Savanna Structure And Function

Ecosystem function can be interpreted in two ways. It can refer to the flow of energy and nutrients through an ecosystem or to the flow of species populations through time, i.e. the persistence of species populations and their properties, what Holling calls the resilience of the system (Holling 1973, 1986; Solbrig 1993). The usual way of looking at ecosystem function is to consider only the flow of energy and nutrients. We first discuss how species characteristics control the flow of energy and materials in the savanna ecosystem, and then address how species characteristics give the savanna ecosystem its resilience.

The herbaceous component of the savanna can be considered as the controlling element of the system and the one that regulates fundamental ecological processes such as water balance, productivity, mineral cycling, fires and herbivory. The common species have wide geographical distributions, yet each species has its own phenology and microdistribution (Solbrig et al. 1992). Although the herbaceous stratum is considered to be continuous, it is only so at the height of its growth, since the actual basal area of the grasses may be only 10-20% (Sarmiento 1984).

8.2.1 Dynamics of savanna resources

Water and nutrients are the basic resources that limit productive processes in tropical savannas. Soil moisture regimes, in turn, are affected by (1) the total amount and seasonal distribution of rainfall and the proportion of this water that enters the soil (2) the water-holding capacity of the soil, which is largely a function of soil particle size and depth, and (3) the amount of évapotranspiration. which is related in complex ways to climate, soil particle size, surface characteristics and the type of vegetation at the site. In turn, savanna community structure and species composition are highly correlated with soil-water dynamics along moisture gradients (Silva and Sarmiento 1976a,b>.

Savanna soils vary widely in particle size, structure, profile and depth, reflecting the interaction of geology, geomorphology and climate, as well as the influence of topography, the kind of vegetation cover and animal activity (Young 1976; Montgomery and Askew 1983}. Three factors play an important role in pedogenesis: topography, parent material and age.

The principal influence that topography has over the ecosystem is on the regulation of drainage, and ultimately over the water balance. In turn, through their action on pedogenesis, the agents that produce the relief indirectly determine the physico-chemical characteristics of the soils, so that relief also translates into the chemical and nutritional characteristics of savanna soils (Sarmiento 1984).

Dystrophic savanna soils derived from the weathering of acid crystalline rocks or from ancient sedimentary deposits generally have low reserves of weatherable minerals. The predominance in these soils of 1:1 lattice clays and iron and aluminum oxides results in low effective cation exchange capacity and small amounts of total exchangeable bases, particularly calcium and magnesium (Jones and Wild 1975; Lopes and Cox 1977; Mott et al. 1985). Phosphorous levels are sometimes also very low, and soils rich in sesquioxides have a high capacity for fixing phosphorous. Some highly weatherable soils also have high levels of exchangeable aluminum (Lopes and Cox 1977; Haridasan 1982).

The nutrient status of the soil in tropical savannas is related principally to the age of the sediments (Cole 1986). For example, in the Orinoco savannas, the poorest soils (oxisols and ultisols) are those derived from the oldest deposits, since these materials have been subjected to predogenic processes for prolonged periods of time.

With the exception of extremely acid soils, the amount of organic matter is the main determinant of cation exchange capacity. In wet savannas, high rainfall and an extended wet season favor plant production, with a consequent input of organic matter into the soil. Because of the almost yearly frequency of fire, the organic matter input is almost exclusively the result of below-ground production, since fire effectively mineralizes most of the aerial matter produced (Sanford 1982; Menaut et al. 1985). High temperature and humidity favor microbial activity. However, microbial activity is limited by the low levels of assimilable carbon, high C: N ratios, lignin content, and, in some cases, high amounts of condensed tannins and secondary chemicals. Microbial activity may be stimulated by root exudates and by water-soluble compounds produced by earthworms (Lavelle et al. 1983; Menaut et al. 1985).

Mound-building termites, especially earth-eating species, modify the physico-chemical properties of the soil in their nests by selecting fine particles in their construction and by increasing the nutrient content of the soil in the nests, especially Ca, K, Mg and P, through their feeding activities (Pomeroy 1983; Lopez-Hernandez et al. 1989). Termites are efficient foragers and can denude the area surrounding their nests of organic matter and its nutrients. In the American savannas ants of the genera Atta and Acromyrmex behave in a similar manner to termites, removing litter from a large area and concentrating its nutrients in their underground nests. According to Coutinho (1984). a well-developed ant colony processes a ton of material in a year. Termites and ants create a patchy nutrient distribution that in turn is perpetuated by the vegetation, especially trees that grow preferentially on these mounds.

The nutrient dynamics of tropical savannas is now well known. Several authors (Medina 1982, 1993; Sarmiento 1984; Menaut et al. 1985) have summarized existing knowledge on nutrition partitioning between various compartments in the savanna ecosystem and proposed models for the cycling of nitrogen and other elements. The principal conclusion of these studies are that fire represents the principle source of nutrient loss from the system, that internal cycling accounts for the greatest proportion of nutrient fluxes, and that the most important compartment is the organic matter in the soil. The deficit in nitrogen must be covered through rainfall input and free-nitrogen fixation.

The flow of energy and nutrients through savanna ecosystems is tightly linked to the flow of water through the soil-plant-atmosphere continuum (Figure 8.2). Pulses of energy and nutrient input to the biotic components of the ecosystem result from pulses of production of plants. There are two levels of moisture pulses: (a) the alternation of dry and wet seasons, and (b) changes in PAM due to irregularities of rainfall during the wet season. In the wet neotropical savannas, there is an important distinction between savanna trees and grasses in this respect: grass production is tightly linked to the rainfall pulses whereas tree production is not (Sarmiento 1984; Cole 1986; Frost et al. 1986; Walker 1987). Trees rather depend on total annual rainfall to replenish underground water reserves. In contrast, in the drier, sandy savannas of southern African, trees seem to be depending on the rainfall pulses as do the grasses. In neotropical savannas, grasses represent a very high fraction of the total plant biomass, therefore most of energy and

Litterfall And Decomposition Plant

Figure 8.2 Nutrient cycling in savannas. The deep roots of trees obtain water and nutrients from the deep layers of the soil. Tree leaf and litter fall and decompose on the upper layers of the soil (upper 30 cm approximately) where they are utilized primarily by the grass layer. Some water and nutrients may percolate to lower layers, but it is insignificant compared to the pumping action of trees

Figure 8.2 Nutrient cycling in savannas. The deep roots of trees obtain water and nutrients from the deep layers of the soil. Tree leaf and litter fall and decompose on the upper layers of the soil (upper 30 cm approximately) where they are utilized primarily by the grass layer. Some water and nutrients may percolate to lower layers, but it is insignificant compared to the pumping action of trees nutrients flowing through the system are linked to the rainfall pulses. After several days without rain, soil moisture is reduced and not available to the grasses. The consequent closure of stomata and the loss of photosynthetic tissue if drought persists will reduce the uptake of carbon dioxide. The flux is reestablished as soon as it rains, and adequate soil moisture availability allows transpiration water fluxes from the soil to the atmosphere through the plants (Sarmiento el al. 1985).

A number of estimates of the productivity of tropical savanna grasses have been carried out. Most of these studies were made assuming that productivity, the gain of new organic matter by vegetation, approximated to the measured increase in above-ground biomass. However, this assumption has proved to be incorrect and has led to an underestimation of true productivity by a factor of two or three (Sarmiento 1984; Long et al. 1989, 1992) for three reasons: (1) below-ground production can be as high or higher than above-ground biomass; (2) the methods used assumed that death of tissues occurs only after the peak of production has been reached; (3) the researchers did not consider that different species reach their peak of production at different times. Recent studies (Table 8.2) in tropical grasslands that took these considerations into account have obtained values that are 5-10 times higher than those from previous studies, and an approximate figure of 1000 to 2300 g m"2 year 1 for tropical forests (Ajtay et al. 1979).

An accurate appraisal of tropical savanna productivity is essential to understand the input of organic matter into the ecosystem and the amount and material available for producers and decomposers, including those in

Fable 8.2 Estimates of productivity of tropical savanna grasses

Site

Apn1

Bpn2 igm~: yr"1)

Pn3

Rainfall (mm)

Source of data

Fete Ole (Senegal)

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Responses

  • Ermias
    What are the pedogenic processes associated with savanna?
    3 years ago
  • senja
    What is the structure and climate and function of a savanna?
    2 years ago

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