THE ROLE OF SEED SIZE IN SEEDLING ESTABLISHMENT IN DRY SOIL-CONDITIONS - EXPERIMENTAL-EVIDENCE FROM SEMIARID SPECIES

1 The hypothesis that large seed size provides an advantage for seedlings establishing in low soil moisture was tested under both field and glasshouse conditions, using a range of sympatric species of different seed sizes and growth forms from the semiarid woodland flora of western New South Wales, The hypothesis predicted that the relative advantage of large seeds should be greater under dry-soil than better-watered conditions. 2 In the field, seedlings from large seeds had higher percentage emergence and survival than small seeded seedlings, but there was no evidence of a relatively greater advantage under drier soil conditions. Daytime temperatures were very high during the field experiment and even the most favourable watering treatment showed quite low rates of emergence and survival. This low survival may have reduced the ability to detect a greater advantage of large seeds over small seeds under drier than under better- watered conditions. 3 In the glasshouse, the difference in survival from emergence between seedlings from small and large seeds increased under increasingly dry soil conditions, thus supporting the hypothesis. In addition, survival time of seedlings under drying soil conditions was positively associated with seed size. 4 The difference in the field results compared with the glasshouse experiment suggests that although large seeds do provide an advantage for seedlings establishing in low soil moisture conditions, this may be realized only under particular field conditions, depending on the characteristics of the rainfall event and the soil conditions. Thus, only a subset of establishment events may provide selection pressure for large seed size.