Effects of seed dormancy and emergence time on the survival and early growth of Eucalyptus delegatensis and E-amygdalina

Seed of two tree species, Eucalyptus amygdalina Labill. and E. delegatensis R.T.Baker, was sown on 12 separate dates at two sites close to which both occurred. The species have parapatric distributions but form ecotonal stands. One site was harsh by comparison to the other. Recruitment, survival and growth were recorded at regular intervals following sowing and were related to temperature and soil moisture. The non-dormant seed fraction of both species responded similarly to environmental cues, with rapid emergence occurring when the mean daily temperature exceeded 6 degrees C and the soil moisture index was below 30 mm. Differences in the pattern of emergence between the species resulted from differing degrees of seed dormancy. On the harsh site, spring emergence gave superior survival and growth than did autumn emergence. Consequently, spring emergents were dominant at the end of the experiment. At the milder of the two sites, little difference in survival or growth was observed between emergence times and consequently seedlings emerging in the autumn were dominant at the end of the experiment. Differences between the mortality rate of very young seedlings of each species were detected at some times of year. The combination of differences in seedling mortality, and patterns of emergence as a result of seed dormancy suggest that differences in the regeneration niche may act to reinforce species boundaries, and combined with spatial and temporal variation in regeneration conditions may promote coexistence in ecotonal stands.