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Savanna (changes)

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Contents

Idea

To collect changes both in biodiversity and climate to the biome savanna due to global warming. Wikipedia defines it as:

A savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 carbon fixation grasses.

Some classification systems which also recognize a grassland savanna from which trees are absent. This article deals only with savanna under the common definition of a grassy woodland with a significant woody plant component.

acacia

It is often believed that savannas feature widely spaced, scattered trees. However, in many savannas, tree densities are higher and trees are more regularly spaced than in forest. Savannas are also characterized by seasonal water availability, with the majority of rainfall confined to one season. Savannas are associated with several types of biomes. Savannas are frequently in a transitional zone between forest and desert or prairie. Savanna covers approximately 20% of the Earth’s land area. The largest area of savanna is in Africa.

Details

This quote is also from Wikipedia:

It is often believed that savannas feature widely spaced, scattered trees. However, in many savannas, tree densities are higher and trees are more regularly spaced than in forest. Savannas are also characterized by seasonal water availability, with the majority of rainfall confined to one season. Savannas are associated with several types of biomes. Savannas are frequently in a transitional zone between forest and desert or prairie. Savanna covers approximately 20% of the Earth’s land area. The largest area of savanna is in Africa.

Climate change

There exists the possibility that human induced climate change in the form of the greenhouse effect may result in an alteration of the structure and function of savannas. Some authors have suggested that savannas and grasslands may become even more susceptible to woody plant encroachment as a result of greenhouse induced climate change. However, a recent case described a savanna increasing its range at the expense of forest in response to climate variation, and potential exists for similar rapid, dramatic shifts in vegetation distribution as a result of global climate change, particularly at ecotones such as savannas so often represent.

Here is a diagram from William Hoffmann in his paper “Positive feedbacks of fire, climate, and vegetation and the conversion of tropical savanna”, showing changes in the forest fire danger index (FFDI):

forest fire risk savanna

Hoffmann also explains that there are some effects like the ENSO that might have a large impact on the results:

In the simulations presented here, the use of climatological sea surface temperatures eliminates an important source of variability in tropical climates. In many regions of the tropics, extreme fire years are associated with events such as el Nino [Siegert et al., 2001]. Because of the nonlinearity in the fire response to climate (Figure 2), an increase in the fire danger index should have a larger impact in years with severe fire weather than in milder years, so the mean effect of climate change on fire frequency may be larger than predicted here.

This diagram is also from the same paper by Hoffmann and shows changes in the tree population due to feedbacks:

tree change savanna

The contribution of the vegetation-climate-fire feedback to the decline in tree cover in the cerrado region. The case without climate feedbacks shows the decline in tree cover, based on a clearing rate of 1.7% yr and a background fire interval of 3 years. Projected tree cover is relative to the tree cover present at the onset of clearing at year 0.

background So fire almost interval no of trees 3 after years. 100 Projected years tree and cover with is feedbacks relative it to will be a steeper decline in the tree number cover present at the onset of clearing trees. at year 0.

References

Abstract: We combine general circulation modeling (GCM),remote sensing, and field results to identify a positive feedback loop in which clearing of tropical savannas results in warmer and drier climate, accelerated fire frequencies, and further tree cover loss. The GCM simulations indicate that ongoing clearing of tropical savannas increases temperatures and wind speeds and decreases precipitation and relative humidity, substantially increasing fire frequency. Using NOAA-12 satellite images and meteorological data, we estimate that complete savanna clearing will increase fire frequency by 42%. Combining these data with long-term fire studies, we demonstrate that this fire-mediated feedback may already be contributing to declining tree densities in the world’s savannas and will become increasingly important as vegetation change continues in the coming century.

Abstract: Forests and savannas are the major ecotypes in humid tropical regions. Under present climatic conditions, forest is in a phase of natural expansion over savanna, but traditional human activities, especially fires, have strongly influenced the succession. We here present a new model, FORSAT, dedicated to the forest–savanna mosaic on a landscape scale and based on stochastic modelling of key processes (fire and succession cycle) and consistent with common field data. The model is validated by comparison between the qualitative emergent behaviour of the model and results of biogeographical field studies. Three types of forest succession are shown: progression of the forest edge, formation and coalescence of clumps in savanna and global afforestation of savanna. The parameters (frequency of savanna fires, climate and soil fertility) appear to have comparable effects and there is a sharp threshold between a forest edge progression scenario and the cluster formation one. Moreover, pioneer seed dispersal pattern and recruitment are determinant: peaked curves near a seed source and far dispersal combine to increase the fitness of the pioneers.

Abstract: Tropical savannas have been heavily impacted by human activity, with large expanses transformed from a mixture of trees and grasses to open grassland and agriculture. The National Center for Atmospheric Research (NCAR) CCM3 general circulation model, coupled with the NCAR Land Surface Model, was used to simulate the effects of this conversion on regional climate. Conversion of savanna to grassland reduced precipitation by approximately 10% in four of the five savanna regions under study; only the northern African savannas showed no significant decline. Associated with this decline was an increase in the frequency of dry periods within the wet season, a change that could be particularly damaging to shallow-rooted crops. The overall decline in precipitation is almost equally attributable to changes in albedo and roughness length. Conversion to grassland increased mean surface air temperature of all the regions by 0.5°C, primarily because of reductions in surface roughness length. Rooting depth, which decreases dramatically with the conversion of savanna to grassland, contributed little to the overall effect of savanna conversion, but deeper rooting had a small positive effect on latent heat flux with a corresponding reduction in sensible heat flux. The authors propose that the interdependence of climate and vegetation in these regions is manifested as a positive feedback loop in which anthropogenic impacts on savanna vegetation are exacerbated by declines in precipitation.

category: ecology