Emergence and survival of buried doublegee (Emex australis Steinh.) seeds

Abstract

The emergence and survival of seed of doublegee (Emex australis Steinh.) buried at various depths were studied over 4 years at 3 climatically contrasting sites in Western Australia. We concluded that there is an optimal burial depth for seedling emergence and seed survival. Seeds placed at 1 cm gave maximum emergence but no seedling emerged from seeds placed at 15 cm or deeper. Emergence occurred mainly during the first autumn from seed less than 1 year old. Over the following 3 years, emergence declined progressively. The waves of emergence reflected the seasonal rainfall pattern, although emergence did not occur after every rainfall. There was no direct relationship between emergence and loss of seed viability. At the coolest site with higher rainfall the retention of viable seeds increased with increasing depth in the soil, whereas at warmer sites with lower rainfall there was an inverse relationship between viability and burial depth. The viable seeds exhumed after 4 years of burial were in different states of dormancy but induced or innately dormant seeds predominated at the cooler higher rainfall site. These types of dormancies and the various environmental factors will determine the number of viable seeds germinating in any given season. It is shown that even when the influx of new seed was stopped for 4 consecutive years, it was still not possible to achieve complete reduction in numbers of doublegee seeds.