Fire in the TropicsOctober 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 tropics1. 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

“Stable”

“Unstable”

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 agricultureConversion 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

• 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– 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– 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 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.

Implications of tropical fires for climate changes:

carbon sink or source?