984a), appar­
ently is capable of absorbing and assimilating both ammonium
and nitrate, and differences in growth, if any, probably depend
on prevailing environmental conditions (van den Driessche
1978).
ABSORPTION, TRANSLOCATION, AND
UTILIZATION OF MINERAL ELEMENTS
Absorption and Translocation
Mineral elements are absorbed mostly in the form of ions
from the soil solution and directly from soil colloids. Root ex­
tension and movement of ions toward the roots by diffusion
and mass flow bring ions in contact with root surfaces.
Absorption occurs mostly through the region just behind the
root tips, although some entry also occurs several centimeters
away. In addition, the surface area of the roots in contact with
the soil is greatly increased, and the absorption capacity of the
roots is greatly enhanced through the association of the roots
with different types of mycorrhizal fungi.
From the root surface, elements move across the epidermal
and cortical cells, mostly in the cell walls, by diffusion or mass
flow, and at the endodermis they enter the symplast by active
transport. Elements then enter into the xylem sap.
Once in the root xylem, nutrients are carried to the shoots by
mass flow in the transpiration stream. As they move upward,
some nutrients move laterally into the phloem, where they are
used or translocated to the roots. Nutrients reaching the leaves,
such as nitrogen, phosphorus, and potassium, may be mobile,
moving from older to younger leaves and out of senescing
leaves. Other minerals, such as calcium and iron, are relatively
immobile; they tend to accumulate as the leaves age.
There are no definite, detailed studies of the pattern of nu­
trient uptake by Douglas-fir throughout the year. It can be
safely assumed, however, that most of the uptake occurs dur­
ing the fall and late winter, and in the spring. During this pe­
riod, there is much root extension, while soil moisture and
temperature are favorable for ion movement.
Quantities Absorbed
.
Substantial amounts of nutrients are absorbed by Douglas-fir
in a rotation. The amounts absorbed annually vary by many
factors, including the nutrient element, the site, and tree age.
For example, Cole and Bledsoe (1976) reported that uptake of
nitrogen, potassium, and calcium by Douglas-fir changed sig­
nificantly with stand age, reaching a maximum at or shortly
after crown closure. Sollins et al. (1980) estimated the nu­
trients absorbed per year by the aboveground biomass of an
old-growth Douglas-fir stand in Oregon as follows: nitrogen,
42.0 kg/ha; phosphorus, 9.1; potassium, 26.9; and calcium,
44.2.
Nutrient Interactions and Utilization
sense, therefore, Douglas-fir is a relatively efficient user of na­
tive nutrients. As with other forest tree species, however, re­
covery of nutrients from applied fertilizers by Douglas-fir
stands is thought to be well below recoveries reported for agri­
cultural crops (Allison 1966). Estimates of such recoveries are
not available, but some calculations for nitrogen are possible
from studies designed for other purposes. For example, short­
term recovery of nitrogen from urea fertilizer in a 45-year-old
stand of Douglas-fir is calculated at less than 6% from data by
Turner (1977). Also, data by Dangerfield and Brix (1979) indi­
cate nitrogen recoveries by Douglas-fir foliage of 9% from
urea and 19% from ammonium nitrate. In contrast, one test
with single saplings over a two-year period showed that the
total tree recovered 25 to 36% of the nitrogen applied as urea
(Heilman et al. 1982). Other recovery data available from
work with seedlings indicate variable recoveries as follows: ni­
trogen, 2 to 77%; phosphorus, 0. 7 to 4%; calcium, 0 to 28%;
and sulfur, 0 to 84% (Radwan and Shumway 1985). Clearly,
there is much need for accurate estimates of nutrient recoveries
by Douglas-fir under field conditions. In such studies, use of
fertilizers containing stable isotopes of the different nutrients
would be especially helpful. In addition, future work must in­
clude careful assessment of practices that might increase
utilization of added nutrients.
NUTRIENT DISTRIBUTION IN THE TREES
Nutrient elements interact in many different and sometimes
complex ways before and after absorption (Hewitt 1963, Ep­
stein 1972). In Douglas-fir, application of nitrogen fertilizer
depressed foliar phosphorus concentration (e.g. Radwan et al.
1984b, Radwan and Shumway 1984) and depleted foliar sul­
fate (Humphreys et al. 1975, Radwan and Kraft, unpublished
data on file at the Forestry Sciences Laboratory, Olympia,
Washington). Also, application of urea to Douglas-fir seed­
lings grown in a low phosphorus soil induced phosphorus defi­
ciency, and some fertilizer mixtures reduced growth response
(Radwan and Shumway 1985). Nutrient interactions, there­
fore, are important and should be considered, especially when
nutrient amendments are planned. Results of fertilization trials
with combination of several compounds are often question­
able, since they may be influenced by undesirable interactions.
Absorption of nutrients is followed by their incorporation,
with products of photosynthesis, into complex compounds that
form the structures and organs of the trees. Some ions, how­
ever, remain in the ionic form indefinitely, whereas others may
be released from various compounds and redistributed within
the plant.
Nutrients absorbed by Douglas-fir are distributed among all
parts of the tree. Concentration and content in the different tree
parts vary by nutrient, and levels of each nutrient depend on
many factors, including soil and site, tree age, tree part, and
season of the year. Table 1 shows nutrient distribution in 20­
year-old trees (Webber 1977) and nutrient concentrations and
contents in 3-year-old seedlings (Radwan and Shumway
1984).
As expected, these data show that the highest levels of the
different nutrients occur in the foliage (and shoots), and nitro­
gen is the principal foliar nutrient. Other data in the literature
indicate that concentrations of nutrients in Douglas-fir foliage
vary with the nutrient, leaf age, season of the year, and posi­
tion in the crown (Lavender and Carmichael 1966, Morrison
1974). Genetic differences in foliar nutrient concentrations
have also been reported between Douglas-fir provenances (van
den Driessche 1973) and among open-pollinated families
(Radwan et al. 1984b). Clearly, such genetic differences may
influence the demand on native soil nutrients, and in turn may
affect site productivity and the need for fertilizer supplements.
Efficiency of Nutrient Utilization
MINERAL DEFICIENCIES
and Recovery of Nutrients
In general, forest trees, including Douglas-fir, can grow on
relatively infertile soils not suited for agricultural crops. In a
Deficiencies of mineral elements occur when the supplying
capacity of the site is insufficient to meet the requirement of
Nutrition
179
TABLE 1.
Nutrient distribution in 20-year-old Douglas-fir trees and nutrient concentrations and contents in
3-year-old seedlings.
Tree Component
20-year-old trees
Foliage
Branches
Wood
Bark
Total
3-year-old seedlings
Shoots
Roots
Phosphorus
Potassium
101
16
37
25
7
Kilograms/hectare
52
23
Nitrogen
23
186
1.23
0.78
Calcium
79
67
Magnesium
Sulfur
10
7
3
5
31
5
23
15
34
3
3
103
195
23 0.10
0.09
Percent
0.61
0.42
0.26
0.20
0.13
O.ll
0.14
0.11
21
20
41
28
17
45
Milligrams/seedling
Shoots
Roots
Total
245
122
367
21
15
In
36
187
the plant for normal growth and development. Foliar chlorosis
has often been observed in individual trees and in some stands
of Douglas-fir. Such chlorosis has been attributed mostly to
nitrogen deficiency, although deficiencies in other elements,
such as calcium, sulfur, and iron, produce general chlorosis
too. Visual symptoms associated with some other deficiencies
have also been described for Douglas-fir (Oldenkamp and
Smilde 1966, Carter et al. 1984, van den Driessche 1984); they
resemble those of other plants (Epstein 1972),
In addition to visual symptoms, mineral deficiencies can be
diagnosed by soil analysis, soil bioassays, fertilization field tri­
als, and foliage (or other tissue) (van den Driessche 1979)
analysis. Only foliage analysis is discussed here, since the
other methods are covered elsewhere in. this volume.
Foliage analysis is performed and results are compared with
established "critical" values for the various elements (or vari­
ous forms of elements, such as soluble nitrogen and sulfate);
an element is considered deficient when its level falls below
the critical value. The general relationship of growth to defi­
cient, critical, optimum, luxury, and toxic foliar levels is
shown in Figure 1.
There is general agreement that a foliar concentration of
about 1.5% is the critical value for nitrogen. Critical values for
other nutrients in Douglas-fir have not been established. Van
den Driessche (1979) suggested that foliar concentrations of
0.14% phosphorus and 0.40% potassium may be "very low"
for Douglas-fir.
The most known and acknowledged deficiency in Douglas­
fir is that of nitrogen. Deficiencies in other nutrients, such as
boron (Carter et al. 1984) and phosphorus, are thought to occur
in some areas along the Pacific coast where soils are low in
these elements. It is also possible that increased growth, now
obtained with nitrogen fertilization, may induce additional nu­
trient deficiencies in the future.
180
Radwan and Brix
52
32
84
65
MINERAL TOXICITIES
In addition to their beneficial effects, all nutrients, native or
introduced, can be toxic to Douglas-fir. Among toxicities to
Douglas-fir attributed to native nutrients, manganese toxicity
is probably the most often suspected. Recent experiments,
however, suggest that manganese toxicity is highly unlikely to
be of much importance to Douglas-fir, because of the natural
high tolerance of the species and the high capacity of the soil to
fix the element (Radwan et al. 1979).
Douglas-fir appears to be quite tolerant also to many intro­
duced nutrients, as is evidenced by tolerance of the species to
application of sewage sludge, which is known for its high con­
tent of nitrogen, phosphorus, calcium, copper, and zinc (Bled­
soe and Zasoski 1981). Still, some toxicities are possible. Ex­
amples of such toxicities are those due to boron when borax is
applied as a fire retardant, and to nitrogen when urea fertilizer
Luxury
. Critical
.c
....
J
o
...
"
Foliar nutrient concentration
Figure 1. Relationship of growth to levels of foliar nutrients.
induces phosphorus deficiency in Douglas-fir grown in low­
phosphorus soil (Radwan and Shumway 1984). In a recent
greenhouse experiment, boron toxicity was observed only
when borax additions to Douglas-fir seedlings exceeded 100
kg/ha (Radwan and Johnson, unpublished data on file at the
Forestry Sciences Laboratory, Olympia, Washington).
PHYSIOLOGY OF GROWTH RESPONSE
TO ADDED NUTRIENTS
The physiological mechanisms by which nutrients affect
Douglas-fir growth have been the subject of only a few studies
and only in relation to nitrogen. Rate of application and source
of nitrogen fertilizer were shown to influence nitrogen metabo­
lism (Ebell and McMullan 1970). The total free amino acid
pool was increased by fertilization and more so with nitrate-N
than with ammonium-N. Most of the ammonium-N was incor­
porated as protein, but nitrate fertilizer resulted in a large accu­
mulation of basic amino acids. Nitrogen fertilization was re­
ported to increase rates of photosynthesis and respiration (Brix
1971, 1972, 1981a). Photosynthesis increased by about 22%
with an increase in foliar nitrogen concentration from 1.0 to
l .7%, and the response was greatest under high light intensity
(Brix 1981a). This was partly explained by an increase in chlo­
rophyll concentration (Brix 1971). Production of foliage was
also enhanced (Brix 1981b), and this accounted for most of the
growth response to nitrogen fertilization; the remainder of the
response was attributed to a photosynthetically more efficient
foliage (Brix 1983).
EFFECTS OF MANAGEMENT PRACTICES
As with other forest tree species (Tamm 1979), the nutrient
cycle in an undisturbed Douglas-fir ecosystem would normally
proceed with little loss of nutrients. Stand management prac­
tices, such as harvesting, site preparation, and fertilization,
however, can greatly influence losses and the normal cycling
of nutrients within the ecosystem. Amounts, availability, and
utilization of nutrients are therefore affected, and effects will
vary with a variety of factors, such as kind and intensity of the
management practice, time since application, and the ecosys­
tem involved.
Our knowledge of nutrient cycling and management effects
has been greatly enhanced since the inception of the Interna­
tional Biological Program nearly two decades ago. Several
symposia and conferences have included these subjects as one
of their major topics (e.g. , Leaf 1979, Ballard and Gessel
1983). Only a brief overview, therefore, will be given here.
Fertilization
Application of synthetic fertilizer adds substantially to the
available nutrient capital of the ecosystem. The effects of the
added fertilizers extend beyond relieving the deficiency of
some nutrient element(s). For example, work with Douglas-fir
and other coniferous species indicates that fertilization affects
leaching and immobilization of nutrients, soil microbial ac­
tivity and decomposition of organic matter, mineralization,
availability and uptake of nutrients other than those contained
in the fertilizer, and soil pH (e. g., Fowells and Krauss 1959,
Cole and Gessel 1965, Mahendrappa 1978, Johnson 1979,
Otchere-Boateng and Ballard 1978, 1981, Radwan et al.
1984b, Radwan and Shumway 1985). Other aspects of fer­
tilization are discussed in another paper in this volume.
The most notable losses of nutrients occur in removal of
logged-off material, leaching, soil erosion and other forms of
soil displacement, volatilization in burning, and denitrifica­
tion. These losses are caused by harvesting and site preparation
activities.
Harvesting
An important effect of harvesting on site productivity is the
direct removal of nutrients with the crop. Indirect effects in­
clude losses by increased leaching, erosion and runoff, and
those related to road building and yarding (Fredriksen et al.
1975, Cromack et al. 1978).
The greater demand for wood as a source of fiber and energy
has brought about an increase in harvest intensity and conse­
quently a greater removal of nutrients from the forest. For in­
stance, harvesting of whole trees has increased greatly in some
regions of North America in the last decade (Phillips and van
Lear 1984). This could have serious effects on site productivity
on infertile soils and with short rotations (Boyle et al. 1973,
Kimmins 1977). A symposium, "Impact of Intensive Harvest­
ing on Forest Nutrient Cycling" (Leaf 1979), addressed this
problem, as did several papers in the IUFRO symposium on
forest site and continuous productivity (Ballard and Gessel
1983). Data on tree biomass and element content in compo­
nents of different trees and ecosystems are now available for
many species on different sites and of different ages. This
material gives a basis for estimating nutrient removal with dif­
ferent harvest intensities (Morrison and Foster 1979). The im­
pact of this removal on future productivity is more difficult to
assess without better knowledge than is available on nutrient
inputs, losses, and immobilization during a rotation and of
methods of evaluating availability of soil nutrients (Johnson
1983, Kimmins 1977).
Data by Turner (1981) for a 95-year-old Doug" s-fir stand
show that about twice as much nitrogen, phosphorus, po­
tassium, calcium, and magnesium would be removed if
branches and foliage 'were included with the stems in the har­
vest. The total content of these elements in the aboveground
N utrition
181
portion of the trees was 445, 80, 254, 433, and 58 kg/ha, re­
spectively. For a Douglas-fir and western larch forest in the
northern Rocky Mountains, conventional harvest was esti­
mated to remove I % of the total ecosystem cations, 6% of ni­
trogen, 4.5% of phos'phorus, and 1 % of potassium, whereas
whole-tree harvest would remove 9% of total cations, 40% of
nitrogen, 50% of phosphorus, and 15% of potassium (Stark
1982).
Johnson et al. (1982) summarized information from seven­
teen ecosystems in Washington and Oregon, mainly for Doug­
las-fir of different ages; they reported on effects of bole,
whole-tree, and vegetation removal on nitrogen losses in rela­
tion to total nitrogen capital of the ecosystems. Only a small
amount of nitrogen (2 to 7.5% or 176 to 276 kg N/ha) was
removed with the bole, but generally twice as much and up to
12% (442 kg N/ha) when the whole tree was harvested. Al­
though whole-tree harvesting is not utilized in the Pacific
Northwest, whole-tree yarding is practiced to some extent with
branches and tops being cut at a central landing. The impact of
such practice is probably similar to that of whole-tree harvest­
ing.
The effect of clearcutting on leaching and erosion losses at
the H. J. Andrews watershed 10 in the Cascade Range in Ore­
gon has been investigated by Cromack et al. (1978). Forest,
stream, and erosion processes were studied intensively before
and after clearcut harvesting of a forest dominated by 450­
year-old Douglas-fir. No roads were built, yarding of trees was
accomplished with skyline cable systems, and slash was not
disturbed or burned. ·About 12% of the total nitrogen in the
system was removed by harvesting. L ss of dissolved nitrogen
in stream runoff amounted to 0.8 kg/ha the first year after
clearcutting, and export of particulate organic nitrogen was 2.0
kg/ha. After logging, 20% of the area was deeply compacted
and 34% was deeply disturbed. Erosion decreased during the
first three years after cutting. The authors estimated that less
than 20% of the total nitrogen capital may be lost by typical
harvest and slash burning practices in forests of the Pacific
Northwest.
The impact of forest management activities, including clear­
cutting, burning, and fertilization, on soil solution chemistry
and leaching has been studied by Cole and associates (1975).
They listed the factors by which clearcutting could affect the
chemistry of the soil solution and rates of ion leaching as fol­
lows: (1) absence of ion uptake by trees; (2) increase in surface
soil temperature and moisture content which could increase
soil microbial activity, organic matter decomposition, and car­
bon dioxide production; and (3) increase of water percolation
through the soil because of decreased evapotranspiration and
water interception by vegetation. All these factors could in­
crease leaching. The higher rate of organic matter decomposi­
tion and associated carbon dioxide production would increase
the level of bicarbonate ions and would provide a mobile anion
182
Radwan and Brix
for the dislocation of cations. In ecosystems where nitrification
rates are high, nitrate could constitute the mobile anion for ca­
tion leaching, as was the case in the Hubbard Brook watershed
(Likens et al. 1970). This also occurred after clearcutting a
hardwood but not a softwood stand in New Brunswick (Krause
1982); the author suggested that presence of nitrification inhi­
bitors might have caused low nitrification rates at the softwood
site. Other studies reported increased nitrate in stream water
after clearcutting, apparently as a result of increased nitrifica­
tion (Brown et al. 1973, Bateridge 1974). Overall, however,
leaching losses after clearcutting of Pacific Northwest forests
will probably be small and may not have a long-term detrimen­
tal effect on site productivity (Cole 1981).
Harvesting may affect soil microbiological activity and
therefore nutrient availability by processes such as mineraliza­
tion, denitrification, and N2-fixation (Jurgenson et al. 1977,
Swank and Waide 1979). This may result directly from re­
moval of carbon and nutrient supply, and iI?directly by chang­
ing soil physical and chemical properties such as water con­
tent, temperature, bulk density, pH, and levels of O2 and
carbon dioxide. The overall impact will depend on the soil,
climate, and harvest activity. Quantitative data, however, are
largely lacking, particularly on long-term effects and for
Douglas-fir sites. Based on the limited information available,
Jurgenson et al. (1977) concluded that no generally detrimental
influence on site quality can be attributed to harvest effect on
soil microflora. With more intensive harvesting and slash dis­
posal, however, it may be required as a safeguard that some
woody residue be left on certain sites, particularly those low in
nitrogen and where long periods of drought occur.
Site Preparation
Site preparation activities include slash burning, use of vari­
ous mechanical techniques, such as piling and windrowing,
and vegetation management with herbicides. During site prep­
aration, much loss (or displacement) of nutrients can occur,
but some gains are also possible depending on the site and the
type of practice and its intensity.
Slash Burning. Slash burning after logging is practiced on
many Douglas-fir sites in the Pacific Northwest. Many investi­
gations of the effect of slash burning on forest soils and their
nutrients have been made, and many views have been ad­
vanced about whether the practice is beneficial, harmful, or
without lasting effect (e.g., Isaac and Hopkins 1937, Tarrant
1956, Dyrness and Youngberg 1957, Neal et al. 1965, Knight
1966, Grier and Cole 1971, Kimmins and Feller 1976,
Kraemer and Hermann 1979, Feller 1982).
In general, slash burning results in many chemical changes,
such as an increase in soil pH and base saturation (Tarrant
1956; Neal et al. 1965, Baker 1966); "available" phosphorus
(Dyrness and Norum 1983); exchangeable potassium, calcium,
and magnesium (Austin and Baisinger 1955, Wells et al.
1979); and a"mmonium-N (up to six months after burning)
(Neal et al. 1965). After burning, gains in nitrogen can also
result from "invasion of burned areas by Nrfixing plants, such
as alder and snowbrush (Youngberg and Wollum 1976), and
through fixation by nonsymbiotic microorganisms (Jorgensen
and Wells 1971).
Nutrient losses from burning are mainly due to volatiliza­
tion, leaching, and erosion and runoff. Nutrients lost by vola­
tilization mostly include nitrogen (Knight 1966, DeBell and
Ralston 1970) and sulfur. Leaching processes and rates after
clearcut of a Douglas-fir forest and slash burning at two inten­
sities were investigated by Cole et al. (1975). For the first two
months after burning, leaching losses beyond the rooting zone
were increased 3.3 to 4.5 times. The A and B horizons, how­
ever, absorbed 70 to 90% of the cations leached from the ash
layer, and losses were small. On the other hand, 90% of the
nitrogen in the fuel was lost by volatilization. Clearly, the most
significant effect of burning was due to nitrogen volatilization
and not to leaching. In another study, Johnson et al. (1982)
compared losses from clearcutting and slash burning in a 42­
year-old Douglas-fir ecosystem. Clearcutting caused loss of
123 kg N/ha (3.8% of tota!), whereas loss by clearcutting and
burning was 299 kg N/ha (9.5% of total); a site with an intense
fire lost 27% of total nitrogen. The authors recommended that,
as a management tool, slash burning should be closely as­
sessed in regard to nitrogen losses and tree production.
Losses of nutrients by erosion and runoff can be increased
by burning, and microbial changes could affect subsequent
mineralization and N2-fixation rates (Ahlgren 1974). In the Pa­
cific Northwest, the strongly aggregated soil and quick inva­
sion by annual vegetation after burning can reduce the impact
of losses by erosion (Mersereau and Dyrness 1972, Wells et al.
1979). Runoff and associated nutrient removal, however, can
be high, particularly where fire has induced water repellency
of the soil surface (DeBano et al. 1976, Dyrness 1976) and has
reduced infiltration rates (Tarrant 1956).
In slash burning, substantial amounts of organic matter are
lost (Austin and Baisinger 1955, Dyrness and Youngberg
1957). Besides loss of nutrients, loss of organic matter will in­
fluence soil moisture holding capacity and availability of some
nutrients (Neal et al. 1965). These effects have not been suffi­
ciently studied in the Pacific Northwest. The percentage of
burned areas where fire has been severe enough to significantly
affect soil organic matter content, however, is likely to be
small (Dyrness and Youngberg 1957), particularly when pru­
dent utilization of burning is practiced (Wells et al. 1979).
Also, the impact will be small when soil and duff moisture is
high, and such a condition should be observed when sites are
burned where the organic layer is particularly important (Sand­
berg 1980, Green et al. 1984).
The effect of clearcutting and burning on ectomycorrhizal
fungi that may influence tree nutrition was reviewed by Perry
and Rose (1983). Several studies have shown reduction of ec­
tomycorrhizal formation on disturbed sites in the Pacific
Northwest, but the long-term impact is difficult to assess. Sim­
ilarly, burning can markedly' alter the makeup of soil microor­
ganisms (Neal et al. 1965, Perry and Rose 1983) and can affect
iron nutrition of and induce chlorosis in Douglas-fir (Perry and
Rose 1983, Radwan and Kraft, unpublished data on file at the
Forestry Sciences Laboratory, Olympia, Washington); but ef­
fects of such changes on long-term soil fertility are still un­
known.
Clearly, the literature shows that effects of burning are nu­
merous and variable; they depend on fire intensity and on char­
acteristics of the ecosystem, such as properties of soil and
slash. In a recent, long-term study in the Oregon Cascades,
Kraemer and Hermann (1979) concluded that broadcast burn­
ing did not have a lasting effect on physical and chemical prop­
erties of soil. Whether this applies to other sites in the Pacific
Northwest is not known. Indeed, knowledge of fire effects re­
mains very incomplete (Wells et al. 1979). In addition, infor­
mation on where and when slash burning is really needed is
still lacking .
Other Treatments. Mechanical site preparation procedures
include piling and windrowing of slash, as well as various
scarification treatments. These methods are used mainly (1) to
facilitate planting or burning and (2) to improve nutrient avail­
ability by providing better conditions for root growth, making
fertile soil layers m re easily accessible to roots of newly
planted seedlings, speeding up decomposition of organic mat­
ter, and reducing competition from weeds for moisture and nu­
trients.
Piling and windrowing of slash may have serious effects on
site nutrient conditions by displacing forest floor and top min­
erai soil along with their nutrients and by increasing loss of
nutrients resulting from an increase in fire intensity. The
equipment used in piling may also increase soil compaction.
This, in turn, may restrict root extension by young trees and
limit their ability to absorb nutrients needed for maximum
growth. Other aspects of the effects of soil compaction are dis­
cussed elsewhere in this volume.
Studies of other mechanical methods of site preparation in
the Pacific Northwest have not usually measured improve­
ments in nutrient availability directly; they have recorded ef­
fects on seedling survival and growth and, at best, reported
seedling nutrient concentrations. Also, these studies have not
generally included investigations of nutrient input and loss to
the site, such as by N2-fixation, leaching, runoff, and denitrifi­
cation, or effects on site nutrition beyond the establishment
phase.
Use of herbicide treatments for site preparation in the Pacific
Northwest is extensive. Aside from assessing treatment effica-
Nutrition
183
·cies, however, herbicide studies have not provided information
:)n nutritional aspects, and most were concerned with herbicide
persistence in the soil and pollution of streams (Newton 1977).
DISCUSSION AND CONCLUSIONS
The foregoing review shows the extent of and gaps in
knowledge of the nutrition of Douglas-fir and the conse­
quences of important management practices. The review also
affords us the opportunity to point out inadequacies in some
present nutritional concepts and methodologies, and to suggest
important research areas to increase forest productivity.
Basic nutritional information now available for Douglas-fir
is inadequate, and corrective measures must be taken if maxi­
mum production is to be achieved. First, we need to know
more about the nutritional requirements of the species at differ­
ent stages of development, and particularly with regard to ni­
trogen, sulfur, phosphorus, and the micronutrients. Second,
there is much to learn about nutrient interactions, especially
when synthetic fertilizers are applied. For example, why and
how nitrogen plus phosphorus can result in less growth re­
sponse than phosphorus alone. Third, the role of the forest
floor in supplying nutrients, especially nitrogen and phos­
phorus, to newly planted seedlings and later to established
trees needs to be recognized and emphasized. In this connec­
tion, we should find out whether Douglas-fir, as some suspect,
can utilize organic forms of some nutrients. Fourth, critical
values of nutrients other than nitrogen, as well as optimum nu­
. trient proportions (Ingestad 1979), need to be established. Not
only do we need to know these values in foliage, but we also
need to learn if other more accessible tissues are as good as or
better than needles in providing such information. Fifth, inter­
action of nutrients with environmental factors, such as water,
light, and temperature, at optimum and stress levels, need to .
be studied with regard to growth performance, nutrient re­
quirements, and critical nutrient values.
At present, work in nutrition of Douglas-fir in the Pacific
Northwest is very limited. Also, plant physiologists and
biochemists generally play a small role. Yet the literature con­
tains much plant and soil analytical data. Analyses are usually
done by commercial laboratories using whatever methods hap­
pen to be available, and data rarely include nutrients such as
sulfur and the micronutrients, which require difficult or expen­
sive methods. Clearly, such research is unlikely to solve prob­
lems or add meaningful information to our knowledge. In­
stead, more profitable areas of research should be explored.
These include: (1) determination of the best times and methods
for sampling plants and soils for analysis, (2). assessment of
novel diagnostic techniques to determine nutrient status of
plants and soils, (3) study of the mechanisms and controlling
factors responsible for different growth patterns, and (4) evalu­
184
Radwan and Brix
ation of methods to increase efficiency of nutrient utilization.
In nutritional analyses of plant tissues, we also suggest to in­
clude, as a minimum, all the mineral macronutrients, and to
report the nutdent content in addition to concentration when­
ever possible to avoid artifacts caused by dilution by growth.
There is no doubt that all management practices affect the
nutrient status, and thus productivity, of any Douglas-fir eco­
system. Harvesting and site preparation techniques can cause
the loss or displacement of nutrients, and arguments for and
against their use have been made by others. Whole-tree har­
vesting, one of the more potentially harmful practices, is not
used in the Pacific Northwest, although whole-tree yarding is
practiced to some extent. In addition, with prudent applica­
tions, most site preparation methods are likely to adversely af­
fect only small areas. To be on the safe side, however, it is
suggested that harvesting continue to be limited to the bole
only until nutritional impacts are further clarified, or unless the
forest manager has considered the economic consequences of
nutrient replacement. The authors also urge careful evaluation
of the necessity to use any site preparation treatment before it
is actually carried out, and doubt very much that all the slash
burning done is really needed. Today, no-till planting is widely
practiced in agriculture, and forest managers might well adopt
a similar strategy of minimum site preparation whenever possi­
ble. This will ensure leaving the litter layer and top soil in
place and help preserve site fertility.
Losses of nitrogen during harvesting or slash burning may
be detrimental to growth of Douglas-fir in succeeding rotations
(e.g., Bigger and Cole 1983) in spite of natural inputs from
precipitation and by N2-fixation. Application of synthetic ni- .
trogen fertilizer or use of Nrfixing plants (Haines and DeBell
1979) can correct such negative impacts. Alder is the most of­
ten mentioned N2-fixer; it is recommended either in mixture Qf
in rotation with Douglas-fir. In using alder, however, we must
recognize that the species has its own nutritional requirements
for growth and development, and that the trees accumulate
many nutrients in the wood to be harvested. Additions of nitro­
gen to the site by alder, therefore, would not be significant un­
less the site has sufficient amounts of nutrients other than nitro­
gen to meet the trees' own requirements for healthy growth. It
should be evident, therefore, that benefits to Douglas-fir from
alder may be limited to sites where the latter species grows
well naturally. In the authors' experience, such sites are those
that are sufficient in all nutrients except nitrogen.
To evaluate short- and long-term consequences of silvicul­
tural treatments, we must know their effects on nutrient cy­
cling and availability. Our knowledge of the complex biologi­
cal, physical, and chemical processes involved in nutrition is
scattered and meager. Simulation modeling, however, may
eventually assist us in using present and future knowledge to
the fullest (Soilins et al. 1983). Some models, now available,
incorporate effects of management alternatives such as rotation
length, harvest intensity, and fertilizer regimes on nutrient cy­
cling and availability, and consequences for tree growth. Nota­
ble examples of these models are FORCYTE (Feller et al.
1983, Kimmins and Scoullar 1983) and FORTNITE (Aber and
Melillo 1982). With more basic information and further de­
velopment, these or other models should be of great assistance
to forest managers. Similarly, there are now only very prelimi­
nary interpretations dealing with nutritional problems in rela­
tion to harvesting and site preparation techniques. When suffi­
cient knowledge concerning the impacts of these practices
becomes available, guidelines and land classification systems
. may be further developed to aid in management decisions.
During the past two decades, recognition of nutrition as an
important factor affecting forest production has significantly
increased. More recently, overviews concerning the effects of
forest management practices on site productivity, and percep­
tions of the means for nutrient management and conservation,
have been published (e.g., Stone 1979, Ballard 1980, Bengt­
son 1981, DeBell 1981). As with other tree species, prospects
for better management of Douglas-fir in the Pacific Northwest
hinge on the success of innovative research that biological sci­
entists will be able to do in the near future. The parameters of
the research objectives suggest a team approach by plant and
soil scientists to cover the different aspects of Douglas-fir nu­
trition. Such an approach would ensure more productive for­
ests in the decades ahead.
Bengtson, G. W. 1981. Nutrient conservation in forestry: A perspective.
Southern J. Appl. For. 5: 50-57.
Bigger, C. M . , and D. W. Cole. 198 3 . Effects of harvesting intensity on nu­
trient losses and future productivity in high and low productivity red alder
and Douglas-fir stands. In R. Ballard and S. P. Gessel (eds . ) IUFRO sym­
posium on forest site and continuous productivity, pp. 167-178 . USDA
For. Servo Gen. Tech. Rep. PNW-163. Pac. Northwest For. and Range
Exp. Stn . , Portland, Oregon.
Binkley, D. , K. Cromack , Jr. , and R. L. Fredriksen. 1982 . Nitrogen accre­
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Bledsoe, C. S . , and R. J. Zasoski. 1981. Seedling physiology of eight tree
species grown in sludge-amended soils. In C. S. Bledsoe (ed. ) Municipal
sludge application to Pacific Northwest forest lands, pp. 93-100. College
of Forest Resources, University of Washington, Seattle.
Boyle, J. R . , J. J. Phillips, and A. R. Ek. 1973. "Whole-tree" harvesting:
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Brix, H. 1971. Effects of nitrogen fertilization on photosynthesis and respira­
tion in Douglas-fir. For. Sci. 17:407-414.
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Brown, G. W., A. R. Gahler, and R. B . Marston. 1973. Nutrient losses after
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188
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