Plant species with physical seed dormancy are common in mediterranean fire-prone ecosystems. Because fire breaks seed dormancy and enhances the recruitment of many species, this trait might be considered adaptive in fire-prone environments. However, to what extent the temperature thresholds that break physical seed dormancy have been shaped by fire (i.e., for post-fire recruitment) or by summer temperatures in the bare soil (i.e., for recruitment in fire-independent gaps) remains unknown. Our hypothesis is that the temperature thresholds that break physical seed dormancy have been shaped by fire and thus we predict higher dormancy lost in response to fire than in response to summer temperatures. We tested this hypothesis in six woody species with physical seed dormancy occurring in fire-prone areas across the Mediterranean Basin. Seeds from different populations of each species were subject to heat treatments simulating fire (i.e., a single high temperature peak of 100°C, 120°C or 150°C for 5 minutes) and heat treatments simulating summer (i.e., temperature fluctuations; 30 daily cycles of 3 hours at 31°C, 4 hours at 43°C, 3 hours at 33°C and 14 hours at 18°C). Fire treatments broke dormancy and stimulated germination in all populations of all species. In contrast, summer treatments had no effect over the seed dormancy for most species and only enhanced the germination in Ulex parviflorus, although less than the fire treatments. Our results suggest that in Mediterranean species with physical dormancy, the temperature thresholds necessary to trigger seed germination are better explained as a response to fire than as a response to summer temperatures. The high level of dormancy release by the heat produced by fire might enforce most recruitment to be capitalized into a single post-fire pulse when the most favorable conditions occur. This supports the important role of fire in shaping seed traits.

/pdf/tanned-or-burned-the-role-of-fire-in-shaping-physical-seed-h5iica012r.pdf

Tanned or burned: the role of fire in shaping physical seed dormancy.

Though observations on re-colonisation of post-fire sites in the Mediterranean Basin are plentiful, there still is an ongoing debate on the interrelation of fire regimes and species traits related to fire adaptation. Most of the studies found are restricted to particular species or claim to present community attributes. Therefore they often lack information for the evaluation of evolutionary evidence and historical contingency of the local fire regime and other abiotic conditions, which may act as selective pressure for plant regeneration strategies. Indeed, knowledge about the success of regeneration mechanisms and their interrelation with ecological factors is essential for the interpretation of the high spatio-temporal variability found in post-fire species performance. Such knowledge would be necessary to assess the potential of different regeneration mechanisms to cope with ongoing land-use and climate change—a crucial scientific challenge.

The challenge of plant regeneration after fire in the Mediterranean Basin: scientific gaps in our knowledge on plant strategies and evolution of traits

An understanding of soil seed bank processes is crucial for understanding vegetation dynamics, particularly in ecosystems experiencing frequent disturbance. This paper examines seed bank dynamics in a tropical savanna in northern Australia, an environment characterised by frequent fire and highly seasonal rainfall. In particular, we examine the contribution of seed bank processes to the high level of resilience shown by grass-layer vegetation in relation to fire. We assess the spatial congruence between seed bank composition and extant vegetation, document temporal variation in the germinable seed bank over the annual dry season, test the effects of laboratory-applied heat and smoke treatments on seed germinability, and determine the effect of experimental fires on seed bank composition. Although dominant species were shared, the composition of the germinable seed bank was significantly different to that of extant vegetation, with approximately half the extant species not being detected in the seed bank. The density and species richness of germinable seeds was significantly greater in the late dry season than the early dry season, with annual grasses showing particularly high levels of seed dormancy in the early dry season. The density and species richness of germinable seeds in the seed bank was significantly enhanced by laboratory-applied treatments of smoke and especially heat, driven by the response of legumes. However, fire had no significant effect on the density or species richness of germinable seeds in the field, indicating soil temperatures during fire were too low to overcome physical dormancy, or burial was too deep to experience adequate heating or smoke exposure. Our results provide a mechanistic understanding of the persistence of annual grasses and forbs in an environment subject to frequent fire and highly seasonal rainfall, and, together with the sprouting capacity of perennial grasses, explain the high resilience of savanna grass-layer plants in relation to fire.

Soil seed banks confer resilience to savanna grass-layer plants during seasonal disturbance

Physical dormancy (PY) was identified in six genera representative of Australian Rhamnaceae and subsequently was broken, based on identification of key seed dormancy characteristics: (1) isolation and classification of embryo features; (2) imbibition experiments to determine the rate and amount of water uptake in seeds; and (3) determination of optimum temperature regimes for germination. All six species had relatively large spatulate embryos. Imbibition studies showed all species possessed PY (i.e. a water-impervious seed coat) that was broken by a hot-water treatment. Alleviation of PY resulted in high germination (<70%) at 7/18°C, temperatures similar to winter in south-west Western Australia. Germination was suppressed at higher temperatures and in the presence of light. The study adds information to our knowledge of seed dormancy in Australian Rhamnaceae, and highlights the benefits of understanding dormancy states in seeds prior to evaluating dormancy-release mechanisms on wild species used in restoration ecology and horticulture.

Physical dormancy in seeds of six genera of Australian Rhamnaceae

We investigated the selective advantage of continuous variation of awn length in the clonal grass Hyparrhenia diplandra (Poaceae) in a West African tropical savanna (Ivory Coast) that burns every year. This species is apomictic, so all seeds produced by one mother plant have the same genotype. H. diplandra produces diaspores with a hygroscopic awn whose length varies within and among plants. In other grass species, awns play a significant role in secondary dispersal on the ground and in seed burial. Using an experimental approach, we quantified (1) the environmental and genetic components of awn length variation, (2) the influence of awn length variation on seed burial depth, (3) the effect of awn length on seed germination and seedling survival rates in burned and unburned plots, and (4) the potential role of fire intensity in selecting diaspores with different awn lengths. Awn length varied from 35 to 70 mm and was highly heritable (h2 = 0.63). Variation within individuals was not related to flowering p...

Evolutionary significance of awn length variation in a clonal grass of fire-prone savannas

Tolerance to high temperatures is an important attribute of seeds of species that inhabit regions that periodically burn In some species, seeds not only survive fire, but require a heat shock in order to germinate Twenty-one taxa, representing a range of ecological attributes, were tested for germination following scarification and boiling-water treatments of up to 90 min duration The black-coloured seeds from the serotinous fruits of Dryandra sessilis (Knight) Domin, Hakea lissocarpha RBr and four species of Eucalyptus were vulnerable to temperatures of 100˚C with increasing duration of exposure associated with reduced germination percentage Seeds of Xanthorrhoea preissii Endl, a species that flowers in response to fire and disperses seed into a post-fire environment unlikely to experience a second fire, also had few seeds that could tolerate even 30 s of boiling-water treatment The reddish coloured, soil-borne, leguminous species seeds showed enhanced germination after short durations of 100˚C, but gradually decreasing proportions of seeds germinated as heat-shock durations were lengthened to 90 min Within the genus Eucalyptus, seeds of relatively large mass and thick seed coats survived longer than seeds of small mass and thin seed coats In contrast, within the legumes, seed mass, wall thickness and cellular structure were not strong indicators of thermal-shock tolerance Differences in germination percentage between scarified and short-term heat-shock treatments indicated that the heat required to break dormancy in Australian legumes may not always be related to breaking an impervious seed coat Australian native legumes appear more tolerant to thermal-shock than species reported from other regions of the world

Tolerance of thermal shock in seeds

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Tolerance of thermal shock in seeds