into the forests, and where the trees are not fire-resistant, year by year savanna gradually encroaches into the area previously occupied by forests (Fig. 10.29).
Savanna trees are particularly fire resistant (Figs. 10.30 and 10.31), being predominantly evergreen, with a thick corky bark and dormant buds, and sprout after fires before the beginning of the rainy season (see Sect. 10.1.2.1). It is estimated that only trees with a bark less than 5 mm thick can be killed by fires in the cerrado (Miranda et al. 1993; Gottsberger and Silberbauer-Gottsberger 2006). Some trees, in areas where regular burning occurs, may restrict formation of their woody stems to below the ground surface in the form of lignotubers or xylopodia, where dormant buds are protected and can readily produce new growth after a fire (Fig. 10.31B). Xylopodia may also serve storage of water and minerals, and the plants forming such organs can be considered to represent the typical life form of xylohemicrypto-phytes (Gottsberger and Silberbauer-Gottsberger 2006; for definitions of life forms see Sects. 3.3.4 and 11.3.1, Table 11.2). Deciduous trees, with phenological cycles related to the seasonality of rainfall, are more sensitive to fire, and they are excluded from regularly burned savannas (Medina and Silva 1990). In the experiment at Calabozo mentioned above, during 20 years of protection, total tree density increased considerably, both of fire-resistant savanna trees and fire-sensitive species from the surrounding semideciduous forest (Table 10.15), and Table 10.16 shows similar findings for a Brazilian cerrado. In dry savannas of Namibia where fires are disastrous and burning is now strictly avoided one can observe Acacia mellifera ssp. detinens spreading out and occupying vast areas, which has become a curse for cattle farmers.
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