/
of
FERTILIZER VERSUS RED ALDER FOR ADDING NITROGEN TO
DOUGLAS-FIR FORESTS OF THE PACIFIC NORTffi EST
'his
2
l
Richard E. Miller and Marshall D. Murray
l
USDA Forest Serv ice
Pacif iC: N9rthwest Forest and Range Experiment Station
Olympia, WA 98502
2
Western Forestry Research Center, Weyerhaeuser Co.
Centralia, WA 98531
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ABSTRACT
ted,'
EOlTotrS
FILE COEl:
Seven possible silvicultural options for supplying additional nitrogen to
intensively managed Douglas-fir forests are described. Conventional fertilization
with repeated applications of urea is compared to several crop rotation and mixed­
stand options using red alder. Our preliminary analysis indicates that using
red alder in some options may be a practical silvicultural alternative to urea
fertilizer. Specific research studies are suggested to clarify or improve the
feasibility of using N -fixing plants to increase amounts of available N in
2
Northwest forests.
INTRODUCTION
Wood yields from forests of the Pacific Northwest can be increased with supple­
mental nitrogen. Field trials in Douglas-fir stands provide direct evidence that
224 kg N /ha applied as urea fertilizer can increase annual volume growth during
the following 4 years by 1.82 to 6. 37 m3 /ha (26 to 91 ft3 /acre) on sites I through
IV respectively ( Turnbull and Peterson 1976). Despite current costs of $104-$178
per hectare ($42-$70 per acre), forest fertilization can be profitable ( Atkinson
1977, Miller and Fight 1979). Because each fertilization with urea at the
conventional 224 kg/ha dosage also expends the equivalent of about 470 liter/ha
(50 gal / acre) of oil for fertilizer manufacture and application, fertilization
costs are closely tied to energy costs. Therefore future fertilization with
nitrogen will probably increase even more than the general level of inflation.
Nitrogen-fixing plants could supplement or replace nitrogen fertilizers in
some locations. Both biological and mechanical application of nitrogen will be
most effective on nitrogen-deficient sites and least effective where other factors
limit growth. Clearly, the silvicultural use of N -fixing plants will not be
2
as simple and certain as the aerial application of fertilizers, because the
benefits of nitrogen-fixing plants can only be achieved if these plants grow
vigorously and support active N Z- fixing bacteria on their roots. Moreover, this
fixed N will not be free; as we shall detail later, supplying biologically fixed
nitrogen will have direct and indirect costs.
In this paper, we shall first suggest some criteria for selecting NZ-fixing
plants for silviculture.
Second, we shall describe a conventional, intensive
management regime using commercial fertilizer as supplemental N. Third, we shall
then describe and compare six hypothetical regimes using red alder for enhancing
yields of Douglas-fir forests, these proposed regimes should apply to other
species combinations. Finally, we will suggest specific information that is
356
needed to clarify or improve the feasibility of using N -fixing plants to increase
2
amounts of available nitrogen in North .,est forests.
CHOICE OF N -FIXING SPECIES
2
We suggest the following cri'teria for selecting appropriate native or introduced N -fixing plants for Northwest silviculture: 2
1. Cheaply and easily established.
2.
Provides annually about 20 -50 kg/ha readily available N to supplement
mineralization in the soil and forest floor. Such nitrogen production
will require active bacteria in root nodules and vigorous plant growth to
provide sufficient carbohydrates as energy for these bacteria to reduce
dominant nitrogen. Therefore, the N2-fixing plants must retain a
dominant position in the stand or remain vigorous under the coniferous
canopy.
3. Extensively distributes fixed nitrogen throughout the stand. The area
of distribution will depend on the extent of the crown and roots and
total height of the plant.
4. Exerts minimum competition on coniferous crop trees for light, moisture,
nutrients, and space. For example, admixed red alder can be excessively
competitive unless it is managed. The rapid juvenile height growth of
alder that leads to early attainment of commercial size and dominant
crown position also ensures a good carbohydrate supply to symbiotic
organisms and wide distribution of the fixed nitrogen in the stand. Yet,
rapidly growing red alder can easily suppress nearby Douglas-fir or
mechanically damage their crowns. Thus equal-aged trees of these
species cannot occupy the same microsite without Douglas-fir being
suppressed (Newton et al.
1968).
5. Has additional value for wood products, wildlife, control of unwanted
vegetation, or improvement of the physical or chemical properties of
the soil.
Based on the preceding criteria, we think native red alder, Alnus rubra Bong., is
a prime candidate for mesic sites of western Oregon, Washington, and British
Columbia.
SILVICULTURAL OPTIONS FOR SUPPLYING ADDITIONAL N
We will describe seven hypothetical options for supplying additional nitrogen to
Douglas-fir forests grown on a 60 -year rotation. These options include:
- Conventional fertilization with urea (option 1).
- Alternating rotations of pure alder grown for 8, 13, and 32 years followed
by Douglas-fir grown for 60 years (options 2, 3, and 4). Such crop
rotation systems have been recently evaluated by Atkinson et al. (1979).
- Even-aged mixtures of red alder and Douglas-fir; one with natural and one
with planted alder (options 5 and 6).
357
TABLE 1.
Option
w
In
co
1.1
2:...1
1/
APPROXIMATE YEAR OF SPECIFIED ACTIVITIES IN SILVICULTURAL OPTIONS ON SITES I AND II
Species Site
prep.
Plant
Herbicide
spray
1
DF
1
1
2
RA
DF
1
l(seeded)
8
3
RA
DF
1
13
4
RA
DF
1
32
5
RA
DF
1
1
1
6
RA
DF
1
1
1
1
7
DF
RA
1
30
1
30
Release
cut
PCT1.1
Fert.
3
F" na1-1
harvest
30
60
5
10 8
13 18
38
68
1
13
18 23
43
13
73
1
32
37
42
11
62
32
92
5
5
10
10
30
30
50
60
5
5
10
10
30
30
40
60
10
30
60
60
5
20,30
40,50
Fi2 t
7
CT-
PCT is precommercial thinning (non-commercial thinning or stocking control).
CT is commercial thinning (income from merchantable material exceeds direct costs of harvesting).
Commercial thinnings of Douglas-fir are scheduled for age 30, 40, and 50 years.
Douglas-fir is assumed to be grown on a 60-year rotation in all options.
An uneven-aged understory of alder or other N -fixing vegetation in the
2
second half of the Douglas-fir rotation (option 7).
We estimated the timing of various activities in these seven regimes (Table 1).
The timing specified in this table is for highly productive land, sites I and II;
slower growth on lower quality lands will lengthen the rotation and period
between activities.
Conventional Fertilization
Option 1 is a conventional, intensive management regime. As many as 1,500 per
hectare of 2- to 3-year-old Douglas-fir are planted after site preparation.
Within 5 to 10 years, one or more herbicide sprays may be required to control
competing vegetation. Ironically, control of red alder is frequently the
objective of these sprays on highly productive sites where the quick establishment
and rapid initial height growth of this species usually necessitates early control
before or shortly after Douglas-fir crop trees are overtopped. A single spray
will seldom kill all alder. Some alder are undamaged; others produce new crowns,
but usually have poor, non-merchantable stems. Ordinarily, subsequent pre­
commercial thinning eliminates most remaining hardwoods and reduces conifer
density to about 1,000 trees per hectare; however, there is an increasing trend
to leave red alder that is not likely to overtop conifers. Urea fertilizer
providing about 225 kg/ha N is applied by helicopter. Timing and number of
fertilizations vary. Although early fertilization during the stage of rapid
height growth and high nutrient demand i piologically desirable, some managers
delay fertilization until late in the rotation to reduce interest charges on
this investment.
The commercial thinnings in this and other options provide early harvests of
merchantable, surplus trees which generate early income and increase the growing
space available to the remaining trees. Depending on site quality and management
objectives, final harvest of intensively managed Douglas-fir occurs at 35 to 100
or more years. We have used a 60-year rotation for Douglas-fir in this and the
remalnlng options, because this approaches the culmination of mean annual
increment in cubic feet (Reukema and Bruce 1977).
Crop Rotations
Options 2, 3, and 4 are hypothetical short rotations of pure alder followed by
a rotation of Douglas-fir. These and other pure stand regimes were proposed and
evaluated by Atkinson et a1. (1979); however, their Douglas-fir rotations were for
45 years instead of 60 years. In option 2, a very dense stand of alder created
by artificial seeding would supply nitrogen to the succeeding Douglas-fir crop.
This alder stand is subsequently killed with herbicide at age 8. Douglas-fir
seedlings are then planted among the dead alder. In options 3 and 4, 2-year­
old alder is planted and grown for pulpwood until age 13 (option 3) or for
pulpwood and saw10gs until age 30 (option 4).
Mixed Stands
Our two even-aged, mixed stand options would provide nitrogen by retaining a
small number of volunteer (option 5) or planted (option 6) red alder in Douglas­
fir plantations. In an earlier investigation, we estimated that 50 to 100
uniformly distributed red alder per hectare could meet the N requirements of
Douglas-fir plantations providing these alder were maintained in a dominant
359
position through most of the rotation (Miller and Murray 1978). Because of the
greater height attained by alder during the first 25 or more years (Newton et al.
1968), both species cannot occupy the same microsite. Therefore, each red alder
retained in the mixed-stand options, S and 6, would eventually displace at least
one Douglas-fir.
Under option 6, approximately 120 alder stems per hectare (9- x 9-m spacing)
are planted with 1,100 Douglas-fir (3- x 3-m spacing). To reduce losses of these
planted alder from competition and animal damage, 2-0 red alder that will be about
1.5 m tall should probably be planted. Tops of these seedlings might be reduced
by pruning to redUC t planting costs, however, without major concern for multiple
/
tops or stem crooks- .
To reduce crown Yidth of alder and improve its stem form,
planting a red alder between the rows of Douglas-fir seems more desirable and less
complicating than substituting the alder in the Douglas-fir planting pattern.
Although a uniform distribution of red alder throughout the Douglas-fir plantation
is desirable; planting alder beside unburned or incompletely burned slash piles
may provide additional benefits.
These slash piles reduce site occupancy by
Douglas-fir and frequently harbor mountain beaver, Aplodontia rufa, which
eliminate nearby Douglas-fir. Planting alder near these piles would increase the
probability of occupying these areas and provide additional space for crown
expansion of alder with less impact on nearby Douglas-fir. Finally, planting
the red alder could be purposely delayed a few years for several reasons:
(1) to give the Douglas-fir an initial age and height advantage, (2) to see if
volunteer alder would eliminate the need for planting and (3) to delay or
possibly eliminate the need for the scheduled release cutting at age 5.
In both options, excess alder are felled or poisoned in an early release
cutting at age 5 and in the precommercial thinning at age 10.
If alder
competition and overstocking were not severe, then these two cuttings could be
combined into a single operation. Stocking control leaves 100 well-distributed
red alder per hectare at an average spacing of 10 m and 1,000 Douglas-fir
spaced at 3. 2 m. Based on our measurement of 7-year-old red alder, maximum
crown diameter of these alder will average about 3 m, so some adjacent Douglas­
fir could still be affected by alder crowns. The alder are gradually removed in
commercial thinnings (Table 2). Prior to the last thinning at age 50, the 50
residual alder per hectare could have an average crown diameter of about 10 m
(Smith 1978) and therefore cover about 40 percent of the area. Although the
average spacing of the 260 residual Douglas-fir per hectare is 6. 3 m, the Douglas­
fir near these alder would either be taller than the alder or would have been
removed in earlier commercial thinnings.
Additional N could be provided by retaining volunteer alders that frequently form
pure, vigorous stands on skid roads and landings. Beside providing more N to
nearby Douglas-fir, pure stands of red alder would accelerate soil improvement
in these disturbed areas. These pure alder stands could be removed in the
commercial thinning when these former access areas are reused.
Understory N -fixers
2
In option 7, red alder is introduced on disturbed soil created by the first
commercial thinning and possibly by additional site preparation. Either l20/ha
2+0 alder seedlings are planted or volunteer red alder that frequentlY establish
l/
Personal communication with Dr. Paul Heilman, Research Soil Scientist,
Washington State University at Puyallup on March 5, 1979.
3 60
TABLE 2.
Tree
age
PROPOSED NUMBER OF TREES IN MIXED D2 GLAS-FIR/RED ALDER STANDS
Y
(OPTIONS 5 AND 6) ON SITES I AND 11/
Afte
treatment
Average spacing
Species
DF
Both
RA
DF
- m­
-----number/hectare------­
- years­
RA
10
PCT
1100
1000
100
3.2
10
30
CT
650
600
50
4.1
14
40
CT
400
350
50
5.3
14
50
CT
250
250
0
6.3
Proposed stocking levels of Douglas-fir based in part upon Reukema and
Bruce (1977).
PCT is precommercia1 thinning and CT is commercial thinning.
after commercial thinning could be used. Some of these red alder would survive
subsequent commercial thinnings at age 40 and 50 and then be felled at final
harvest. Even in a heavily thinned Douglas- fir 1 tand, however, it is doubtful
2 .
that any of these alder would produce sawtimberESTIMATED AMOUNTS OF ADDITIONAL N
We estimated the range in amount of extra N that might be supplied by each of
these options from reported gains of N in the soil and forest floor attributed to
red alder at numerous locations (Table 3). These are conservative gains because
they do not include the greater amounts of N in the vegetation of alder stands.
Estimated average annual net rates of N accumulation vary widely in the original
reports (T b1e 3). Some of these differences are probably due to inherent
imprecision of field estimates. Others undoubtedly reflect true differences in
fixation rates among these locations caused by plant factors (density, vigor, and
stage of development) and environmental factors (initial N content of the soil,
climate, and competing vegetation). The surprisingly high annual fixation rate
reported by Newton et a1. (1968) for the Coast Ranges of Oregon was estimated from
36 stands growing on landings or other scarified sites where all the surface soil
had been removed 1- 62 years previously. Despite this disturbance, the initial
amount of N at these locations was estimated at 2,350 kg/ha. From their investi­
gation, the authors postulated a linear trend of N accretion in the soil during
the first 20 years, after which N additions were apparently small. Subsequent
glasshouse experiments confirmed these high rates of N - fixation (Zavitkovski
2
and Newton 1968).
361
TABLE 3. ESTIMATED NET AMOUNTS F N SUPPLIED BY THE PROPOSED
l?
SILVICULTURAL OPTIONS-
Option
Alder
-vears­
J
References
20
900
8
63
300
500
2,400
Atkinson et al. (1979)
Newton et al. (1968)
3
13
52
300
670
3,900
Atkinson et al. (1979)
New on et al. (1968)
4
32
24
52
57
139
(300)
770
1,682
1,830
4,450
4/
6,00<T-
Franklin et al. (1968)
Atkinson et al. (1979)
Cole et al. (1978)
Tarrant et al. (1969)
Newton et al. (1968)
/
26'1.
/
40&-
1,300
2,000
Tarrant et al. (1969)
Tarrant and Miller (1963)
14
52
400
1,550
Berg and Doerksen (1975)
Berg and Doerksen (1975)
1
cl=-
2
5,6
50 7
1/
/
Amount N
3/
per year
total­
kg/ha
30 Urea applied at ages 20, 30, 40, 50
Based on reported net increases of N in forest floor and soil; does not include
amounts in vegetation or lost from site.
2) Assumes no N -fixing plants established during the rotation; however, S years
1/
/
i/
2
of volunteer alder at 63 kg N/ha per year could provide 315 kg N/ha.
Because of rounding and metric conversions, total may not correspond to simple
extension of yearly rates of fixation.
Based on 20 years of N -fixation at 300 kg/ha before rate decline to 0
2
(Newton et al. 1968).
Equals measured average rate during 40 years; this rate was extrapolated for
10 additional years.
Equals measured average rate during 30 years; this rate was extrapolated for
20 additional years.
3 62
Zavitkovski and Newton (1968) also estimated the sources of N additions in 2- to
14-year-old red alder stands as follows:
Source
Annual N Addition
kg/ha
Percent of total
Litterfall
100
33
Root system and
rhizosphere
185
58
35
9
320
100
Retained in stand
Total
/
Personal communication with Dr. Alan B. Berg, Professor of Silviculture,
Oregon State University. Corvallis on February 27. 1979.
These large annual additions of N in the litterfall from red alder stands were corroborated in 30-year-old stands in western Washington (Gessel and Turner 1974). They estimated 80 to 200 kg/ha per._year of N was deposited in litterfall; in contrast, comparative Douglas-fir stands returned 7 to 22 kg/ha per year.
Althoueh annual litter­
fall may account for only one-third the total N accretion, the amount of N in litterfall from pure alder stands is surprisingly high.
Since litterfall is readily measured, it could provide relatively reliable estimates of N contribution from alder. Although most options using N -fixing plants could provide more total N than the 2
fertilizer option, this may not apply to option 7 (assuming a light stocking of understory alder) and option 2 (8-year rotation of alder using a low estimate of N -fixation rate). An important unknown of all options is the proportion of the
2
supplemental N that is eventually available to conifers after losses and
immobilization. Real losses from the site (volatilization and leaching) could
reduce our estimated amount of N supplied by fertilization; however, such losses
are accounted for in these estimates of net accumulation of N in the soil and
forest floor of red alder stands.
Not all the N supplied by either fertilizer or N-fixing plants is immediately
available for conifer uptake; a portion is at least temporarily immobilized in
living and dead organic matter of the forest floor or soil. Studies using N
isotopes indicate that conifer uptake during one or two growing seasons after
fertilization ranged between 10 and 20 percent of the N applied; about 70 percent
of applied urea was in the soil and forest floor and presumed to be eventually
mineralized (Knowles 1975). Moreover, the rate of this mineralization and thus
N-turnover may be faster in fertilized than in unfertilized Douglas-fir stands
(Miller et al. 1976).
Practically all of the N supplied by red alder and other N-fixing plants is
initially associated with organic compounds. Although soluble nitrate and other
nitrogen compounds may be directly leached from this organic matter (Tukey 1966),
the bulk of the fixed nitrogen eventually becomes available through decomposition.
Because of their high N content and favorable C:N ratio, alder leaves, twigs,and
root residues decompose much more rapidly than coniferous plant residues (Bollen
et al. 1967). Presumably, a large proportion of the biologically applied N is
readily available to conifers.
363
Our attempts to contrast the likely benefits of fertilization, option 1, with the
N -fixation options exemplifies the controversy, inorganic versus organic
2
gardening. Although quantity and quality of soil organic matter were increased
and soil density was decreased under pure (Newton et al. 1968) and mixed stands
(Tarrant and Miller 1963) of red alder, the practical effects of these improvements
must be quantified.
This need to speculate emphasizes the need for additional information. In our
opinion, there is an immediate need for field trials that will provide direct,
quantitative evidence of the effects of red alder and other N -fixing plants.
2
Measuring rates of N -fixation in the laboratory and field provides useful, but
nonetheless, indirec and incomplete information that must eventually be verified
in field trials. For example, answering the question, "How much N is fixed per
year?" must be followed by the questions, "How much is available to conifers? and
What other positive and negative interaction occur between conifers and N-fixers?"
In our opinion, the direct evidence of long-term, empirical research is needed.
ESTIMATED COSTS OF THE SUGGESTED OPTIONS
The pragmatic forest manager will want to know the net economic benefits from our
seven silvicultural options. We wanted to oblige by providing an analysis of
present net worth, but believe that insufficient data exist to compute and compare
the profitability of these seven options. In particular, precise estimates are
needed of gains in cubic volume, by log size and quality for each option. From
such estimates of products and product values, one could subtract costs of
harvesting and then estimate the discounted net revenues. As an interim step,
however, we assessed these seven options on the basis of their estimated direct
costs, N supplied, indirect costs, and uncertainty about the outcome of each regime.
Initial Costs
The initial costs of various silvicultural activities in these options are
provided in Table 4? those costs that are related to managing red alder and
supplying N to Douglas-fir are also tabulated separately.
Discounted Costs
To compare the direct costs of these seven management regimes, we discounted the
initial costs of each activity back to the start of the rotation. Thus, an
activity that occurs late in the rotation costs less than if it occurs earlier
in the rotation. We used an annual discount rate of 7 percent. The 7-percent
rate corresponds to a real rate of interest n excess of inflation. The reSUlting
cost for each activity was then combined to obtain an estimated cost of each
option (Table 5). For example, the estimated direct cost of conventional option
1 is $758/hectare; this includes the total discounted cost of $73 for four
applications of urea at ages 20, 30, 40, and 50 years. Although the discounted
costs for supplying N through biological fixation range from $23 to $280 per
hectare, some or all of these costs will be offset by stumpage revenue from
harvesting red alder (except for option 2).
Dividing these discounted direct costs by the estimated smallest and largest
amounts of N supplied by each option (Table 3), provides a high and low estimate
of cost per kg N (Table 5). Thus conventional fertilization currently supplies
N at a discounted cost of 8 cents per kg. Depending on assumed amounts of N
supplied by biological fixation, corresponding costs range from 2 to 42 cents per kg.
364
Indirect Costs
As previously mentioned, utilization of the N -fixing red alder for pulp or
Z
sa .,timber will reduce the cost of N supplied ln options 3 through 7. The amount
of reduction, however, depends largely on the amount of higher valued Douglas-fir
that was foregone because alder instead of Douglas-fir was utilizing the site (pure
stand options) or microsites (mixed stand options 5 and 6). These indirect costs
of using red alder will be moderately high for option 2 in which no alder is
harvested and for option 3 in which low value alder pulpwood is produced. Although
we anticipate little merchantable alder volume from option 7, maintaining an
understory of red alder in the latter half of the Douglas-fir rotation, we also
anticipate no indirect costs because these trees are not likely to displace any
Doug1asrfir trees.
TABLE 4.
ESTI} TED INITIAL COSTS FOR SPECII ,ED ACTIVITIES
(1979 dollars, per hectare basis)­
Activity
Option
-No. ­
Prepare site, initially
after alder
, after thinning
Cost
$
Reference
All
3,4
7
309
198
99
2
49
}j
Plant 2+1 DF (1100/ha)
All
247
2:./
Plant 2+0 RA (1334/ha)
(840/ha)
(120/ha)
3
4
6,7
198
148
74
3/
3/
2/
49
]j
Seed alder
Control RA (herbicide)
1, 2, 3, 4, 7
2/
1/
2:./
Thin RA, release DF
5,6
185
2:./
Thin DF
All
185
1./
1
148
1./
Fertilizer
1/
]j
Costs that are underlined are for supplying N.
Source:
Data on file at Forestry Sciences Lab, Olympia, Wash.
Source:
Atkinson et a1. 1979.
365
98502.
TABLE 5.
Option
))
]j
1/
ESTIMATED COSTS AND UNCERTAINTY OF VARIOUS SILVICULTURAL OPTIONS FOR
SUPPLYING ADDITIONAL N, PER HECTARE BASIS
N-fixer
N supplied
1/
D'lscounted d'lrect costsAll
of N
per kg N
---------dol1ars-------­
2/
3/
,
Ind'lrect- Uncertalnty-
1
o
900
758
73
2
8
500-2,400
606
78
. 16-. 03
moderate
high
3
13
670-3,900
745
280
. 42-. 07
moderate
moderate
4
32
770-6,000
523
171
. 22-. 03
low
moderate
5
40
1,300-2,000
782
132
. 13-. 08
low
moderate
6
40
1,300-2,000
856
206
. 20-. 13
low
moderate
7
30
405-1,550
708
23
. 06-. 02
o
high
0. 08
o
low
Direct costs of all activities and those to provide nitrogen are discounted
at 7% real interest to present.
Based on delayed income from Douglas-fir (option 2) and lower stumpage
volumes for alder than for the alternative species, Douglas-fir (options 3, 4,
5, 6)
Based on lik1ihood of obtaining the projected additional N.
UNCERTAINTY
Conventional fertilization of Douglas-fir attracts land managers for several
reasons:
(1) a high degree of mechanization - nitrogen can be applied to the forest
at a sustained rate of about 200 hectares per day.
(2) low administrative costs - one forester using part of his time during
several months can lay out, contract, and administer a large fertilization
project. In contrast, supplying N by biological means would require
considerably more administrative effort during an extended period to
secure seed or seedlings, make silvicultural prescrip ons, supervise
planting, and monitor growth of the N -fixing p1ants.­
2
(3) low risk - contract fertilization has low risk; ei 7er the contractor
In contrast,
applies the N as specified or payment is withheld.biological options are much more susceptible to the performance of man
and nature.
366
Thus, more uncertainty is inherent to using red alder and other N -fixing plants
2
to provide supplemental N. Moreover, some biological options appear more uncertain
than others for supplying sufficient N (Table 5). For example, an 8-year rotation
of red alder, option 2, is a short period for fixation; moreover, attaining full
density from artificial seed i ng seems chancy. Option 7, understory red alder, is
also uncertain because such a light-demanding species may not survive with sufficient
number and vigor to supply adequate amounts of N. In these and other biological
options, however, supplemental fertilizers could be used.
RANKING THE OPTIONS
In the absence of reliable quantitative data, any ranking procedure or analysis
of these options ip necessarily based on assumptions about attendant costs and
returns. We all recognize that the accuracy and precision of such evaluations
are only as good as the data used. Having provided high-low estimates of
discounted direct costs and a non-numeric rating of indirect costs and uncertainty
for these silvicultural options (Table 5), we left the final ranking to the
reader.
Nonetheless, our preliminary analysis indicates that using red alder in
some silvicultural regimes may be a practical silvicultural alternative to using
fertilizer. Anticipated increases in future costs of fertilization emphasize
the need to test this preliminary conclusion with more reliable data.
NEEDED RESEARCH
Additional information is needed to clarify or improve the feasibility of using
red alder and other N -fixing plants to supplement or replace nitrogen fertilizers.
2
Although red alder is a promising species, especially because its commercial value
can reduce the costs of using it, red alder's natural range is limited to moist
and mesic sites. Therefore, other native and introduced N -fixing plants should
2
be screened. Additional research should provide information to quantify the costs
and benefits of various silvicultural regimes using such pl nts to improve growth
of associated conifers. Economic analysis should be used to judge the
efficiencies of various regimes and of specific activities in these regimes.
Because insufficient amounts of N will probably limit growth in practically all
intensively managed forests, this research will have much practical significance
to Northwest forestry.
In this secti on, we suggest specific research including long-term field trials
touted earlier in this paper. We also suggest other research to complement these
long-term efforts.
Long-term Field Trials
Alternating rotations of pure stands. --The economics of crop rotation could
be investigated in two simultaneous phases rather than successive phases. For
example, some existing Douglas-fir plantations have been established after a
rotation of natural red alder. Permanent plots should be established in such
plantations and growth compared with and without addition of nitragen fertilizer.
This comparison could indicate the adequacy and residual effect of the preced ing
11 Personal communication with William Voelker, Director Inventory, Analysis
and Planning, Oregon Forestry Department, Salem on February 12, 1979.
il
Ibid.
367
crop of alder for providing N. Ideally, the growth of these plantations could also
be compared to plantations on similar site conditions that had no alder crop
immediately preceding them. Concurrently, the costs and economic returns from
pUlpwood and sawlog rotations of red alder (DeBell et al. 1978) and the costs and
methods of converting the site to Douglas-fir could be examined in other
experimental areas.
Even-aged mixtures. --The number of red alder to retain in mixed stands is a critical decision for the silviculturist. Sufficient red alder are needed to eliminate nitrogen as a growth-limiting factor; yet each alder will replace at least one Douglas-fir that would probably provide a greater economic return. This question is similar to asking what optimum dosage of fertilizer should be
supplied in option 1. A field trial in numerous Douglas-fir stands at Ndeficient locations can provide the answer. Measurements on experimental plots
at each location would quantify the relationship between volume and value growth
per hectare and increasing numbers of equal-aged red alder retained in these
conifer stands. The objective of these trials is to locate the point of
diminishing returns. Although conifer density after initial stocking control on
all plots could be held constant at about 1,000 per hectare, we think alder density
should range to 200 per hectare using levels of 0 , 50, 100, and 200. Subsequent
commercial thinning would reduce conifer but not red alder density on these
experimental plots. A desirable addition to these trials would be to contrast
the effects of red alder with those of conventional fertilization (option 1) in
the same stand.
We have installed such a field trial on Weyerhaeuser Company's White River Tree
Farm near Enumclaw, Washington. We hypothesize that merchantable yields during 60­
year rotation at this location will show the following relations (Figure 1). As
a result of retaining increasing numbers of red alder, Douglas-fir yields will
initially increase then decline. At point B, either (1) the alder density is
sufficient to eliminate N as a growth-limiting factor or (2) too many Douglas-fir
were eliminated or suppressed by alder competition. Repeated additions of N
fertilizer will increase Douglas-fir yields.
For example, point C represents growth
of Douglas-fir receiving 224 kg/ha N; the response to fertilization is calculated
as growth at C less growth at A. Further, the number of alder necessary for an
equivalent response can be estimated from the curve.
The N content in the annual litterfall should also be measured in these stands, and
the contribution of alder litter to the total litterfall should be determined. We
anticipate that
(1) the litterfall from 100 to 200 dominant red alder per hectare will
annually contribute about 20 to 50 kg/ha N or about one-fourth that
contributed by fully stocked stands of alder (Zavitkovski and Newton
1979, Gessel and Turner 1974).
(2) the Douglas-fir litter from the mixed stands will have higher N
concentrations than that from pure stands and therefore will be more
readily decomposed.
The above procedures will quantify the relationship between growth per hectare
by Douglas-fir, red alder, and both species over a range of red alder admixture.
Similarly, the litter sampling will quantify an important component of nitrogen
addition over this same range of red alder. The more N-deficient the sites tested,
the more precisely the N response surface can be established.
3 68
-
Q)
co +-I (.) Q) .r::. Q) Co -
o
..J W
-
>-
c
A
w
:::>
..J
o
>
o
50
100
150
200
ADMIXED RED ALDER
(number per hectare)
Figure 1. Theoretical relations in merchantable yields during a 60-year-rotation
yield on a N-deficient site with increasing numbers of red alder
retained in a Douglas-fir plantation.
Uneven-aged interp1anting. --Introducing red alder into Douglas-fir stands
after the first commercial thinning appears econemica11y attractive because of
low direct and indirect costs. If sufficient N could be provided by N -fixers
2
under the canopy of thinned stands, this option could be feasible in both natural
stands and plantations.
Berg and Doerksen (1975) estimated N additions of 225 and 876 kg/ha by a 17-year­
old red alder stand that established naturally under a heavily thinned, 60-year­
old stand of Douglas-fir (Table 3). The very heavy thinning in this stand
removed about half the initial basal area, and the alder grew vigorously for about
12 years.
In contrast, alder failed to establish in an adjacent lightly thinned
stand. We have observed vigorous stands of red alder under moderately thinned
(about 30 percent of the initial basal area was removed) 40-year-old Douglas-fir
in western Washington. Although we did not measure N additions to the sites, we
can reasonably assume some N -fixation had occurred.
2
3 69
N -fixation rates are probably reduced under the canopy of conifers. For example,
2
Sprent and Silvester (1973) estimated that annual N -fixation by Lupinus
z
arboreus. in thinned stands of Pinus radiata was unllkely to exceed 40 kg / ha N or about one-quarter of that in the open. Shading reduced root development, nodule yields, and rate of N0-fixation. The preceding information suggests that a red alder understory could be
established under Douglas-fir after moderate-to-heavy thinning; however, the
vigor and duration of the alder is closely related to the amount of available
light. Moreover, N -fixation rates are probably reduced under the canopy of
2
conifers. Few understory red alder are likely to reach merchantable size
because of limited light and breakage during repeated commercial thinning
operations, therefore, initial screening is needed for other N2-fixing species and
endophytes that will survive and fix adequate nitrogen under a reasonable range
of canopy densities.
Suitable Species for Sites of Low Productivity
Volume response of Douglas-fir to nitrogen fertilizer is generally inversely
related to site quality, i. e., more response in cubic meters on land of lower
inherent productivity. Yet the adverse growing conditions on such sites may limit
the survival or N -fixing capacity of most plants. For example, water stress
2
reduced N -fixation by several legumes (1) by reducing photosynthesis and thus the
2
supply of carbohydrate to the root nodules and (2) by direct effects on nodule size
and activity ( Sprent 1976).
Field trials are needed to determine the growth and N -fixation capacity of
2
various plants on environmentally harsh, N-deficient sites.
The screening should
include native nitrogen fixers that are likely to be genetically adapted to local
conditions as well as introduced legumes like alfalfa (Medicago sativa L), birds­
foot deervetch (Lotus corniculatus L. ), and especially crownvetch (Coronilla
varia L) which formed well-stocked stands during a 9-year period after sowing on
three harsh sites in the western Cascade
Range of southwest Oregon (Miller and
.
Zalunardo 1979).
Response of Red Alder
We should know more about the growth and form of red alder grown at varying
densities; quantitative information is needed.
Stem form. --By increasing the stumpage revenue from red alder, we can reduce
the net costs of using this species to supply nitrogen. The economic value of
red alder is closely related to stem size, straightness, and branching. How much
can various silvicultural practices produce positive or negative changes in
quality of red alder? What is the relative effectiveness of artificially pruning
alder or maintaining nearby conifers to reduce branching and encourage straight
boles in red alder?
Stump sprouting. --We may want to minimize sprouting from alder stumps in some
regimes and encourage it in others. For example, if stump sprouts are effective
hosts for N -fixation, then retaining sprouts instead of more strongly competing
2
trees could be a feasible silvicultural option. What factors influence the
growth of alder sprouts?
370
CONCLUSIONS
Our preliminary comparison of seven options for supplying additional nitrogen
to intensively managed stands of Douglas-fir indicates that using red alder in some
regimes is probably si1vicu1tura11y and economically feasible.
Clearly, we
cannot be as certain about the results of substituting N -fixing plants for
2
commercial fertilizers. With conventional fertilization, treatment of thousands
of acres can be accomplished during a fe v weeks. In contrast, biological
fertilization has both the advantages and disadvantages of being a continuous
treatment depending on the survival and growth of plants during many years.
Despite the added uncertainty of these biological options for supplying N, there
are compelling reasons for further investigations of these alternatives. First,
quantitative information about the costs, benefits, and techniques of using N ­
2
fixing plants for nitrogen fertilizers. For example, the financial attractiveness
of using red alder will increase ( 1) if social concerns about herbicides increase
the costs of controlling red alder, (2) if the stumpage values of red alder
improves relative to those of Douglas-fir, or ( 3 ) if the cost of ferti1iz r and
application increase substantially more than the general rate of inflation.
Additional quantitative information is needed to clarify or improve the
feasibility of using N2-fixing plants to supplement or replace nitrogen fertilizers.
Although red alder is a financially attractive species, other native and cultivated
plants should be evaluated.
Long-term field trials are needed to provide input
into economic analyses of various regimes and specific activities in these regimes.
Species are needed that can fix nitrogen under reduced light conditions and in the
adverse environment of below-average site qualities that are frequently N­
deficient. Through improved hardwood silviculture, we may be able to increase
the quantity and quality of red alder and thereby reduce the indirect costs of
replacing some Douglas-fir with this species. Because insufficient amounts of
nitrogen will probably limit growth in practically all intensively managed forests,
this research will have practical significance to forestry in the Northwest.
371
LITERATURE CITED
Atkinson, William. 1977. An economic analysis of Douglas-fir response to
nitrogen fertilizer. Pages 27-35 in Part IV of Regional Forest Nutrition Research
Project Biennial Report 1974-1976. ColI. For. Resour. , Univ. Wash. , Seattle.
Atkinson, W. A. , B. T. Bormann, and D. S. DeBell. 1979.
Crop rotation of Douglas­
fir and red alder: A preliminary biological and economic assessment.
Bot. Gaz.
[In press. ]
Berg, Alan and Allan Doerksen.
1975.
Natural fertilization of a heavily thinned
Douglas-fir stand by understory red alder. For. Res. Lab. Res. Pap. 56. Oreg.
State Univ. , Corvallis.
3 p.
Bollen, Walter B. , C. Chen, K. C. Lu, and R. Tarrant. 1967.
Influence of red
alder on fertility of a forest soil: Microbial and chemical effects. Res. Bull.
12. For. Res. Lab. , Oreg. State Univ. , Corvallis. 61 p.
Cole, D. W. , S. P. Gessel, and J. Turner. 1978. Comparative mineral cycling
in red alder and Douglas-fir. Pages 327-336 in Utilization and management of alder,
D. G. Briggs, D. S. DeBell, and W. A. Atkinson, compilers.
USDA For. Servo Gen. Tech. Rep. PNW-70 . Pac. Northwest For. and Range Exp. Sta. , Portland, Oreg. DeBell, D. S. , R. F. Strand, and D. L. Reukema.
1978.
Short-rotation production
of red alder: Some options for future forest management. Pages 232-244 in
Utilization and management of alder. D. G. Briggs, D. S. DeBell, and W. A.
Atkinson, compilers.
USDA For. Servo Gen. Tech. Rep. PNW-70 . Pac. Northwest
For. and Range Exp. Sta. , Portland, Oreg.
Franklin, Jerry F. , C. T. Dyrness, Duane G. Moore, and Robert F. Tarrant. 1968.
Chemical soil properties under coastal Oregon stands of alder and conifers.
Pages 157-172 in Biology of alder. J. M. Trappe, J. F. Franklin, R. F. Tarrant,
and G. M. Hansen, eds. Pac. Northwest For. and Range Exp. Sta. , Portland, Oreg.
Gessel, S. P. and J. Turner. 1974. Litter production by red alder in western
Washington. For. Sci. 29(4):325-330.
Knowles, R. 1975.
Interpretation of recent l5N studies of nitrogen in forest
systems. Pages 53-65 in Forest soils and forest land management, Proc. 4th
North Am. For. Soils Conf.
Laval Univ. Press, Quebec.
Miller, R. E.
Douglas-fir.
D. S. DeBell,
PNW-70.
Pac.
and M. D. Murray.
1978. The effects of red alder on growth of
Pages 283-306 in Utilization and management of alder. D. G. Briggs,
and W. A. Atkinson, compilers.
USDA For. Servo Gen. Tech. Rep.
Northwest For. and Range Exp. Sta. , Portland, Oreg.
Miller, Richard E. and Roger D. Fight. 1979.
Fertilizing Douglas-fir forests.
USDA For. Servo Gen. Tech. Rep. PNW-83. Pac. North,,,est For. and Range Exp. Sta. ,
Portland, Oreg. 56 p. plus appendix.
Miller, Richard E. , Denis P. Lavender, and Charles C. Grier.
1976. Nutrient
cycling in the Douglas-fir type - silvicultural implications. In Proc. , 1975
Annu. Conv. , Soc. Am. For.
p. 359-390 .
37 2
Miller, Ric.hard E. , and Ray Zalunardo.
1979. Long-term grm"th of eight intro­
duc.ed legumes at three forest loc.ations in south "est Oregon. USDA For. Servo
Res. Pap. Pac.. Northwest For. and Range Exp. Sta. , Portland, Oreg.
[In press. ]
Newton, Mic.hael, B. A. El Hassan, and Jaroslav Zavitkovski.
1968.
Role of red
alder in western Oregon forest suc.c.ession. Pages 73-84 in Biology of alder.
J. M. Trappe, J. F. Franklin, R. F. Tarrant, and G. M. Hansen, eds. Pac.. Northwest
For. and Range Exp. Sta., Portland, Oreg.
Reukema, Donald L. and David Bruc.e.
1977. Effec.ts of thinning on yield of
Douglas-fir: Conc.epts and some estimates obtained by simulation. USDA For.
Servo Gen. Tec.h. Rep. Pffi\T-58. Pac.. North .,est For. and Range Exp. Sta. , Port­
land, Oreg.
Smith, J. Harry G. 1978.
Growth and yield of red alder: Effec.ts of spac.ing and
thinning. Pages 245-263 in Utilization and management of alder. D. G. Briggs,
D. S. DeBell, and \\T. A. Atkinson, c.ompilers. USDA For. Servo Gen. Tec.h. Rep.
Pffi\T-70. Pac.. Northwest For. and Range Exp. Sta. , Portland, Oreg.
Sprent, Janet I. 1976. Water defic.its and nitrogen-fixing root nodules. Pages
291-315 in Water defic.its and plant growth, Vol. IV. Ac.ademic. Press, London. Sprent, Janet I. and W. B. Silvester. 1973. Nitrogen fixation by Lupinus
arboreus grown in the open and under different aged stands of Pinus radiata. New Phytol. 72 (5): 991-1003. Tarrant, R. F. , K. C. Lu, W. B. Bollen, and J. F. Franklin. 1969. Nitrogen
enric.hment of two forest ec.osystems by red alder. USDA For. Servo Res. Pap. Pffi\T-76. Pac.. Northwest For. and Range Exp. Sta., Portland, Oreg. 8 p. Tarrant, Robert F. and Ric.hard E. Miller. 1963. Ac.c.umu1ation of organic. matter and soil nitrogen beneath a plantation of red alder and Douglas-fir. Soil Sc.i. Soc.. Am. Proc.. 27: 231-234. Tukey, H. B., Jr. 1966. Leac.hing of metabolites from above-ground plant parts and its imp1ic.ations. Bull. of the Torrey Botanic.a1 Club 93 (6): 385-401. Turnbull, K. J. and C. E. Peterson. 1976. Analysis of Douglas-fir growth response to nitrogenous fertilizer. Part 1: Regional trends. lnst. For. Prod. Contrib. No. 13. Co11. For. Resour., Univ. Wash. , Seattle.
15 p. Zavitkovski, J. and Mic.hae1 Newton. 1968. Effec.t of organic. matter and c.om­
bined nitrogen on nodulation and nitrogen fixation in red alder. Pages 209­
223 in Biology of alder. J. M. Trappe, J. F. Franklin, R. F. Tarrant, and G. M. Hansen, eds. Pac.. North "est For. and Range Exp. Sta., Portland, Oreg. Zavitkovski, J. and Mic.hae1 Newton. 1971. Litterfa1l and litter ac.c.umu1ation in red alder stands in western Oregon. Plant and Soil 35: 257-268. 373
Reproduced from SYMBIOTIC
NITROGEN FIXATION IN THE MANAGEMENT OF
TEMPERATE FORESTS:
proceedings of a workshop held April 2-5,
O SU;
C. T.
J.
C.
Gordon,
FOREST SERVICE,
U.S.
Wheeler,
and D.
Department of A griculture,
GPO 989-177 1979
A. Perry, eds., by the
for official use.