Germination depth vs. temperature requirements in naturally dispersed seeds of Leucospermum cordifolium and L. cuneiforme (Proteaceae)

Abstract

Myrmecochory (ant dispersal) and the requirement for both low and high diurnal soil temperatures during seed germination after fire, are important ecological determinants in the seedling recruitment phase of Leucospermum R. Br. species. A crucial unknown parameter linking these two factors is the effective burial depth (i.e. germination depth) of seeds stored in the subterranean seed bank. The germination depth of seedlings was effectively determined in a nursery trial by measuring the length of the hypocotyl between the radicle end and ground level. Using this method the germination depth of Leucospermum cordifolium (Salisb. ex Knight) Fourcade and L. cuneiforme (Burm. f.) Rourke seedlings emerging during winter, after summer burns in three localities was estimated. Average estimated germination depths varied between 30 and 45 mm. Daily minimum and maximum temperatures were measured at various soil depths during winter in burnt and unburnt mesic mountain fynbos. Temperatures measured at 10, 20 and 40 mm depths during winter were close to the known optimum daily low and high germination temperatures in L. cordifolium , in burnt fynbos, but not in unburnt fynbos. These temperatures in burnt fynbos prevailed especially during early winter. It was concluded that a high proportion of the Leucospermum cordifolium seed bank will germinate only during the first early winter following a fire. Other aspects of natural seed dispersal were studied. These included seedling dispersal numbers and patterns around burnt mother plants. Evidence for seed dispersal and burial by ants was found.