Savanna

Water—its availability, its timing, its distribution—is the primary factor shaping the dynamics of the savanna ecosystem. The savanna experiences recurrent episodes of drought lasting 4-8 months out of the year. During the xeropause, or "dry spell," plant activities—growing, dying, decomposing—continue, but at vastly reduced rates. Studies have shown that resistance to drought is more important to savanna vegetation than resistance to fire. The plants that thrive in the savannas employ many strategies to exploit available water and to survive the xeropause. The mechanisms of survival endow the savanna with its characteristic appearance.

The common savanna grasses grow in tussock form; from protected underground growing points the seasonal grasses grow in a bunch 12 in (30 cm) high or higher. A dense root system allows the individual plant to survive the annual drought, when the aerial (aboveground) grasses die. Typical savanna grasses are the sedges (Latin family name, Cyperaceae), the true grasses (Gramineae), and the bunch grasses (for example, the genera Andropogon and Stipa). The grasses are chiefly of the C4 group; that is, they follow the C4 pathway of photosynthesis, which benefits from high light intensity (such as is found in the tropics), high temperatures, and high evaporation rates. The dominance of C4 grasses is a useful way to demarcate savannas from temperate grasslands, where the grasses are predominantly of the C3 group.

The primary water recruitment strategy of savanna tree species is to maintain an extensive root system. The root system may extend deep underground, sometimes reaching the water table, or it may be a shallow, lateral system designed to harvest water over a broad area. The leaves of the trees are often tough and fibrous; they may be leathery, sandpapery, or hairy—all features that enable them to husband water. Most leaves are lost during the dry period. Thorns, which may represent leaves that have been reduced through evolution to save water, are common on African savanna tree and shrub species. Many savanna tree types are unfamiliar to North Americans. The more familiar ones are Eucalyptus, Acacia, and Adansonia, the last of which includes the storied baobab tree. Seeds grow within thick casings that allow them to survive until the first rainfall before germinating. And in the midst of this thorny, corky, leathery protection, delicate, showy flowers bloom briefly on grasses and shrubs.

Having survived the dry season, savanna plants next must survive the rainy season, which is not simply a respite from drought but a completely different life episode. For many savanna grasses, the entire reproductive cycle must be accomplished during the rainy season. As the new leaves, which serve photosynthesis, and new flowers are borne in close succession, the energy needs of the grasses zoom upward. These energy-consuming activities must then be reined in and shut down to a semidormant state in preparation for the next dry period. This general pattern has shown some partitioning, with precocious species blooming even before the start of the rainy season, early and intermediate bloomers blooming serially during the rainy season, and late bloomers blooming at the end of the rainy season or after the start of the dry season. The temporal niching strategies of similar species may take advantage of different nutrient availability, or may be driven by some other, unknown factor. For each species, however, the cycle of growth and dormancy is driven by water availability, not by genetics.

In contrast to the grasses, savanna trees may conduct the entirety of their reproductive cycle during the dry season. Such a strategy would maximize the amount of foliage available for photosynthesis during the rainy season.

Besides water, other primary factors that affect the savanna ecosystem are fire and soil type. Fire triggers the growth of seeds, protected in seed beds underground during the dry season. Fire also limits the growth of trees, maintaining the distinction between savanna and forest. In particular, juvenile trees that have not reached a certain height are susceptible to fire; the lack of young trees contributes to the open appearance of a savanna. Some fires result from lightning strikes, but the majority are set by humans as part of hunting or agricultural pursuits. Fire improves soil by adding the nutrients calcium, magnesium, and potassium, which occur in the ashes, to the soil. The timing of fire—early or late in the dry season—is critical, however, and the ideal time seems to differ for different plant associations.

Soil determines whether the deep roots will grow to their potential length. Different soils have different moisture-holding and drainage capacities. The soils underlying savannas cover a wide range of types, and it is thought that at least some of these soils are inhospitable to tree growth, thereby maintaining the characteristic physiognomy of the savanna. Soil type and bedrock geology have a major controlling influence over the plant communities that will grow in them. Depending on their structure, degree of porosity, and so forth, the major soil types may determine whether a savanna is classified as moist or arid, independent of the amount of rainfall. There is usually a noticeable disconformity in soil type at the boundary between forest and savanna, and again at the boundary between savanna and desert.