Seed dormancy and persistent sediment seed banks of ephemeral freshwater rock pools in the Australian monsoon tropics

Abstract

Background and aims: Rock pools are small, geologically stable freshwater ecosystems that are both hydrologically and biologically isolated. They harbour high levels of plant endemism and experience environmental unpredictability driven by the presence of water over variable temporal scales. This study examined the hypothesis that the sediment seed bank in monsoon tropical freshwater rock pools would persist through one or more periods of desiccation, with seed dormancy regulating germination timing in response to rock pool inundation and drying events.

Methods: Seeds were collected from seven dominant rock pool species, and germination biology and seed dormancy were assessed under laboratory conditions in response to light, temperature and germination stimulators (gibberellic acid, karrikinolide and ethylene). Field surveys of seedling emergence from freshwater rock pools in the Kimberley region of Western Australia were undertaken, and sediment samples were collected from 41 vegetated rock pools. Seedling emergence and seed bank persistence in response to multiple wetting and drying cycles were determined.

Key results: The sediment seed bank of individual rock pools was large (13 824 ± 307 to 218 320 ± 42 412 seeds m(-2) for the five species investigated) and spatially variable. Seedling density for these same species in the field ranged from 13 696 to 87 232 seedlings m(-2). Seeds of rock pool taxa were physiologically dormant, with germination promoted by after-ripening and exposure to ethylene or karrikinolide. Patterns of seedling emergence varied between species and were finely tuned to seasonal temperature and moisture conditions, with the proportions of emergent seedlings differing between species through multiple inundation events. A viable seed bank persisted after ten consecutive laboratory inundation events, and seeds retained viability in dry sediments for at least 3 years.

Conclusions: The persistent seed bank in freshwater rock pools is likely to provide resilience to plant communities against environmental stochasticity. Since rock pool communities are often comprised of highly specialized endemic and range-restricted species, sediment seed banks may represent significant drivers of species persistence and diversification in these ecosystems.

Keywords: Ephemeral freshwater wetland; ethylene; hydrophytes; sandstone rock pools; sediment seed bank; seed dormancy; seed germination; seedling emergence patterns.

Fig. 1.

Location of vegetated rock pools on exposed sandstone pavement within the study area. Eight major clusters of vegetated rock pools were present (annotated), with 1–4 located to the north and 5–8 located to the south of the Morgan River. Grey areas indicate the surrounding matrix of savannah woodland.

Fig. 2.

Annual temperature profiles for the top 10 mm of sediment in Kimberley sandstone rock pools, with daily rainfall data overlaid (Australian Bureau of Meteorology; http://www.bom.gov.au/climate/data/). June–November dry season temperatures in 2012 are presented for both a shallow pool (3·2 m² surface area with a depth of 25 mm and a sediment depth of 15 mm; A) and a deeper pool (4·8 m² surface area with a depth of 43 mm and a sediment depth of 13 mm; B), and October–April wet season temperatures for the deeper pool (C). Temperature probes were washed out of the rock pool on 16 March 2012 during a flood event.

Fig. 3.

Seedling emergence (mean ± s.e.) from the sediment seed bank of five rock pool species over ten consecutive inundation events, with sediments irrigated to either field capacity (hashed columns) or waterlogged (white columns) and incubated for 4 weeks at 35 °C on a 12/12 h light/dark cycle. Annotated lettering indicates the within-treatment significance of seedling emergence between inundation events.

Fig. 4.

Seedling emergence (mean ± s.e.) from the sediment seed bank of five rock pool species in response to annual inundation over 3 years of dry storage (Treatment), and in control sediments stored dry at 35 °C with inundation events occurring after 12, 24 and 36 months (Control). Sediments were maintained dry at 35 °C during storage, and incubated at 35 °C during inundation events. Annotated lettering indicates the within-treatment significance of seedling emergence between inundation events.

Fig. 5.

Germination percentage (mean ± s.e.) of the seeds of rock pool species in response to up to 6 months of dry after-ripening (35 °C and 50 % relative humidity). Seeds of each species were plated either on water agar (white columns), water agar with KAR₁ (hashed columns), or water agar after exposure to ethylene gas (black columns). Annotated lettering indicates the within-treatment significance of germination percentage during after-ripening.