Past Fire Frequencies: Treerings, Charlocal & Sediments
Fire is present in all ecosystems, from tundras to deserts.
All vegetation burns (at different frequencies) under certain circumstances.
Fire is a dominant ecological process in some ecosystems: chaparral,
grasslands, xeric forests (ponderosa pine, pinyon-juniper woodland, oak woodland).
METHODS OF RECONSTRUCTING FIRE FREQUENCY
1. FIRE-SCARRED TREES (Heinselman, 1973):
Produced by repeated burnings of individual trees. Initial burning makes trees more
susceptible, and older trees burn easier, so there is an exponential decline in abundance
of older trees.
- Fire scars sampled with Chain Saw or Increment Borers
- Fire history and extent reconstructed by collecting hundreds of trees in an area.
- Production depends on fire intensity. Intense fires may destroy all the trees in an area.
Note historic decline
CHARCOAL IN LAKE SEDIMENTS:
SEDIMENTARY CHARCOAL = f(P,D,R,I)
Exponential decline away from fire
Particles range in size from pollen to large branches
superb, it's already oxidized
difficult: charcoal vs. "carmalized" wood, opaque minerals
processing can break-up charcoal, increasing number of pieces
- count fragments in pollen preparations, surface area, or volume
- count burned macrofossils (positive identification by breaking)
- mounting sediment in epoxy and sectioning (Clarke, 1988; Smith and Anderson, 1995)
Charcoal Production Factors (Albini, 1983)
WILDFIRE CHARCOAL = f(B,M,H,S)
Biomass (fuel type, fuel volume)
Meteorological conditions: temperature, moisture, wind
Habitat conditions (hills vs. flats)
Size and intensity of fire
In general, smaller (microscopic) particles represent a regional
geograpical area, and larger (macrofossil-sized) particles represent
a local geograpical area (Carcaillet et al., 2001; MacDonald et al., 1991)
after MacDonald et al., 1991
Changing charcoal frequency is an INDEX of fire frequency
Note historic decline
CLIMATIC CHANGE AND FIRE FREQUENCY
Increased aridity, higher temperature, greater wind speed can increase both the frequency
and intensity of fires. For example, the maximum charcoal percentages during the "early
Holocene xerothermic" at Lost Trail Pass Bog
(Mehringer et al.1977)
- Charcoal in loess more plentiful during cold dry glacials (Wang et al., 2005)
- Global Warming
Cold climate favors more undergrowth and more frequent low-intensity fires.
During Medieval Warm Period fire frequency peaked due to intense droughts and periodic stand-replacing fires
(Pierce et al., 2004).
Climatic change can also produce vegetation changes that create greater availability of
fuel. For example the transition from open vegetation to forest at Lake Cleveland
(Davis et al., 1986) and at Balsam Meadow (Davis et al., 1985)
ANTHROPOGENIC CONTROL OF FIRE FREQUENCY
Reasons to burn vegetation: (Williams & Reed, 1994)
a. wildlife and livestock management: game drives, improvement of forage
b. improve campsites and trails, remove dense vegetation, prevent wildfire
c. warfare: conceal movements, confound enemies
New Guinea: Goeff Hope (1980) suggests grasslands in high-elevation rainforest result from fires
Australia: Gerdip Singh (Singh & Geissler, 1985), at Lake George increased charcoal frequency during the last
interglaciation is coincident with earliest human artifacts, vegetation change at that
time may have been caused by fire.
Minnesota: Eric Grimm (1983), position of prairie/forest border determined by
anthropogenic fires during the Late Holocene (fire shadows)
Western North America: Kay (1995 ff.) All ecosystems were impacted by Indian burning,
restoration management toward pre-Columbian ecosystem states would require frequent
California: Tulare Lake (Davis, 1992) Charcoal accompanies transition from Great-Basin woodland to
valley grassland human & climate?
Arizona, San Pedro Valley (Davis et al., 2001) Burning of wetlands much less frequent
during historic period.
Soot (industrial) in pollen preparations
Fires destroy desert varnish of rocks by spalling
HISTORIC DECREASE IN FIRE FREQUENCY
Grazing and Timber Cutting
Governmental Fire Suppression