Menaut and Cesar (1979)
1 Apn - Above ground primary productivity J Bpn - Below ground primary productivity 3 Pn - Total productivity
1 Apn - Above ground primary productivity J Bpn - Below ground primary productivity 3 Pn - Total productivity the soil. Such knowledge is also necessary to understand the potential effect of removal of vegetation as a result of fire, herbivory and human activity. The new values indicate that the efficiency of light conversion in tropical savanna grasses is higher than previously estimated, which is of economic importance. Finally, an accurate assessment of tropical savanna productivity is indispensable to establish a baseline against which the effects of future changes in global C02 levels may be assessed.
8.2.2 Demographic and physiological characteristics of savanna species
To understand the ways in which savanna species respond to natural stresses and human disturbances, account must be taken of the existence of a great diversity in life history characteristics and physiology among savanna species. Many different species with apparently similar characteristics can coexist in the same community. However, their patterns of growth and reproduction are different (Sarmiento and Monasterio 1983). Some grass species in wet savannas start to grow with the first rains or shortly after a late fire, and after a spurt of growth go into a reproductive phase. Others grow more gradually, develop their shoots slowly, and enter into their reproductive phase towards the middle or the end of the rainy season. This temporal displacement in the peaks of growth and reproduction may partly explain the ability of species which superficially are very similar in morphology to coexist in the same environment.
The soil-available moisture and soil-available nutrients are not the same in the early, middle and late season (Sarmiento 1984), which suggests that early, middle and late species may have different physiological capabilities. Preliminary studies by Goldstein and Sarmiento (1987) indicate that each of the phonological groups, but especially early bloomers, have different ecophvsiological attributes, and that these differences are of adaptive value. Precocious and early growers tend to maintain lower transpiration rates, have higher water-use efficiencies (ratio of carbon assimilated to water loss) and higher turgor pressures than intermediate and late-growing species. So, for example, Leptocoriphium !anatum, a precocious species, showed maximum daily transpiration rates of 7.5 nmol m 2 s compared with 12 nmol m 2 s 1 for Trachypogon vestitus, a late species. Maximum photo-synthetic rates at high photon flux densities and low vapour pressure densities are between 25 and 32 |imol m 2 s~' and do not differ significantly among species (Goldstein and Sarmiento 1987). Maintenance of positive turgor pressure should allow the precocious species to maintain continuous growth under high water-stress conditions. The observed differences in gas exchange characteristics could explain the high growth rates of the precocious species during the transition from the dry to the wet season. The differences in ecological behavior among perennial C4 savanna grasses may also be determined by other traits related to growth and morphology such as the proportion of photosynthate allocated to leaves and underground organs. The higher initial growth rates of the early-growing species may also be strongly associated with the higher proportion of photosynthate and nutrient reserves allocated to roots and below-ground organs (Medina and Silva 1990). At the end of the dry season growth may be supported more by stored nutrients and carbohydrate reserves than by current absorption. F.ariy growers may behave as stress-tolerant species and be rapidly out-competed when nutrient and water availability increases. Studies by Raventos and Silva (1988) show that late growers are competitively superior to early growers during the wet season.
Herbaceous and shrubby nitrogen-fixing Leguminosae are an important floristic component of savannas, particularly in South America. However, they frequently contribute less than 1% of the total biomass of the herbaceous layer. Yet in nitrogen-deficient savannas these species play an important role in nitrogen cycling, covering a substantial fraction of the nitrogen losses caused by fire (Medina and Bilbao 1991).
While definitions of stability have previously carried implicit assumptions of an equilibrial or steady state as a preference point, more current definitions of stability recognize that a range or cloud of system states may be used for reference (Solbrig 1993). That range may contain regular cycles at different temporal scales, threshold responses and apparently chaotic behaviors with underlying order (e.g. "strange attractors"). When cycling among system states is a characteristic system behavior it becomes essential to differentiate measures of short- and long-term stability, bccause while a short-term measure may indicate instability, a longer-term measure may indicate stability. While savannas may oscillate or fluctuate among a range of states, they can still be stable systems.
Measures of stability that are based upon floristic composition may provide different results than measures based upon functional group compositions owing to similarity of function of species within a functional group. The more alike species are in their functions, the less critical it is to maintain a particular species, as long as all critical ecosystem functions are preserved. Thus, the level of functional identity within groups must be known in order to interpret the significance of changes in floristic composition.
Resilience, that is the capacity of the system to maintain its overall functional identity, is dependent on the ability to withstand unusual combinations of environmental factors, usually called disturbances. These may be the result of extreme values of an otherwise natural event, such as an extremely dry year, an intensification in the frequency of fires, or fire suppression, where fire is a regular event, or they may be the result of an entirely new circumstance, such as the appearance of a new pathogen or herbivore. There are several important modifiers of disturbance responses. These modifiers must be taken into consideration to interpret species responses accurately. They are (5) time since disturbance, (2) direct and indirect interactions among species following or preceding the disturbance, (3) abiotic variables such as soil depth, soil fertility or rainfall, and (4) the occurrence of other regular events causing mortality, such as fire.
In particular, time-dependent variables such as rainfall may confound responses to disturbance. When rainfall changes over time since a disturbance, the effects of the change in rainfall must be disentangled from the effects of the disturbance in order to interpret the response. It is important to recognize that the response of the savanna to a disturbance depends upon the initial state of the system. In other words, system dynamics are sensitive to initial conditions. The history of disturbance also has an important impact, through selective forces, upon the presence of species that are adapted to subsequent disturbances of the same kind. Indeed, savannas may be intrinsically stable relative to other systems because they have evolved with habitual fires, herbivory and drought. Thus, the continued persistence of savannas may necessitate their presence to preserve stability.
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