ENVIRONMENTAL EFFECTS

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Forest fires only consume forest litter (leaves, branches) and grass

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Forest fires are low intensity slow moving fires

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Forest litter is rebuilt and stable after a few years without fires

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The essential role of fire in mushroom yield is unclear

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Forest fires            

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     Forest fires in Northern Thailand occurs mostly in dry dipterocarp and mixed deciduous forests, two types of forest that can be found on the Doi Suthep-Pui range for example. These forests have between 1000 and 3500 trees per hectare depending on type, slope, humidity, etc. It amount to 25 to 100 tonnes per hectare of wood and around 5 to 10% can ideally be used as fuel for wildfires. These fires are typically surface fires that never affect the canopy and only consumes dead leaves, branches, twigs, seeds and grasses. Crown fires (spreading from tree to tree) are only known in cases of illegal logging and deforestation.

     Forest fires typically have 0.2 to 0.9 m of flame height (up to 4 m) reaching 300-500ºC and spread at an average 2 m/min (up to 3.5 m/min). The head fire moves 2 to 15x faster than back fire and 2 to 4x faster than flank fire. The quick moving fire with low fuel density only affect the top soil layer of a burned forest where it reach 200 to 400ºC. A couple of centimeters in the ground, the temperature is 30ºC.

     The intensity of fires is controlled by fuel characteristics (moisture, composition, compaction, continuity, ...), atmospheric conditions (temperature, humidity, wind) and topography (slope, aspect, elevation, ...). Since fires only consume forest litter and grasses, the biomass fuel is fully recovered after a few years at which point it reaches stability (dead material accumulates in the dry season and an equal amount rot in the rainy season). Around 60-70% of available fuel is burned during a standard forest fire. The average forest fire is 1 to 15 ha based on satellite data and most of them are between 1 and 3 ha with thousands of burning events per season.

     While surface fires feed on twigs, dead leaves, plants, grass and undergrowth, it still affect the canopy, reducing the cover from ~90% to 70% in areas experiencing yearly fires. Young trees (<1.3 m high, less than 1 cm in diameter) do not survive fires and larger trees have a slower growth (a few mm less per year) than in areas spared by wildfires. In addition to killing seedlings and saplings, there is a loss of biodiversity associated with regular fires and overall lower soil nutrients levels.

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Mushrooms

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     In Northern and North-East Thailand, one of the main and most discussed reason for starting fires in dry deciduous forests is to promote the production of false earthstars mushrooms (Het Thawp, Het Kra Bueang, Het Pho Fai, Het Pho Hnang). The immature fruiting bodies of these mushrooms grow in May-June and collected to be sold on local market at 300 to 500 ฿/kg with the excess preserved in saline solution for exportation.

     At this stage, the role of fire in the production of these mushrooms is still debated. It is thought that fire is not an absolute requirement and serves a role to create very dry soil conditions when the forest litter has been removed. The absence of leaves and grasses after a fire also facilitate harvesting conditions. To this date, all experiments in vitro to promote the growth of false earthstars mushrooms have failed.