Discussing adaptations of plants to fire as an important natural ecological factor it is useful to distinguish between:
• pyrophilous plants, which obtain an advantage in the competition with other plants, and
• true pyrophytes, which essentially need fire at least at some stage in their life cycle.
Smaller plants may survive fires since the temperature at the soil surface may reach values of ~ 75 °C only for a few minutes, and 1 - 5 cm below the surface temperatures may already be much lower (Walter and Breckle 1984). Thus the terminal buds in the center of tussock grasses are well enough protected, and regeneration can also occur from below-ground plant organs. Taller fire resistant plants, apart from specialized savanna trees with thick bark and dormant buds (see Sect. 10.3.3), are often tree ferns or monocotyledonous plants (like palms, Yucca or Xanthorrhoea, Fig. 10.27). These plants do not have a cambium at the periphery of their stems, as found in dicotyledonous shrubs and trees. In some Eucalyptus species in Australia, survival is guaranteed by formation of below-ground stem-thickenings ("lignotubers"), and reproduction by seeds is facilitated by removal of dry litter during fires and by killing the predators of young seedlings (Walter and Breckle 1984).
The genuine pyrophytes are literally dependent on fire. In the Cyperaceae Bul-bostylis spadicea in the Brazilian cerrados flowering is stimulated by fire (Gotts-berger and Silberbauer-Gottsberger 2006). A member of the Australian Liliaceae (or Xanthorrhoeaceae), Xanthorrhoea, only flowers after a fire. Among the woody plants of the cerrados in Brazil, Coutinho (1976) (see also Gottsberger and Silber-bauer-Gottsberger 2006) distinguished the following responses of flowering to fires:
• species which quantitatively and qualitatively depend on fire and where fire elicits flowering at any time during the seasons,
• species where fire elicits flowering only during the dry season or in combination with short days,
• species which do not react to fire and flower during the dry season or after induction by short days,
• species which are damaged by fire and normally flower during the rainy season or after induction by long days.
Among the Australian Proteaceae there are many species where the fruits can only open and disperse seeds after a fire, e.g. Banksia ornata, Hakea platysperma and Xylomelum pyriforme, as well as the conifer Actinostrobus.
The evolution of such fire resistant and fire demanding plants also implies that fire is necessary to stabilize the ecological equilibria in ecosystems which have always been regularly subject to fire. In fact, it has been noted in some conservation areas and national parks that total prevention of fires has had adverse effects (Walter and Breckle 1984).
10.3.3 Burning by Man: Losses and Gains
During severe drought periods in savannas the decomposition of above-ground dead organic matter by microorganisms is very much reduced. This cover prevents new growth. Perennial grasses die back and seedling mortality under such a dense layer of dead plant material is high. Eventually the whole grass layer may die, as shown in a long-term experiment over 20 years, where specific areas were protected from fire and grazing at the biological field station at Calabozo in the Venezuelan Llanos (Medina and Silva 1990).
Rapid mineralization by fire removes the dead biomass and also has nutritional effects. Burning decreases soil acidity, and promotes mineralization of nitrogen. After an episode of fire, rates of nitrification increase for several years, followed by a decline of nitrification and increase in ammonium availability (Stewart et al. 1993). However, fire not only enriches the soil with minerals, it may also lead to losses especially of N and S in the form of volatile oxides (Table 10.12). Most of these losses are from vegetation rather than soil (Stewart et al. 1993). For N the range of such losses is:
• 4.5-5.6 kgha-1year-1 in Australia, •8-10 kgha-1year-1 in Africa, and
Input by rain
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