Interaction between seed dormancy-release mechanism, environment and seed bank strategy for a widely distributed perennial legume, Parkinsonia aculeata (Caesalpinaceae)

Abstract

Background and aims: Parkinsonia aculeata (Caesalpinaceae) is a perennial legume with seeds that have hard-seeded (physical) dormancy and are potentially very long-lived. Seed dormancy is a characteristic that can both help maximize the probability of seedling establishment and spread the risk of recruitment failure across years (bet-hedging). In this study, dormancy-release patterns are described across the diverse environments in which this species occurs in order to test whether wet heat (incubation under wet, warm-to-hot, conditions) alone can explain those patterns, and in order to determine the likely ecological role of physical dormancy across this species distribution.

Methods: A seed burial trial was conducted across the full environmental distribution of P. aculeata in Australia (arid to wet-dry tropics, uplands to wetlands, soil surface to 10 cm deep).

Key results: Wet heat explained the pattern of dormancy release across all environments. Most seeds stored in the laboratory remained dormant throughout the trial (at least 84 %). Dormancy release was quickest for seeds buried during the wet season at relatively high rainfall, upland sites (only 3 % of seeds remained dormant after 35 d). The longest-lived seeds were in wetlands (9 % remained dormant after almost 4 years) and on the soil surface (57 % after 2 years). There was no consistent correlation between increased aridity and rate of dormancy release.

Conclusions: The results suggest that physical dormancy in P. aculeata is a mechanism for maximizing seedling establishment rather than a bet-hedging strategy. However, seed persistence can occur in environmental refuges where dormancy-release cues are weak and conditions for germination and establishment are poor (e.g. under dense vegetation or in more arid micro-environments) or unsuitable (e.g. when seeds are inundated or on the soil surface). Risks of recruitment failure in suboptimal environments could therefore be reduced by inter-year fluctuations in microclimate or seed movement.

Fig. 1.

The seven seed burial locations mapped over the mean annual rainfall across Australia (see Table 1 for details).

Fig. 2.

The proportion of P. aculeata seeds (mean ± s.e., three replicates) that remained dormant throughout the trial that were buried early in the wet season (seeds sourced from Auvergne) and late in the wet season (seeds sourced from Taemus) at upland sites at (A) Alroy Downs, (B) Auvergne, (C) Beatrice, and (D) Millstream. Dormancy at time of burial is circled for each seed source. Horizontal bars indicate when conditions were wet and warm.

Fig. 3.

The proportion of seeds (mean ± s.e., three replicates) that remained dormant throughout the trial at various habitats at (A) Taemus, (B) Leura (uplands only), (C) Alroy Downs and (D) Millstream. There were no significant habitat affects on dormancy release at these locations. Horizontal bars indicate when conditions were wet and warm. Lowland and wetland sites were not inundated during the course of this study.

Fig. 4.

The proportion of seeds (mean ± s.e., three replicates) that remained dormant throughout the trial in contrasting habitats at (A) Alcoota, (B) Auvergne and (C) Beatrice. Values represented by different letters indicate that dormancy release within the census dates varied significantly between habitat types (P < 0·05). Horizontal bars indicate when conditions were wet and warm (thick bars) and when wetland sites were inundated (thin bars).

Fig. 5.

The effect of seed burial depth on the proportion of seeds that remain dormant at upland sites at (A) Auvergne and (B) Taemus. Values represented by different letters indicate that dormancy release within the census dates varied significantly between burial depths (P < 0·05). Data for 0 d since burial refers to the ‘control’ seeds. Horizontal bars indicate when conditions were wet and warm.

Fig. 6.

A contour-surface plot of the relationship between rate of dormancy release and temperature and soil moisture (averaged over the burial duration). The parameter estimates were taken from the multiple regression model (Table 2).

Fig. 7.

Seed dormancy levels (seed persistence) after approx. 1 year for seeds sourced from Auvergne and Taemus and buried 2-cm deep at upland sites in each region. Regions are ordered by increasing aridity during the burial period (total rainfall for each burial period is given for each location). Detailed time series data is provide in Figs 3–5.