19.3.2  Fire ephemerals: plants that grow from seed banks

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Fire ephemerals, sometimes referred to as fire weeds (e.g. the grass Stipa compressa, native hibiscus Allyogyne spp. and members of the family Gyrostemonaceae (Figure 19.8B), form conspicuous components of the early post-fire floras of many ecosystems. Their seeds germinate only after intense fires from seed banks in the soil that are both extensive and very long lived. Even in the absence of human intervention when intervals between fires can be many years, such seed can survive to exploit a post-fire regime. Some species of fire ephemerals (e.g. Stipa) are monocarpic; that is, they complete a growth cycle and return seed effectively to the soil within six to nine months of a fire. Others (e.g. Gyrostemon spp.) are polycarpic; they commence flowering in the second season after fire and then reproduce over the following two or three seasons before dying. Since it is unlikely that sufficient fuel will accumulate in a habitat to sustain two fires in close succession, most fire ephemerals complete their life cycles by ‘avoiding fire in time’. During these brief lives, fire ephemerals play a critical role in resource capture. Important inorganic nutrients released by fires are retrieved and assimilated into the organic biomass of a new generation of parent plants. Absence of fire ephemerals could allow large-scale losses of nutrient capital through leaching.

All fire ephemerals are characterised by extremely high rates of seedling growth in comparison with other classes of species recruiting in the same ash bed. They also exhibit extreme plasticity in adult size, depending on: (1) how densely they germinate, (2) competition between new seedlings and (3) patchiness of nutrient distribution in ash and decomposing plant material within their rooting catchments. Thus, in a small area of habitat where ash has been deposited unevenly, the largest individuals of a fire ephemeral such as the grass Stipa compressa might be 500 to 1000 times heavier than the smallest. Despite such plasticity in size, there is remarkably little variation in the proportion of dry matter committed by individuals to flowers and seeds just prior to their death. This proportionality ensures that the size of a seed bank is geared to resource availability across a habitat.

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Table 19.1

Harvest index (the proportion of a specific plant resource (e.g. dry matter, potassium) invested in seed at maturity) has been used to compare relative reproductive effort of a range of fire ephemeral species from southwestern Australian. That is, harvest index quantifies how effectively each species passes
its dry matter or specific nutrients on to seeds of the next generation (Pate et al. 1985). Table 19.1 illustrates strong sequestering of phosphorus in each of the listed fire ephemerals, followed broadly by nitrogen, potassium, dry matter, magnesium and calcium. Because parent ecosystems of these species are considered to be limited primarily by availability of phosphorus, channelling of up to 90% of a plant’s phosphorus pools into seeds enhances survival of seedlings after fire. Little is known of the regulatory processes of senescence at a cellular level that release nutrients from vegetative parts for retranslocation to seeds (Chapter 16).

Abundant seed production by many species that are fire ephemerals points to selection for high numbers of relatively small seeds, each well stocked with long-term energy reserves of oil or starch but less well endowed with mineral reserves. These features enhance seed longevity and ensure that dense populations become established effectively after hot fires. Modest reserves of some nutrients in seeds are likely to be well compensated for by high availability of nutrients in the ash bed in which the seed germinates (Section 19.2). Supporting evidence shows that fire ephemerals flourish particularly well in burnt habitats but grow very poorly if transplanted into unburnt habitats of poorer mineral resource availability.

 

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