USE OF NITROGEN -FIXING PLANTS TO IMPROVE
AND MAINTAIN PRODUCTIVITY OF FOREST SOILS
by
Sharon G.
Haines and Dean S.
DeB ll
1980
In proceedings,
"Impact of
Intensive Harvesting
on Forest Nutrient Cycling,"
August 13-16,
1979
IMPACT OF INTENSIVE HARVESTING ON FOREST NUTRIENT CYCLING
THIS SYMPOSIUM was held at the State University of New York College of Environmental Science and Forestry Syracuse, New York, August 13-16, 1979. SPONSORED by the USDA Forest Service, and USDOE, Fuels from Biomass Systems. HOSTED by the State University of New York College of Environmental Science and Forestry. COPIES OF THIS SYMPOSIUM PROCEEDINGS ARE AVAILABLE FROM THE Northeast Forest Experiment Station USDA Forest Service 370 Reed Road Broomall, Pennsylvania. 19008 State University of New York College of Environmental Science and Forestry Syracuse, New York 13210 This material was prepared with the support of U.S. Department of Energy Grant No.
ET-78·G·Ol·3424, and USDA Forest Service Agreement No. 23·075. Any opinions,
findings, conclusions, or recom,mendations are those of the authors and do not necessarily
reflect the views of the Department of Energy or the Forest Service.
279
USE OF
I TRO
GEN-F IXING PLANTS TO IMPROVE
AND MAI NTAI N PRODUCTIV I TY OF F OREST SOILS
Sharon G.
Haines and Dean S.
1/
DeBell­
Abs tract.--Nitrogen-fixing plants can be used alone,
in mixture with, or in rotation with other crop species.
Much of the information required for effective use of
nitrogen-fixing s pecies in fores try is lacking, and
only limited applications are immediately pos sible.
This paper evaluates and synthesizes the available
literature with respect to potential applications
in fores t management.
six cropping s ystems are
proposed,
and species that seem adaptable for each
sys tem are identified.
Additional keywords:
legumes,
actinomycete-nodulated
angios perms, crop rotations , s pecies mixtures,
biolbgical nitrogen fixation.
I NTRODUCTION
Forest s oils are characteris tically low in available nitrogen,
and therefore tree growth is often les s than optimal, even in
unmanaged ecosystems.
This situation can be greatly magnified in
those ecosys tems which are intens ively managed for fiber or timber
production. Silvicultural practices that affect available soil
nitrogen can have a major impact on s ite productivity. The degree
of impact will vary with the silvicultural practice itself, as
well as with other preventative or ameliorative measures taken to
temper any negative effects on productivity. . Concern about accel­
erated nutrient removal as a res ult of intens ified harvesting
techniques has been wides pread ( Keeves 1966, White 1974) . Removal
of nitrogen, a nutrient already in short s upply, has stimulated
debate about s ubs equent s oil productivity (Bormann et al. 1967) .
Concern was for nitrogen removed from the site in harvested biomas s
(Boyle e t al. 1973) , as well as for leaching losses following
accelerated organic matter decompos ition ( Cole and Ges s el 1965).
The impact of s uch los ses can be alleviated by addition of nitrogen
and organic matter to the system t hrough use of nitrogen-fixing
species.
The mos t obvious beneficial effect of biological fixation on
forest nutrient cycling is the addition of nitrogen.
Other poten­
tial benefits may be as or more important. For example, nitrogen­
11
Project Leader, Soils Research, I nternational Paper Company,
Southlands Experiment Forest, Bainbridge, Georgia, and Project
Leader, I ntens ive Culture of Douglas -fir and As s ociated Species ,
Forestry Sciences Laboratory, Pacific Northwes t Forest and Range
Experiment Station, Forest Service, U. S. Department of Agriculture,
Olympia, Washington. Mrs. Wynona McCranie's editorial as sis tance
and typing of the final manus cript are gratefully acknowledged.
280
fixing s pecies add organic matter which may increas e the avail­
ability of other nutrient s , as well as nitrogen .
The us e of
nitrogen-fixing plants can also improve the s oil physical environ­
ment by decreasing bulk density and increas ing moisture retention
and thereby benefit companion and s ubs equent tree crops . Disease
control and reduced competition from non-crop plants are hypothe­
sized benefits. Certain s pecies have potential for eros ion control
and prevention of topsoil los s es following s evere site disturbances .
Leffel (1973) speculates that a certain degree of allelopathic
weed control may be obtained when legumes with high alkaloid con­
tent decompose in the soil.
Historically, nitrogen-fixing plants ( viz. , legumes ) have been
used in agriculture for forage crops and green-manures, as well as
food crops. Wides pread use of legumes has been attributed to
several factors:
( i) Legumes are "soil builders ", improve tilth,
and help prevent erosion; ( ii) They increase soil nitrogen and
organic matter levels when turned under as green manures; ( iii)
Legumes favorably influence s oil microorganisms respons ible for the
the efficient decay of organic matter; ( iv) Legumes utilize atmos­
pheric nitrogen which is unavailable to nonlegumes ( with the notable
exception of some nonleguminous nitrogen-fixing s pecies); ( v) Legumes
store large quantities of protein in plant tis sue and make high
quality forage for livestock and wildlife.
The utility of nitrogen-fixing plants in forestry can be based
on s imilar reas oning. Moreover, mos t of the above factors appear
to apply equally well to the actinomycete-nodulated angios perms
which are common in forest ecosystems. Specific objectives for
us ing s uch species in a total forest management program include:
( i) restoration of degraded s ites ( e.g., mine s poils, s evere ero­
sion, severe burning); ( ii) amelioration or improvement of sites
where some growth factor ( primarily nitrogen) is naturally limiting;
and ( iii) preventative maintenance on s ites where growth is cur­
rently at adequate or optimal levels.
Often consideration of potential uses of nitrogen-fixing plants
is limited to comparing biological nitrogen fixation with chemical
Economics of chemical fertilization appear
fertilizer application.
more favorable at this time than supplying nitrogen via biological
means ( Atkinson and Hamilton 1978, Atkinson et al. 1979), but
future cos ts and availability of nitrogen fertilizers are uncertain.
Although nitrogen additions are generally the maj or reason for
considering use of nitrogen-fixing plant s , the previously mentioned
benefits with respect to other productivity factors s hould not be
ignored.
If the nitrogen-fixing s pecies is the principal crop,
there also may be advantages with respect to levels of management
needed to ins ure a succes s ful crop. In addition, there may be
advantages with respect to other forest uses or problems ( e.g. ,
cover and food for wildlife). Thus, many factors in addition to
nitrogen merit s ome cons ideration in as s es sing the potential for
use of nitrogen-fixing plants .
281
At the outset, the reader should be forewarned that far more
people have philosophized about potential uses of nitrogen-fixing
species in forestry than have established well-designed research
studies or attempted operational use of such plants.
Most of the
forestry literature deals with opportunistic evaluations of natural
occurrences (or unplanned comparisons in man-made situations)
coupled with speculation regarding future potential for planned
use of the nitrogen-fixing species in management programs .
This
paper is no exception.
In fact, the purpose is to evaluate and
synthesize the available literature with a view toward identifying
some promising species and potential cropping systems for use in
forest management.
Finally, factors that may limit application
be
discussed,
as
well as research needed if speculated oppor­
will
tunities are to be realized in actual management programs.
NITROGEN-FIXING PLANTS FOR USE
IN
FORESTRY
Based on their estimated nitrogen-fixation rates and their
adaptation to forest environments, legumes and actinomycete­
nodulated angiosperms appear to have the greatest potential for
use in forest management.
These plants also are the only ones
which offer any possibility for near-term practical application.
Use of blue-green algae and symbiotic algal associations in forestry
is at best decades away.
Even then, rates of nitrogen fixation
for the latter are low and any use would probably be confined pri­
marily to wetland ecosystems.
Thus, the scope of this paper will
be restricted to use of leguminous and actinomycete-nodulated
angiospermous nitrogen-fixing plants.
Legumes
Approximately 500 genera of legumes containing 11, 000 species
have been identified, with 4, 000 species native to North America
(Fernald 1950) .
Small (1933) identified 99 genera and 358 species
of native or naturalized legumes which occur in southeastern United
States forests.
Obviously, many of these species have little appli­
cability for forestry purposes; however, a number of species may
prove suitable if satisfactory management techniques can be devel­
oped .
For purposes of this discussion, legumes will be categorized
as :
(i) agronomically important legumes with potential for forestry
use, and ( ii) native or naturalized legumes known to occur in tem­
perate forest ecosystems.
Legumes growing on agricultural soils have been studied exten­
Nutrient requirements of agronomically important legume
sively .
species have been determined, as well as those of the Rhizobia
Intensive breeding programs
bacteria associated with such plants.
have developed strains of plants with favorable morphological
characteristics and microbial strains whioh efficiently fix
nitrogen.
Rates of fixation by legumes on agricultural soils have
been relatively well-documented ( e. g . , Hoveland et al. 1976).
Data
are unavailable on fixation rates for such species growing on forest
soils which are characteristically more acid and less fertile.
282
Rhizobia that efficiently fix nitrogen in limed, fertilized soils
may be almost totally ineffective on forest soils.
Without fertili­
zer, forage type legumes do not make sufficient growth to improve
the nitrogen status on infertile forest soils (Jorgensen 1978).
Additionally, seeding and fertilization regimes appropriat e for
agricultural purposes (e. g. , forage production) may be inappropri­
ate for forest management objectives.
Selected agriculturally important legumes with potential for
forestry use are identified (Table 1). Species included are those
few known to be capable of growing on forest soils as a result of ..
Others were used in research aimed at
direct experimentation.
erosion control along forest roads in the West (Dyrness 1967, 1975;
Berglund 1976).
Also identified are those species suspected of
being valuable because of traits such as tolerance of low fertility
and acidic conditions and ability to grow on droughty sites.
While native or naturalized legumes are capable of growing on
forest soils, few are known to have management potential on the
basis of actual experimental evidence.
Genera such as Cassia and
Lespedeza, widely used as game foods, are noteworthy exceptions.
The first obstacle to be overcome with native and naturalized
species is the lack of readily available seed for experimentation.
Plans for seed collections from plants growing in the wild must
take into consideration the difficulties of finding adequate con­
centrations of seed-bearing plants which have not been devastated
by insects, disease, or adverse weather.
Breeding programs for
native species cannot be considered until much more information on
adaptability and effectiveness in nitrogen accretion is available.
Data are available as to the impact of forest management acti­
vities on the frequency and distribution of some native legumes
(Stoddard 1936, Cushwa et al. 1970). Little is known about the
nitrogen accretion attributable to these species in forest eco­
systems.
Data are unavailable on benefits of increased stocking
levels of native species which tend to occur in low concentrations
naturally.
Some research on the nutrient quality and digestibility
of native legumes has been reported (Short and Epps 1976), as well
as the frequency with which these species occur in wildlife diets
(Martin et al. 1951).
egume seeds may be the best source of pro­
tein .and phosphorus among temperate forest forages.
Selected
native and naturalized legumes are characterized herein (Table 2).
Species listed are those identified by Landers (1978) as potentially
valuable in temperate forest management.
Actinomycete-nodulated Angios?erms
Actinomycete-nodulated nitrogen-fixing plants have not been
cropped nor utilized by man to the extent that legumes have been
used in agriculture.
Rather interestingly, however, there are
reports of use of Alnus and Casuarina (two of the major
actinomycete-nodulated genera) in rotational agriculture of Taiwan
and New Guinea (Silvester 1977). Nevertheless, it is widely
Table !.--Agriculturally ilftportant leguminous .pecies having potential utility in fore.t management systems!/
Site Conditions
Tolerated or Required
Species
L.
Aeschynomene
wet soils I
Longevity
can tolerate flooded condi tions
annual
following establishlnent.
Centrosema
drought.y.
spp.
Moo re
relatively infertile
moist to well-drained acidic
Lespede:z:a spp.
can grow on eroded
acid
Reference.
Hodges et a1.
aOl.ls
80ils
aOl1s
wbleh are l()"tol
(1975)
(1978)
perennial
Whyte And Trwnble
perennial
Wells
4nnua1 or perennial
KalIs et. a1.
perennial
Dyrness
(1953)
(1978)
(1970)
in phosphorus
corniculatus
L.
deep or l'IOderately deep
well-drained loams or
clay loaa. developed on tuffs and brecClas
Lupinus anqustifolius
L.
slightly acid and moderately fertile .andy
Annual
Lupinus hispanicus asp.
poor ly to moderately veIl-drained soils of
blcolor
low fertility
Lupinus luteus XelL
moderately acid,
sandy soils of low
well-dralned loams
or clay 10alaS developed on tutr. and
aCid.
breccias
Ornithopus .pp.
moist •
sandy soils of low
Trlfoliwa incarnatwn L.
well-drained san<ly to clayey soils of IhOder4te
(1953)
'l 79)
Wells
annual
Whyt.e and Trumble
(1978)
Haines et a1.
deep or modera.tely deep.
•lightly
Whyte a.nd TnlDlb1e
annual
fertility
L.
1975)
(1976)
Hain•• et a1.
to loaay 80i 1.
M....dicago
(1967.
Berglund
(1953)
(1979)
perennial
Berglund
annual
Whyt.e and TruJl\ble
Miller and Zalunardo
annual
Haines et a1.
Jorgensen
Sawyer
moist.
clayey to silty soils of .-oderat.
perennial
Oyrness
Sawyer
well-drained sandy to loamy soil. of
annual
-..derate acidity
Berglund
White
Trlfoll.tml vesl.CUlOS1Ulft savi
well-drained sandy and clayey soils of
annual
annu41
(1978)
(1979)
Haines et .1.
(1978)
Spec-les listed have been tested on forest sites or ",re known to grow on acidic soils wit.h low fert.ility.
(l978)
(1978)
Haines et al.
(1979)
Haines et a1.
(1978)
Jorgensen
.!/
(1978)
(1978)
Haines et a1.
Sawyer
sandy to lOUlY soils
(1979)
(1978)
Jorgensen
moderate acidity And low fertility
Y!£!!. .!l..!!2!! Roth
iIIl .
(1976)
Hain•• et &1.
Jorgensen
Sawyer
1975)
(1976)
(1978)
Haines et
Trifoliwn subteraneum L.
(1979)
(1967.
Berglund
fenility
(1978)
(1978)
(1978)
Haines et;1.
repens L.
(1978)
(1979)
(1953)
fertility
acidit.y
N
CO
(1976)
284
l'
Table 2, .. N.tlve or n.turaliud h9U_lno\a .peei.. haying potential utility in lornt .u.naqulent system.s-'
TYPiCal Length/
Seecies
H.bit.t
LonSievit;i
moist. to poorly drained
annual
Ell.
.oU.
!e.!E.!. a.ricana KadlC.
Bloat to poorly drained
Baptisu spp.
flxceuively dUln
Growth rom
vine
HeJ. ht
5-20
10-)0
perennial
References
LAndus
(1918)
Landers (1978)
soils
perennul
.
erect
)-15
Cu.h..... et .1.
0970;
lAnders (978)
undy solh
Cass14 spp.
("'"
IDOderate to well-drained.
."nual
erect
1.5-6
Whyh and Trumble
Cush..,. fit at.
sand}' solls
d.
Halls fit
{1953)
(l970·
(1970)
Landers U91S)
Vir9in14n\.n
perennul
....ell drained 80ih
vine
5-15
.£..!ll.2.!.!! lIIariana L.
excessively to lflOderately
SOlh
well-dra ined. sand}'
IPP,
Whyte and Tnmble
(1953)
Cuah..,. et al. (1970)
lAnders (1978)
(L. ) Bent.h.
dup or lIIOderate1y dup,
5-10
perennul
cush... et AI.
Landers
perennla I
enct to
)5 50
prostrate
well-drained loal!ls or
(1970)
(1918)
Oyrness (1967. 1915)
Hiller and Zalunardo
(19191
clay lous
Crotala.na spp.
[)es:r.odiulft sPF.
lIIOderately drained, s.ndy
annual or
erect to
or 9ravelly scrill
perennial
prostrate
!!IOderate to well-dralned,
annual or
er.ct to
sandy to 10&lllY soils
perennlA 1
prostrat.
1-15
Whyt. and Trulllb le (1953)
L4r1nd ra (1918)
0.4-20
Whyte and Trumble (19S))
Cushwa et oil.
Lander.
Galacti.l spp. IIIOdl!rAte-ly drained, .. ndy
Salls
perennial
• an.:!}'
pe-renn;'.}
erect or
Indi90hra carolinian.
H111.
II'()d.rately drainl!d,
wspedeu spp. moderau to well-drained,
annual or
I!rect to
nndy 10&111 to clay lou
soih
per.nnlal
prostrate
exceuivel)' drained, sandy
perennial
Lupinus spp. ,pp. spp.
'pp.
.icrophYlla
ISo land. e x Smith)
Hacbr. (Ra(.)
e)llltata
Rydb.
Strophost):: les .pp. (1910)
5-15
Whyte and Trumble
1-)0
.....ll-drained _oils
.xc.. _ivdy drained.
.rect to
2-.
annual or
erect to
perennial
decull\be>r.t
annual or
.rect to
bi.nnial
decuJllbent p.rennial
vine
peunnial
lAnden
1-10
0.3-15
11)-40
erect or
O.S-l;)
per,ennial
pro.trate
moi.t to poor1}' duined
Whyte and Trulllb1e
Annu.l
erect
Whyte and Tr\L"ftble
10-20
Landen
(1918) 7-20
Llndu·s
(1978)
)-z\\
Landera
11918)
(1978)
lhOdeute1y to ....ell ..<!Uln.d
annu.1 or
perennial
St)::lounthes
IL.l 8.S.F. !!'Odeutely drained, .and}·
perennial
er..('t
1-5
Lander.
TephroUA .pp. iIIOderatel ' druneod,
perennial
erect to
2-'
Whyte and TruMhle
IAndy
10111
well-drained soil.
prostnt"
annul1 to
perennial
Vic1. Ipp.
..... U-drll.n.d IOU.
annual to
perenn1.al
1.
Adapted trc- LAnders 1978.
erect to
decuabent
vine
CU.h..... et oil.
l-lv
5-15
11(5))
(1910)
Whyte and Trulllbb
Land.n
(195))
(1970)
(1918)
.011_
(l953)
(1978)
WIlden
loila (l95)}
(19181
CU.h..... et oil.
drained sandy loih (195))
(1978)
Wh 'U and TruJl\b1e
L&ndera
(195)
(1978)
Whyte and TnUl\b1e
4nders
vine
lnOdeutely to t)(cusively
(1970)
Wh}'te and Tru-.ble
L.anders
lIOderateIy drain.d,
nndy soils
d.
Landers (1978)
decumbent
....ell-drained soils Whyte and TruDlble (195))
Cushwa et
sandy lo1ls
¨IPP,
(195))
Landers (19781
sandy soill
RhYncOOSiA
erect
Cu.hw& .t al.
Lande-rs (1979)
soils
.oih
Medicalio Ipp.
2-15
vine-
(1910)
(1978)
(195)
(1978)
Whyte and Trull'lhle
Landers 119:08)
(195))
285
suspected that the total contribution of actinomycete-nodulated
plants to the nitrogen economy of natural ecosystems (and parti­
cularly forested areas in temperate zones) exceeds that of legumes
(Silvester 1976; Torrey 1978; Gordon and Dawson 1979). This group
of plants consists of about 160 species in at least 13 genera of
7 families ( Bond 1976).
Most genera and even some individual
species occur naturally or have been introduced over broad geogra­
phic ranges. Moreover, they appear adaptable to rather,diverse
site conditions, and commonly can grow on adverse sites where most
other species cannot survive. Thus, the biological potential for
use of these plants in forest management appears most promising.
The major actinomycete-nodulated genera available for use in
temperate and subtropical climates are listed ( Table 3) .
Estimated
rates of nitrogen acc etion, natural habitats, and examples of use
or suggested applications in forestry are also summarized. Readers
interested in greater detail are referred to comprehensive reviews
on this subject (Silver 1969; Youngberg and Wollum 1970; Silvester
1976, 1977; Torrey 1978) and to the specific references listed for
each genus ( Table 3) .
A brief examination of the summa rized information will reveal
that some genera offer greater potential in forestry applications
than others. The more promising genera have higher nitrogen accre­
tion rates, wide ecological amplitude, and have been previously
used or suggested in forestry situations.
Thus, the genera Alnus,
Ceanothus, and Elaeagnus appear to offer the greatest potential
for use in northern United States climates ( e.g., Northeast, North
Central, and West). In addition to the above genera, Casuarina
and Coriaria may have application in southern ' (e.g., Southeast,
South Gulf, and Pacific Southwest) forestry. Although little is
known about the ecological significance of Myrica and estimates of
accretion are rather low, the authors believe that it too merits
consideration in forestry applications. Many species in the genus
Myrica are adaptable to a wide range of site conditions and may be
suitable as understory plants in managed forests; one species had
the most efficient nodules of all nitrogen-fixing species compared
by Bond (1967). Both legume and actinomycete-nodulated genera and
species will be discussed further as appropriate in the forthcoming
section on management systems.
POTENTIAL CROPPING SYSTEMS
Six different systems or strategies for using nitrogen-fixing
plants in forest management programs have been identified. Five
of these are modifications of two basic systems for using a
nitrogen-fixing species with a non-nitrogen-fixing principal tree
crop--alternate cropping (crop rotation) qnd mixed-species cropping.
The modifications are associated with opportunities for commercial
utilization of the nitrogen-fixing plant and also with the timing
of its establishment and growth in relation to the principal forest
crop. The sixth system is continuous use of a commercially valu­
able nitrogen-fixing tree as a single species crop.
Table 3.--Actinc:.ycet_nodulated. an9ioape
ltulllber of
Genu.!'
lIodulated:
Species
ER.w.ted
Mltrog.tn Accretion
--k
t!!
Other For••t
Cen.ra
IWbitata
.....
Jl
JUnua
IJenera having poteatial utility in forest _n.q
fro.
BencH tting
References
Aaaociationa
N/ha yr40-325
Poat-qlaclal ar_.
8Oih,
sand
hl11a
poor and .reded.
bog.
and other _t
alt.a6 ai .. _at.a, aandy and gra_lly
El£!!.
, PaeucSotauQa6
.and
Crocker
Dickaon
"UI.r
Tarrant and
Newton .t .1.
Tarrant and Trappe
alUi •• diaturbld alt••
(978)
DeBell and bd_n
'50-200
Co.atal an.s,
lIone r
..It _rabe., sand
dunes, hot and dry sit••
Silvester (l976
rt«1
Paeudotauq... Pima
Subalpine shrub, dry cMpPoaral.
31
(1911)
Zavitkovaki and St.v.ns
"illar and Murray
24
(1957)
(1963)
(1968)
underatory in dry torests
1(77)
tIollw. and Younqberg'
Oelvich. et a1.
(1972)
(1979)
(1964)
(1965)
Zavit)c.ovski and Newton
(1968)
YouQ9berq ( d Wollua (1970.
1976)
JCroct.al
C.rcocarl?U!
u
IID.data
High altitudes
infertile soil.
90-200
Poor. UrdY6 gr..v.lly acil. in lowland.
and subalpine habiut.:
al.o on claya.
.nd
McCrain
Nona reported.
VI_is et a1.
Pinus
--
Barr!. and Morr,ltIon
(1975)
(1964)
Koeppel and Wollu. (1971)
_t .r..... . lluviu.. -.orr.ine, alpina
(1958l
Silve.t.
(l976.1977)
Silv.star
(1977)
acr... nd volcanic debria
lIo
Loe••,
data
alluvial terna. grav.l acrees;
.rid. spars.ly veg.uted land; low
Ilona report.:!
tu.sock gr.... l.nd
PoR.-qlacial ar••a, ..,.ndy. gravelly.
12
and calcareoua
Stabilizes site tor
- Alnus,
and sub.equent
esta.bliahlaent ot Pop.Jlus
Croc)c.er and Major
(1955)
Crocker and Dickson
(l9S7)
and
Ela.aJ!!UII
No
16
<1&..
"Yrical'
poor aoU.
(1)
2-15J 179
'-9
27
Jugana
Di.turbed are ... aand dun.s.
Sand:.
calcareous aoil.
coa.tal dunes
Acid
boq••
and
Re-tore.tat:1.0n ot dry
baaina
in
Rus.ia;
qener. p!anted
und dunea. str('a. banka.
.auntain aloPfts. aine wastes. and.
poo r, c
Funk et a1.
Pinus
unknown
Akker-ns (1971)
Danielyan
(1972)
Silv.ster (1976)
Bond
cted soil s
(1979)
(1967)
SHYer
(1969)
UC'!a.Iu.
(1971)
Silver .lRd "'-que
(19701
Titfney and a.rrera
., 1; 50+
!!!!.!!!.!!.
Shephudia
(1)
110 data
Underatory in pine fore.t••
and undy 1101.1., C!isturbed soils
1/
- Adapted (rca tabl.,s ancl t:ext. presenteeS 1.n revi.,...
and poo r aites, sandy
by
and only
sites.
Silv"'5tec fl976. 1977)
2/
- OtMr genera r.ported to have nodule. includ. Accto.t.lphflos. Colloeu.J.
l'
puaiee
Along .tre.aJllS. near lak_, also dry
.aila
1976)
dry
ecoded
and Torrey
and RubUS.
Silvester
Dalton and Zobel
(1977)
None reporte<l
G&rdner and Bond
fl9S7)
Cd••
Youngberq and WollUlll (1970)
(l978) plus other spacific references cited
Nitrogen fixation
by
by
Arctostaphyloa is doubtful
one specle. haa been reported in _ch of thoe lattf'r two qoeru-rll and both are contineod to the tropic:••
This genu. includ•• COIIptonia and
(1916)
(l919)
IIone reported
qenera. (Sil .......t.r
N
0:>
(j\
287
In this section, each system is defined and known or potential
Suitable species and
advantages and disadvantages are identified .
examples of situations where the system has been tested or appli­
cation proposed are cited .
Crop Rotation Systems
In crop rotation systems, the nitrogen-fixing crop is grown
by itself for a given period, followed by one or more crops of non­
nitrogen-fixing (and usually the principal) species, and then grown
again .
Problems associated with incompatibility of growth patterns
of 2 crops [e . g . , Pseudotsuga menziesii .(Mir b . ) Franco var .
menziesii and Alnus rubra Bong . (Newton et ale 1968) ; Robinia
pseudoacacia L . and cunifers (Kellogg 1934) 1 are avoided in these
systems .
Each crop receives full sunlight, and growth and nitrogen
fixation can therefore proceed at maximal rates for the species
and existing site conditions.
In addition to applications for
fertility improvement, crop rotation has been used in agriculture
for at least 2,000 years to ameliorate problems associated with
soil-borne pathogens (Glynne 1965) .
If "diseases of the site"
[such as the root rots of Fames annosus, Phellinus (Poria) weirii,
Phytophthora spp . , and Armillaria mellea1 become more serious in
future rotations with increased management and utilization inten­
sity, alternate cropping systems may attain similar importance in
forestry .
For example, Nelson and co-workers ( 1978) have suggested
that Alnus rubra could control Phellinus weirii in Pseudotsuga
menziesii forests via crop rotations which may reduce survival of
the fungus in stumps and roots following logging and occupy the
site in place of susceptible conifers until the pathogen has died
out .
The principal disadvantage o f crop rotation is the opportu­
nity cost for the time the land may not be producing a commercial
crop or at least not the crop of highest value .
Two crop rotation systems--green manuring and alternate crop­
ping of commercial species--are discussed below .
They differ
primarily in commercial utilization of the nitrogen-fixing crop .
Green manuring . --In green-manure cropping, a nitrogen-fixing
In
plant cover is grown and no portion of the crop is harvested.
agriculture, legume crops such as Trifolium spp . or Vicia spp. are
The term "green
turned under while green or soon after maturity.
manuring" is used in this paper in a broader sense to include
nitrogen-fixing crops that are killed prior to or die out soon
after establishment of the principal commercial crop .
This system often can be used to ameliorate adverse sites that
previously could not support adequate growth of other plant species.
Alnus glutinosa (L . ) Gaertn . (Lowery et a1 . 1962 ) , Robinia
pseudoacacia (Finn 1953) , and Myrica spp . (Silver 1969) have been
used or suggested for use on coal mining wastes in Pennsylvania
Lupinus arboreus has been used to aid
and the midwestern states.
sand dune stabilization in New Zealand (Gadgil 1971 ) .
There are
For example,
many examples of green manuring in natural ecosystems .
288
Dryas drurnmondii Richards. and Alnus sinuata (Reg.) Rydb. are pio­
neer invaders of post-glacial sites in Alaska (Crocker and Major 1955 ), and Casuarina equisetifoli9_ L. and Coriaria arborea Lindsay
have contributed significantly as seral vegetation following vol­
canic activity on Krakatau Island, Indonesia and in New Zealand,
respectively (Silvester 1976). Disadvantages of green manuring
center primarily upon the previously-mentioned opportunity costs
incurred when land is not producing the commercial crop.
The legume genera that appear to be most suitable for use with
this system in temperate areas include Vicia spp. and Lupinus spp.
(Mulder et al. 1977). Lupinus spp. may be particularly useful in·
forest conditions as they are able to grow and fix nitrogen at low
pH, can obtain phosphorus from relatively insoluble phosphates,
and, by virtue of a deep rooting habit, can obtain water not avail­
able to other species. Among actinomycete-nodulated genera, Alnus,
Coriaria, and Casuarina appear to be most promising.
The best example of modified use of green manuring in forestry
is that involving Lupinus arboreus in New Zealand to stabilize sand
dunes and enhance subsequent pine growth G
( adgil1977). After
stabilization, Pinus radiata is interplanted in established Lupinus
cover. The Lupinus is subsequently killed with 2,4-D, b ut it may
reestablish itself from seed remaining in the soil. When the latter
occurs naturally or is so manipulated by management, the Lupinus­
Pinus association is more fittingly termed a mixed species system
and will be identified as such later in this paper.
A green manuring system involving Alnus rubra and Pseudotsuga
(
menziesii has been conceptualized by Atkinson and others 1979)
for the Pacific Northwest. Dense stands of Alnus would be estab­
lished by seeding and grown for 8 years. During this time an esti­
mated 660 kg-N/ha would be added to the site. The Alnus would then
be killed with herbicide and Pseudotsuga menziesii seedlings
planted among the standing dead Alnus. This system may therefore
provide additional benefits associated with non-competitive "shade"
for the young trees, a factor of particular importance in stressful
environments. At current costs and stumpage values, Atkinson and
his co-workers considered this to be a profitable program but not
as lucrative as chemical fertilizer application.
Alternate cropping with two commercial species.--A crop rota­
tion system which includes a legume or grass-legume forage crop at
frequent intervals among grain crops (e.g., Zea mays L ) is prac­
ticed in agriculture. This s"stem differs from green manuring in
that tha nitrogen-fixing crop has some value, though usually not
as much as the non-nitrogen-fixing crop. Advantages and disadvan­
tages are similar to those m entioned for green manuring, but oppor­
tunity costs are less due to the value of the nitrogen-fixing crop.
Suitable species for this system are limited by definition to
those having commercial value or at least potential for utilization.
In the southern and midwestern United States, oppor tunities may
289
exist for production of leguminous forage or food crops on well­
prepared sites for a few years after harvest and prior to estab­
lishment of tree crops.
Wildlife cover and food species could also be considered in
this c ategory if the landowner obtained monetary returns from
hunting leases or fees.
Moore ( 1978) successfully established
Aeschynomene americana L. on a wet , infertile sand in South Florida ,
and determined that a maintenance program incl uding fertil izati n
and light disking would be needed for economical forage production.
Hal ls and his colleagues ( 1970) were less successful in establishing
Cassia and Lespedeza on a cutover Pinus palustris Mill. site.
They
concluded that investment in establishment and maintenance was too
high to permit economical utilization of these species as managed
wildl ife food on such sites.
Lespedeza , however , is being managed as
a wildlife food species on International Paper Company ' s Southlands
Experiment Forest.
On most forest sites nitrogen-fixing trees are , for all prac­
tical purposes , the only species of potential use.
These include
Alnus rubra , Alnus glutinosa , and Robinia pseudoacacia in temperate
areas.
Casuarina spp. and perhaps Acacia spp. may be suitable in
southern Florida or California.
The report of Carmean and his co-workers ( 1976) regarding
growth of hardwoods on a site previously supporting Robinia
pseudoacacia is the best example of opportunities for crop rotation
in forestry.
Liriodendron tulipifera L. , Liquidambar styraciflua L. ,
and Juglans nigra L. made far superior growth , when planted after
Robinia than when planted on sites which had supported other native
hardwoods , Pinus spp. , or herbaceous vegetation.
Improved hardwood
growth was attributed primarily to increased foliar nitrogen levels.
Earlier work by Ike and Stone (1958) indicated net annual increases
of 44 kg-N/ha under Robinia.
Robinia has also been used on mine
spoils to improve site conditions sufficiently to permit establish­
ment of other species.
There are also several reports that suggest other opportunities
1).
in forestry , especially with Alnus species (Tarrant and Trappe 197
Atkinson and his associates ( 1979) conceptualized some systems for
rotating Alnus rubra and Pseudotsuga menziesii crops in the Pacific
The
Northwest and assessed biological and economical impacts.
assessment was based upon projected yields of Pseudotsuga menziesii
and Alnus rubra , patterns of nitrogen accretion over time in Alnus
stands , and an estimate of the effects of nitrogen added by Alnus
on Pseudotsuga growth.
Estimated yields from such systems were
higher than for programs involving only Pseudotsuga , but financial
returns were lower.
The authors concluded that future changes such
as expanded wood markets (particu larly for Alnus ) and increased
fertilizer costs could enhance profitability of he alternate crop­
ping systems and recommended pilot-scale testing of the concepts.
290
similar opportunities seemingly exist in the midwestern,
eastern, and southern United states for Alnus glutinosa, and Robinia
pseudoacacia, provided that problems associated with insect stem­
borers in the latter species can be resolved. Both species grow
rapidly at young ages (Lowery et al. 1 962) and could be utilized
for fuelwood, pulpwood, posts, or small sawlogs. Although s uitable
only to a limited geographic range, Casuarina spp. could also be
used in this system. This genera makes rapid early growth; its
wood is exceptionally strong and is an excellent fuelwood.
Mixed Species Systems
Mixed species systems involve a sharing of site resources
between the nitrogen-fixipg species and principal crop during all
or selected portions of a rotation. They may involve simultaneous
planting (as with nurse crop or companion crops) of both species
or interplanting (sub quent planting) of the nitrogen-fixing
species after the principal crop is established. Advantages of
mixed species systems include the absence of any period during which
the principal crop is not growing on the site. Benefits, however,
may accrue from the opportunity of the nitrogen-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 thinnin'g) . Disadvantages include allocation of a por­
tion of site resources (space, moisture, nutrients, light) to other
than the principal crqp. Far more serious considerations with
some mixtures are compatibility problems due to differential growth
patterns or to differences between the environment "created" by
the principal crop and that needed by the nitrogen-fixing species.
The following three systems are variations of the mixed species
theme. They differ in whether the nitrogen-fixing species has
commercial value and also in t he time that both species share site
resources.
Mixtures of two commercial tree species.--This system is dis­
tinguished from the other two mixed systems in that both species
have commercial value, disparate though such values may be. More­
over, there may be additional variants of this system in that the
nitrogen-fixing crop ma be planted simultaneously with the prin­
cipal crop or interplrulted in subsequent years.
It may also be
harvested prior to or at the same time as the principal crop .
The advantages discussed previously for mixed species systems
in general are all applicable to this system. In addition, the
nitrogen-fixing crop has value in its own right. Disadvantages
related to occupa tion of growing space are presumably greater than
for other mixed species systems, because the nitrogen-fixing species
occupies the tree layer rather than the shrub or ground layer of
vegetation. The need for information regarding just how many
individual nitrogen-fixing trees are needed per hectare is more
crucial for the above reason and also because options for control­
ling density of nitrogen-fixing trees are greater.
291
Suitable species for this system are the same as those listed
for rotation of two commercial crops (Alnus rubra, Alnus glutinosa,
Incompatibility in early
Robinia pseudoacacia and Casuarina spp. ) .
growth rates with the principal crop is likely to be a problem
with all these species, particularly if light-demanding conifers
are the principal crop.
However, interplanting several years after
the principal crop is established may do much to alleviate this
concern.
The Alnus rubra-Pseudotsuga menziesii plantation at Wind River
in the southern Washington Cascades is perhaps the classic example
of this system.
When the plantation was first studied at age 30
by Tarrant (1961) , the Pseudotsuga trees,were beginning to emerge
from the. Alnus canopy and were growing faster than trees in an
A more detailed study of
adjacent, pure Pseudots ga plantation.
soil properties (Tarrant and Miller 1963) indicated that Alnus has
substantially increased organic matter and nitrogen in the upper·
soil horizons.
At age 48 the mixed plantation had only two-thirds
as many Pseudotsuga stems as the pure stand, but volume of
Pseudotsuga alone in the mixed stand was 7 percent higher than that
More­
in the adjacent pure plantation (Miller and Murray 1978) .
over, the dominant Pseudotsuga of the mixed stand were 20 percent
taller and more than 30 percent larger in diameter than those in
the pure stand.
If per hectare volumes of both Alnus and
Pseudotsuga are considered, production in the mixed stand was
nearly double that of the pure st and.
Although simultaneous plant ings of Robinia pseudoacacia and
conifers inadvertently led to failure due to overtopping, Kellogg
(1934) suggested that understory plantings of ' Robinia in native
Pinus spp. plantations or mixt ures with rapid growing hardwoods
may be useful .
Indeed, McIntyre and Jeffries (1932) documented
the Robinia pseudoacacia-Catalpa bignonioides W. mixture as being
beneficial.
Finn (1953) found that several planted hardwood species
had superior growth when associated with Robinia that had been
planted simultaneously or interplant ed later.
Liriodendron
tulipifera, Juglans nigra, and Prunus serotina Ehrh. showed the
greatest response but several other species (Liquidambar
styraciflua, Fraximus pennsylvanica Marsh. , and Quercus spp. ) also
benefited.
Working on strip mine spoils in Kentucky, Plass (1977) showed
10-year growth of 5 hardwoods and 5 conifers was improved when
associated with a simultaneously planted nurse crop of Alnus
glutinosa.
No data are given however, on Alnus growth patterns as
related to the other species nor are there any speculations as to
whether Alnus will attain merchantable size.
Benefits from growing Alnus rubra and Populus trichocarpa
Torr. & Gray together in coppice culture has been documented in the
Pacific Northwest (DeBell and Radwan 1979) .
Two-year coppice yields
of the mixed plantings obtained 4 years after establishment were
substantially higher (35 percent plus) than yields from pure
29 2
The higher yields obtained in mixed
plantings of either species .
culture were attributed primarily to enhanced growth of Populus .
Growth of Alnus was similar to that obtained in pure plantings of
the species .
Nitrogen contents of Populus twigs and soil were
higher in the presence of Alnus.
Such findings may have application
for short-rotation systems for fiber and energy production in other
parts of the country .
For example, similar results seem probable
through combinations of Alnus glutinosa with Platanus occidentalis L . ,
Populus deltoides Bartr . var . deltoides and Liquidarnbar styraciflua.
Non-commercial nitrogen-fixing species in understory during
most of tree rotation . --In this system, the non-commercial nitrogen­
fixing species is planted simultaneously with or soon after the
It may be the dominant vegetation canopy for a
principal cro p .
short time, but usually is a component of the understory for most
of the stand's life .
The nitrogen-fixer may survive and grow
throughout the entire rotation if the trees are spaced far enough
apart.
It is more probable, however, that the nitrogen-fixing
species will be shaded out and reappear intermittently after thin­
ning operations, provided that seeds remain dormant in the forest
floor or mineral soil .
Advantages of this system are similar to those listed for the
In addition,
previous system (mixture of two commercial species) .
the nitrogen-fixing species in this system does not compete with
the principal crop for light, except perhaps in the first few years .
Other benefits may be associated with wildlife cover and food or,
in some cases, disease control .
The disadvantage is that the under­
story may be using moisture and nutrients that would otherwise be
absorbed by the principal crop.
Conversely, presence of the
nitrogen-fixing understory may decrease such competition compared
to that which would occur if other understory species occupied the
site.
A more important potential problem with this system is that
most nitrogen-fixing species (especially domesticated ones) are
intolerant of shade and nitrogen-fixation rates are reduced by
shading.
There are a number of species that may be used with this
system if stands remain relatively open .
These include Lupinus spp .
and Vicia spp . and perhaps most of the actinomycete-nodulated
genera, especially Elasagnus, Coriaria, Myrica, Ceanothus, and
The latter four genera are sometimes found natu­
perhaps Purshia .
rally under open pine forests in temperate climates, particularly
on dry sites .
The best example of this system is the intentional use of
Elaeagnus urnbellata Thunb . in widely-spaced Juglans nigra planta­
After 9 years, Juglans
tions in the midwest (Funk et al. 1979) .
grown in mixture with Elaeagnus averaged 70 percent taller and 76
How
percent larger in diameter than Juglans grown in pure stands .
long the Elaeagnus will persist in the understory is unknown .
Rather interestingly, all attempts to stimulate growth of Juglans
with nitrogen fertilizers have met with negligible success (Ponder
1976).
293
The common occurrence of Coriaria arborea as an understory
shrub in Pinus radiata plantations in New Zealand (Silvester 197E)
suggests its suitability for a mixed species system .
Silvester
(1977) showed definite benefits to early Pinus growth from Coriaria
but indicated that vigorous Coriaria may suppress growth in
recently planted Pinus stands.
On better sites where nitrogen is
not severely limiting, Coriaria is eventually shaded out by other
shrub species as stands mature. However, Silvester reports that
following thinning, Coriaria may reenter the Pinus stands and that
Coriaria persists as long as it receives at least 30 percent full
sunlight. Unless seeds remain dormant and viable over long periods
in the forest floor or mineral soil, an adjacent seed-supplying
stand or direct seeding would be essential.
A dense understory of Alnus rubra was established after heavy
thinning in a 62-year-old Pseudotsuga menziesii stand and added
nearly 880 kg-N/ha before it began to die out (Berg and Doerksen
1975). Use of Alnus species in the present system, however, would
require either planting or an adjacent seed source.
Another example of this system also occurs in the Pinus
radiata forests of New Zealand, the first phase of which was used
as an example of green manuring. After stabilizing sand dunes,
Lupinus arboreus is crushed or sprayed and Pinus radiata is planted
(Gadgil 197l). A new growt h of Lupinus is soon established and
persists until Pinus canopy closure. Seeds of Lupinus persist in
the soil, and germination is stimulated by the activity of machinery
and increased light associated with the first (10-12 years) and
second (18-22 years) thinnings. Thus, Lupinus is perhaps the best
example of intermittent occupation of the site by an understory
nitrogen-fixing species. Seemingly, opportunities may exist for
similar use of other legumes and actinomycete-nodulated plants
(Myrica, Ceanothus, and Purshia) with planned management programs.
Non-commercial nitrogen-fixing species in understory during
early years only.--In this system, the nitrogen-fixing species is
established, simultaneously with or soon after the principal crop .
It acts as a nurse plant d uring the stand's early life, is shaded
out after the canopy closes, and does not become reestablished at
later stages.
Nitrogen is accumulated in the soil by the nitrogen-fixing
species while available resources (space, light, other nutrients,
and water) exceed demands of the principal crop. Full sunlight,
especially in widely spaced stands, may result in high nitrogen­
fixation rates. Channeling site r esources to an ideal nitrogen­
fixing species by purposeful management may also limit encroach­
ment of non-desirable vegetation which would otherwise volunteer
on the site. The greatest potential disadvantage of this system
is that the period of greatest nitrogen need of the principal tree
crop may not coincide with the period of occupancy by the nitrogen­
fixing species .
Thus, benefits for later stand use are dependent
upon high nitrogen accretion in early years, good nitrogen retention
on the site, and ready availability of the nitrogen for later stand
needs.
294
High levels of insolation in this system make use of most
nitrogen-fixing species possible. Probably many legumes would be
suitable, provided that soil fertility and pH are at favorable
levels. The latter restrictions can be rather important for many
domesticated varieties (Jorgensen 1978) in forest environments,
but may be less so with native legumes and actinomycete-nodulated
plants. Within the latter group, several shrubby species of Alnus
merit consideration.
The best example of this system in forestry is the recent
study in which 5 legumes (3 Trifolium spp. and 2 Vicia spp.) were
tested in a 2-year-old Platanus occidentalis plantation (Haines
et al. 1978). After 4 years, total height and volume of Platanus
in the legume-containing plots exceeded that in the check plots by
more than twofold and threefold, respectively. Trifolium
subteraneum L. and Trifolium incarnatum L. also provided the best
weed control, maintained a low profile, and did not shade or vine
around the young Platanus trees.
Legumes have been similarly used for several years in the
Hevea braziliensis Muell. Arg. plantations of Malaya. Nitrogen
fixation by creeping legumes d uring a 5-year period was estimated
at nearly 880 kg-N/ha (Broughton 1977) .
In such plantations,
control of root rot diseases such as Fornes lignosus (Klotzsch)
Bres. is a major benefit also (Fox 1965) .
Despite the fact that
legumes died out after the sixth year, increases in rubber yields
persisted for about 20 years.
Attempts to establish legumes (Aeschynomene, Melilotus,
Indigofera, Desmodium, Lespedeza, and Vigna) in newly planted
Pinus elliottii Engelm. var. elliottii on infertile sands were
unsuccessful (Anonymous 1974) .
Even with disking, liming, and
fertilization, none of the legumes established themselves to any
marked degree.
Nitrogen-fixing Trees as the Principal Crop
This system differs from the others in that the nitrogen-fixing
tree is the crop species of prime interest. Advantages of such
culture lie in the fact that most nitrogen-fixing trees grow very
,
rapidly (especially at young ages) , are adaptable over a wide range
of sites, and appear to require lower management inputs for excel­
lent growth than most non-nitrogen-fixing trees. Thus, nitrogen­
fixing species appear particularly well-suited for short-rotation
production of fuel and fiber and possibly small sawlogs and posts.
Disadvantages include the fact that current stumpage values are
usually lower for these species than for others which might occupy
the site. Also questions persist about potential detrimental
effects on soil pH and base status over the long term, particularly
with Alnus (Bollen et al. 1967, Franklin et al. 1968) .
Alnus species offer by far the greatest opportunities for
commercial production in temperate climates. Robinia pseudoacacia
may also have potential in temperate climates if insect problems
29 5
can be resolved. A number of Casuarina and Myrica species have
potential for local consideration in tropical and subtropical
environments (Gordon and Dawson 1979) .
Yield data collected in young, natural stands of Alnus rubra
(Zavitkovski and Stevens 1972, Smith and DeBell 1974) show that
15-25 tons/ha/yr of aboveground biomass are possible. Such data
are indicative of the potential of this species. Recently esti­
mates of yields that might be obtained with spacing alone in con­
ventional management systems (pulplogs and sawlogs) have been
published (DeBell et al. 1978) .
Where winter hardiness is important ; Alnus glutinosa may have
utility. This species has been planted on mine spoils , and recently
has received serious attention as a fiber source (particularly for
silage cellulose systems) in the Southeast (Saucier 1977) .
Nitrogen-fixing trees deserve con s ideration in research and
development programs on "biomass for energy" plantations. They
may also have future importance in areas where synthetic nitrogen
. fertilization has been a necessary requisite to adequate growth
of other commercial species.
FUTURE CONSIDERATIONS
The future use of biological nitrogen fixation in forest man­
agement programs will depend , in part, on development of essential
information via research, but other factors also impact implemen­
tation.
For example , changes in the availability and cost of
chemical nitrogen fertilizer and associated application costs may
enhance or detract from the potential utility of nitrogen-fixing
plants in forest management systems.
One frequently overlooked
obstacle to use of nitrogen-fixing species is the fact that foresters'
experiences with these plants have in many cases been limited to
Pueraria thunbergiana (S. & Z.) Benth. or sane similarly "noxious
weed." Thus , many forestex's need to be educated as to the benefits
that accrue from use of nitrogen-fixing plants.
Gaps in the knowledge of biological nitrogen fixation in
forest ecosystems are large. Areas requiring attention include :
(i) screening of the most promising native and domesticated species
and genera, (ii) methods of establishing these plants on forest
sites (including specie s propagation) , (iii) development of effec­
tive inocula and inOCUlation techniques , (iv) assessment of nitro­
gen fixed and its benefits to the tree crop , and (v) assessment of
auxiliary effects of nitrogen-fixing plants on the forest environment.
Critical ecological requirements will vary with species and
with the cropping system proposed , particularly light requirements.
Species needing full or a large percentage of insolation will have
little utility in developed stands where shading is heavy. Transfer
of agr icultural legumes to forest ecosystems will depend on satis­
factorily resolving fertility, moisture , and pH (as well as light)
296
requirements of these species.
Use of native legumes and
actinomycete-nodulated plants should present fewer problems.
Optimal site preparation and fertilization prescriptions are
needed.
Methods of establishment (broadcast seeding, strip seeding,
containerized seedlings, etc. ) must be eval uated.
For example, a
legume that grows so tal l and rank that it severely shades and
crowds newly planted seedlings may still have utility when strip
planted between the tree rows.
Prior to testing native species, seed col lection an4 treatment
techniques must be developed. Survival mechanism in some native
species is seed dispersal as a result of seed pod "explosion. "
While this type of dehiscence may be beneficial once the species is
established, it creates problems during seed collection.
Seeds
cannot be coll ected before they mature, but must be harvested
Many native legumes have extremely hard
before they are dispersetl.
seed coats, requiring drastic treatment before germination occurs.
Some seeds germinate only after exposure to high temperatures
(e. g. , pr scribed burning Martin et al. 1975) .
Development of the most effective inocula for agronomic legumes
has taken decades ; foresters cannot expect to discover the best
Those strains discarded by agronomists in their
inocula overnight..
breeding programs may be the ones of most value on forest soils.
Development of inocula fbr native species has received only casual
consideration.
Techniques for assessing fixation rates and nitrogen accretion
in forest ecosystems are not refined.
Fixation rates for native
and agricultural species growing on forest soils must be determined.
Direct comparisons on the same site of biological nitrogen fixation
and chemical nitrogen fertilization are needed.
Assessment of beneficial and detrimental auxi iiary effects of
nitrogen-fi xing plants on the forest environment should be made.
Beneficial effects include increased organic matter, decreased bul k
density, and possible disease control.
Conversely, increased soil
pH needed to establish some nitrogen-fixing plants may be detrimental
These plants may attract insects or animals to a
to the tree crop.
greater degree than would occur if only the tree crop were present
on the site.
CONCLUDING REMARKS
Nitrogen-fixing plants can provide a useful and versatile tool
for improving and maintaining forest prod uctivity.
Cropping sys­
tems involving these plants impact other productivity factors as
wel l as add nitrogen.
Present use of nitrogen-fixing species is
largely confined to special situations, such as mine spoils, road
cuts and fills, and wildlife management areas.
A number of factors
limit immediate implementation on most other lands. A comprehen­
sive program of applied r esearch and development, however, could
soon provide information needed to exploit this tool for more
genera l use in forest management.
297
LITERATURE CITED
In Progress
Anonymous. 1974. Legumes in young pine plantations .
report, cooperative research in forest fertilization, p . 81­
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