Savannas II The Environmental Factors Water Mineral Nutrients and Fire

Very important anthropogenic factors determining the existence of savannas are her-bivory by cattle (see anthropogenic hypothesis in Sect. 9.3) and fire. The major natural environmental factors are:

• water (hydrological hypothesis, Sect. 9.3),

• mineral nutrients (edaphic hypothesis, Sect. 9.3),

• fires caused by natural events

(Hogberg 1986a). These are discussed in this chapter.

10.1 The Water Factor

A model of the water budget of savannas explaining the various inputs and losses is shown in Box 10.1. The key elements, of course, are precipitation for the input and evapotranspiration by the vegetation plus free evaporation for the losses. The strong seasonality often encountered in savannas is mainly determined by the water factor (Fig. 9.10). Woody plants and grasses in savannas have different requirements from the annual precipitation, dependent on the distribution of rainfall, soil availability of water over the year and the water-capacity of the soil (Table 10.1). As a result the phenological behaviour of savanna trees and grasses is also different (Sects. 10.1.1.1 and 10.1.2.1). This strongly affects overall fluxes of carbon, water and energy in the seasons as documented by an important comprehensive investigation in a Brazilian cerrado (Miranda et al. 1997) some comparisons of which are depicted in Fig. 10.1. The leaf area index (see Sect. 3.4.1) was 1.0 and 0.4 in the wet and dry season, respectively, and during the light periods of the days loss of H2O-vapour (transpiration rate), downward fluxes of CO2 and ecosystem surface conductance were much higher in the wet season than at the end of the dry season. With the different hydraulic requirements of grasses and trees it is appropriate to consider them separately in the following two sections.

Fig. 10.1 Canopy fluxes of water vapour (Jh2o) (A), carbon ( Jco2) (B) and surface conductance (C) over a Brazilian cerrado towards the end of the wet season (April 1993, solid lines) and at the end of the dry season (September 1993, dotted lines). Negative values of Jco2 and Jh2o give the downward fluxes from the atmosphere to the canopy. (Drawn after data of Miranda et al. 1997; with kind permission of Blackwell Science, Oxford, UK)

Table 10.1 Different requirements of grasses and woody plants in savannas with respect to the water factor. (After Walter and Breckle 1984)

Grasses

Woody plants

(i) Amount of precipitation

Lower amounts of annual precipitation Larger amounts of annual precipitation

(ii) Annual distribution of precipitation Precipitation must occur Precipitation may occur during the growth period during the rest period

(iii) Annual distribution of water uptake

During the rest period no water The soil must provide enough water is taken up from the soil for a minimal water-uptake also in the dry season

(iv) Soil-water capacity

The water capacity of the soil The water capacity of the soil does not must be high: need to be high:

the plants do not limit their transpiration as the crumb size of the soil may be large, long as the soil provides enough water, and then the leaves dry rapidly the soil may be stony and rocky; the root system develops far in both horizontal and vertical direction

Box 10.1

Model of the water budget of savannas with a continuous vegetation-soil compartment separated from the atmosphere. Rom. = losses, ital. = input. (Sarmiento 1984; reprinted by permission of Harvard University Press).

Box 10.1 (Continued)

Box 10.1 (Continued)

10.1.1 Grasses

10.1.1.1 Phenology

Due to the mostly only superficial rooting of grasses phenological differentiations are highly important for adaptations of grasses relating to the water factor (Table 10.1). The following phenological groups are observed among savanna grasses:

• perennial with a seasonal semi-dormant-period,

• annual, ephemeral with a short cycle,

• perennial with a seasonal dormant period,

• continuous growth and flowering.

The first group is most frequent. It is represented, for example, by Trachypogon plumosus and Leptocoryphium lanatum (Fig. 10.2). The phenological diagrams for the two species, in contrast to the tree Curatella americana, Dilleniaceae (see Sect. 10.1.2.1), show flowering and new shoot production during the rainy season, but closer to the end of the rainy season in T. plumosus than at the beginning as

Reproductive Biomass

Green Biomass

Shoot Production

Reproductive Biomass

Green Biomass

Shoot Production

Reproductive Biomass

Green Biomass

Shoot Production

Reproductive Biomass

Green Biomass

Shoot Production

Reproductive Biomass

Green Biomass

Shoot Production

Reproductive Biomass

Green Biomass

Shoot Production

Rainy season

Dry season

Trachypogon plumosus

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