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Spatial Legume Composition and Diversity across Seeded Landscapes

^a Dep. of Agron., Kansas State Univ. Agric. Res. Cent., Hays, KS 67601-9228
^b Dep. of Agron., Iowa State Univ., Ames, IA 50011-1010

* Corresponding author (kharmone{at}oznet.ksu.edu)

Received for publication March 11, 2000.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Pastures typically have diverse landscapes, and resulting soil conditions and plant composition can vary within small area units. This study was performed to quantify the spatial variation in legume contribution to the plant community when seeded into established perennial cool-season grass pastures. Pastures were interseeded with an 11-legume mixture and divided into three stocking methods (continuous, rotational, and nongrazed), with each stocking method containing five landscape positions (summit, backslope, toeslope, opposite backslope, and opposite summit). Grass and legume components were sampled three times annually between 1996 and 1998. Backslopes had greater legume dry matter (DM) composition (161 g kg^-1) than either summit (62 g kg^-1) or toeslope positions (7 g kg^-1), and total legume concentrations increased over years. Legume composition consisted mostly of red clover (Trifolium pratense L.), birdsfoot trefoil (Lotus corniculatus L.), and white clover (T. repens L.). Species richness and Shannon–Weiner diversity index (H^'_dm) for the legume functional group were also greatest on backslope landscape positions, especially in continuously and rotationally stocked pastures. Legume DM composition showed a positive linear relationship with legume species richness in continuous, rotational, and nongrazed pastures (r² = 0.76). The H^'_dm showed a positive linear relationship with legume DM composition in only rotationally stocked paddocks (r² = 0.88). The legume component at grazed backslope sites filled a niche left unoccupied by the grass component. Species and site recommendations for pasture improvement and management should be made based on landscape position and stocking method.

Abbreviations: DM, dry matter • H^'_dm, Shannon–Weiner diversity index (DM basis)

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
VARIATION IN LEGUME COMPONENTS as they relate to richness and diversity has not received much attention in plant communities used for animal production. Many studies involving richness and diversity are in native habitat or in revegetated areas void of production animal influence (Tilman, 1996; Tilman et al., 1996; Vermeer and Berendse, 1983; Wedin and Tilman, 1996; Wilson and Tilman, 1991).

Tilman (1996) discovered that productivity of overall vegetative community was more stable than individual species productivity when greater species numbers were present during normal and drought years. Greater species diversity acts as a buffer through certain species compensating for lack of production from other species. Based on this idea, greater legume richness and diversity may aid legume contributions to pasture swards used for animal production.

Information is also lacking that quantifies changes in legume composition across pasture landscapes, especially under grazed conditions. Information regarding landscape positions and plant growth mostly involves landscape effects on row crop and cereal grain production from tilled acreage (Afyuni et al., 1993; Cassel et al., 1996; Miller et al., 1988).

Ahlgren et al. (1946) observed evident variations or patterns in legume growth within different pasture areas following legume establishment. Changes in legume production and persistence in pastures may also be seen because of grazing or other environmental stress (Marten et al., 1990; Peterson et al., 1992). Legume species that survive following establishment will compete with the grass companion for soil nutrients, water, and light.

More resistance to invasion and establishment is found when greater numbers of species are already present in swards (Tilman, 1997) while new species that do establish tend to occupy unfilled niches. Frequency and magnitude of disturbances in canopies also may influence species richness and biomass and the susceptibility for other species to establish (Grime, 1979; Huston, 1994). Taylor and Allinson (1983) were able to establish legumes into established grass sods by disturbing the canopy with only three evenly spaced clipping periods rather than discing or chemically suppressing the grass canopy.

Following trends in legume pasture components across the landscape may lead to a greater understanding of the spatial changes of legumes in pasture systems used for animal production. The effect of legume species richness and diversity on legume production and the location of species across the landscape could aid in better manipulation and management of the pasture system for sustained production. The objectives of this study were to quantify the spatial difference in legume components across seeded landscapes under different stocking treatments and to determine the relationship between legume species richness and diversity and subsequent composition and production.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Research was conducted at the Iowa State University Rhodes Research Farm (41°52'N, 93°10'W) in central Iowa. Six pasture sites were selected, three with both north- and south-facing slope aspects and three with both east- and west-facing aspects. The three pastures within each orientation served as the three replications for the study. Each of the six pastures dissected the landscape to include summit, backslope, toeslope, opposite backslope, and opposite summit landscape positions (Fig. 1). Both the summit and backslope positions consisted of Downs (fine-silty, mixed, superactive, mesic Mollic Hapludalf) and Fayette (fine-silty, mixed, superactive, mesic Typic Hapludalf) soils with 2 to 24% slopes. Toeslope positions consisted of Colo (fine-silty, mixed, superactive, mesic Cumulic Haplaquoll) and Ackmore (fine-silty, mixed, nonacid, mesic Aeric Fluvaquent) soils with 0 to 5% slopes (Oelmann, 1981). Pastures ranged from 0.81 to 1.41 ha and were dominated by smooth bromegrass (Bromus inermis Leyss.), Kentucky bluegrass (Poa pratensis L.), and reed canarygrass (Phalaris arundinacea L.), with isolated orchardgrass (Dactylis glomerata L.) and tall fescue (Festuca arundinacea Schreb.).

View larger version (34K): [in this window] [in a new window]   Fig. 1. Schematic of one of six typical pasture layouts. Backslopes faced east and west in three pastures (one shown above) and north and south in the other three pastures. Shaded areas indicate experimental units for stocking system and landscape position combinations within which vegetative data were collected three times each season from 1996 to 1998.

The legume mixture included species with perennial, biennial, winter annual, and summer annual cycles of growth. The legumes were alfalfa (Medicago sativa L.), biennial yellow sweetclover [Melilotus officinalis (L.) Pall], biennial white sweetclover (Mel. alba Medik.), birdsfoot trefoil, white clover, red clover, kura clover (T. ambiguum Bieb.), cicer milkvetch (Astragalus cicer L.), berseem clover (T. alexandrinum L.), striate lespedeza [Kummerowia striata (Thunb.) Schindler], and annual white sweetclover (Mel. alba Medik.). Seed were treated with fresh inoculant for each respective species. Each legume was seeded at a density of 43 seeds m^-2, and the seeding density of the entire legume mixture was 473 seeds m^-2 at 9.1 kg ha^-1 pure live seed.

Following seeding in the spring of 1995, pastures were mowed to a height of approximately 15 cm or to the height of the legume to reduce grass competition. Biomass was evenly removed from all pastures early in April 1996 by controlled burning. New growth was allowed to accumulate before stocking treatments were applied in 1996. Pastures were reseeded with the same seed mixture and at the same rate on 6 Mar. 1997.

Following initial seeding, each of the six pastures were divided into three strips across the landscape positions, and one of three stocking treatments were applied within each strip. Stocking treatments consisted of continuous stocking, intermittent rotational stocking, and a nongrazed control. Each stocking system treatment of a pasture included a summit, backslope, toeslope, opposite backslope, and opposite summit position (Fig. 1). This resulted in a total of three replications for each landscape, stocking, and slope aspect combination.

Stocking treatments were applied beginning in May 1996. Continuous stocking began in the middle of May each season and continued until early to mid-August. Rotational stocking took place in mid-May, early July, and late October or early November each season. High stocking densities were used to achieve an approximate 15-cm forage height within five to nine grazing days. Although stocking method was the main emphasis, average annual stocking rates for continuous and rotational stocking were similar (4.1 and 3.8 animal unit months, respectively). Nongrazed control had no animal inputs and remained idle throughout each growing season. Accumulated dead matter in the nongrazed control pastures was removed by rotary mowing and raking before the onset of the 1997 growing season, but pastures were not accessible because of snow cover and early spring precipitation before the onset of plant growth in 1998. Removal of standing dead matter was not possible without damaging fresh growth, so accumulated DM was allowed to remain during the 1998 growing season and then hand-separated at harvest.

Stocking system and landscape position combinations served as the experimental unit. One permanent 9- by 9-m plot for sampling was established within each stocking and landscape position combination in a pasture (Fig. 1). Destructive sampling took place in mid-May, late July, and early October each season from 1996 to 1998. Vegetative composition was determined by hand-clipping all vegetation within one 0.4-m² square frame in the 9- by 9-m plot from all landscape and stocking combinations at each harvest. Samples were placed in cloth bags and refrigerated at 4°C until hand separated.

All green tissue was separated into grass (composite of all grasses) and legume (individual species) components and placed in forced-air drying ovens for 48 h at 60°C. Grass and individual legume species were then weighed and recorded. Total forage mass was determined by adding the dry mass of all grass and legume vegetation from each 0.4-m² sample and converting to kilograms per hectare.

Species richness and diversity were determined on the separated legume component only. Legume species richness was calculated by adding the total number of different legume species found in each 0.4-m² separated sample. Legume species diversity was determined by using the Shannon–Weiner diversity index (H^'_dm) on legume species DM data from the sorted samples. The H^'_dm was calculated following mathematical manipulation presented by Zar (1974), which simplified the equation if proportions of species were not previously calculated. The calculation is as follows:

where n = total of all legume species, f = total of the ith species, and k = total number of species in a sample.

Fixed independent effects of aspect, landscape position, stocking system, harvest date, and year were assessed by analysis of variance using a generalized linear models approach (SAS Inst., 1985). Data were analyzed as a randomized complete block design with a strip strip-plot configuration and repeated harvests over time. Pasture within an aspect was considered to be random and served as the source of error. Sources of variation, error terms, and degrees of freedom for each source are listed in Table 1. Mean comparisons were calculated using least significant differences (LSD), and Fisher's protected F-test was used for significant differences mentioned in the text. Because of the inherently high level of variation among experimental units, significance tests were based on a P < 0.10 level.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

Aspect
Slope aspect had no direct effect on available DM herbage of the six pasture sites and no interaction with any other independent variable (data not shown). Total forage mass, grass forage mass, legume forage mass, legume composition on a DM basis, legume species richness, and legume diversity were similar regardless of slope aspect. Steepness of slope and length of slope may not have been great enough to alter the microenvironment at different aspects and cause or direct changes in vegetative dynamics. Therefore, summit and opposite summit and backslope and opposite backslope data were combined into one summit and one backslope value.

Year Effect
Total and grass forage mass were very similar during 1996 and 1997. In 1998, forage mass was much greater and probably resulted from abundant precipitation in May and June. The most evident trends between years was for the legume component. A steady increase in legume herbage was observed for all stocking treatments (Table 2). Continuously stocked pastures increased legume herbage each successive year while the rotational and nongrazed pastures showed a great increase between the second and third years. Increases in legume herbage were reflected in the sward legume proportion. Legume proportion steadily increased each successive year for the grazed pastures while the increase was significant for the nongrazed pastures between 1997 and 1998 (Table 2).

Similar to findings of George (1984) and Gettle et al. (1996), birdsfoot trefoil and red clover were successful in establishment. White clover may not have persisted as well in this study in nongrazed or rotationally stocked paddocks because of the height of surrounding forage. Springer (1997) found a negative relationship in which white clover ground cover decreased by 10% for every 5-cm increase in bermudagrass sward height.

Blaser et al. (1956) indicated that alfalfa and sweetclover were the most aggressive legume species tested for germination and seedling growth. However, red clover, birdsfoot trefoil, and white clover were the dominant species in this study. Initial seedling vigor may not have been as important for legume establishment as other factors in this study, such as adaptation to soil conditions or grass competition. Adaptation of the different species to soil, environmental, and grazing conditions between years is not surprising. The white clover population in a specific sward was shown to change between plants of different genetic backgrounds within a 2-mo period (Gustine and Huff, 1999). Soil data collected at the present study site will hopefully lead to greater understanding of legume and soil interactions.

Landscape and Stocking Effects
In general, the effect of landscape showed decreasing trends in total and grass forage mass at backslope positions compared with summits and toeslopes (Table 4). Legume herbage, proportion, richness, and diversity, however, showed increasing trends at backslope positions compared with summits and toeslopes (Table 4). Likewise, grazed pastures showed trends for having less total and grass forage mass compared with the nongrazed pasture (Table 2). Continuously and rotationally stocked pastures, however, showed trends for having greater legume proportions, numbers of species, and diversity than the nongrazed areas (Table 2).

Significant landscape position x stocking system interactions altered some of the main-effect trends. Backslope landscape positions had less total and grass herbage than the summits in the grazed pastures (Table 4), but no difference in total or grass herbage was found between backslopes and summits in the nongrazed control. Declines were greatest on backslopes for continuous stocking. This effect was likely linked to the regrowth period between grazing events for the continuous system. Grazing animals prefer gentle slopes rather than steep slopes for resting (Ide et al., 1998) and, subsequently, could have spent more time grazing on backslopes. This, however, was not quantified in the present study.

No interaction occurred between stocking method and landscape position for available legume herbage. Legume DM availability surprisingly was not different between the three stocking methods (Table 2). Pastures with animal pressure had the same amount of legume DM available as those without any defoliation. Backslopes had greater legume forage mass (292 kg ha^-1) than summit (113 kg ha^-1) or toeslope positions (14 kg ha^-1) (Table 4).

Overall, backslopes had the lowest total available DM herbage and grass available DM herbage in the two grazed methods. This follows similar trends observed in cultivated or row-crop field studies where grain yield or total DM production declined on steeply sloped hillside areas compared with less severely sloped summits (Wright et al., 1990). Row crop and cereal crop yields have also been shown to be greater in toeslope positions compared with backslope positions (Afyuni et al., 1993; Miller et al., 1988). In Wisconsin, steeply sloped areas within a pasture (26–35% slope) also had less available forage mass for grazing than areas with 15 to 25% slope (Ahlgren et al., 1944).

Perez Corona et al. (1995) also showed that lower slope positions had greater total forage mass production than upper slope areas, which contribute water and nutrient inputs to lower slopes. Lower slope areas also tended to have greater available grass herbage than upper slope areas (Perez Corona et al., 1995). Similar trends were found for toeslopes in this study. Drought or low precipitation also causes differences in grain yield and total biomass at different landscapes (Afyuni et al., 1993; Cassel et al., 1996). Landscape differences in forage mass were present regardless of year in this study, even with normal growing season precipitation. This indicates that moisture alone may not be responsible for all of the landscape effect.

Backslopes would benefit most from legume introduction by supplying N and DM to areas with less forage mass. Legume DM herbage differed proportionally for each stocking scheme and landscape combination (Table 4), but an interaction resulted for legume DM proportion. Legume DM proportion changed as a result of the changing total and grass herbage available for the stocking scheme and landscape combinations. Compared with summit positions, backslopes increased legume DM proportion in the continuous and rotational pastures by 143 and 114 g kg^-1, respectively, but legume DM proportion was not different in the nongrazed pastures (Table 4). This information contradicts the observation of Ahlgren et al. (1946) that legume growth on steep slopes was less vigorous than under normal, less-sloped field conditions. Others have found forb concentrations to be greater and to dominate swards on upper slopes compared with lower slopes (Perez Corona et al., 1995), which more closely reflects the findings of this study.

Legume proportion was lowest at toeslopes in all stocking methods: 13, 7, and 0 g kg^-1 for the continuous, rotational, and nongrazed paddocks, respectively (Table 4). Tesar and Sheperd (1963) also found that poorly drained organic soils in Michigan had adequate first-season legume production, but winterkill, root disease, and heavy competition from grasses reduced legume composition during the second season of growth. Legume stands in that study were mostly eliminated before the third season (Tesar and Shepherd, 1963). Those same factors could have contributed to lower legume composition on the more poorly drained toeslope soils in this study.

Even though backslopes had greater legume proportions than the toeslope in nongrazed pastures, season-long competition from grass still inhibited overall legume persistence. Summits and toeslopes in all stocking treatments would have received greater production benefit from fertilization management as a grass sward rather than a mixed sward because legume composition remained well below 20% in any year (Kresge, 1964).

The same general trend for DM proportion held true for species richness. Species richness was greatest on backslope landscape positions for all stocking methods. Backslopes in rotationally stocked pastures contained an average of 1.4 more legume species than adjacent summits and 2.0 more legume species than the toeslope position while continuously stocked backslopes contained 0.9 more legume species than the summits and 1.0 more legume species than the toeslope (Table 4).

Species diversity indicates the distribution of DM among the different legume species present in the sample. Achieving maximum species diversity of legumes would mean that all legume species in a sample had equal DM proportions for each legume. Main effects of landscape and stocking method were evident for H^'_dm, but no interactions were found. The H^'_dm showed that the continuously stocked (H^'_dm = 0.13) and rotationally stocked (H^'_dm = 0.13) paddocks had greater diversity than did the nongrazed control (H^'_dm = 0.07) (Table 2). Backslopes (H^'_dm = 0.15) had greater legume diversity than summits (H^'_dm = 0.08) and the toeslope (H^'_dm = 0.08) (Table 4). Dry matter proportion of legume species was more evenly distributed at backslopes and in the grazed paddocks.

The increase in legume species richness at backslope positions under different stocking methods corresponds well with legume composition on a DM basis. Using means for the two aspects at each landscape position, species richness had a positive linear relationship with legume DM proportion for all stocking treatments combined, with r² = 0.76 (Fig. 2).

View larger version (15K): [in this window] [in a new window]   Fig. 2. Relationship between species richness and sward legume dry matter (DM) proportion for the continuously stocked, rotationally stocked, and nongrazed pastures, averaged over three harvest periods each season from 1996 to 1998.

It was speculated that legume proportion on a DM basis would be closely related to species diversity because species diversity using H^'_dm is a combination of total legume species and the proportion of each species in the stand. Only rotationally stocked paddocks had strong relationships between DM diversity and legume proportion on a DM basis (r² = 0.88) (Fig. 3). The continuously stocked pastures had a poor coefficient-of-determination value (r² = 0.25) for legume DM proportion. Dry matter diversity realistically showed no relationship with legume proportion in nongrazed pastures (r² = 0.01). Species diversity is often an indicator of site productivity and sustainability. However, our evidence suggests that using legume diversity for long-term evaluation of pasture improvement through legume seeding might be useful only in rotational stocking systems.

View larger version (17K): [in this window] [in a new window]   Fig. 3. Relationship between legume species dry matter (DM) diversity and legume DM proportion for continuously stocked, rotationally stocked, and nongrazed pastures, averaged over three harvest periods each season from 1996 to 1998. Trend line and coefficient of determination indicate relationship for rotationally stocked pastures only.

View larger version (13K): [in this window] [in a new window]   Fig. 4. Total forage mass and its relationship to legume species richness, averaged over three harvest periods each season from 1996 to1998.

To a certain extent, legume species richness increased with both increasing disturbance and decreasing site productivity, contrary to earlier studies. The difference may be that earlier studies compared total species richness with total biomass (Vermeer and Berendse, 1983), whereas the present study compared legume species richness as a functional group with total forage mass.

Grazing disturbance tended to lower the accumulated available DM. Concurrently, legume DM increased, as did the richness and diversity of the legume components that contributed to that DM. The legume component also increased in proportion over years, especially in the grazed areas. Therefore, grazing disturbance reduced competition from grasses, allowed legumes an opportunity to persist, and increased legume contributions to total forage mass.

The legume component at grazed backslope sites filled a niche left unoccupied by the grass component and was able to contribute DM to the pasture sward. Overseeding legumes into all areas of highly variable pastures should not be recommended, especially if the grass is not disturbed or suppressed. Areas with high grass forage mass, namely summits and toeslopes, may contribute greater production through better grazing and fertilizer management rather than overseeding with legumes. The grass component at summits and toeslopes is dominant and utilizes most resources for its own production, leaving little room for unoccupied niches, even with greater disturbance from grazing. Pasture vegetation was most similar at like landscape positions, and stocked pastures should be fenced according to the landscape to best manage available vegetation. Species and site recommendations for pasture improvement through frost-seeding legumes may be made based on landscape position and stocking method.

	ACKNOWLEDGMENTS

 
The authors would like to thank the Leopold Center for Sustainable Agriculture for financial support and technical contributions toward the completion of this project.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Journal Paper no. J-18729 and Project no. 2899 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, and supported by Hatch Act and State of Iowa.

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