Fire

Figure 8.3 Diagram showing the coupling of the seasonal rhythms of green biomass, dry biomass, nutrient and light availability at the ground level with the two main factors of savanna functioning: rainfall and fire

As the season progresses, PAM in the upper layers of the soil becomes more favorable, but PAN less so, as more nutrients are removed by the growing vegetation. Roots of species initiating growth after the precocious ones will grow deeper into the soil. Also, the increased biomass changes light conditions within the canopy, which in turn reduces tillering and promotes elongation of existing tillers (Deregibus et ai 1985). Light conditions at ground level become unfavorable for seedling germination and growth. Late-growing species move their photosynthetic surface upwards, as the tillers elongate and the basal leaves senesce and decompose. Their seeds "show dormancy which has evolved natural selection as an\| seedlmç, that has no dormancy and germinates late in the season will produce seedlings that cannot attain the minimum size necessary to withstand the dry season before the end of the rainy period and dies (Silva and Castro 1989).

Towards the end of the rainy season, PAM again deteriorates as rainfall becomes more scattered, but the accumulation of litter and dead underground biomass leads to an improvement in PAN. Light environment within the canopy is now very poor for growth as the canopy has reached its maximum extent. Savannas differ in phenological diversity, with some groups becoming extremely dominant whereas other groups may be totally absent. Some results show that this variation is related to factors such as rainfall and soil which determine the length of the season with available moisture (Sarmiento 1983).

In addition to the phenological differences, grass species belonging to different tribes exhibit differences in their photosynthctic types. Most tropical savanna grasses belong to the Paniceae, Andropogoneac or Chlori-deac and exhibit a C4 type of photosynthesis. However, some savanna grass species exhibit the C3 photosynthetic type, especially those growing in hyper-seasonal savannas (Medina and Motta 1990). Because of their better forage quality, such species are of great importance (Klink and Joly 1990). C4 grass species, in turn, can be divided into malate and aspartate formers. The former grow preferentially in humid savannas with dystrophic soils, while the latter dominate in dry and semi-dry seasonal savannas (Medina and Huber 1992; Baruch and Fernández 1993).

Trees are an important component of savanna ecosystems. Tree species from different savannas and different continents differ in their morphological and physiological characteristics. In wet neotropical savannas trees tend to be evergreen, have relatively high root/shoot biomass ratios and large, highly sclerophyllous leaves. Their roots usually explore deeper soil horizons than grasses, and therefore in wet savannas are exposed to more uniform water levels. Transpiration raves are generally high even during the dry season, and leaf water potentials are maintained above the turgor loss point because of sufficient soil water availability and the hydraulic properties of the vascular system (Goldstein et al. 1990). In dry savannas with more fertile soils deciduous trees are the norm, making it difficult in these savannas to separate attributes that enhance a tree's capacity to withstand water stress from those that ameliorate nutrient stress.

The principle difference between trees from different savannas is in the length of their active season and the degree of xerophytism they exhibit. In wet oligotrophic savannas with water available in the subsoil, evergreen trees with an extensive deep root system and scleromorphic foliage predominate (Walter 1973; Sarmiento et al. 1985). In environments with a relatively extended wet season and rich soils, trees with large deciduous leaves and relatively small and superficial root systems dominate and in semi-arid savannas, trees with small, scleromorphic leaves arc the norm. These differences have been explained in terms of water and nutrient economy (Medina 1982; Sarmiento et al. 1985).

A complicating factor is nutrient economy. Very wet savannas occur mostly over highly oligotrophic soils. The scleromorphic characteristics of leaves in trees from oiigotrophic savannas may be an indication of nutrient deficiency, rather than water economy. It has been suggested (Sarmiento et al. 1985) that the large leaves of savanna trees growing on oiigotrophic soils have evolved as a mechanism for augmenting transpiration and thereby increasing nutrient uptake.

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