Natural Re-Seeding and Trifolium repens Demography in Grazed Hill Pastures. I. Flowerhead Appearance and Fate, and Seed Dynamics

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Abstract
The appearance and fate (removal by livestock versus natural ripening and eventual seed deposition) of Trifolium repens flowerheads were measured for 3 years under different sheep grazing managements and soil fertility levels in a range of topographical (slope/aspect) zones in summer-moist New Zealand hill country. The spatial distribution of T. repens at the individual plant level (in 100 cm2 quadrats) and the treatment level were measured and related to flowerhead production. Variation in flowerhead production between fertilizer levels largely paralleled variation in T. repens content of the sward, but there was a stong trend for greater flowering activity per unit of T. repens under rotational grazing than under set stocking. At the individual plant level, variation in density of T. repens foliage explained only 5-10% of the variation in numbers of flowerheads. Flowering appeared to be strongly influenced by micro-environment and genotype. Removal of flowerheads, mostly unripe, by grazing stock was relatively consistent at 89-97% of flowerheads produced. On average, flowerheads were removed within 12 days of their appearance, with small treatment differences unlikely to influence the viability of seed in consumed flowerheads. Estimated seed yields were 275, 96 and 145 seeds m-2 (1.4, 0.5 and 0.8 kg ha-1) for the 3 years, 75% of which were hard seeds. Annual seed input depended much more on the number of flowerheads produced and the proportion of flowerheads grazed than on variation in yield per flowerhead. Half-lives of buried seeds were estimated at about 1 year or less, wtih only 13% of seeds still surviving after 18 months in a simulated seed return experiment. High turnover rates in the buried seed pool indicate that seed reserves are likely to be relatively recent in origin and an ineffective source of new plants within existing swards.