growth and seed yield of three perennial grains within monocultures and mixed stands

Ž .Agriculture, Ecosystems and Environment 68 1998 1–11

Growth and seed yield of three perennial grains withinmonocultures and mixed stands

Jon K. Piper )

The Land Institute, 2440 E. Water Well Road, Salina, KS 67401, USA

Accepted 6 June 1997

Abstract

ŽThis study examined growth and seed yield of three perennial species, Desmanthus illinoensis Illinois bundleflower, a. Ž . Ž .legume , Leymus racemosus mammoth wildrye, a C grass , and Tripsacum dactyloides eastern gamagrass, a C grass , in3 4

monoculture, biculture, and triculture treatments, on two soils differing in initial fertility, and over 5 yrs. There weresignificant effects of site, treatment, and year on both aboveground biomass and seed yield. On average, bundleflowermonoculture produced the greatest aboveground biomass, although the three-species mixture produced the peak biomassŽ 2. Ž814 grm in any given year. Among treatments, Illinois bundleflower monoculture yielded the most seed maximums122

2.grm . Overall, biomass and seed yield were higher at the more fertile Site 1, but species differed in their dependence onsoil fertility. Among species, bundleflower performed fairly independently of soil fertility, wildrye grew poorly on the lessfertile soil, and gamagrass persisted at both sites although it grew less well at Site 2. In most cases, mixtures produced as

Ž .well as the best-yielding monoculture. In 26 of 30 instances, biomass relative yield totals RYTs were statistically )1.0and, in 19 of 21 cases, RYT for seed yield was statistically )1.0. In general, the overyielding effect appeared stronger atSite 2 than at Site 1, with seed yield RYT appearing to increase with time at Site 2. The results show that the seed yield ofperennials can be high, and that some species can persist in mixture for several years. The data for 5 yrs point to the need tofollow long-term patterns of yield and interspecific interactions within perennial grain polycultures in order to maintainspecies diversity and to make reasonable predictions. q 1998 Elsevier Science B.V.

Keywords: Desmanthus illinoensis; Leymus racemosus; Perennial grains; Polyculture; Relative yield total; Soil; Tripsacum dactyloides

1. Introduction

Several studies have demonstrated that thereestablishment of perennial cover on retired crop-land can reduce soil erosion, increase root turnover,and increase the accumulation of surface litter. The

) Corresponding author. Department of Biology, Bethel Col-lege, North Newton, KS 67117, USA. E-mail: [email protected]

greater root biomass associated with establishedŽ .perennial grasses Richter et al., 1990 commonly

gives annual C inputs into the soil that can be severalŽtimes greater than those into cultivated soils Ander-

son and Coleman, 1985; Buyanovsky et al., 1987;.McConnell and Quinn, 1988 while reducing rates of

Žnutrient leaching relative to annual crops Paustian et.al., 1990 . In the Great Plains of the US, active soil

organic matter, available nutrients, water-stable ag-gregates, and polysaccharide content may recover

0167-8809r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0167-8809 97 00097-2

( )J.K. PiperrAgriculture, Ecosystems and EnÕironment 68 1998 1–112

Žunder perennial grasses within a few decades Mc-.Connell and Quinn, 1988; Gebhart et al., 1994 ,

although the total soil organic matter pool may takeŽlonger to reach that of the virgin state Parton et al.,

.1987; Burke et al., 1995 .Moreover, studies using experimental mixtures of

grassland plant species have shown that plant biodi-versity can be associated with higher productivity aswell as greater efficiency of soil nutrient extractionŽ .Tilman et al., 1996 . This phenomenon is reflectedin agronomic mixtures that outproduce their respec-tive monocultures because of differences in locationand timing of resource use that reduce overlap inresource demand between interspecific neighbors.For example, roots of different species may exploredifferent soil layers, or develop at different times, orspecies may have complementary nutrient require-ments, as in grassrlegume mixtures that produce

Žhigher dry matter yields e.g., Barnett and Posler,.1983; Posler et al., 1993 . Similarly, intercrops may

be released from competition for light, and showgreater overall productivity, if canopies of compo-nent species develop at different times or the canopy

Žarchitecture minimizes mutual shading Davis et al.,.1984; Clark and Francis, 1985 . Finally, differences

in length of the growing period or in the peakŽperiods of nutrient uptake among species e.g., Piper,

.1993a can also reduce direct competition and thusŽ .promote overyielding Smith and Francis, 1986 .

Successful polycultures consist of species thatcomplement one another spatially, seasonally, or in

Ž .nutrient requirements, so that they either a useŽ .land, labor, or resources more efficiently; b in-

Ž .crease yield; c reduce loss to insects, diseases, andŽ . Žweeds; or d reduce yield variation Moreno and

.Hart, 1979; Francis, 1986; Vandermeer, 1989 . Un-fortunately, it may not be possible to predict, fromits performance in monoculture, how a species willbehave in polyculture. For example, some specieshave different patterns of nutrient uptake when grown

Žin association with other species Goodman and Col-.lison, 1982 , and shorter plants may be shaded out

by taller neighbors in polyculture, although they arevigorous in monoculture. Moreover, the relative per-formance of monocultures and mixtures may differwith site and time.

Environments that differ in soil fertility, water-holding capacity, and exposure to wind and sunlight

are likely to influence not only plant performancebut also the outcomes of plant species interactionsŽPickett and Bazzaz, 1978; Boryslawski and Bentley,

.1985; Tilman, 1987 . Similarly, physiological differ-ences between species may also affect the outcomeof interactions in various environments. Because ofpossible differences between environments in theoutcome of interactions, then, it is crucial to studyspecies interactions on soils of widely different fertil-

Ž .ity Connell, 1983; Smith and Francis, 1986 anddifferent climatic conditions.

The relative success of different species, whengrown in mixtures, can change with time. Speciesthat dominate a mixture initially, when levels of bothsunlight and available soil resources are high, maylater be suppressed if they are poor competitorswhen available soil nutrients and water decline andshading increases.

Grain-producing mixtures of perennial grasses,legumes, and composites could protect the soil andprovide the restorative properties of a perennial coverwhile yielding significant amounts of edible grain.Several promising candidates for perennial grain

Žagriculture have been identified Wagoner, 1990;.Soule and Piper, 1992 , though further selection and

breeding for intercrop compatibility and grain yieldis needed. The present study investigated the perfor-mance of three perennial species, differing in photo-

Ž .synthetic pathway C vs. C and ability to fix N,3 4

that show promise for perennial grain production. Todetermine the effects of site, species composition,and year on growth and seed yield, the study wasconducted in two environments and over 5 yrs. Assuch, the study may be seen as a model system topredict how perennial grains will interact with soiltype, differences in species composition, and time.

2. Materials and methods

2.1. Species

Ž . ŽDesmanthus illinoensis Michx. MacM. Illinois.bundleflower, Mimosaceae is a nitrogen-fixing

species that forms deep taproots in its first year. It isnative to the Great Plains, with a range extendingnorthward to Minnesota, east into Florida, and as far

Žwest as New Mexico Great Plains Flora Associa-

( )J.K. PiperrAgriculture, Ecosystems and EnÕironment 68 1998 1–11 3

.tion, 1986 . In favorable years, plants approach 2 mheight after 4 months. In central Kansas, it flowersfrom late June onward. Maximum seed yield has

2 Ž .ranged from 163 to 197 grm Piper, 1993b , withŽhigh nutritional quality 38% protein, 34% carbo-

. Ž .hydrate Piper et al., 1988 , suggesting its potentialas a grain legume. The bundleflower accession usedhere was originally collected from a wild populationin Ellsworth, KS.

Ž . ŽLeymus racemosus Lam. Tsvelev mammoth.wildrye, Poaceae is a rhizomatous C species native3

to Bulgaria, Romania, Turkey, and southwestern partsof the former Soviet Union. Its grain has been gath-ered by Asian and European people, especially indrought years when annual grain crops failedŽ .Komarov, 1934 . Reproductive tillers grow to about1.5 m high, and maximum seed yields have ranged

2 Ž .from 51 to 83 grm Piper, 1993b . Most growthand uptake of soil water and nutrients occurs inspring and autumn, and seeds mature by late JuneŽ .Piper, 1993a . The wildrye used here was from astand of US Natural Resource Conservation Servicevariety ‘Volga wild rye’ planted at The Land Insti-tute, Salina, KS, USA in 1989.

Ž . ŽTripsacum dactyloides L. L. eastern gamagrass,.Poaceae is a large C bunchgrass native from the4

southeastern United States and Great Plains south-Žward to Bolivia and Paraguay Great Plains Flora

.Association, 1986 . The canopy height ranges from 1to 2 m; reproductive tillers can exceed 2 m high.Although gamagrass is an excellent forage species, italso shows much promise as a grain crop for humanconsumption. The grain is nutritious, containing 27

Ž .to 30% protein and 7% fat Bargman et al., 1989 ,and has baking properties similar to those of maize,

Žbut its seed yield is low typical range for uncleanedspikelets is 40–100 grm2; seed mass is approxi-

Ž ..mately 25% of spikelet mass Piper, 1993b . Thegamagrass used in this study was derived from seedcollected originally from a natural population justwest of Salina, KS.

2.2. Study sites and experimental design

The study took place within experimental plotsestablished at The Land Institute, located 4.8 km SE

Žof Salina, KS, USA Section 5 T15S R2W Hutchin-X X .son Quadrangle, 38844 N, 97834 W . Plots were

established in March 1991 on two sites at 3 kmŽapart. Site 1 was on a level Cozad silt loam Coarse-.silty, mixed, mesic Fluventic Haplustolls , previ-

ously in continuous wheat, then planted to alfalfaŽ .Medicago satiÕa L. in 1990. Site 2 was the southface of a hillside on a Kipson–Clime complex soilŽfine to loamy, mixed, mesic, Udorthentic Haplus-

.tolls that had experienced erosion. This area wasŽplanted to native grasses primarily Andropogon ger-

w xardii Vitman big bluestem , Bouteloua curtipendulaw x w x .Michx. Torr., and Sorghastrum nutans L. Nashin 1982, but was continually cropped before then.Relative to Site 2, Site 1 soil initially had lower pH,

Žhigher concentrations of available and total poten-.tially mineralizable N to 60 cm depth, and higher K

Ž .near the surface Table 1 . Average annual precipita-tion is 735 mm, with almost 75% falling during theApril to September growing season.

Plots were planted in May in a replacement seriesdesign in which initial overall density was constant.

Ž .Six cropping system treatments were used: i threeŽ w x w xmonocultures bundleflower Di , wildrye Lr , and

Table 1Ž .Initial levels mgrkg of selected soil nutrients at two experimental sites. Ns18 observations per depth per site

Ž . Ž .Site Depth cm pH Organic C Available N Total N Available P Bray KŽ .NH qNO4 3

b bb1 0–30 5.94 11300 21.6 1090 10.8 363ab30–60 6.74 6900 10.9 700 6.8 195

60–100 7.69 5600 7.8 500 2.6 169b a2 0–30 6.91 10640 8.4 930 15.7 276

aa30–60 7.45 6600 6.0 620 6.8 229aa60–100 7.78 5300 6.8 510 12.0 222

a,b Ža b .Indicates a significantly higher level P-0.05, P-0.001, Student’s t-test between sites at this depth.


w x. Ž .gamagrass Td , ii two 1:1 alternating mixturesŽ w xgamagrass with wildrye TdrLr and gamagrass

w x. Ž .with bundleflower TdrDi , and iii a random mix-Ž .ture of the three species in a 1:1:1 ratio TdrLrrDi .

All treatments were replicated three times, in a ran-Ždomized complete block design, at each site Ns36

.plots .Bundleflower and gamagrass plants were started

as seedlings in the greenhouse whereas wildrye plantswere transplanted to the sites from a nearby fieldstand. Plots were 7.31 m wide by 9.00 m long, withrows oriented east to west, containing 96 plants perplot. Rows were 0.91 m apart, with plants placed0.75 m apart within rows, resulting in a density of1.47 plantsrm2. All species were planted at the samedensity. This initially wide spacing was used toallow for plant horizontal spread in subsequent years.The outer two plants on all sides were left as aborder to minimize edge effects, leaving a 21.8 m2

data area in the center of each plot. Plots werehand-weeded in the first year to assist establishment.Four-meter borders between plots, to control forinterference between plots, consisted of alfalfa atSite 1 and native grass sod at Site 2.

2.3. Measurements

Aboveground biomass of bundleflower was esti-mated in late summer by determining total basal

Ž .stem diameter mm for each plant, then convertingŽ .diameter to mass using the expression: biomass g

Ž w 2 x. Ž 2s0.556 basal stem area mm y4.65 r s0.933,.P-0.0001, Ns98; J.K. Piper, unpublished . The

aboveground biomass of wildrye was estimated bymeasuring and then summing the total length of

Ž .reproductive tillers cm just before seed harvestŽ Ž . Ž w x.mass g s0.120 total tiller length cm q7.57;r 2 s0.757, P-0.0001, Ns25; J.K. Piper, unpub-

.lished . Aboveground biomass of gamagrass plantswas estimated in late winter by measuring basal

Ž .crown circumference cm , then converting basalarea to aboveground biomass using the expression:

Ž . Ž w 2 x. Ž 2mass g s0.315 basal area cm y18.55 r s.0.903, P-0.0001, Ns41; J.K. Piper, unpublished .

To measure seed yield, bundleflower pods wereharvested when ripe. Air-dry pods were threshed,then seed was cleaned using an office-sized cleaner.Because wildrye tends to shatter upon ripening, en-

tire rachises were clipped as seed began to ripen inlate June and collected in paper bags in the field.Harvest of mature rachises was repeated weekly untilharvest was complete. Rachises were stored in agreenhouse until dry, then threshed and cleaned.

Ž .Harvest of gamagrass seed i.e., entire spikeletsbegan in late July, and was repeated every one totwo weeks until complete. Air-dry seed of all specieswas weighed to "0.01 g in the laboratory.

To assess whether there were yield advantages inŽ .polyculture, the relative yield total RYT , a com-

monly used measure of overyielding, was used: it isthe sum of the fractions of the various components

Žrelative to their yields in monoculture Mead and.Willey, 1980 . If intraspecific competition is stronger

than interspecific competition, or facilitation is oc-curring, plants should yield relatively better in mix-ture than in monoculture, resulting in an RYT)1.

2.4. Statistical analyses

Results were analyzed using the MGLH proce-Ž .dure of SYSTAT for Windows SYSTAT, 1992 ,

testing for effects of site, treatment, year, and inter-actions. The analysis was treated as a split–blockanalysis at each site and combined over both sites.Treatments were assigned randomly within blocks.Comparisons were made using Tukey’s HCD Proce-dure. The significance level for all tests was P-

0.05.

3. Results

3.1. Biomass yield

3.1.1. Site effectsOverall, the greater soil fertility of Site 1 was

reflected in greater plant production, based on thebiomass formulas presented in the Materials and

ŽMethods, relative to Site 2 means327 vs. 2622 .grm ; F s42.17, P-0.0001 . Among species,1,4

Žwildrye appeared most sensitive as indicated by.biomass response to site quality, bundleflower was

least sensitive, and gamagrass was intermediate inresponse to soil quality.


3.1.2. Treatment effectsAcross sites and years, there were significant

Ždifferences among treatments F s72.09, P-5,20.0.0001 , with bundleflower monoculture producing

Žthe greatest aboveground biomass mean s 4112 . Žgrm and wildrye monoculture the least means

2 . A113 grm . The ranking of treatments was Di )

TdrDiAB ) TdrLrrDiAB ) TdAB ) TdrLr B ) Lr C.In addition, there was a significant site by treatment

Ž .interaction F s4.33, Ps0.008 , with treatment5,20

groupings, but not their ranking, differing betweensites.

3.1.3. Year effectsLarge annual differences in precipitation, damage

by small mammals in 1 yr, and the decline of onespecies led to a significant year effect on above-

Ž .ground biomass F s1158.07, P-0.0001 as4,16Žwell as a year-by-site interaction F s28.28, P-4,16

. Ž0.0001 . The years 1991 and 1994 were dry 1991:.60.6 cm; 1994: 58.9 cm annual precipitation ,

whereas 1992, 1993, and 1995 were wetter thanŽnormal 1992: 92.7 cm, 1993: 147.5 cm, 1995: 87.5

. Žcm . Rodent probably Sigmodon hispidus Audubon.and Bachman, Cricetidae damage in 1993 affected

seed yield of one species and growth and seed yieldŽ .of another see Section 4 . During the winter of

1993–1994, small mammals grazed numerousbundleflower crowns just below ground level at Site1, leading to reduced bundleflower size, and thuslower seed yield, in the 1994 and 1995 growingseasons. This grazing occurred more in biculture andtriculture treatments than in monoculture plots. Inaddition, during the summer of 1993, small mammalgrazing on gamagrass reproductive tillers precludedgamagrass seed harvest in all plots at Site 2, al-though plant size appeared unaffected.

Treatment effects occurred in all site–year combi-Ž .nations but Site 2 in 1991 Fig. 1 . In nearly every

case, total biomass of mixtures was similar to that ofthe most productive monoculture. Wildrye wasclearly the least vigorous of the three species, andwas suppressed in biculture with gamagrass. After1993, wildrye was only a minor contributor to thetotal biomass of polycultures. By 1994, wildrye haddeclined greatly at Site 2 and from biculture plots atSite 1; however, there was some evidence of recov-ery, via tillering, of this species at Site 1 in 1995. In

Fig. 1. Mean estimated total aboveground biomass of six croppingsystem treatments at two sites in 5 yrs. In each group of bars, thefirst three bars represent monocultures, the fourth and fifth barsare bicultures, and the sixth bar indicates the three-species treat-ment. For each year–site combination, bars with the same letter

Ž .do not differ at P -0.05 ANOVA, Tukey’s HCD Procedure .

contrast, bundleflower persisted well in mixture withgamagrass throughout the study.

3.2. Seed yield

3.2.1. Site effectsAs with aboveground biomass, site differences in

soil quality were reflected in overall higher seedŽproduction at Site 1 than Site 2 F s54.27, P-1,4

.0.0001 . Seed yield of wildrye appeared highly de-pendent on site quality whereas bundleflower yieldwas fairly independent of soil fertility.

3.2.2. Treatment effectsThere were significant differences among treat-

Ž .ments F s77.66, P-0.0001 , with bundle-5,20

flower monoculture the overall highest seed yieldingŽ Atreatment and grass monocultures the lowest Di )


Fig. 2. Mean total seed yield of six cropping system treatments attwo sites in 5 yrs. For each year–site combination, bars with the

Žsame letter do not differ at P -0.05 ANOVA, Tukey’s HCD.Procedure . Treatment and shading designations are as in Fig. 1.

B BC C CD D.TdrDi ) TdrLrrDi ) TdrLr ) Td ) Lr .There was, however, a significant site by treatment

Ž .interaction F s5.00, Ps0.004 , such that the5,20

w ild ry e an d g am ag rass m o n o cu ltu res ,gamagrassrwildrye biculture, and triculture treat-ments yielded better at Site 1 and the bundleflower

monoculture and gamagrassrbundleflower treatmentyields were independent of site. The overall rankingof treatments was the same at each site.

3.2.3. Year effectsBundleflower flowered and set seed in each year,

whereas both grass species remained vegetative inthe establishment phase, which lasted 1 yr. The lowprecipitation in 1991 appeared to reduce bundle-flower yield at Site 2 more than at Site 1. Themammal damage that led to reduced bundleflowersize also lowered seed yield in the 1994 and 1995growing seasons. In the summer of 1993, smallmammal grazing on gamagrass reproductive tillersprecluded seed harvest in all plots at Site 2. As a

Žconsequence, there were highly significant F s4,16.156.98, P-0.0001 year effects as well as year-by-

Ž .site F s20.07, P-0.0001 and year-by-treat-4,16Ž .ment F s16.79, P-0.0001 interactions.20,80

Examining treatment seed yields for each site ineach year showed that there were significant treat-

Ž .ment effects on seed yield in most instances Fig. 2 .Bundleflower seed yield peaked in 1992, with meanhighs around 120 grm2 at the two sites, then lev-elled off in 1994 and 1995. Bundleflower monocul-ture yield exceeded yield of the other treatments atSite 1 in 1992 and at Site 2 from 1992 to 1995. AtSite 2, the bundleflower monoculture consistentlyyielded best among treatments, followed in generalby the gamagrassrbundleflower biculture and three-species mixtures. At Site 2, in contrast, bundleflowermonoculture consistently outyielded the other mono-culture and mixture treatments from 1992 to 1995.

Table 2Ž .Relative yield totals aboveground biomass for three perennial species mixtures in 5 yrs. Values are means"SE, Ns3

Mixture 1991 1992 1993 1994 1995

Site 1a aGamagrassrwildrye 1.63"0.04 1.57"0.42 1.14"0.06 1.14"0.18 0.82"0.05

a a aGamagrassrbundleflower 1.14"0.08 0.78"0.06 1.19"0.01 0.88"0.02 1.00"0.07a aGamagrassrwildryerbundleflower 1.79"0.04 0.84"0.08 1.62"0.05 1.00"0.26 1.08"0.15

Site 2a a aGamagrassrwildrye 2.09"0.35 1.66"0.30 1.01"0.09 1.44"0.09 1.47"0.06

aGamagrassrbundleflower 2.37"1.40 0.79"0.08 1.01"0.03 0.84"0.02 1.08"0.06a a a aGamagrassrwildryerbundleflower 2.00"0.28 1.45"0.11 1.36"0.04 1.39"0.19 1.65"0.08

a Ž .Significantly different from 1.00 P-0.05, Student’s t-test .


Table 3Ž .Relative yield totals seed yield for three perennial species mixtures in 4 yrs. Note that in 1993, the reproductive tillers of T. dactyloides

were grazed by rodents prior to harvest at Site 2 so no data is available. Values are means"SE, Ns3.

Mixture 1992 1993 1994 1995

Site 1aGamagrassrwildrye 1.17"0.08 1.10"0.11 0.84"0.05 1.11"0.19

Gamagrassrbundleflower 1.05"0.03 1.13"0.16 0.99"0.13 1.06"0.22aGamagrassrwildryerbundleflower 0.86"0.01 1.25"0.14 1.05"0.28 1.02"0.18

Site 2aGamagrassrwildrye 1.03"0.17 y 1.54"0.22 1.52"0.11aGamagrassrbundleflower 1.00"0.11 y 1.90"0.41 2.28"0.32aGamagrassrwildryerbundleflower 0.91"0.12 y 1.22"0.14 1.37"0.11

a Ž .Significantly different from 1.00 P-0.05, Student’s t-test

The yield pattern of wildrye monoculture differedgreatly from that of bundleflower, yielding lower

Ž .than most other treatments in most years Fig. 2 . Ingeneral, wildrye performed relatively poorly in mix-ture, its growth particularly reduced by gamagrass.In 1994, wildrye monoculture at Site 1 yielded morepoorly than all other treatments. The exception wasat Site 1 in 1993 where wildrye in monocultureyielded 71.2 grm2.

Seed yield of gamagrass also varied considerablyŽ .among sites, treatments, and years Fig. 2 . It came

to dominate mixtures at Site 1, largely as a result ofpoor wildrye competitive ability and bundleflowerdamage from mammals, whereas it and bundleflowerpersisted well in mixture throughout the study at Site2. Its highest monoculture yield occurred at Site 1 in

Ž 2 .1993 60.3 grm .

3.3. OÕeryielding

For estimated aboveground biomass, in nearly allŽ .i.e., 26 of 30 instances, RYT was statistically

Ž .G 1.0 Table 2 . The three-species mixtureoveryielded more often than either two-species mix-ture. In general, the overyielding effect appearedstronger at Site 2 than at Site 1. Wildrye contributed

Ž .little to RYT after 1993 see Fig. 1 , but there waslittle evidence of increasing or decreasing RYT withtime.

Relative yield totals were not as high for seedŽ .yield as they were for biomass Table 3 . In 19 of 21

cases, however, RYT was statistically G1.0. As

with aboveground biomass, RYTs for seed yieldappeared somewhat higher at Site 2. At Site 1, RYsfor wildrye and bundleflower contributed little toRYT after 1993. At Site 2, seed yield RYT appearedto increase with time.

4. Discussion

4.1. Growth and reproduction in two enÕironments

Although overall growth and seed yield werehigher at the more fertile Site 1, individual compo-nent species responded to different environments indifferent ways. Differences in N-fixing ability andphotosynthetic pathway among the experimentalspecies led to predictable differences in the waysspecies interacted with soil type.

Of the three species, bundleflower was least de-pendent on soil fertility, particularly available N. Theonly dramatic site effect occurred in the establish-ment year, suggesting that the relatively fertile soiland lower water stress at Site 1 favored its establish-ment in that dry year when water was probably morecritical than soil nutrient status. Site differenceslargely disappeared in subsequent years, however,suggesting that this legume is able to compensate forlow soil N without reducing its growth and seedyield.

Among species, wildrye displayed the greatestdifferences between sites, indicating that this C3

species was the most soil quality dependent of the


three. Growth and yield of wildrye were consistentlylower at Site 2, even in wet years, indicating greaterdependence of this grass on high soil available Nconcentrations. Low favorableness of Site 2 for thisspecies was further evidenced by the near disappear-ance of wildrye there by 1994. Wildrye may havedifficulty persisting under low soil N concentrations.

Gamagrass appears tolerant of low soil N regimes,although its growth and seed yield were noticeablylower at Site 2 than at Site 1 in each year. PerennialC grasses may have lower soil N requirements than4

many C grasses. In grasslands, C grasses may3 4

retain their dominance because of their ability towithstand low soil N conditions. In contrast to the C3

wildrye, the C gamagrass may be better able to4

extract soil available N and is therefore more tolerantŽof relatively low N levels see Wedin and Tilman,

.1990; Tilman and Wedin, 1991; Piper, 1993a . Sitedifferences in soil texture, or associated soil physicalproperties, may have also affected the performanceof the plant species.

In addition, there were also differences in theperformance of mixtures between the two sites. Pho-tosynthetic pathway and ability to fix atmospheric Nare two factors likely to influence the outcome ofinteractions on different soils, leading to the predic-tions that N-fixing species and C grasses should4

compete better on less fertile soils whereas perennialC grasses should persist better on more fertile soils.3

These patterns are observed in some tallgrass prairiesites, where poor soils support relatively high propor-tions of N-fixing species in addition to C grasses,4

Ž .but few C grasses Piper, 1995 . On deeper, more3

fertile soils, legumes may be rare, but the C3

graminoid component may average 20% of above-ground biomass.

Ž .Soria et al. 1975 found that overyielding byintercrops of maize, bean, and cassava consistentlyexceeded 1.0, but was higher under low rather thanhigh soil fertility conditions. Similarly, in the presentstudy, both biomass and seed yield RYTs for themixtures were generally higher at Site 2 than Site 1.Because a low-N soil should favor N-fixing species,and not allow grasses competitive exclusion, thebenefits from intercropping legumes and nonlegumesshould be greatest in low N regimes. This suggeststhat the benefit of polyculture is greater on soils ofdiminished fertility.

4.2. Growth and reproduction in monocultures andmixtures

Much of the emphasis in multiple cropping sys-tems research involves identifying methods or plant-ing designs that allow or even promote species coex-istence. In the present study, significant biomassoveryielding, as measured by RYT, occurred for 11of 30 cases and significant seed overyielding wasshown in 3 of 21 cases. At Site 2, biomass of thethree-species mixture significantly overyieldedmonocultures in 4 of 5 yrs. This indicates that, ingeneral, intraspecific competition was similar to orslightly more intense than interspecific competitionfor these species. In latter years, the higher seedyield RYTs were accounted for largely by high RYsfor gamagrass, especially in gamagrassrwildrye bi-culture, and the weaker performance of wildrye andbundleflower in polyculture. The rodent damage tobundleflower in polyculture at Site 1 exacerbatedthis effect in 1994 and 1995. It is interesting that inno case did the best-yielding mixture outperform the

Ž .best-yielding monoculture usually bundleflower . Inmost instances, however, polyculture yield was simi-lar to the yield of the best monoculture. In nearlyevery instance, mixtures outperformed wildryemonoculture.

Different mechanisms were likely responsible fordifferences in competitive outcomes. In the establish-ment year, both grass species were relatively shortwhereas all bundleflower individuals were taller. Insubsequent years, as the grasses became well estab-lished, canopies of all species were more similar inheight. This suggests that changing light relationsmay have influenced species competitive relations indifferent years. Wildrye generally yielded better inmonoculture because it did not compete well againstthe more vigorous gamagrass. Gamagrass tended togrow largest and yield best in biculture with wildryeat both sites probably because the more shallow-rooted wildrye with its vertical canopy left morelight and soil resources available to gamagrass. Con-versely, gamagrass tended to yield less well inmonoculture because, there, the intensity of intraspe-cific competition exceeded that of interspecific com-petition. Differences in gamagrass abovegroundbiomass between the two biculture treatments, i.e.,biomass generally lower in gamagrassrbundleflower


Ž .than in gamagrassrwildrye mixture Fig. 1 , sug-gests that the response was species-specific, ratherthan merely a response to lower gamagrass densityin mixture.

In the ecological literature, reservations have beenexpressed about the various difficulties in interpret-ing the results of 1:1 replacement-series experimentsŽ .Connolly, 1986, 1988 . The main problem is that, inmost cases, the planting density chosen can influencethe outcome of two-species competition studies. Themethod is prone to misinterpretation especially whenthere are large differences in size of the species inthe mixture, such that the design has an inherenttendency to favor the larger species. Because thepresent study may therefore have favored such rela-tively large species as gamagrass or bundleflower,inferences about competitive relationships among thethree experimental species should be cautiously de-rived.

Perennial grasses are likely to benefit from associ-ation with legumes in polyculture. Nitrogen transferfrom legumes to grasses may occur via leakage and

Ž .excretion from roots Simpson, 1965 , following de-Ž .cay of nodules and roots Haynes, 1980 , and byŽdirect mycorrhizal exchange van Kessel et al., 1985;

.Eissenstat, 1990 . Perennial grasses may receive from46 to 80% of their N directly from companion

Žperennial legumes Brophy et al., 1987; Farnham and.George, 1994 . Thus, it was not altogether surprising

that bundleflower and gamagrass grew well togetherat both sites for the duration of this study. A longerterm study would be needed to determine whetherthis mixture is truly stable.

4.3. Growth and reproduction oÕer successiÕe years

Perennial grain polycultures that are to producewell for several growing seasons will need to accom-modate changes in soil nutrient status with time.Where species differ in competitive ability, the un-even sharing of resources between components inmixture tends to become accentuated with time andcan lead to the suppression of the less vigorouscomponent. Mixtures of species adapted to soils ofdifferent nutrient status can even display reversals ofdominance over time. A species’ effects on soil maylead to a N supply rate for which it is not a goodcompetitor. For example, N-fixing plants are typical

of low N soils during early succession in manynatural ecosystems. These plants are good competi-tors under low N supply rates, but are usually dis-

Žplaced under more productive conditions Vitousek.and Walker, 1987; Brown and Byrd, 1990 . This led

to a prediction that, because of differences in nutrientuse efficiency, a decline in soil N with time shouldeventually favor the C grass and the legume but not4

the C grass. In fact, wildrye, which may be depen-3

dent on high soil N, declined with time and its seedyield fell drastically after the third year. Additionalstudies with this, or an analogous species, will needto account for changes in soil nutrients status withtime. On the other hand, it was encouraging that onemixture, gamagrass with bundleflower, appeared rel-atively stable over the 5 yrs.

Grazing by rodents in summer and winter 1993was a complicating factor. This type of damage had

Ž .not been observed previously 1985–1992 in plotsof these plant species at The Land Institute. Theextremely wet growing season of 1993 supportedrecord high populations of several small mammal

Žspecies in the summer and following winter D..Kaufman, personal communication , so this damage

may have been a unique event. Damage to bundle-flower stems was greater in mixtures with gama-grass, where the rodents presumably had winter pro-tection because of the gamagrass leaf canopy.

5. Conclusions

Analyses of how interactions between species varyin different environments can provide the raw mate-rial for understanding the mechanisms of year-to-yearcoexistence for perennial grain species. For perennialgrain mixtures, the entire suite of plant traits relevantto system productivity is unknown but would includecanopy size, phenology, shade tolerance, root mor-phology, and insect and disease resistance. Breedingmethods will need to consider multiple year evalua-tions of species mixtures to examine the potential forgrowth and yield improvement under low input con-ditions. Cropping system effects on plant perfor-mance have important implications for plant breed-ing in that multiple year conclusions reached inmonoculture may not transfer to polyculture. Im-provement of species sensitive to cropping system


Ž .e.g., mammoth wildrye may need to take placeonly within multispecies systems. Moreover, the oc-currence of site by treatment interactions suggeststhat the results obtained in one environment may bespurious, and may not predict outcomes in otherenvironments. This suggests that the mix of speciesin the polyculture would be adjusted for soil type.

The environmental problems arising from modernfarming methods are likely to be resolved in the longterm only with new and innovative research ap-proaches. Research in sustainable agriculture facesthe dilemma of resource utilization for adequate seedyield versus sufficient conservation for environmen-tal protection. Diverse stands of perennial grains,sown on land that is productive but vulnerable toerosion, have the potential to reconcile these twopriorities. This study showed favorably high seedyields in some instances and a beneficial associationbetween two candidate perennial grains but alsopointed to the need to follow year-to-year dynamicsto predict better long-term patterns of growth andyield.

Acknowledgements

For assistance in the field and with data entry,thanks are expressed to Heather Brummer, AudreyBarker, Portia Blume, Abigail Breuer, RebeccaGeisen, Tonya Haigh, Michelle Mack, Ted Schuur,and Jennifer Tressler. Marty Bender assisted withstatistical analyses, and Charles Francis, Tom Lee,Judith Soule, Jacob Weiner, Charles Yamoah, andtwo anonymous reviewers provided helpful com-ments on an earlier draft. A portion of the study wasfunded by the Charles A. and Anne Morrow Lind-bergh Foundation.

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