BM5 - USBM-005
Additional Site Information

Thomas T. Veblen, Thomas Kitzberger, and Joseph Donnegan


Dating Method: Crossdated

Sample Storage Location: Biogeography Lab, Department of Geography, 
University of Colorado, Boulder, CO, USA

Reference: T.T. Veblen, T. Kitzberger, and J. Donnegan. 2000. Climatic 
and human influences on fire regimes in ponderosa pine forests in the 
Colorado Front Range. Ecological Applications 10: 1178-1195.

Abstract: In the northern Colorado Front Range, fire suppression 
during the 20th century is believed to have created a high hazard of 
catastrophic fire in ponderosa pine (Pinus ponderosa) forests.  Since 
the early 1990s, resource managers have increased the use of prescribed 
fires to re-create fire regimes and forest structures similar to those 
of the pre-Euro-American settlement period in order both to reduce fire 
hazard and to improve forest health.  To improve understanding of 
historic fire regimes, we conducted a study of fire history along an 
elevational gradient from ~1830 to 2800 m in ponderosa pine forests in 
the northern Front Range.  Fire-scar dates were determined from 525 
partial cross-sections from living and dead trees at forty-one sample 
sites.  Fire frequencies and fire intervals were analyzed in relation 
to changes in human activities and inter-annual climatic variability as 
recorded in instrumental climatic records and tree-ring proxy records. 
	Prior to modern fire suppression, the low elevation, open 
ponderosa pine forests of the northern Front Range were characterized 
by frequent surface fires, similar in frequency to many other ponderosa 
pine ecosystems in the West.  In contrast, in higher elevation forests 
(above ~2400 m) where ponderosa pine is mixed with Douglas fir 
(Pseudotsuga menziesii) and lodgepole pine (Pinus contorta), the fire 
regime was characterized by a much lower fire frequency and included 
extensive stand-replacing fires as well as surface fires.  In the mid-
1800s there was a marked increase in fire occurrence that can be 
related both to Euro-American settlement and increased climatic 
variability.  This episode of increased fire left a legacy of dense, 
even-aged stands in higher elevation ponderosa pine forests whereas 
increased stand densities in low elevation forests are attributed 
mainly to fire exclusion during the 20th century. 
	Warmer and drier spring-summers, indicated in instrumental 
climatic records (1873 to 1995) and in tree-ring proxy records of 
climate (1600 to 1983), are strongly associated with years of 
widespread fire.  Years of widespread fire also tend to be preceded 2 
to 4 years by wetter than average springs that increase the production 
of fine fuels.  Alternation of wet and dry periods over time periods of 
2 to 5 years is conducive to fire spread, and is strongly linked to El 
Niņo-Southern Oscillation (ENSO) events.  The warm (El Niņo) phase of 
ENSO is associated with greater moisture availability during spring 
that results in a peak of fire occurrence several years following El 
Niņo events.  Conversely, dry springs associated with La Niņa events 
were followed by more widespread fire during the same year.  
	 The 1600 to 1920 fire-scar record indicates that individual 
years during which high percentages of the 41 sample sites 
synchronously recorded fire have occurred at least several times per 
century.  The association of these years of widespread fire with very 
strong ENSO events demonstrates the importance of ENSO-related climatic 
variabililty in creating extreme fire hazard at a landscape scale.

Fire History Graphs: Fire History Graphs illustrate specific years when 
fires occurred and how many trees were scarred. They are available in both 
PDF and PNG formats. The graphs consist of 2 parts, both of which show the 
X axis (time line) at the bottom with the earliest year of information on 
the left and the latest on the right.
The Fire Index Plot is the topmost plot, and shows two variables: sample 
depth (the number of recording trees in each year) as a blue line along 
the left Y axis, compared with the percent trees scarred shown as gray bars 
along the right Y axis.
Below, the Fire Chronology Plot consists of horizontal lines representing
injuries by year on individual sampled trees. Symbols are overlain that 
denote the years containing the dendrochronologically-dated fire scars or
injuries. The sample ID of each tree is displayed to the right of each
line. The Composite Axis below represents the composite information from
all individual series. The symbols used to represent the fire scars or
injuries, and the filters used to determine the composite information,
are shown in the legend.
These graphs were created using the Fire History Analysis and Exploration
System (FHAES). See http://frames.nbii.gov/fhaes/ for more information.