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

Agriculture, Ecosystems and Environment 68 Ž1998. 1–11

Growth and seed yield of three perennial grains within monocultures and mixed stands Jon K. Piper

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The Land Institute, 2440 E. Water Well Road, Salina, KS 67401, USA Accepted 6 June 1997

Abstract This study examined growth and seed yield of three perennial species, Desmanthus illinoensis ŽIllinois bundleflower, a legume., Leymus racemosus Žmammoth wildrye, a C 3 grass., and Tripsacum dactyloides Žeastern gamagrass, a C 4 grass., in monoculture, biculture, and triculture treatments, on two soils differing in initial fertility, and over 5 yrs. There were significant effects of site, treatment, and year on both aboveground biomass and seed yield. On average, bundleflower monoculture produced the greatest aboveground biomass, although the three-species mixture produced the peak biomass Ž814 grm2 . in any given year. Among treatments, Illinois bundleflower monoculture yielded the most seed Žmaximums 122 grm2 .. Overall, biomass and seed yield were higher at the more fertile Site 1, but species differed in their dependence on soil fertility. Among species, bundleflower performed fairly independently of soil fertility, wildrye grew poorly on the less fertile soil, and gamagrass persisted at both sites although it grew less well at Site 2. In most cases, mixtures produced as well as the best-yielding monoculture. In 26 of 30 instances, biomass relative yield totals ŽRYTs. were statistically ) 1.0 and, in 19 of 21 cases, RYT for seed yield was statistically ) 1.0. In general, the overyielding effect appeared stronger at Site 2 than at Site 1, with seed yield RYT appearing to increase with time at Site 2. The results show that the seed yield of perennials can be high, and that some species can persist in mixture for several years. The data for 5 yrs point to the need to follow long-term patterns of yield and interspecific interactions within perennial grain polycultures in order to maintain species 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 the reestablishment of perennial cover on retired cropland can reduce soil erosion, increase root turnover, and increase the accumulation of surface litter. The

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

greater root biomass associated with established perennial grasses ŽRichter et al., 1990. commonly gives annual C inputs into the soil that can be several times greater than those into cultivated soils ŽAnderson and Coleman, 1985; Buyanovsky et al., 1987; McConnell and Quinn, 1988. while reducing rates of nutrient leaching relative to annual crops ŽPaustian et al., 1990.. In the Great Plains of the US, active soil organic matter, available nutrients, water-stable aggregates, and polysaccharide content may recover

0167-8809r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 8 8 0 9 Ž 9 7 . 0 0 0 9 7 - 2

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J.K. Piper r Agriculture, Ecosystems and EnÕironment 68 (1998) 1–11

under perennial grasses within a few decades ŽMcConnell and Quinn, 1988; Gebhart et al., 1994., although the total soil organic matter pool may take longer to reach that of the virgin state ŽParton et al., 1987; Burke et al., 1995.. Moreover, studies using experimental mixtures of grassland plant species have shown that plant biodiversity can be associated with higher productivity as well as greater efficiency of soil nutrient extraction ŽTilman et al., 1996.. This phenomenon is reflected in agronomic mixtures that outproduce their respective monocultures because of differences in location and timing of resource use that reduce overlap in resource demand between interspecific neighbors. For example, roots of different species may explore different soil layers, or develop at different times, or species may have complementary nutrient requirements, as in grassrlegume mixtures that produce higher dry matter yields Že.g., Barnett and Posler, 1983; Posler et al., 1993.. Similarly, intercrops may be released from competition for light, and show greater overall productivity, if canopies of component species develop at different times or the canopy architecture minimizes mutual shading ŽDavis et al., 1984; Clark and Francis, 1985.. Finally, differences in length of the growing period or in the peak periods of nutrient uptake among species Že.g., Piper, 1993a. can also reduce direct competition and thus promote overyielding ŽSmith and Francis, 1986.. Successful polycultures consist of species that complement one another spatially, seasonally, or in nutrient requirements, so that they either Ža. use land, labor, or resources more efficiently; Žb. increase yield; Žc. reduce loss to insects, diseases, and weeds; or Žd. reduce yield variation ŽMoreno and Hart, 1979; Francis, 1986; Vandermeer, 1989.. Unfortunately, it may not be possible to predict, from its performance in monoculture, how a species will behave in polyculture. For example, some species have different patterns of nutrient uptake when grown in association with other species ŽGoodman and Collison, 1982., and shorter plants may be shaded out by taller neighbors in polyculture, although they are vigorous in monoculture. Moreover, the relative performance of monocultures and mixtures may differ with site and time. Environments that differ in soil fertility, waterholding capacity, and exposure to wind and sunlight

are likely to influence not only plant performance but also the outcomes of plant species interactions ŽPickett and Bazzaz, 1978; Boryslawski and Bentley, 1985; Tilman, 1987.. Similarly, physiological differences between species may also affect the outcome of interactions in various environments. Because of possible differences between environments in the outcome of interactions, then, it is crucial to study species interactions on soils of widely different fertility ŽConnell, 1983; Smith and Francis, 1986. and different climatic conditions. The relative success of different species, when grown in mixtures, can change with time. Species that dominate a mixture initially, when levels of both sunlight and available soil resources are high, may later be suppressed if they are poor competitors when available soil nutrients and water decline and shading increases. Grain-producing mixtures of perennial grasses, legumes, and composites could protect the soil and provide the restorative properties of a perennial cover while yielding significant amounts of edible grain. Several promising candidates for perennial grain agriculture have been identified ŽWagoner, 1990; Soule and Piper, 1992., though further selection and breeding for intercrop compatibility and grain yield is needed. The present study investigated the performance of three perennial species, differing in photosynthetic pathway ŽC 3 vs. C 4 . and ability to fix N, that show promise for perennial grain production. To determine the effects of site, species composition, and year on growth and seed yield, the study was conducted in two environments and over 5 yrs. As such, the study may be seen as a model system to predict how perennial grains will interact with soil type, differences in species composition, and time.

2. Materials and methods 2.1. Species Desmanthus illinoensis ŽMichx.. MacM. ŽIllinois bundleflower, Mimosaceae. is a nitrogen-fixing species that forms deep taproots in its first year. It is native to the Great Plains, with a range extending northward to Minnesota, east into Florida, and as far west as New Mexico ŽGreat Plains Flora Associa-

J.K. Piper r Agriculture, Ecosystems and EnÕironment 68 (1998) 1–11

tion, 1986.. In favorable years, plants approach 2 m height after 4 months. In central Kansas, it flowers from late June onward. Maximum seed yield has ranged from 163 to 197 grm2 ŽPiper, 1993b., with high nutritional quality Ž38% protein, 34% carbohydrate. ŽPiper et al., 1988., suggesting its potential as a grain legume. The bundleflower accession used here was originally collected from a wild population in Ellsworth, KS. Leymus racemosus ŽLam.. Tsvelev Žmammoth wildrye, Poaceae. is a rhizomatous C 3 species native to Bulgaria, Romania, Turkey, and southwestern parts of the former Soviet Union. Its grain has been gathered by Asian and European people, especially in drought years when annual grain crops failed ŽKomarov, 1934.. Reproductive tillers grow to about 1.5 m high, and maximum seed yields have ranged from 51 to 83 grm2 ŽPiper, 1993b.. Most growth and uptake of soil water and nutrients occurs in spring and autumn, and seeds mature by late June ŽPiper, 1993a.. The wildrye used here was from a stand of US Natural Resource Conservation Service variety ‘Volga wild rye’ planted at The Land Institute, Salina, KS, USA in 1989. Tripsacum dactyloides ŽL.. L. Žeastern gamagrass, Poaceae. is a large C 4 bunchgrass native from the southeastern United States and Great Plains southward to Bolivia and Paraguay ŽGreat Plains Flora Association, 1986.. The canopy height ranges from 1 to 2 m; reproductive tillers can exceed 2 m high. Although gamagrass is an excellent forage species, it also shows much promise as a grain crop for human consumption. The grain is nutritious, containing 27 to 30% protein and 7% fat ŽBargman et al., 1989., and has baking properties similar to those of maize,

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but its seed yield is low Žtypical range for uncleaned spikelets is 40–100 grm2 ; seed mass is approximately 25% of spikelet mass ŽPiper, 1993b... The gamagrass used in this study was derived from seed collected originally from a natural population just west of Salina, KS. 2.2. Study sites and experimental design The study took place within experimental plots established at The Land Institute, located 4.8 km SE of Salina, KS, USA ŽSection 5 T15S R2W Hutchinson Quadrangle, 38844X N, 97834X W.. Plots were established in March 1991 on two sites at 3 km apart. Site 1 was on a level Cozad silt loam ŽCoarsesilty, mixed, mesic Fluventic Haplustolls., previously in continuous wheat, then planted to alfalfa Ž Medicago satiÕa L.. in 1990. Site 2 was the south face of a hillside on a Kipson–Clime complex soil Žfine to loamy, mixed, mesic, Udorthentic Haplustolls. that had experienced erosion. This area was planted to native grasses Žprimarily Andropogon gerardii Vitman wbig bluestemx, Bouteloua curtipendula wMichx.x Torr., and Sorghastrum nutans wL.x Nash. in 1982, but was continually cropped before then. Relative to Site 2, Site 1 soil initially had lower pH, higher concentrations of available and total Žpotentially mineralizable. N to 60 cm depth, and higher K near the surface ŽTable 1.. Average annual precipitation is 735 mm, with almost 75% falling during the April to September growing season. Plots were planted in May in a replacement series design in which initial overall density was constant. Six cropping system treatments were used: Ži. three monocultures Žbundleflower w Di x, wildrye w Lr x, and

Table 1 Initial levels Žmgrkg. of selected soil nutrients at two experimental sites. N s 18 observations per depth per site Site

Depth Žcm.

pH

Organic C

Available N ŽNH 4 q NO 3 .

Total N

Available P ŽBray.

K

1

0–30 30–60 60–100 0–30 30–60 60–100

5.94 6.74 7.69 6.91b 7.45a 7.78

11300 6900 5600 10640 6600 5300

21.6 b 10.9 b 7.8 8.4 6.0 6.8

1090b 700a 500 930 620 510

10.8 6.8 2.6 15.7 a 6.8 12.0 a

363b 195 169 276 229a 222a

2

a,b

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

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gamagrass w Td x., Žii. two 1:1 alternating mixtures Žgamagrass with wildrye w TdrLr x and gamagrass with bundleflower w TdrDi x., and Žiii. a random mixture of the three species in a 1:1:1 ratio ŽTdrLrrDi .. All treatments were replicated three times, in a randomized complete block design, at each site Ž N s 36 plots.. Bundleflower and gamagrass plants were started as seedlings in the greenhouse whereas wildrye plants were transplanted to the sites from a nearby field stand. Plots were 7.31 m wide by 9.00 m long, with rows oriented east to west, containing 96 plants per plot. Rows were 0.91 m apart, with plants placed 0.75 m apart within rows, resulting in a density of 1.47 plantsrm2 . All species were planted at the same density. This initially wide spacing was used to allow for plant horizontal spread in subsequent years. The outer two plants on all sides were left as a border to minimize edge effects, leaving a 21.8 m2 data area in the center of each plot. Plots were hand-weeded in the first year to assist establishment. Four-meter borders between plots, to control for interference between plots, consisted of alfalfa at Site 1 and native grass sod at Site 2. 2.3. Measurements Aboveground biomass of bundleflower was estimated in late summer by determining total basal stem diameter Žmm. for each plant, then converting diameter to mass using the expression: biomass Žg. s 0.556 Žbasal stem area wmm2 x. y 4.65 Ž r 2 s 0.933, P - 0.0001, N s 98; J.K. Piper, unpublished.. The aboveground biomass of wildrye was estimated by measuring and then summing the total length of reproductive tillers Žcm. just before seed harvest Žmass Žg. s 0.120 Žtotal tiller length wcmx. q 7.57; r 2 s 0.757, P - 0.0001, N s 25; J.K. Piper, unpublished.. Aboveground biomass of gamagrass plants was estimated in late winter by measuring basal crown circumference Žcm., then converting basal area to aboveground biomass using the expression: mass Žg. s 0.315 Žbasal area wcm2 x. y 18.55 Ž r 2 s 0.903, P - 0.0001, N s 41; J.K. Piper, unpublished.. To measure seed yield, bundleflower pods were harvested 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 in late June and collected in paper bags in the field. Harvest of mature rachises was repeated weekly until harvest was complete. Rachises were stored in a greenhouse until dry, then threshed and cleaned. Harvest of gamagrass seed Ži.e., entire spikelets. began in late July, and was repeated every one to two weeks until complete. Air-dry seed of all species was weighed to "0.01 g in the laboratory. To assess whether there were yield advantages in polyculture, the relative yield total ŽRYT., a commonly used measure of overyielding, was used: it is the sum of the fractions of the various components relative to their yields in monoculture ŽMead and Willey, 1980.. If intraspecific competition is stronger than interspecific competition, or facilitation is occurring, plants should yield relatively better in mixture than in monoculture, resulting in an RYT ) 1. 2.4. Statistical analyses Results were analyzed using the MGLH procedure of SYSTAT for Windows ŽSYSTAT, 1992., testing for effects of site, treatment, year, and interactions. The analysis was treated as a split–block analysis at each site and combined over both sites. Treatments were assigned randomly within blocks. Comparisons were made using Tukey’s HCD Procedure. The significance level for all tests was P 0.05.

3. Results 3.1. Biomass yield 3.1.1. Site effects Overall, the greater soil fertility of Site 1 was reflected in greater plant production, based on the biomass formulas presented in the Materials and Methods, relative to Site 2 Žmean s 327 vs. 262 grm2 ; F1,4 s 42.17, P - 0.0001.. Among species, wildrye appeared most sensitive Žas indicated by biomass response. to site quality, bundleflower was least sensitive, and gamagrass was intermediate in response to soil quality.

J.K. Piper r Agriculture, Ecosystems and EnÕironment 68 (1998) 1–11

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3.1.2. Treatment effects Across sites and years, there were significant differences among treatments Ž F5,20 s 72.09, P 0.0001., with bundleflower monoculture producing the greatest aboveground biomass Žmean s 411 grm2 . and wildrye monoculture the least Žmean s 113 grm2 .. The ranking of treatments was Di A ) TdrDi AB ) TdrLrrDi AB ) Td AB ) TdrLr B ) Lr C . In addition, there was a significant site by treatment interaction Ž F5,20 s 4.33, P s 0.008., with treatment groupings, but not their ranking, differing between sites. 3.1.3. Year effects Large annual differences in precipitation, damage by small mammals in 1 yr, and the decline of one species led to a significant year effect on aboveground biomass Ž F4,16 s 1158.07, P - 0.0001. as well as a year-by-site interaction Ž F4,16 s 28.28, P 0.0001.. The years 1991 and 1994 were dry Ž1991: 60.6 cm; 1994: 58.9 cm annual precipitation., whereas 1992, 1993, and 1995 were wetter than normal Ž1992: 92.7 cm, 1993: 147.5 cm, 1995: 87.5 cm.. Rodent Žprobably Sigmodon hispidus Audubon and Bachman, Cricetidae. damage in 1993 affected seed yield of one species and growth and seed yield of another Žsee Section 4.. During the winter of 1993–1994, small mammals grazed numerous bundleflower crowns just below ground level at Site 1, leading to reduced bundleflower size, and thus lower seed yield, in the 1994 and 1995 growing seasons. This grazing occurred more in biculture and triculture treatments than in monoculture plots. In addition, during the summer of 1993, small mammal grazing on gamagrass reproductive tillers precluded gamagrass seed harvest in all plots at Site 2, although plant size appeared unaffected. Treatment effects occurred in all site–year combinations but Site 2 in 1991 ŽFig. 1.. In nearly every case, total biomass of mixtures was similar to that of the most productive monoculture. Wildrye was clearly the least vigorous of the three species, and was suppressed in biculture with gamagrass. After 1993, wildrye was only a minor contributor to the total biomass of polycultures. By 1994, wildrye had declined greatly at Site 2 and from biculture plots at Site 1; however, there was some evidence of recovery, via tillering, of this species at Site 1 in 1995. In

Fig. 1. Mean estimated total aboveground biomass of six cropping system treatments at two sites in 5 yrs. In each group of bars, the first three bars represent monocultures, the fourth and fifth bars are bicultures, and the sixth bar indicates the three-species treatment. For each year–site combination, bars with the same letter do not differ at P - 0.05 ŽANOVA, Tukey’s HCD Procedure..

contrast, bundleflower persisted well in mixture with gamagrass throughout the study. 3.2. Seed yield 3.2.1. Site effects As with aboveground biomass, site differences in soil quality were reflected in overall higher seed production at Site 1 than Site 2 Ž F1,4 s 54.27, P 0.0001.. Seed yield of wildrye appeared highly dependent on site quality whereas bundleflower yield was fairly independent of soil fertility. 3.2.2. Treatment effects There were significant differences among treatments Ž F5,20 s 77.66, P - 0.0001., with bundleflower monoculture the overall highest seed yielding treatment and grass monocultures the lowest Ž Di A )

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monoculture and gamagrassrbundleflower treatment yields were independent of site. The overall ranking of treatments was the same at each site.

Fig. 2. Mean total seed yield of six cropping system treatments at two sites in 5 yrs. For each year–site combination, bars with the same letter do not differ at P - 0.05 ŽANOVA, Tukey’s HCD Procedure.. Treatment and shading designations are as in Fig. 1.

TdrDi B ) TdrLrrDi BC ) TdrLr C ) Td CD ) Lr D .. There was, however, a significant site by treatment interaction Ž F5,20 s 5.00, P s 0.004., such that the w ild ry e a n d g a m a g ra ss m o n o c u ltu re s , gamagrassrwildrye biculture, and triculture treatments yielded better at Site 1 and the bundleflower

3.2.3. Year effects Bundleflower flowered and set seed in each year, whereas both grass species remained vegetative in the establishment phase, which lasted 1 yr. The low precipitation in 1991 appeared to reduce bundleflower yield at Site 2 more than at Site 1. The mammal damage that led to reduced bundleflower size also lowered seed yield in the 1994 and 1995 growing seasons. In the summer of 1993, small mammal grazing on gamagrass reproductive tillers precluded seed harvest in all plots at Site 2. As a consequence, there were highly significant Ž F4,16 s 156.98, P - 0.0001. year effects as well as year-bysite Ž F4,16 s 20.07, P - 0.0001. and year-by-treatment Ž F20,80 s 16.79, P - 0.0001. interactions. Examining treatment seed yields for each site in each year showed that there were significant treatment effects on seed yield in most instances ŽFig. 2.. Bundleflower seed yield peaked in 1992, with mean highs around 120 grm2 at the two sites, then levelled off in 1994 and 1995. Bundleflower monoculture yield exceeded yield of the other treatments at Site 1 in 1992 and at Site 2 from 1992 to 1995. At Site 2, the bundleflower monoculture consistently yielded best among treatments, followed in general by the gamagrassrbundleflower biculture and threespecies mixtures. At Site 2, in contrast, bundleflower monoculture consistently outyielded the other monoculture and mixture treatments from 1992 to 1995.

Table 2 Relative yield totals Žaboveground biomass. for three perennial species mixtures in 5 yrs. Values are means " SE, N s 3 Mixture

1991

1992

1993

1994

1995

Site 1 Gamagrassrwildrye Gamagrassrbundleflower Gamagrassrwildryerbundleflower

1.63 " 0.04 a 1.14 " 0.08 1.79 " 0.04 a

1.57 " 0.42 0.78 " 0.06 a 0.84 " 0.08

1.14 " 0.06 1.19 " 0.01a 1.62 " 0.05a

1.14 " 0.18 0.88 " 0.02 a 1.00 " 0.26

0.82 " 0.05a 1.00 " 0.07 1.08 " 0.15

Site 2 Gamagrassrwildrye Gamagrassrbundleflower Gamagrassrwildryerbundleflower

2.09 " 0.35a 2.37 " 1.40 2.00 " 0.28 a

1.66 " 0.30 0.79 " 0.08 1.45 " 0.11a

1.01 " 0.09 1.01 " 0.03 1.36 " 0.04 a

1.44 " 0.09 a 0.84 " 0.02 a 1.39 " 0.19

1.47 " 0.06 a 1.08 " 0.06 1.65 " 0.08 a

a

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

J.K. Piper r Agriculture, Ecosystems and EnÕironment 68 (1998) 1–11

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Table 3 Relative yield totals Žseed 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, N s 3. Mixture

1992

1993

1994

1995

Site 1 Gamagrassrwildrye Gamagrassrbundleflower Gamagrassrwildryerbundleflower

1.17 " 0.08 1.05 " 0.03 0.86 " 0.01a

1.10 " 0.11 1.13 " 0.16 1.25 " 0.14

0.84 " 0.05a 0.99 " 0.13 1.05 " 0.28

1.11 " 0.19 1.06 " 0.22 1.02 " 0.18

Site 2 Gamagrassrwildrye Gamagrassrbundleflower Gamagrassrwildryerbundleflower

1.03 " 0.17 1.00 " 0.11 0.91 " 0.12

y y y

1.54 " 0.22 1.90 " 0.41 1.22 " 0.14

1.52 " 0.11a 2.28 " 0.32 a 1.37 " 0.11a

a

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

The yield pattern of wildrye monoculture differed greatly from that of bundleflower, yielding lower than most other treatments in most years ŽFig. 2.. In general, wildrye performed relatively poorly in mixture, its growth particularly reduced by gamagrass. In 1994, wildrye monoculture at Site 1 yielded more poorly than all other treatments. The exception was at Site 1 in 1993 where wildrye in monoculture yielded 71.2 grm2 . Seed yield of gamagrass also varied considerably among sites, treatments, and years ŽFig. 2.. It came to dominate mixtures at Site 1, largely as a result of poor wildrye competitive ability and bundleflower damage from mammals, whereas it and bundleflower persisted well in mixture throughout the study at Site 2. Its highest monoculture yield occurred at Site 1 in 1993 Ž60.3 grm2 .. 3.3. OÕeryielding For estimated aboveground biomass, in nearly all Ži.e., 26 of 30. instances, RYT was statistically G 1.0 ŽTable 2.. The three-species mixture overyielded more often than either two-species mixture. In general, the overyielding effect appeared stronger at Site 2 than at Site 1. Wildrye contributed little to RYT after 1993 Žsee Fig. 1., but there was little evidence of increasing or decreasing RYT with time. Relative yield totals were not as high for seed yield as they were for biomass ŽTable 3.. In 19 of 21 cases, however, RYT was statistically G 1.0. As

with aboveground biomass, RYTs for seed yield appeared somewhat higher at Site 2. At Site 1, RYs for wildrye and bundleflower contributed little to RYT after 1993. At Site 2, seed yield RYT appeared to increase with time.

4. Discussion 4.1. Growth and reproduction in two enÕironments Although overall growth and seed yield were higher at the more fertile Site 1, individual component species responded to different environments in different ways. Differences in N-fixing ability and photosynthetic pathway among the experimental species led to predictable differences in the ways species interacted with soil type. Of the three species, bundleflower was least dependent on soil fertility, particularly available N. The only dramatic site effect occurred in the establishment year, suggesting that the relatively fertile soil and lower water stress at Site 1 favored its establishment in that dry year when water was probably more critical than soil nutrient status. Site differences largely disappeared in subsequent years, however, suggesting that this legume is able to compensate for low soil N without reducing its growth and seed yield. Among species, wildrye displayed the greatest differences between sites, indicating that this C 3 species was the most soil quality dependent of the

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three. Growth and yield of wildrye were consistently lower at Site 2, even in wet years, indicating greater dependence of this grass on high soil available N concentrations. Low favorableness of Site 2 for this species was further evidenced by the near disappearance of wildrye there by 1994. Wildrye may have difficulty persisting under low soil N concentrations. Gamagrass appears tolerant of low soil N regimes, although its growth and seed yield were noticeably lower at Site 2 than at Site 1 in each year. Perennial C 4 grasses may have lower soil N requirements than many C 3 grasses. In grasslands, C 4 grasses may retain their dominance because of their ability to withstand low soil N conditions. In contrast to the C 3 wildrye, the C 4 gamagrass may be better able to extract soil available N and is therefore more tolerant of relatively low N levels Žsee Wedin and Tilman, 1990; Tilman and Wedin, 1991; Piper, 1993a.. Site differences in soil texture, or associated soil physical properties, may have also affected the performance of the plant species. In addition, there were also differences in the performance of mixtures between the two sites. Photosynthetic pathway and ability to fix atmospheric N are two factors likely to influence the outcome of interactions on different soils, leading to the predictions that N-fixing species and C 4 grasses should compete better on less fertile soils whereas perennial C 3 grasses should persist better on more fertile soils. These patterns are observed in some tallgrass prairie sites, where poor soils support relatively high proportions of N-fixing species in addition to C 4 grasses, but few C 3 grasses ŽPiper, 1995.. On deeper, more fertile soils, legumes may be rare, but the C 3 graminoid component may average 20% of aboveground biomass. Soria et al. Ž1975. found that overyielding by intercrops of maize, bean, and cassava consistently exceeded 1.0, but was higher under low rather than high soil fertility conditions. Similarly, in the present study, both biomass and seed yield RYTs for the mixtures 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, the benefits from intercropping legumes and nonlegumes should be greatest in low N regimes. This suggests that the benefit of polyculture is greater on soils of diminished fertility.

4.2. Growth and reproduction in monocultures and mixtures Much of the emphasis in multiple cropping systems research involves identifying methods or planting designs that allow or even promote species coexistence. In the present study, significant biomass overyielding, as measured by RYT, occurred for 11 of 30 cases and significant seed overyielding was shown in 3 of 21 cases. At Site 2, biomass of the three-species mixture significantly overyielded monocultures in 4 of 5 yrs. This indicates that, in general, intraspecific competition was similar to or slightly more intense than interspecific competition for these species. In latter years, the higher seed yield RYTs were accounted for largely by high RYs for gamagrass, especially in gamagrassrwildrye biculture, and the weaker performance of wildrye and bundleflower in polyculture. The rodent damage to bundleflower in polyculture at Site 1 exacerbated this effect in 1994 and 1995. It is interesting that in no case did the best-yielding mixture outperform the best-yielding monoculture Žusually bundleflower.. In most instances, however, polyculture yield was similar to the yield of the best monoculture. In nearly every instance, mixtures outperformed wildrye monoculture. Different mechanisms were likely responsible for differences in competitive outcomes. In the establishment year, both grass species were relatively short whereas all bundleflower individuals were taller. In subsequent years, as the grasses became well established, canopies of all species were more similar in height. This suggests that changing light relations may have influenced species competitive relations in different years. Wildrye generally yielded better in monoculture because it did not compete well against the more vigorous gamagrass. Gamagrass tended to grow largest and yield best in biculture with wildrye at both sites probably because the more shallowrooted wildrye with its vertical canopy left more light and soil resources available to gamagrass. Conversely, gamagrass tended to yield less well in monoculture because, there, the intensity of intraspecific competition exceeded that of interspecific competition. Differences in gamagrass aboveground biomass between the two biculture treatments, i.e., biomass generally lower in gamagrassrbundleflower

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than in gamagrassrwildrye mixture ŽFig. 1., suggests that the response was species-specific, rather than merely a response to lower gamagrass density in mixture. In the ecological literature, reservations have been expressed about the various difficulties in interpreting the results of 1:1 replacement-series experiments ŽConnolly, 1986, 1988.. The main problem is that, in most cases, the planting density chosen can influence the outcome of two-species competition studies. The method is prone to misinterpretation especially when there are large differences in size of the species in the mixture, such that the design has an inherent tendency to favor the larger species. Because the present study may therefore have favored such relatively large species as gamagrass or bundleflower, inferences about competitive relationships among the three experimental species should be cautiously derived. Perennial grasses are likely to benefit from association with legumes in polyculture. Nitrogen transfer from legumes to grasses may occur via leakage and excretion from roots ŽSimpson, 1965., following decay of nodules and roots ŽHaynes, 1980., and by direct mycorrhizal exchange Žvan Kessel et al., 1985; Eissenstat, 1990.. Perennial grasses may receive from 46 to 80% of their N directly from companion perennial legumes ŽBrophy et al., 1987; Farnham and George, 1994.. Thus, it was not altogether surprising that bundleflower and gamagrass grew well together at both sites for the duration of this study. A longer term study would be needed to determine whether this mixture is truly stable. 4.3. Growth and reproduction oÕer successiÕe years Perennial grain polycultures that are to produce well for several growing seasons will need to accommodate changes in soil nutrient status with time. Where species differ in competitive ability, the uneven sharing of resources between components in mixture tends to become accentuated with time and can lead to the suppression of the less vigorous component. Mixtures of species adapted to soils of different nutrient status can even display reversals of dominance over time. A species’ effects on soil may lead to a N supply rate for which it is not a good competitor. For example, N-fixing plants are typical

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of low N soils during early succession in many natural ecosystems. These plants are good competitors under low N supply rates, but are usually displaced under more productive conditions ŽVitousek and Walker, 1987; Brown and Byrd, 1990.. This led to a prediction that, because of differences in nutrient use efficiency, a decline in soil N with time should eventually favor the C 4 grass and the legume but not the C 3 grass. In fact, wildrye, which may be dependent on high soil N, declined with time and its seed yield fell drastically after the third year. Additional studies with this, or an analogous species, will need to account for changes in soil nutrients status with time. On the other hand, it was encouraging that one mixture, gamagrass with bundleflower, appeared relatively stable over the 5 yrs. Grazing by rodents in summer and winter 1993 was a complicating factor. This type of damage had not been observed previously Ž1985–1992. in plots of these plant species at The Land Institute. The extremely wet growing season of 1993 supported record high populations of several small mammal species in the summer and following winter ŽD. Kaufman, personal communication., so this damage may have been a unique event. Damage to bundleflower stems was greater in mixtures with gamagrass, where the rodents presumably had winter protection because of the gamagrass leaf canopy.

5. Conclusions Analyses of how interactions between species vary in different environments can provide the raw material for understanding the mechanisms of year-to-year coexistence for perennial grain species. For perennial grain mixtures, the entire suite of plant traits relevant to system productivity is unknown but would include canopy size, phenology, shade tolerance, root morphology, and insect and disease resistance. Breeding methods will need to consider multiple year evaluations of species mixtures to examine the potential for growth and yield improvement under low input conditions. Cropping system effects on plant performance have important implications for plant breeding in that multiple year conclusions reached in monoculture may not transfer to polyculture. Improvement of species sensitive to cropping system

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J.K. Piper r Agriculture, Ecosystems and EnÕironment 68 (1998) 1–11

Že.g., mammoth wildrye. may need to take place only within multispecies systems. Moreover, the occurrence of site by treatment interactions suggests that the results obtained in one environment may be spurious, and may not predict outcomes in other environments. This suggests that the mix of species in the polyculture would be adjusted for soil type. The environmental problems arising from modern farming methods are likely to be resolved in the long term only with new and innovative research approaches. Research in sustainable agriculture faces the dilemma of resource utilization for adequate seed yield versus sufficient conservation for environmental protection. Diverse stands of perennial grains, sown on land that is productive but vulnerable to erosion, have the potential to reconcile these two priorities. This study showed favorably high seed yields in some instances and a beneficial association between two candidate perennial grains but also pointed to the need to follow year-to-year dynamics to predict better long-term patterns of growth and yield.

Acknowledgements For assistance in the field and with data entry, thanks are expressed to Heather Brummer, Audrey Barker, Portia Blume, Abigail Breuer, Rebecca Geisen, Tonya Haigh, Michelle Mack, Ted Schuur, and Jennifer Tressler. Marty Bender assisted with statistical analyses, and Charles Francis, Tom Lee, Judith Soule, Jacob Weiner, Charles Yamoah, and two anonymous reviewers provided helpful comments on an earlier draft. A portion of the study was funded by the Charles A. and Anne Morrow Lindbergh Foundation.

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