FUTURE POTENTIAL FOR USE OF SYMBIOTIC
NITROGEN FIXATION IN FOREST MANAGEMENT
Dean S. DeBell
USDA Forest Service
Pacif ic Northwest Forest and Range Experiment Station
Olympia, WA 98502
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ABSTRACT
Management of the forest's nitrogen supply will become more important as forest
production and utilization methods intensify. Foresters have three major oppor­
tunities to affect the nitrogen system--conservation, application of synthetic
fertilizers, and use of nitrogen-fixing plants. These oppo tunities have some
similarities, but they also differ in many ways. The extent to which nitrogen­
fixing plants will be used is dependent on economic, sociopolitical, and bio­
logical factors. Many nitrogen-fixing species and systems appear to have promise
for forestry programs. Unfortunately, practical applications of these systems
are limited by several obstacles. Information on establishing and managing
nitrogen-fixing systems is urgently needed; so are convincing demonstrations of
benefits derived therefrom. Opportunities also exist for significant research on
cultural and genetic manipulation of host plants and nodule-forming endophytes
to enhance nitrogen fixation. If the potential for use of nitrogen-fixing plants
in forest management is to be realized, a comprehensive research and development
effort, preferably a cooperative program involving many organizations and
scientific descip1ines, is needed.
INTRODUCTION
Future use of symbiotic nitrogen fixation in forestry will depend on many factors­
-economic, sociopolitical, and biological. Any evaluation of the potential for
such use is only as good as our understanding of these factors. Unfortunately,
rapid economic and sociopolitical changes throughout the world make long-range
prediction difficult; moreover, our level of biological knowledge regarding
intentional manipulation of nitrogen-fixing plants in forest ecosystems is only
elementary at best.
I have therefore decided not to attempt to appraise the
magnitude of future use of nitrogen fixation in forest management. Instead, I
will discuss some factors that will influence the potential use of nitrogen­
fixing plants in forestry. Economic and sociopolitical considerations will be
discussed briefly, but my discussion will concentrate on biological matters--the
nitrogen system and other factors related to forest productivity. The major theme
will be that use of symbiotic nitrogen fixation involves more than simply adding
nitrogen (N) to forest ecosystems, and that the manner in which N is added as
well as other biological impacts of the N -fixing system are integral and signifi­
2
cant components of the system's potential. I will also mention some specific
N -fixing systems that could be used in forest management. Finally, I will
2
describe obstacles that limit application and suggest some things we might do in
order to realize the potential of symbiotic N fixation in managed forests.
451
ECONOMIC CONSIDERATIONS
A valid economic assessment must be more than a comparison of synthetic fertilizers
and N -fixing plants on the basis of cost per unit of total or available N. There
2
are many costs and benefits associated with application of both techniques, and
these need to be evaluated fully. Costs and benefits of N fertilizer application
have been discussed previously and are fairly straightforward (Miller and Fight
1979) ; now it is desirable to review costs and benefits associated with use of
N -fixing systems.
2
The costs fall into two general categories: (1) direct costs of establishing and
managing the N -fixing system and (2) indirect costs that may be incurred because
2
certain site resources are utilized by the N -fixing system rather than the
2
principal crop. The importance of the indirect costs will depend on the degree of
resource uti1izaiton and its net impact on the prinicpa1 crop; the periods of such
impact must be integrated for the entire rotation or management program.
Values or benefits obtained as a result of the N -fixing system must also be
2
included. Increased growth and yield of the principal crop is the primary benefit.
Other considerations may include indirect benefits associated with reduced costs
of weed control, reduction in soil-borne diseases, and in some instances, reduced
fire hazard. Yield of the N -fixing crop can also be of major importance if this
2
crop has commercial value.
SOCIOPOLITICAL CONSIDERATIONS
The potential will also be influenced by several factors which can be lumped under
the heading, sociopolitical. Such factors include events and alliances in world
politics as well as national decisions made by various countries with regard to
production and allocation of timber and energy resources.
The relationship between energy supply and synthetic N fertilizer has been
discussed many times. Energy and timber resource decisions, however, will also
affect other aspects of the potential for use of N -fixing systems. These include
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the demand for various wood products, the need for intensified forest management,
the application of certain forest practices, and the economic value of certain
N -fixing trees. For example, the development of a furniture manufacturing
2
industry based on red alder (Alnus rubra Bong. ) in the western United States
which parallels that of the eastern furniture industry might be favored by
petroleum shortages, increased transportation costs, and an orientation of eastern
forest management toward noncommodity uses. Because of expanded markets and
increased stumpage values for red alder, the profitability of using the species
in mixture or in crop rotation with conifers would be greatly enhanced.
Attitudes, goals, and the political strengths of various environmental groups may
also have both positive and negative impacts on potential applications of N -fixing
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systems. Governmental regulations regarding the use of certain herbicides ln
forestry may be an important factor; so will attitudes regarding the relative
hazards of synthetic fertilizers and N -fixing systems. Although most environ­
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mental groups may line up on the side of biological N fixation today, this is
more likely a function of limited understanding or misinformation than an
appreciation of all the factors involved. It is important that we foster a
better understanding among the environmental community of the advantages and
disadvantages of both N -fixing systems and synthetic N fertilizers.
2
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BIOLOGICAL CONSIDERATIONS
Although nonbio10gica1 matters, such as political and social decisions, may
influence the magnitude of economic potential and application of N -fixing systems
2
at any given time, the limits of the potential are defined by the Eio10gica1
system itself.
In this section, I will discuss biological considerations involving
N as well as other growth factors that may be impacted by N -fixing plants. I will
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also present an overview of N management opportunities.
The N Supply System
Nitrogen is a major factor limiting forest growth in most timber-producing regions
throughout the world. This limitation will become increasingly important as
forest management and utilization intensifies, and nutrient husbandry will become
a significant aspect of forestry plans and operations. As a prelude to assessing
N -fixation in a biological context, I will review the basic characteristics of
This review will provide a perspective
t e N supply system in forests (Figure 1) .
on the advantages and limitations associated with N2-fixing systems and can also be
used to compare N -fixation with other opportunities in N management.
z
Litterfa1l and root
Chemical
Precipitation
Organic N
In
Forest
Floor
And
Mineral Soil mineralization
Inorganic
immobilization
N
uptake
/
J
Leaching
Volatilization
t
Nitrogen
In
Tree Crop
And Other
Vegetation
t
Burning
Erosion
Harvest
Burning
Figure 1.
The nitrogen supply system
Organic N in the forest floor and mineral soi1. --The amounts of N present in
the forest floor and mineral soil of most commercial forest land varies from 1 000
kg/ha to ZO 000 kg/ha. Amounts average about 5 000 kg/ha in the Douglas-fir
(Pseudotsuga menziesii (Mirb. ) Franco var. menziesii) region and about Z 000 kg/ha
in the southern pine (Pinus spp. ) forests. At least 90 percent of these N reserves
(Wollum and Davey 1975) and probably 98 percent or more in many situations exist
in organic form and are not readily available for tree growth. Losses of organic
N may occur from the system via erosion and fire. Thus, management policy
regarding harvest and site preparation practices can have significant effects on
this portion of the system. Gains in organic N occur primarily through litter
fall, root exudation and death, and biological fixation by and death and decom­
position of N -fixing plants.
Z
453
Minera1ization-immobi1ization.--The organic N component is slowly mineralized
to inorganic N. The annual rate of mineralization has been estimated at about
5 percent for agricultural soils but is considerably less in many forest soils.
Inorganic N is available to plants and also to soil microorganisms. Assimilation
by the microbes will return the mineralized N to the organic N pool; this is
shown in Figure 1 as immobilization. Net mineralization rate on a low-quality
Douglas-fir site was estimated to be less than 1.0 percent of total N present
( Heilman and Gessel 1963 ) . The general response of Douglas-fir to fertilizer N
suggests that, even on more productive sites, mineralization rates are not much
higher. Rates are probably higher in the southern pinery where temperature and
moisture conditions are more favorable and perhaps do approach 5 percent. A
number of factors influence mineralization rate. The more important are the
nature of soil organic matter, temperature, soil moisture, soil animals, and
microbial populations (Wo11um and Davey 1975 ) . Foresters can alter rate of
mineralization and immobilization through management practices, such as drainage
of wetlands, thinning, site preparation, and cultivation techniques. Mineralization
rates can also be enhanced--at least in some situations--by application of
chemical fertilizers ( Broadbent 1965, Overrein 197 2) . High mineralization rates
have been observed in the presence of N -fixing plants ( Bollen and Lu 1968, Cole
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et a1. 1978 ) , but whether this is due slmp1y to high decomposition rates for litter
from such plants or also to enhanced mineralization of native soil N is open to
question ( Ike and Stone 1958 ) .
Inorganic N in soi1.--The amount of inorganic N in soil normally does not
exceed 5 to 10 percent of total soil N ( Wo11um and Davey 1975 ) and exists as
ammonium, nitrate, or nitrite. Nitrogen in this pool is readily available for
plant uptake; moreover, continuous losses may occur from this pool via leaching
of nitrates and nitrites and volatilization of nitrogenous gases. Such losses
are probably insignificant in most temperate forest soils under natural conditions,
but may attain importance after site preparation or fertilizer application. For
example, gaseous ± 7 sses from urea applications may amount to 30 percent or more
of the N app1ied.In addition to mineralization of organic soil N, gains to
the inorganic N pool may occur directly through precipitation and additions of
fertilizer. Only minor amounts enter as precipitation at any given time and
therefore are presumed to have little immediate effect on plant uptake.
Application of fertilizer N, of course, can influence this pool directly, and this
is perhaps the greatest biological advantage that chemical fertilizers have over
N -fixing plants.
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Uptake by higher p1ants.--Uptake rates obviously will vary by season and
vegetation. For maximum growth of trees, sufficient amounts of inorganic N must
be available at appropriate times. Information on physiology of uptake is
limited for most commercial forest tree species, especially as it may relate to
fertilizers and application prescription. Only a minor portion of applied
fertilizer N is normally taken up by the forest tree crop. Baker and his
colleagues ( 1974 ) reported that fertilizer recovery rates in young pine plan­
tations ranged from 3 to 24 percent of N application and averaged only 14 percent.
Seemingly, studies of factors influencing uptake could have a substantial impact
on enhancing the efficiency of N fertilizer application. If a better under­
standing of factors influencing nutrient uptake was coupled with similar know­
ledge aimed at reducing losses, fertilization prescriptions could be developed
which could at least double present fertilizer efficiency.
Unpublished data and manuscript by V. G. Marshall ( Canadian Forestry Service )
and D. S. DeBell on file at Forestry Sciences Laboratory, Olympia, Washington.
1/
454
Nitrogen in higher plants. --A major goal in N management is to get optimum
Such amounts are a function of availability,
amounts of N into the forest tree crop.
uptake rates, and competition from soil microhes and les.ser vegetation. The two
principal avenues for losses of N from the system occur at rotation age--harvest
and burning of residual slash. Nitrogen depletion associated with harvesting
depends on degree of utilization and length of rotation. Two opportunities exist
for direct input to the system as higher plant N--use of commercial N -fixing
2
trees as the principal crop and foliar absorption of N solutions (Wittwer and
Teubner 1959) . Foliar applications show some promise; but many problems, including
foliar burning, remain (Miller and Young 1976) . When markets exist for their
wood and fiber, N -fixing trees--such as black locust (Robinia pseudoacacia L. )
2
and certain alder (Alnus spp. ) and Casuarina species--can be used. Such species
usually have rapid early growth rates and may furnish high per acre yields.
Moreover, in addition to meeting their own N needs, they usually improve the site
for companion species or subsequent tree crops.
Other Biological Aspects
Although growth is markedly influenced and often limited by available N supply,
many other factors also influence forest productivity. Some of these other factors
may be affected by N -fixing plants (Figure 2) --both favorably and unfavorably.
2
Nitrogen
Disease, insects and
other animals
Growth and
Yield of
Tree Crop
Soil conditions
(moisture, temperature,
bulk density, organic matter,
mineral nutrients)
Figure 2 .
Competition from
other vegetation
(light, moisture,
nutrients)
Allelopathic or
stimulatory chemicals
Growth factors affected by N -fixing plants
2
Such subsidiary benefits and adverse effects are an important part of the potential
of N -fixing systems. Many of the successful and continued uses of N -fixing plants
2
2
in agronomic systems are based upon benefits of equal or greater importance than
N
se.
For example, white root disease (Jomes lignosus) is a major factor in
use of legumes in combination with other ground covers in rubber (Hevea braziliensis
Muell. Arg. ) plantations in Malaysia (Fox 1965) . Forest pathologists tell us
that similar disease problems may become increasingly important in forest
production as management intensifies. Long-term studies to determine the influence
of alder in mixture or crop rotation with Douglas-fir on Phellinus (Poria) root
rot are already underway in the northwestern United States (Nelson et al. 1978) .
455
In two recently reported forestry studies, growth benefits to forest trees from
N -fixing plants exceeded that which could be obtained with equivalent or
2
conventionally applied amounts of fertilizer N. Such was the case with clover­
sycamore (Trifolium-Platanus occidentalis L. ) cultures in the southern United
States (Haines et al. 1978) and with autumn olive--black walnut (Elaeagnus umbellata
Thunb.-Juglans nigra L. ) mixtures in the midwestern United States (Funk et al. 1979) .
This additional improvement has been attributed to control of weed competition and
improved soil moisture relationships; perhaps improved temperature conditions or
unknown stimulatory chemicals also may be involved. The fact that some N -fixiD
2
plants improve general soil tilth by increasing soil organic matter and decreasing
bulk density has been documented (Tarrant and Miller 1963) .
Adverse effects of N -fixing plants include competition with the principal crop
2
for various site resources (e.g., light, moisture, nutrients) , but the possibilities
are not limited to competition alone. Other potential concerns include: (1)
­
creasei+soil acidity and reductions in other nutrients (e.g., exchangeable Ca
and Mg ; available P and B) as repqrted for red alder stands (Bollen et al. 1967,
Franklin et al. 1968) , (2) increased problems with damaging agents, and (3) perhaps
allelopathic effects.
Overview of Opportunities for N Management
The basic management options--chemical fertilization, biological fixation, and
N conservation---may affect the N supply system in different ways. From a
biological standpoint, they may often provide complementary opportunities as well
as being alternatives which may compete for a limited number of corporate or
agency dollars. The opportunities and limitations of each are examined below.
Chemical fertilization.--The potential for application of N fertilizer is
highest when N deficiencies are due primarily to slow mineralization and thus a
small pool of available N. Moreover, a short-term increase in this pool should
lead to dramatically increased rates of uptake by trees. Thus, a rapid-growing
forest canopy and more or less full soil occupancy by tree roots are prerequisites
for maximum gains from broadcast applications of fertilizers. The ability of N
fertilizer to stimulate growth more or less immediately is especially advanta­
geous for stands which have just been thinned or will be harvested within the
next decade. The opportunity to add other nutrients at the same time is another
advantage of fertilizer application that may become more important in future
rotations.
On sites and in stands where chemical fertilization does have high potential, we
must learn to use it more effectively. Poor recovery of fertilizer by tree crops
is now a major limitation to use, expecially in young plantations. Gaseous and
leaching losses may be involved in low recovery, and such losses may also lead
to environmental problems. Uncertainty regarding future cost and availability of
N fertilizer can be an important drawback, especially if decision makers want to
base present harvest levels on the assumption that growth of forest stands will
be increased by future applications of fertilizer.
BiologicalN2fixation.--Opportunities for use of N -fixing plants are
2
greatest when'total amounts of N and organic matter in Ehe system are low.
Nitrogen-fixing plants also have potential advantages early in a stand's
development when chemical fertilizers may not be effective. An N -fixing crop
2
could boost uptake as well as build up soil N during this period and presumably
provide benefits long after the N -fixing plant was shaded out. Benefits from
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N -fixing plants often accrue slowly but they are long lasting; thus, the potential
2
for use is high when these traits can be used to advantage. Commercial value
456
of the N -fixing species also enhances opportunities for their use. Direct
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utilizatlon of N -fixing trees for wood, fiber, and energy may provide a number of
2
advantages--including rapid early growth and very favorable energy input/yield
output relationships--either as a sole crop or as a component of a mixed species
forest. Such effects are in addition to benefits that accrue to companion or
subsequent crops. Fianlly, N2-fixing plants may be particularly useful where other
factors also limit tree growtli, and these limitations can be alleviated by such
plants.
Limitation of N -fixing plants includes a general lack of knowledge and experience
2
regarding their use and indirect costs. Biological problems include incompatibility
in growth patterns of the principal crop and N -fixing plant and the low light
2
intensity beneath the canopy of most well-stocked forest stands.
Nitrogen conservation.--Management techniques may interact with various
processes in the N s.ystem, such as immobilization-mineralization relationships,
uptake rates, and growth; they also may interact to enhance or reduce effects of
chemical fertilization or biological N fixation. The main effect of most practices,
however, is to deplete the total amount of N in the system. Development or
modification of management policies and practices to curb depletion is most
important on sites where total N supplies are low. The most significant impacts
occur when old stands are harvested and new stands begun--i.e., harvesting, slash
disposal and site preparation. Policy decisions with respect to intensity of
utilization and length of rotation can also have significant impacts on the N
system. Short rotations, for example, require considerably more energy input per
yield output and may double rates of N depletion compared with longer rotations.
A comparable statement could be made for complete tree utilization.
Compared with N fertilizer application and symbiotic N -fixation, N conservation
?
has attracted only limited attention. There are seve al reasons. Nitrogen
conservation, unfortunately, does cost money and benefits are neither immediate
nor dramatic. Some managers do not believe the need for such practices has been
demonstrated (Tucker 1978) . Thus, when compared with the immediate increases in
growth obtained with fertilizer applications, the economic appeal of N conservation
is low. More detailed information on this topic is presented by Wollum and Davey
(1975) , Jorgensen and others (1975) , Miller and others (1976) , and in the recent
symposium proceedings on principles of maintaining productivity on prepared sites
(Tippin 1978) .
NITROGEN FIXING SYSTEMS--SOME POSSIBILITIES
Opportunities for immediate and near future applications of symbiotic Nifixation
in forestry--and particularly in intensive timber production--appear to be
limited to systems involving legumes and actinomycete-nodulated angiosperms.
Intentional manipulation of free-living bacteria, blue-green algae, and symbiotic
algal associations may be interesting possibilities, but manipulation in forestry
is decades away. Comprehensive reviews of legume systems for agriculture are
available (Lie and Mulder 1971, Hoveland et al. 1976, Hardy and Gibson 1977) .
Several summaries regarding actinomycete-nodulated angiosperms and other non­
leguminous N -fixing plants also have been prepared (Silver 1969; Youngberg and
2
Wollum 1970; Silvester 1976, 1977; Torrey 1978) .
Gordon and Dawson (1979) have
-fixing
trees
and
shrubs
in commercial forestry.
potential
uses
of
N
discussed
2
More recently, Haines and DeBell (1979) synthesized available information on
legumes and actinomycete-nodulated angiosperms in terms of use in forest management.
Six different systems were proposed and described. In this paper, I will discuss
457
only three of these systems--those which appear to have the greatest potential
for application in the near future.
Nitrogen-fixing Plants in the Understory During Early Stand Life
In this system, the N -fixing species is established simultaneously with the
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principal crop or soon after. It ac-ts as a nurse plant during the stand's early
life but is shaded out after the canopy closes and does not become reestablished
at later stages. Nitrogen is accumulated in the soil by the N -fixing species
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while available resources (space, light, other nutrients and water) exceed demands
of the principal crop. Channeling site resources to an ideal N -fixing species by
purposeful management may also limit encroachment of nondesirabt e vegetation which
would otherwise grow as volunteers on the site. Full sunlight, expecia11y in
widely spaced stands, may result in high rates of N -fixation. Potential
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disadvantages include the fact that the greatest need of the principal tree crop
may not coincide with the period of occupancy by the N -fixing species. Thus,
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benefits for later stand use are dependent on high N accretion in early years, good
N retention at the site, and ready availability of the N for later stand needs.
The study by Haines and associates (1978) in which five legumes (three clovers and
two vetches) were tested in a Z-year-01d sycamore plantation is an excellent
example of this system in forestry and has already been discussed at this
conference. The black walnut-autumn olive mixtures reported by Funk and coworkers
(1979) is another fine example, but the duration of autumn olive in the under­
story is still unknown. After 9 years, walnut grown in mixture with autumn olive
averaged nearly twice as tall as walnut grown in pure stands. Surprisingly, trials
with synthetic N fertilizer have had only negligible effects on growth of black
walnut (Ponder 1976) .
An optimum management strategy might be to combine this system with applications
of fertilizer. During early years, N -fixing plants could provide N, organic
Z
matter enrichment, and weed control. Nitrogen accumulated in the soil during this
early phase would probably continue to meet stand needs through intermediate
stages. When the stand has fully occupied the site and is closer to rotation age,
the potential for biologically and economically effective application of chemical
fertilizers is at its peak.
Mixtures of Two Commercial Species
This system involves an overstory mixture of two species, an N -fixing species
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and a nitrogen-demanding species, both of which have some commercial value. In
most cases, the nitrogen-demanding species has the highest value and is the
principal crop. The N -fixing species may be established simultaneously with the
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principal crop or interplanted in subsequent years; it may be harvested prior to or
at the same time as the principal crop. Advantages include the absence of any period
during which the principal crop is not growing on the site: for this reason, it
may appear more attractive than alternate cropping systems. Nevertheless, if
space considerations are integrated over time, portions of the site's growing
capacity not utilized by the principal crop may be similar. Benefits, however,
may accrue from the opportunity of .the N')-fixing species to provide " inputs" to the
principal crop during the entire rotation or when demand is greatest (e.g., during
the years of fastest growth or after a thinning) . Disadvantages include allocation
of a portion of site resources (space, moisture, nutrients, light) that might
otherwise be utilized by the principal crop. Far more serious considerations in
mixtures of some species are problems of compatibility caused by differential
'growth patterns of forest tree species. Because of such incompatibility and lower
value of the N -fixing trees, managers will want to carry the minimum number of
Z
458
N -fixing trees per acre consistent with providing the desired benefits. Unfortunate­
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ly, information on numbers of trees needed to obtain given benefits is not available.
This system is best illustrated by the red alder-Douglas-fir plantation at Wind
River in the southern Washington Cascades. At age 30, the Douglas-fir were
growing faster and soil organic matter and soil N were higher in the mixed stand
than in an adjacent pure Douglas-fir plantation (Tarrant 1961, Tarrant and Miller
1963) . A recent evaluation of tree and stand growth in the same plantation at age
48 by Miller and Murray (1978) revealed that Douglas-fir volume of the mixed stand
was 7 percent higher than that of the pure stand. If per hectare volumes of both
species are considered, total production in the mixed stand was nearly double
that of the pure stand.
With some species mixtures, the N -fixing species may be harvested and utilized
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for pulp logs, fuelwood, fence posts, or small saw logs several years or decades
before the principal crop is cut. If this occurs, foresters may find it desirable
to fertilize the principal crop 5 or 10 years prior to final harvest.
Direct Utilization of Nitrogen-Fixing Trees in a Continuous Cropping System or in
Crop Rotations With Other Species
Nitrogen-fixing trees may be grown as a pure crop in a continuous cropping system
or in crop rotation with a nitrogen-demanding species. In crop rotation systems
the N -fixing crop is grown by itself for a given period, followed by one or more
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crops of nitrogen-demanding (and usually the principal) species, and then grown
again. Problems associated with compatibility of growth patterns of two crops
[e.g., Douglas-fir and red alder (Newton et al. 1968) , black locust and conifers
(Kellogg 1934) J are thereby avoided. Each crop rece ves full sunlight, and growth
and N -fixation can therefore proceed at maximal rates for the species and existing
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site conditions. Additional advantages of both crop rotation and continuous
cropping lie in the fact that most N -fixing trees grow very rapidly (especially
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at young ages) , are adaptable over a wide range of sites, and appear to require
lower management inputs for excellent growth than do most nitrogen-demanding trees.
Thus, the N -fixing species appear particularly well suited for short-rotation
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production of fuel and fiber. In some cases, small saw logs and posts are also
possible products. If " diseases of the site" such as the root rots of Fomes
annosus, Phellinus (Poria) weirii, Phytophthora spp., and Armillaria mellea
become more serious with increased management and utilization intensity and in
future rotations, alternate cropping systems aimed at ameliorating such problems
could attain importance in forestry. Disadvantages include the fact that current
stumpage values. of N -fixing trees are usually lower than for other species that
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might occupy the site; as a result, foresters often re,gard them as weeds and have
had little experience managing them.
The possibilities of crop rotation in forestry can be illustrated with the recent
report of superior performance of planted hardwoods following a Z3-year-old black
locust plantation (Carmean Lt al. 1976) . Yellow-poplar (Liriodendron tulipfera L.)
sw.eetgum (Liquidambar styraciflua L.) , and black walnut grew far better following
the black locust cover than after pine, fully-stocked native hardw.oods, or
herbaceous vegetation. Red oak (Quercus rubra L.) grew equally well after pine
and black locust, both of which were superior to the other vege.tative covers.
I mproved growth after locust was attributed primarily to improved N status and
occurred despite the fact that the locust plantation was originally established
only on the most s,everely eroded portion of the site. Black locust has also been
used on mine spoils to improve site conditions sufficiently to permit establishment
of other species
459
General Comments on NZ-Fixing Systems
There are many systems for using N -fixing plants in forest management. Each
2
system has potential to contribute significantly; each has unique advantages and
disadvantages that may be important for specific sites and forest types. Systems
that include N -fixing plants in only a portion of the rotation or in crop
2
rotation with other species might be complemented with applications of synthetic
N fertilizer during the decade prior to final harvest. Foresters in the southern
United States initially have concentrated on legumes early in the rotation;
foresters in the Pacific Northwest have generally focused on the actinomycete­
nodulated plants.
This, however, appears to be largely due to natural occurrences
or immediate opportunities to transfer agricultural technology rather than to any
clear-cut superiority of the species-system combination for management programs
Considering the varied opportunities that these systems may
in those regions.
offer, I think it would be premature for researchers or foresters to limit their
interest to any one species or system at this time. A broad outlook seems more
appropriate with our existing level of knowledge and experience.
REALIZING THE POTENTIAL
Assuming that symbiotic N -fixation has potential for immediate as well as future
2
application, what must be done to realize this potential? i. e. , what are the
factors limiting implementation? In many instances the most limiting factors are
not the need for more scientific research or breakthroughs regarding N -fixation
2
, but rather some fundamental biological information as well as appropriate
demonstrations of benefits.
Some Limitations and Obstacles
Lack of management information and experience. --Foresters lack information
and experience related to propagation, establishment, and culture of N -fixing
2
species.
There are several promising species among the native actinomycete­
nodulated angiosperms and legume cultivars, but we often know very little about
propagating them on a mass scale or manipulating the system to establish them
naturally at appropriate times and in desired numbers. Neither do we have
knowledge or experience with management techniques to encourage N - fixation and
accumulation after the species is established. Stone (1978) spo e in a similar
vein regarding research priorities for use of N -fixing species in southern
2
industrial forests. Although comments were directed primarily at use of legume
cultivars, I think they have general applicability:
The initial aim must be finding combinations of species and culture that assure rapid and dependable establishment in most soils and seasons, and maintenance of effective fixation until stand closure occurs. Although such research may appear unimaginative or pedestrian to some, it seems the most likely route to successful use of nitrogen-fixing species in industrial forests.
This route will be neither likely nor successful, however, if the researchers allow themselves to be unimaginative or pedestrian in outlook, or reluctant to expand beyond conventional agronomic approaches. We can learn much from the agronomists, particularly about legume cultivars.
Unfortunately, the background of experience that we can draw upon is much less
in the case of actinomycete-nodulated plants.
460
Lack of appropriate demonstrations.--Convincing demonstrations of benefits
in planned trials on typical sites are not available, with a few notable
exceptions. This is not to suggest that the benefits are not there, or even that
they are difficult to assess. Rather it is a function of the level of effort
that forest researchers have devoted to the subject. Many reports in he
forestry literature are based on opportunistic evaluations of natural
occurrences, coupled with speculation regarding potential applications in
forest management. Although some of us may believe that substantial benefits
will be obtained in actual practice, forest managers in today's economy must
rely on much more than faith. As a result, they are not jumping at the
opportunity to include N -fixing species, like alder, in forest management
2
programs.
An exception, however, has occurred in the u , of autumn-olive in black walnut
Well-replicated studies were
plantations in the midwestern United States.installed at five locations, which were representative of and spanned the range
of site conditions encountered in most walnut plantations. Substantial increases
in walnut growth were documented, and the research has been eagerly accepted and
is now utilized by most walnut growers.
Appropriate demonstrations of growth response can therefore do much to generate
interest of forest managers in pilot-scale trials and also in funding applied
research. I believe they merit high priority in future research and development
problems.
Negative attitudes.--General attitudes regarding certain N2-fixing species
also hinder implementation. Some of the N -fixing species, introduced and native,
2
are regarded as weeds by many people, including forest managers. Scotch broom
(Cytisus scoparius L.) and kudzuvine (Pueraria) have become so widespread that they
are considered a nuisance. Nitrogen-fixing tree species often grow faster than the
desired crop and suppress its growth. Foresters have spent much time and energy
combating such problems. Moreover, intentional use of N -fixing plants might
2
result in greater control problems in adjacent areas where no benefits are
obtained. Thus, many foresters in the northwestern United States are not
particularly enthused when they hear scientists extolling the virtues of red alder
or ceanothus. Imagine the difficulties a forester in Georgia would have in
proposing kudzuvine for use in loblolly pine (Pinus taeda) management, even if it
were one of the most efficient and adaptable N -fixing legumes available!
2
Nevertheless, these attitudes may change if convincing demonstrations of benefits
and methods for control are available.
Research Opportunities
Stressing the need for effort on the previous matters should not imply that
opportunities for substantial gains through N -fixation research
se are any
less attractive. The strides already made artd reported at this conference are
impressive and so are the _otential impacts of future work. Species screening
tests and research on cultural methods to enhance N -fixation and accretion rates
z
seem to offer the greatest opportunities for signiflcant impacts on forest
practice in the near future. Genetic improvement of host plants could have
impacts in the not-too-distant future. The possibilities for added benefits
through cultural control or genetic manipulation of the endophyte are also of
interest, but successful application is' probably decades away, particularly for
1/ David T. Funk, North Central Forest Experiment Station, Carbondale,
Illinois, personal communication.
46l
perennial, native N2-fixing species and on sites where well-adapted native
endophyte organisms are ubiquitous. Such work should he encouraged; so should
investigations that would provide better understanding of the similarities and
differences between use of synthetic N fertilizers and N -fixing plants. The latter
investigations would include effects on the N system (e.g., mineralization rates,
equivalency of biological N and chemical N) as well as auxiliary benefits and
adverse side effects.
In our enthusiasm for assessing and realizing the potential benefits of N -fixing
2
plants, we should not neglect the consideration of adverse side effects. Some
items of possible concern with use of N -fixing plants include increased soil
acidity and reduced base status as has S een reported with Alnus. (Bollen et a1. 1967,
Franklin et a1. 1968). Increased nitrification, denitrification, and elevated
nitrate content of ground and surface waters may be a potential concern. Moreover,
increased susceptibility to or attraction of damaging agents (e.g., sapsuckers
and bears) has been observed in some instances. The tendency of many N -fixing
2
plants to "escape" from the system in which they were introduced and cause problems
elsewhere is a common occurrence; examples include kudzuvine in the South,
Russian-olive in the Midwest, and Scotch broom in the northwestern United States.
As I gathered information for this paper, the piecemeal nature of research on
N -fixation as it applies to forest management became very obvious. Despite
2
much past research effort, there is little data that can now be used by forest
managers. Many aspects--both beneficial and detrimenta1--of N -fixing systems
need thorough evaluation under a range of forest conditions. S imilar information
is needed for synthetic N fertilizers if chemical applications are to be properly
assessed and compared with N -fixing systems. I think a systems analysis approach
2
is desirable for evaluating and optimizing use of both options for adding N. Such
work will involve substantial investments of time and money and require talent
of several scientific disciplines. Seemingly, these needs could be efficiently
and effectively met by a cooperative research and development program. Successful
cooperatives exist for tree improvement and forest fertilization in many forest
regions. Although the time may not be ripe for establishing a formal cooperative
for N -fixation research, informal exchanges of information and perhaps some
z
coordlnated studies involving teams now doing fundamental research and teams
in applied silviculture would be useful.
SUMMARY
Intensive forestry will provide many opportunities for managers to enhance as
well as degrade the N supply system. The most direct and significant opportunities
are:
(1) application of synthetic N fertilizers, (2) use of N -fixing plants,
2
and (3) implementation or revision of other practices and policies to either
enhance existing N supplies or reduce their depletion (N conservation) . The
potential contributions of these practices differ in many respects. All can
play an important role in future forest management--N conservation to limit
depletion of existing supplies, and chemical fertilizers and N -fixing plants
2
to supplement or buildup N supplies. In many instances, symbiotic N -fixation
2
and chemical fertilization may complement each other as well as being alternative
techniques for adding N.
Symbiotic N -fixation has potential advantages over chemical fertilizer
2
application on sites where total N supplies and soil organic matter content are
low and in recently established stands. When commercially va1uah1e N -fixing
2
trees are involved, N -fixation provides an opportunity for direct input of N to
2
462
the crop and thereby avoids problems associated with competition for the added N from other vegetation and soil microbes. Moreover, beneficial effects of N -fixing
2
plants often persist long after the plants disappear. Nitrogen-fixing plants may also provide benefits other than N. These auxiliary benefits include improved soil conditions as well as control of competing vegetation and soil-horne diseases. In agronomy, such factors frequently are as important as N, and they may assume similar importance in forestry as management intensifies. Applications of synthetic N fertilizer can add N directly to the available N pool and thus may be advantageous on sites where native N supplies are reasonably high and slow mineralization and low availability are the major problems. This assumes, of course, that the stand has developed to the point that it can abs,orb and effectively use a significant amount of inorganic N at the time it is added. The opportunity to increase volume growth of stands that will soon be harvested is a significant one. Chemical fertilization also provides an opportunity to add other nutrients that may be limiting crop growth. The potential for future use of N -fixing plants in forestry seems great and the
2
opportunities are diverse. Although symbiotic N2fixation does offer an alternative to chemical applications of N to forest; the potential of N -fixing systems involves much more. Possible systems for using N -fixing p t ants in forestry are 2
many, and each has unique attributes. Several factors now limit effective use of these two options and realistic assessments or comparisons of them. Most of the obstacles can be overcome, however, with a comprehensive and cooperative research and development effort. LITERATURE CITED
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