Savannas are ancient ecosystems. They are the location of human evolution, and humans are an integral component of these ecosystems. Humans have influenced the determinants of savannas for thousands of years via modification to nutrient availability from fire and clearing for agriculture. Human cultures have used fire as a vegetation management tool and introduced animal husbandry systems, changing grazing and browsing pressures and modified tree-grass competitive balances (e.g., Figure 4). A contemporary impact is now being experienced via climate change and its influence on rainfall distribution, temperature increases, and climate conditions conducive to fire and increased atmospheric CO2 concentration. Human usage ofthe savanna biome is increasing, which can lead to degradation of vegetation and soil resources, resulting in nutrient losses and shifts in water balance and availability. Brazilian cerradao contains over 800 species of trees and shrubs alone; approximately 40% of the cerradao and llanos have now been cleared or altered for agricultural uses with crops such as coffee, soybeans, rice, corn, and beans. Soil management is critical given their low nutrient status, acidity and friability. Alterations in grazing pressure and fire suppression in managed savannas have also resulted in woody dominance, which ultimately reduces grazing production, severely impacting communities relying on cattle-derived incomes and reducing local biodiversity. This thickening or woody encroachment is being observed in areas subjected to extensive grazing activities in both African and Australian savannas.
Clearing for alternative land uses can also result in exotic species invasions, a problem for much of the world's savannas. African savanna, especially in South Africa, are being invaded by woody species, often Acacia or Eucalyptus species from Australia, introduced for fuel wood or timber production. Low herbivory of these species results in high growth rates and water use. The development of thickets reduces deep drainage, ground-water recharge, and streamflow, consequently affecting water supplies. In an attempt to increase the grazing potential of north Australian and South American savanna, fast-growing African grasses such as Andropogon gayanus have been introduced. They are more productive than native species; however, they develop far larger and more flammable fuel loads. At infested sites in north Australia, resultant fire intensity is 5 times that observed from native grass savanna and impacts on tree mortality and recruitment. This in turn will result in a demographic bottleneck, long-term loss in tree cover, and the instigation of a grass-fire cycle. Introductions of African grasses such as Brachiaria, Melinis, and Andropogon species have occurred in the llanos of Colombia and Venezuela and the cerrado of Brazil. These grasses are used as fodder for cattle and are displacing native species, causing a loss in biodiversity of these savannas.
Climate change will alter the distribution of rainfall, thus influencing PAM and PAN. Shifts in temperature regimes and atmospheric CO2 concentration may also alter the relative growth rates of trees and grasses, modifying competitive balances. Trees (C3 photosynthetic pathway) can potentially utilize high CO2 concentrations more efficiently than grasses (C4 photosynthetic pathway) due to increased carbon allocation to roots and lignotubers plus greater water use and nutrient use efficiency apparent at high atmospheric CO2 concentrations. As CO2 concentrations increase, physiological differences between trees (carbon-rich lifeforms) may be favored over grasses (carbon-poor) and trees may gain a competitive edge. Tree saplings may grow to fire-tolerant sizes faster, limiting the impact of fires that maintain grasses in savanna.
All of the above examples involve human impacts acting on one or more of the determinants of savanna structure and function. Clearly, increased knowledge of their interactions will provide improved understanding of savanna processes and enable better management in a rapidly changing world. Savannas may be ideal ecosystems for agro-forestry applications, rather than traditional cropping systems. Small shifts in fire regime may dramatically increase productivity; thus, savanna systems could be used for carbon sequestration and greenhouse gas mitigation schemes, providing alternative livelihoods and aiding in the maintenance of biodiversity.
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