Transcript Tropical Fire Ecology - Natural Resource Ecology and Management

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:
– 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
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?
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)
Three types of fires in the tropics
1. Deforestation fires (slash-burn)
2. Maintenance fires (grasses and early
secondary growth)
3. Accidental forest fires (escaped from
farm lands)
Tropical Shrublands: Brazilian Cerrado
African Savannas
“Unstable”
“Stable”
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
Dominican Republic
Highlands of SE Mexico
Slash and burn agriculture
Conversion to pasture
Escaped fires!
El Nino Fires
of 1998
Effects of El Nino Fire on Biomass:
Mexican Tropical Cloud Forests
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
Amazon rainforest
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.
Amazon
Effect of forest fragmentation
& logging on fire dynamics
• Changes microclimate:
– increased desiccation, wind turbulence
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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
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
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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
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
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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?
TTYGroup again!
Explain how Amazon fire dynamics and
climate change are related to each other on:
- local scales, and
- regional scales
Conclusions
• Because recycling of ET is responsible for 2550% 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.
Implications of tropical fires for climate
changes:
carbon sink or source?