Dormancy and the fire-centric focus: germination of three Leucopogon species (Ericaceae) from South-eastern Australia

Abstract

Background and aims: Germination studies of species from fire-prone habitats are often focused on the role that fire plays in breaking dormancy. However, for some plant groups in these habitats, such as the genus Leucopogon (Ericaceae), dormancy of fresh seeds is not broken by fire cues. In the field, these same species display a flush of seedling emergence post-fire. Dormancy and germination mechanisms therefore appear complex and mostly unknown. This study aimed to identify these mechanisms by establishing dormancy class and testing the effects of a set of typical germination cues, including those directly related to fire and entirely independent of fire.

Methods: To classify dormancy, we assessed seed permeability and embryo morphology, and conducted germination experiments at seasonal temperatures in incubators. To test the effects of fire cues on germination, factorial combinations of smoke, heat and dark treatments were applied. Ageing treatments, using burial and seasonal incubation, were also tested. Germination phenology was established.

Key results: Seeds were dormant at release and had underdeveloped embryos. Primary dormancy of the study species was classified as morphophysiological. Seasonal temperature changes overcame primary dormancy and controlled timing of germination. Fire cues did not break primary dormancy, but there was a trend for smoke to enhance germination once this dormancy was overcome.

Conclusions: Despite the fact that fire is a predominant disturbance and that many species display a flush of emergence post-fire, seasonal temperatures broke the primary physiological dormancy of the study species. It is important to distinguish between fire being responsible for breaking dormancy and solely having a role in enhancing levels of post-fire germination for seeds in which dormancy has been overcome by other factors. Biogeographical evidence suggests that morphological and physiological factors, and therefore seasonal temperatures, are likely to be important in controlling the dormancy and patterns of post-fire germination of many species in fire-prone regions.

Fig. 1.

Experiment 3: mean percentage germination of previously buried seeds after 30 weeks at two different temperatures for Leucopogon exolasius, L. setiger and L. esquamatus. Error bars represent the s.e.m.

Fig. 2.

Experiment 4: temporal patterns of germination of fresh seeds during 60 weeks under the seasonal temperature regime for Leucopogon exolasius, L. setiger and L. esquamatus. Data are the cumulative percentage, each week, of the total number of seeds germinated.

Fig. 3.

Experiment 5: effect of different ageing treatments on mean percentage germination for (A) Leucopogon exolasius, (B) L. setiger and (C) L. esquamatus after 30 weeks under the seasonal temperature regime, starting at warm temperatures. Error bars represent the s.e.m. Means with the same letter are not significantly different (SNK test, P > 0.05) (n/a denotes treatment not applied).

Fig. 4.

Experiment 6: treatment effects on seeds, previously aged by 18 months burial, then treated with smoke and/or heat. Columns represent mean percentage germination after 30 weeks under the seasonal temperature regime, starting at warm temperatures for (A) Leucopogon exolasius, (B) L. setiger and (C) L. esquamatus (n/a denotes treatment not applied).