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Watersheds and Fire
Where conditions are not too dry or too wet and
where accumulated carbon from photosynthesis
will not oxidize slowly or rot, fire cycles Carbon.
Watersheds and Fire
Where fire cycles Carbon, the effect on the
watershed is natural only if fire regimes are
natural (within the natural range of variability).
Types of natural fire regimes:
• Stand replacing forest fires where few if any trees
survive and most new plants grow from seed.
• Over burning where all plants burn, overstory
species die or resprout, and most understory plants
survive to grow rapidly.
• Over burning where all plants are burned along
with a lot of litter but few plants die.
• Under burning where trees survive and most
understory plants survive to grow rapidly.
Short-term Watershed Effects are
Proportional to Fire Intensity
• Decreased ET increasing runoff
• Hydrophobic soils repelling water and increasing
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runoff and erosion
Released nutrients, creating water pollution
problems
Erosion of bare soil releasing sediments to streams
and lentic wetlands
Increased or decreased coarse woody debris for
energy dissipation and sediment detention
Diminished shade allowing insolation and thermal
pollution
Long-term Effects are Inversely
Proportional to Fire Naturalness
• Changes to water flow adjusting the size of the
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channel
Erosion of bare soil diminishing soil water storage
and site productivity.
Sediment slugs entrenching or otherwise adjusting
the form of the stream or lentic wetlands
The temporal sequence of increased or decreased
coarse woody debris for energy dissipation and
sediment detention
Changes to vegetation, including invading species,
interacting with the watershed and future fires
Riparian Fire/watershed
Hypotheses
• Fire is as natural in riparian systems as it is in
upland systems
• Fire frequency is lower where green leaves, water,
and moist soil increase humidity and cause upland
fires to stop or burn around riparian areas
• A high water table and floodplain access
diminishes riparian fire intensity and frequency
• Accumulated woody fuel in the absence of fire
may increase fire intensity, heat, and consumption
of coarse woody debris needed by streams for
energy dissipation and sediment retention.
Pinyon Juniper Fire/watershed
Hypotheses:
• PJ has expanded into many sagebrush areas with
the diminished fire frequency of the past 130 years
• PJ and adds fuel increasing fire heat fire size and
watershed effects
• PJ diminishes the understory decreasing release of
native perennial plants after fire
• Increased ET before the fire and decreased soil
volume after a PJ and fire history in a watershed
watershed can dehydrate springs and riparian areas
Invasive Annuals Fire/watershed
Hypotheses:
• Annuals, especially cheatgrass, increase fire
frequency and size while they decrease fire
perimeter/unit area
• The loss of shrubs alters snow catch and the
seasonality of ET
• Frequent fires increase the exposure of soil to
wind and water erosion.
• Annual weeds facilitate perennial weeds which
are not good at protecting soil.
Trigger Event ?
Alternative State
--Annuals & Frequent Fire Cycle-