fire in the tropics october 21, 2010
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Fire in the Tropics October 21, 2010. Fire in the tropics: natural or human tool?. Natural disturbance in some tropical and subtropical ecosystems, but… Currently fire is largely a “ land treatment tool ” in the tropics: - PowerPoint PPT PresentationTRANSCRIPT
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Fire in the TropicsOctober 21, 2010
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Fire in the tropics: natural or human tool?
• Natural disturbance in some tropical and subtropical ecosystems, but…
• Currently fire is largely a “land treatment tool” in the tropics: – Forest clearing for land use change
(conversion to agricultural land, pastures, urban expansion, road construction, etc.).
– Maintenance of grazing lands– Utilization of seasonal forests and
savannas
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Tropical Ecosystems are Diverse…
• Lowland tropical rain forest• Montane coniferous,
broadleaved and mixed forest
• Dry shrublands/woodlands• Pine forest, dry deciduous
forests• Savannas / Grasslands
How do fire regimes in these ecosystems
compare with temperate ecosystems that we have
studied this semester?
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How do historical fire regimes in the tropics compare with those of
temperate regions?• Lowland tropical rain forest– Temperate rainforest, eastern deciduous
• Tropical montane coniferous and T. broadleaved forests– Subalpine
• Tropical dry shrublands/woodlands– Chaparral
• Tropical pine forest, Tropical dry deciduous forests– SW ponderosa, SE pine-oak scrub (& degraded states)
• Tropical savannas / grasslands– Prairie, savanna complex (& degraded states)
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Three types of fires in the tropics1. Deforestation fires (slash-burn)2. Maintenance fires (grasses and early
secondary growth)3. Accidental forest fires (escaped from
farm lands)
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Tropical Shrublands: Brazilian Cerrado
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African Savannas
“Stable”
“Unstable”
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Tropical pine, oak, and pine-oak forests – Fire regime• frequent, moderate intensity (“fire climax”)• Adaptations to fire: thick bark, resprouting, serotiny• Human modification of fire regiome
Carribbean Pine in Honduras
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Dominican Republic
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Highlands of SE Mexico
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Slash and burn agricultureConversion to pasture
Escaped fires!
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El Nino Fires of 1998
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Effects of El Nino Fire on Biomass:Mexican Tropical Cloud Forests
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The Amazon Rainforests• Historical fire regime– Fires = rare– Only during mega droughts –
time scale of 1000’s of years
• Major causes of fire– 1970s: forest colonization,
agriculture, logging, urban development
– Expanding road network– Increasing populations– Habitat fragmentation
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Amazon rainforest
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TYPGroup
1) How does habitat fragmentation and logging affect fire dynamics?
2) Explain how positive feedbacks can occur to increase fire frequency and severity in Amazon rainforests.
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Amazon
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Effect of forest fragmentation & logging on fire dynamics
• Changes microclimate: – increased desiccation, wind turbulence
• Increased tree mortality & canopy-gap formation• Increased dead wood and leaf litter (edges)• High fire ignitions – pastures, crops• Selective logging – canopy damage, slash– Roads – forest colonization, hunting, land speculation– Increased vulnerability to fire, intensity, spread
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Positive Feedbacks in Fire dynamics
• Initial burn = surface fire– Low intensity surface fire– High duration: kills ~40% trees (small dbh, thin bark!)– Canopy cover reduced 65%, increased fuel loads
• Recurring fire– Greater intensity – Kills ~40% of remaining trees (also large trees!)– Canopy cover reduced <35% = drying– Encroachment of weedy vines, grasses = flammable!
• Alters forest composition and structure– Seedlings and seed killed (lack of adaptations to fire!)– Promotes establishment of pioneer species– Positive feedback on fire cycle
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Fire and Climate Change in the Amazon Rainforest
• Loss of forest cover – alters local/regional climate– Vegetation breeze forest dessication
• Forests pasture/savanna reduces ET – Decrease rainfall and cloud cover– Increase albedo and surface temps– Probably the regional hydrologic system collapse = less rain
• Smoke plumes– Hypersaturates atmosphere with cloud condensation nucliei– Bind with water molecules, not big enough to form raindrops– Absorb solar radiation = warming, less cloud formation– Large fires can create rain shadows (100’s kms downwind)
• Global warming and increase in El Nino frequency– Increase fire frequency– Large-scale dieback of forests shrublands, or even deserts?
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TTYGroup again!
Explain how Amazon fire dynamics and climate change are related to each other on:- local scales, and- regional scales
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Conclusions
• Because recycling of ET is responsible for 25-50% of Amazonian precipitation, regional rainfall is likely to decline in concert with increasing deforestation.
• Unless fundamental changes occur in the way human-dominated landscapes are managed, increasing expanses of Amazonian forests will be subjected to fire regimes for which they are not evolutionarily equipped to survive.
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Implications of tropical fires for climate changes:
carbon sink or source?