19.2.2  Fires and ecosystem composition

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Fire can be a major determinant of the composition of plant communities and it sometimes defines the boundaries between dissimilar plant communities (Section 19.4). Species with little tolerance to fire can be pushed rapidly to extinction within a single generation while others profit from resources liberated through the impact of fire. For example, light levels beneath a canopy improve after fire-intolerant species are eliminated, raising light and heat capture by emergent seedlings and epicormic shoots of fire-tolerant species.

Reduced water and nutrient withdrawal by plants killed by fire leaves these resources available for establishment of fire-tolerant plants. Furthermore, ash released from burnt plant material (‘ash-bed effect’) supplies a flush of particular inorganic nutrients for new plant growth (Renbuss et al. 1972). However, nutrient enrichment might be short lived and depends on the nutrient in question. Calcium, magnesium and potassium can be leached through sandy soils after fires and made unavailable for new seedlings. Analysis of burnt and unburnt sites at Wilsons Promontory, Victoria, revealed major nutrient perturbations in the surface 2 cm of soil; burning improved phosphate concentrations by at least 30% while nitrogen was depleted by about 30% (Adams et al. 1994). Post-fire colonisation by Leptospermum and Kunzea spp. was rapid in these conditions. Shrublands and forests are often enriched in nitrogen-fixing species after fire because of temporary nitrogen deficits and sometimes specifically through fire-induced germination (Figure 19.2). Moreover, natural inhibitors of germination (allelochemicals) can be destroyed by intense heat. Hence the opportunity created by fire can be quickly seized by species with seeds or vegetative structures able to survive burning.