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Ecology and conservation in tropical dry ecosystems "Cerrado savanna…

- - - - "Savannas and grasslands dominate the terrestrial tropics
        and cover approximately 20% of the global land surface" 2
      - TGBs store approximately 15% of the carbon on Earth 3 4
      - "30% of total global terrestrial net primary productivity" 3 4
      - "Represent approximately 85% of the global land area burnt" 3 4
      - "One-fifth of the global human population depends directly on them for their livelihoods" 5
      - "C4 grasses species comprise only 3% of vascular plant species [but] account for 25% of terrestrial photosynthesis" 7
    - - "The evolution of TGBs began approxi-mately 30 million years ago (Mya), when grasses evolved the novel C4 photosynthetic pathway that leads to higher productivity under low atmospheric CO2 concentrations, and seasonally dry and hot conditions" 6
    - - What Limits Woody Cover in C4 Grasslands and Savannas?
        
        Grass-Fire-Grazing Feedback
        
        "However, heavy grazing can also produce grazing lawns, which seldom burn yet have very low tree densities" 14
        
        "Grass–fire cycle: fires burn the herbaceous layer, consuming grass biomass and killing stems of young trees, altering rates of tree growth and survival of adult trees, and promoting grasses" 8 9
        
        "The C4 combination of high productivity, rapid curing in the dry season, and low decomposition rates contributes to the very high fire frequencies in these ecosystems, the most frequent fires on Earth [...] a formidable obstacle for tree recruitment" 12
        
        Grassy Biomes in Climates Suitable to Forests
        
        "Highly variable tree densities along precipitation gradients seem characteristic of most savannas" 13
        
        "[There are] massive discrepancies between [DVGM] simulated climate potential and actual vegetation, especially in C4 grassy biomes [...] much of the global mismatch between actual and potential vegetation could be explained by fire" 12
        
        Bottom-Up Controls
        
        Water Supply (Walter's Two-Layer Hypothesis)
        "The root niche differentiation hypothesis of Walter (1971) argued that tree cover is determined by the amount of water percolating beyond grass roots to deeper soil layers where trees have exclusive access" 12
        
        Nutrient Supply
        "Closed rainforests occur on nutrient-poor soils in the tropics [so] it is unlikely that absolute nutrient content of soils generally limits forest distribution" 12
        
        Top-Down Controls
        
        Fire Regime
        "[There is] unequivocal evidence that tree densities, biomass, and species composition in many savannas are not at equilibrium with resources but rather depend on the fire regime [indeed] fire exclusion often resulted in a complete biome switch, from humid savanna to closed forest" 12
        
        Herbivory
        "[Is suggested that] megaherbivores maintained open grassy ecosystems in many parts of the world before Pleistocene extinctions. In parts of Africa, elephants are major agents of adult tree mortality and may even structure biome distribution" 12
        
        Multifactor Controls
        "Simulated adult tree densities were most limited by seedling establishment in arid savannas and by sapling escape in frequently burned savannas. Sensitivity analyses showed that the latter was strongly influenced by fire responses and by sapling growth rates between fires" 15
        "Grasses compete with seedlings and saplings, reducing their growth rate, whereas frequent fires and/or intense herbivory prevent saplings from escaping the grass layer" 12
        
        Seedling Establishment
        
        "In frequently burnt savannas, seedlings can acquire the ability to resprout within their first growing season [...] Clonal propagation from root suckers is also common, reducing dependence on seedlings [...] Seedlings are less tolerant of burning in more arid savannas and can be killed by fire years after establishment" 12
        
        "Wet season droughts are the most common cause of germinant mortality [...] in arid regions, the establishment phase is probably the critical demographic bottleneck in arid savannas [...] experimental studies have shown increased survival and growth of woody seedlings with grass removal" 15
        
        Sapling Escape
        
        "[Fire-trapped saplings] are very tolerant of fire, resprouting from fire-protected roots and lignotubers. Saplings can be trapped in the flame zone for decades [its] probability [to] escape is determined by the frequency and intensity of fires causing stem death (“topkill”), and *growth rates to escape height above which stems survive fire" 12
        
        "Browsing is analogous to fire in that, once saplings have grown beyond a height of 2–3 m, they are beyond the reach of most mammal herbivores. Chronic browsing would be needed to prevent saplings from growing to escape height in areas of high herbivore density' 12
        
        "High rainfall periods increase grass productivity, promoting fires and suppressing sapling escape in mesic savannas. In arid savannas, high rainfall periods have the opposite effect, promoting tree increases because moisture usually limits establishment and sapling growth" 12
        
        Adult Mortality
        
        "Adult trees are not immune from fire damage, and species vary in their ability to repair damaged bark causing variation in death rates"
        
        "Major reductions in tree biomass may be caused by large, infrequent disturbances [e.g.] severe droughts, occurring at multidecadal intervals"
      - "Switches can occur when changes in environmental drivers facilitate gains or losses in woody cover beyond the upper (>80%) functional limit of canopy covers" 10
    - - "44% of African projects listed on the [...] REDD+ [...] are in savanna or grassland habitat" 1
      - "[...] the use of structural characteristics, including minimum woody cover thresholds, is an inappropriate method to define whether an ecosystem is forest and, thus, may qualify for CDM or REDD+" 1
      - "Four key problems in the application of the current form of CDM and REDD+ to biomes other than forest" 1
        
        Defining a baseline vegetation state is difficult given that TGBs naturally vary in woody cover and can change rapidly in response to several nonanthropogenic drivers
        
        The CDM and REDD+ schemes define ‘degradation’ as the loss of trees and carbon stocks. Hence, TGBs where there have been natural declines in woody cover are inappropriately classified as degraded
        
        Managing TGBs as forests directly compromises ecosystem function because REDD+ programs aim to suppress disturbance
        
        TGBs are often considered anthropogenic in origin and, hence, qualify for afforestation or ‘reforestation’. This promotes the replacement of native, ancient, and endemic flora with plantations of exotic species (e.g., Pinus and Eucalyptus)
    - - "Increased atmospheric CO2 has the potential to drive a reorganization of tropical vegetation, through not only the forcing effects of CO2 on climate, but also the promotion of woody plants (woody encroachment) over grasses, in turn altering both woody biomass dynamics and TGB extent" 11
      - "Reduced transpiration with increasing CO2 would indirectly favor trees by allowing deeper percolation of moisture into soils [...] This would favor woody plant establishment because of their deeper roots" 16
      - "Woody plants would be able to escape fire and herbivory more readily owing to the direct effects of CO2 fertilization on growth rates, resprouting ability [...] and herbivore defense in seedlings and saplings" 17