P.S. Cocks

Nitrogen fixation by subterranean clover (Trifolium subterraneum L.) growing in pure culture and in mixtures with varying densities of lucerne (Medicago sativa L.) or phalaris (Phalaris aquatica L.)

The proportions of biologically fixed (Pfix) plant nitrogen (N) and the total amounts of N2 fixed by subterranean clover (Trifolium subterraneum L.) growing in pure culture and in mixtures with different densities (5, 10, 20, or 40plants/m2) of newly sown phalaris (Phalaris aquatica L.) or lucerne (Medicago sativa L.) were followed over 3 years in a field study using the 15N natural abundance technique. The amount of fixed N in subterranean clover was linearly related to shoot biomass. Over the 3-year period, subterranean clover fixed 23–34 kg N/t shoot biomass compared with 17–29 kg N/t shoot biomass in lucerne. Based on above-ground biomass, pure subterranean clover fixed 314 kg N/ha over the 3 years compared with 420–510 kg N/ha by lucerne–clover mixtures and 143–177 kg N/ha by phalaris–clover mixtures. The superior N2 fixation by the lucerneŒsubterranean clover mixtures was due to the N fixed by the lucerne and the presence of a higher subterranean clover biomass relative to that occurring in the adjacent phalaris plots. In the first year, 92% of subterranean clover shoot N was derived from fixation compared with only 59% of lucerne. The reliance of clover upon fixed N2 remained high (73–95%) throughout the 3 years in all swards, except in pure subterranean clover and lucerne in August 1996 (56 and 64%, respectively). Subterranean clover usually fixed a higher proportion of its N when grown in mixtures with phalaris than with lucerne. The calculated Pfix values for lucerne (47–61% in 1995 and 39–52% in 1996) were consistently lower than in subterranean clover and tended to increase with lucerne density. Although lucerne derived a lower proportion of its N from fixation than subterranean clover, its tissue N concentration was consistently higher, indicating it was effective at scavenging soil mineral N. It was concluded that including lucerne in wheat-belt pastures will increase inputs of fixed N. Although lucerne decreased subterranean clover biomass, it maintained or raised Pfix values compared with pure subterranean clover swards. The presence of phalaris maintained a high dependence on N2 fixation by subterranean clover, but overall these swards fixed less N due to the lower clover herbage yields. Perennial and annual legumes appear compatible if sown in a mix and can contribute more N2 to the system than where the annual is sown alone or with a perennial grass. These findings suggest that increases in the amount of N2 fixed can be achieved through different legume combinations without interfering greatly with the N fixation process. Different combinations may also result in more efficient use of fixed N2 through reduced leaching. Further work looking at combinations of annuals possibly with different maturity times, different annual and perennial legume combinations, and pure combinations of perennial (e.g. lucerne) could be investigated with the aim of maximising N2 fixation and use. Grazing management to encourage clover production in mixtures with phalaris will be necessary before the potential of subterranean clover to contribute fixed N2 in these swards is fully realised.

Hardseededness in annual clovers: variation between populations from wet and dry environments

Annual legumes rely on hardseededness, a form of seed dormancy, to spread the risk of mortality associated with germination and to encourage germination at the optimal time in a season. This paper examines seed softening strategies of a number of clover (Trifolium) species collected over a range of environments. Three hypotheses are tested. The first is that within species, long-term hardseededness increases with aridity due to the greater chance of failure to reproduce in dry environments. It is tested by comparing seed softening patterns of populations of a number of clover species collected from a range of environments. The second hypothesis, that annual legumes from dry environments have a smaller subset of seed softening strategies than legumes from wetter environments, is tested by comparing the range of seed softening patterns from all species found at collection sites. The third hypothesis is that within-year patterns of seed softening that prevent germination after summer rainfall may partially substitute for long-term hardseededness. It is tested by examining the relationship between within-year hardseededness and between-year hardseededness. Accessions of the same species from different collection sites differed in both within-year pattern of seed softening and long-term hardseededness but there was little evidence that these differences were between ecotypes. Different species from the same collection site did not have similar seed softening strategies, and seed softening strategies of clovers from dry sites were as variable as those from wetter environments. Within-year pattern of seed softening does not appear to substitute for between-year hardseededness as a dormancy strategy.

Effect of phalaris (Phalaris aquatica L.) and lucerne (Medicago sativa L.) density on seed yield and regeneration of subterranean clover (Trifolium subterraneum L.)

A mixture of 3 subterranean clover (Trifolium subterraneum L.) cultivars (cvv. Goulburn, Seaton Park, and Dalkeith) was grown with 5–40 plants/m2 of phalaris (Phalaris aquatica L.) or lucerne (Medicago sativa L.) for 3 years at Wagga Wagga, NSW (147°21´E, 35°03´S). Clover seed yield was assessed each year, the number of regenerating clover seedlings was counted, herbage yield of clover and the perennials was measured, and the change in perennial density recorded. The influence of perennials on water availability in spring and light during clover seed set was also examined. The impact of perennial density on clover was assessed using regression analyses. In each of the 3 years, subterranean clover seed yield was negatively related to perennial density, but phalaris suppressed clover seed yield more than equivalent densities of lucerne in 2 of the years. Clover seed yield was positively related (R2 = 0.30–0.85) to clover biomass in spring in all 3 years (except for subterranean clover in phalaris in 1994) and to the proportion of total photosynthetically active radiation reaching the clover canopy beneath the perennial (R2 = 0.33–0.83) in 2 of the 3 years. There was a positive relationship between clover seedling regeneration in autumn and size of the summer seed bank in both years in lucerne (R2 = 0.40–0.76) and in 1 year in phalaris (R2 = 0.76) and a negative association between perennial density and clover seedling regeneration. The rate at which the surface profile (0–37 cm) dried in spring was independent of phalaris and lucerne density over the 3 years and did not differ from that of pure clover. The late season clover cultivar, Goulburn, constituted between 57% and 79% of the seed bank following seed set, substantially more than the earlier flowering cultivars, Seaton Park and Dalkeith. Neither perennial density nor species changed the relative competitiveness of the 3 cultivars. The high seed yield of Goulburn and the lack of a perennial-induced change in surface soil water in spring suggest that growing perennials in association with subterranean clover does not increase the level of moisture stress during clover seed set. The good performance of Goulburn is attributed to superior competitive ability for light in late spring. The basal area occupied by lucerne and phalaris increased with perennial density in each of the first 2 years. However, in the third year, phalaris basal area was similar at all densities. The findings suggest that sowing low densities of lucerne should assist in promoting seed yield and regeneration of subterranean clover. Management of phalaris, which has the capacity to substantially increase plant size at low densities, may require additional strategies such as more frequent grazing in spring.

Ecotypic variation for seed dormancy contributes to the success of capeweed (Arctotheca calendula) in Western Australia

The seed dormancy characteristics of 2 capeweed (Arctotheca calendula (L.) Levyns) ecotypes from Western Australia were studied to determine aspects of seed dormancy that contribute to the success of this species in southern Australia. Short- and long-term dormancy pattern of buried and soil surface seed, effect of summer tem- peratures on afterripening, and effect of temperature on seed germination were investigated using seed produced in a common environment. There were large differences in the seed dormancy pattern of the 2 ecotypes studied. On the soil surface, >95% of seed of the Mt Barker ecotype became non-dormant and germinated in the first year, the remainder germinating the following season. In contrast, only 5% of Mullewa seed germinated in the first year, with 75% germinating in the second year and 20% of seed remaining dormant after 2 years. Cycling of dormancy was observed for buried seed of both ecotypes, with periods of non-dormancy corresponding with the likely timing of the break of the season. Dormancy cycling was also apparent in seed stored under constant conditions in the laboratory. Burial prevented ger- mination of both ecotypes; however, the ability to resist germination while buried was lost in 30% of the Mt Barker seed in the second season. Differences in the duration of dormancy of soil surface and buried capeweed seed have evolved as an adaptation to the different environments likely to be experienced by plants at their site of collection. All seeds possessed primary dormancy at maturity, with any afterripening during the first year occurring by the end of summer. Afterripening was enhanced by exposure to typical soil surface temperatures, providing some pro- tection against germination during early summer rainfall. Protection from late summer rains is insured by the inabil- ity of seed to germinate at temperatures >30∞C and a relatively slow rate of germination. These features of capeweed seed dormancy, combined with the ability to evolve genetically distinct populations suited to particular environ- ments, help explain why capeweed is so widespread and abundant across southern Australia.

Annual clovers (Trifolium spp.) have different reproductive strategies to achieve persistence in Mediterranean-type climates

The aim of this work was to determine whether different species of annual clover (Trifolium spp.), obtained from the same environment, have different reproductive strategies (combinations of reproductive traits) to achieve ecological success. A better understanding of the traits that improve persistence should allow agronomists to narrow the selection criteria for new clover cultivars for ley-farming systems in southern Australia. Seeds of 18 annual clover species were obtained from 3 Australian and 6 Mediterranean sites and were subsequently grown in a common garden in Western Australia. Reproductive traits, including time of flowering, weight per seed, fecundity, pollen to ovule ratio, and pattern of seed softening, were observed. Accessions of different clover species from the same site of collection had different reproductive strategies. Across a range of collection sites, accessions of the same species demonstrated the same broad reproductive strategy; however, some traits, e.g. the timing of flowering, varied within species across collection sites. Principal component analysis suggested that there are 3 broad reproductive strategies demonstrated by these clover species. At one extreme were the relatively large-seeded clovers (T. subterraneum, T. clypeatum, and T. stellatum). The associated cost of these large seeds is reduced fecundity. The large-seeded clovers do not have high long-term hardseededness (the predominant form of seed dormancy in clovers). The relatively small-seeded clovers were all characterised by high fecundity. Many of the small-seeded clovers have high levels of long-term hardseededness, which allow the risk of failure to be spread across seasons (T. spumosum, T. hirtum, T. lappaceum, T. angustifolium, and T. tomentosum). Some of the small-seeded clovers (T. glomeratum, T. nigrescens, T. campestre, T. cernuum, and T. suffocatum) are generalists, producing as many seeds as possible in each season, with very little hardseededness. There are several possible explanations for the apparent success of such different reproductive strategies among clover accessions of different species at the same site. A plant may achieve the same goal by trading one reproductive trait for another. For example, it may either produce many small seeds to spread the risk of failure or produce fewer large seeds with an inherent competitive advantage. Alternatively, temporal and spatial variation may favour clovers with a number of different reproductive strategies. It is likely that a mixture of species with different reproductive strategies will maximise production and persistence of legume-based pastures in ley-farming systems.

Lucerne, phalaris, and wallaby grass in short-term pasture phases in two eastern Australian wheatbelt environments. 2. Effect of perennial density and species on subterranean clover populations and the relative success of 3 clover cultivars of different maturity

The effect of the density of 3 perennial species, phalaris (Phalaris aquatica L.), wallaby grass (Austrodanthonia richardsonii Kunth), and lucerne (Medicago sativa L.), on seed set, regeneration, and the relative competitiveness of 3 cultivars of subterranean clover (Trifolium subterraneum L.) was examined in 2 environments in the south-eastern Australian wheatbelt. Seed yields of subterranean clover were inversely related to perennial density at both sites over the first 2 years, the relationship varying with perennial species. Phalaris depressed the seed yield of clover more than lucerne and wallaby grass in the second and third year at equivalent densities. Clover seed yield was positively related to clover herbage yield in late spring at both sites, and inversely related to perennial herbage yield. Clover seed yield displayed an increasing linear relationship with the proportion of light reaching the clover understorey in spring, which in turn was inversely related to perennial density and perennial herbage yield. Clover seedling regeneration in mixed swards in autumn was positively related to the size of the summer seed bank, but negatively related to perennial density. Clover seedling survival following a premature germination at Kamarah was inversely correlated to the density of phalaris and lucerne in the sward. The relative competitiveness of the 3 subterranean clover cultivars varied between sites, with climatic conditions (rainfall and growing-season length) having a greater effect on the relative cultivar performance than companion perennial species or density. The later maturing subterranean clover cv. Goulburn became the dominant cultivar at the wetter site, constituting 72% of the seed bank, but declined to only 3-8% of the seed bank at the drier site. The proportion of the early flowering cultivar Dalkeith in the seed bank increased over time at the drier site and was highest (53%) in plots with the highest perennial density. We concluded that although perennial pasture species will depress clover seed yield and subsequent regeneration, these effects could be minimised by reducing perennial densities and exploiting variations in competitiveness between perennial species as identified in this study. Sowing earlier maturing subterranean clover cultivars would only be an advantage in increasing clover content in low-rainfall environments. The findings suggest that clover seed reserves and regeneration could also be increased by using grazing management to reduce the level of shading of clover by perennials, a factor associated with reduced clover seed yield.

Hardseededness in annual clovers: variation within populations and subsequent shifts due to environmental changes

The first experiment investigated the possibility of variation in seed softening patterns from progeny of individual clover plants from the same population. Patterns of seed softening were attained by exposing seeds on the soil surface over a single summer-autumn period. Logistic curves were fitted to these softening patterns. Comparison of parameters of logistic curves demonstrated that there was within-population variation for hardseededness at maturity, amount of hard seeds entering the seedbank after 6 months on the soil surface, and the time at which seeds softened. The existence of such variation within populations is required for natural selection in response to environmental changes. The second experiment compared hardseededness in populations of 2 clovers from different grazing and phosphate fertiliser treatments from a long-running grassland management trial in Syria. Differences in hardseedness (after 6 months exposure on the soil surface) between treatments demonstrated the importance of dormancy to the reproductive ecology of these clovers. Additional keywords: seed dormancy, pasture, annual legume, genetic variation evolution, Trifolium dasyurum, T. scabrum, T. stellatum, T. tomentosum, T. campestre. A ess s H. C. N l C H. C. n et

Influence of stocking rate and phosphate fertiliser application on the composition of annual legume seedbanks within a Mediterranean grassland

This paper tested the general hypothesis that differences in grazing intensity and phosphate application lead to adjustments in the structure of an annual legume community in natural and diverse grassland at Tel Hadya, Syria, due to changes in competitive relationships between species. The management treatments imposed were a factorial design of 0 or 60 kg/ha of superphosphate fertiliser (applied annually) and relatively low or high continuous sheep stocking intensities. These management treatments were applied for 13 years across three replicate paddocks for each treatment before the annual legume seed banks were sampled. The seed was sorted and representative plants were grown in a common garden in Perth, Australia, to assess reproductive and morphological traits. We found that phosphate application had more influence on species composition than stocking rate and favoured species with relatively competitive reproductive strategies (plants with relatively large seeds, low fecundity and early maturity). Contrary to our expectations, a higher stocking rate did not result in an increase in the proportion of small seeds in the legume seed bank or favour species with other strongly ruderal-type reproductive strategies (high fecundity and high seed dormancy). In paddocks without phosphate application, the high and low stocking rate paddocks were dominated by different species (Trifolium tomentosum and T. campestre), each with a similar reproductive strategy. Plant architecture and/or differences in animal selectivity may account for these differences. The study highlights the importance of phosphate fertiliser for maintaining seedbanks of competitive-type (large seed size, highly productive with good early vigour) annual legume species.