La Cascada la Virgen - ARCVI001
Additional Site Information

Thomas T. Veblen, Thomas Kitzberger, Ricardo Villalba, 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, R. Villalba and J. Donnegan. 
1999. Fire history in northern Patagonia: the roles of humans and 
climatic variation. Ecological Monographs 69(1): 47-67.

Abstract:  The effects of humans and climatic variation on fire history 
in northern Patagonia, Argentina were examined by dating fire scars on 
458 trees at 21 sites in rain forests of Fitzroya cupressoides and 
xeric woodlands of  Austrocedrus chilensis from 39 to 43 degrees S latitude.  
Climatic variation associated with fires was analyzed on the basis of 
20th-century observational records and tree-ring proxy records of 
climatic variation since ~1500 A.D.  In the Austrocedrus woodlands, 
fire frequency increases after ~1850 coincident with greater use of 
the area by Native American hunters.  Increased burning, particularly 
in the zone of more mesic forests, is also strongly associated with 
forest clearing by European settlers from ~1880 to the early 1900s.  
The marked decline in fire frequency during the 20th century coincides 
with both the demise of Native American hunters in the 1890s and 
increasingly effective fire exclusion.
          Strong synchroneity in the years of widespread fire at sample 
sites dispersed over a north-south distance of ~400 km indicates a 
strong climatic influence on fire occurrence at an annual scale.  Tree-
ring reconstructions of regional precipitation and temperature show a 
steeply declining influence of climatic variability on fire occurrence 
from annual to multi-decadal scales. It is the inter-annual variability 
in climate, rather than variations in average climatic conditions over 
longer periods, that strongly influences fire regimes in northern 
Patagonia.  Although climatic variability overrides human influences on 
fire regimes at an interannual scale, human activity is an equally 
important determinant of fire frequency at multi-decadal scales. 
	Climatic conditions conducive to widespread fire in both xeric 
Austrocedrus woodlands and Fitzroya rain forests are typical of the 
late stages of la Niņa (cold phase of the Southern Oscillation) events 
as indicated by trends in the Southern Oscillation Index and eastern 
tropical Pacific sea surface temperatures during the 1-2 years before 
and after fire-event years.  Years of extreme fire occurrence are 
associated both with dry winter-springs of La Niņa events and with the 
warm summers following El Niņo events.  Years in which the southeast 
Pacific subtropical anticyclone is intense and located further south 
than normal are years of enhanced drought and fire.  Similarly, years 
of widespread fire in northern Patagonia are associated with variations 
in mean sea-level atmospheric pressure at ~50-60 degrees S latitude in the 
South American-Antarctic Peninsula sector of the Southern Ocean as 
reconstructed from tree rings for 1746 to 1984.  Precipitation, and 
hence fire regimes, in northern Patagonia are significantly influenced 
by high-latitude blocking events, which drive westerly cyclonic storms 
northward.  Variations at decadal- to centennial-time scales in major 
circulation features, such as ENSO activity and the meridionality of 
regional air flow at high latitudes, as well as changes in the degree 
of coupling of these feature influence climate and fire regimes of 
northern Patagonia. 

Comments:
Species sampled: Austrocedrus chilensis
No. dated series: 16
No. years with scars: 9

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.