The one common feature of all tropica! savannas is climatic seasonality. The rhythm of the wet and dry seasons regulates the rhythm of growth and reproduction of the herbaceous and woody vegetation. This rhythm is driven by two major factors: rain and fire (Fig. 8.3). Fire, taking place towards the end of the dry season, generates two pulses: increased nutrient availability and high PAR radiation at ground level, both favorable to grass growth. The onset of rains represents the third pulse. With the onset of the rainy season the upper profile of the soil slowly gets saturated, depending on the amount of rainfall, local topography and soil properties. Plant growth progresses rapidly mostly from the regrowth of perennial grasses, but also from the germination of their seeds (Fig. 8.3).
Although contrasting, wet and dry seasons are not totally homogeneous intervals of time, but are yearly oscillations in PAM and PAN that in turn regulate the growth rate, that will then affect the light regime within the canopy and the amount of standing live and dead biomass, that in turn affects the probability of fire. The large number of coexisting herbaceous and woody species are in part adaptive responses to the seasonal changes in the values of PAM, PAN and light that circumscribe different niche spaces utilized by species with different morphologies, physiologies and life histories. For example, since the PAM and PAN status is more favorable near ground level, early-growing species will have primarily superficial roots, low stature and globose form.
'circa one year rainfall green biomass
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