Final Project Report for National Geographic Committee for Research and Exploration
Grant number 7366-02: “Prehispanic Dendrochronology in Mexico.”
David W. Stahle1, Matthew D. Therrell1, Rodolfo Acuña Soto2, Jose Villanueva Diaz3
1
Department of Geosciences,
University of Arkansas, USA
2
Universidad Nacional Autonoma de Mexico,
Ciudad Universitaria, Mexico
3
Instituto Nacional de Investigaciones Forestales y Agropecurias (INIFAP)
Gomez Palacio, Mexico
Matthew D Therrell
113 Ozark Hall,
University of Arkansas,
Fayetteville, AR 72701, USA
479-575-5809
e-mail: therrell@uark.edu
2004
Abstract
This project is an expansion of University of Arkansas Tree-Ring Laboratory (TRL) research
funded by the National Science Foundation (NSF) to develop tree-ring chronologies and climate
reconstructions in Africa, Mexico, and the United States. The goals of this project include the
development of new tree-ring chronologies and climate reconstructions 500- to 1000-years long
in Mexico, and the use of these data in conjunction with documentary evidence to examine the
history of extreme climatic events and their impact on nature and society. The results of this
research include the development of several new climate-sensitive tree-ring chronologies in
Mexico, a 528-year long tree-ring reconstruction of annual maize yield in central Mexico, and
new information about the impact of drought and other extreme climate events in prehispanic
and colonial era Mexico. These results are detailed in several new papers including “Tree-ring
reconstructed maize yield in central Mexico: 1474-2001,” which is currently in revision for
Climatic Change; “Aztec Drought and the Curse of One Rabbit,” which will be published in the
Bulletin of the American Meteorological Society this September; “When Half of the Population
Died: The Epidemic of Hemorrhagic Fevers of 1576 in Mexico,” which is in press with FEMS
Microbiology Reviews; and “Tree-rings and ‘El Año del Hambre’ in Mexico,” which has been
submitted to Dendrochronologia.
Introduction
Tree-ring reconstructions of paleoclimate can provide an interesting perspective on the
role of climate extremes in human history (e.g., Stahle et al., 1998a; Acuña-Soto et al., 2002).
There is an extensive historical documentary record in central Mexico, regarding the impacts of
extreme climate such as drought, crop failure, and famine. But until recently there have been few
high resolution climate records such as tree-ring reconstructions from Mexico, which could be
used for comparison with the rich historical evidence. As part of an ongoing project funded by
the NSF, and in cooperation with Inter-American Institute for Global Change Collaborative
Research Network 03, Treelines Project, which includes the University of Arkansas Tree-Ring
Laboratory and our colleagues at the National Laboratory of Dendrochronology in Mexico have
developed a network of Douglas-fir (Pseudotsuga menziesii (Mirb) Franco), and Montezuma
baldcypress (Taxodium mucronatum Ten.) tree-ring chronologies from over 20 sites in 14 states
of Mexico (Figure 1; Stahle et al. 2000a; Therrell et al. 2002; Villanueva et al. 2003; 2004).
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Figure 1. Hundreds of tree-ring
chronologies have been
developed in the United States
over the last century, but
despite the vast forest resources
in Mexico the density of
chronologies there is quite low
compared to similar ecosystems
just across the border. In
collaboration with Dr. Jose
Villanueva Diaz, the University
of Arkansas Tree-Ring
Laboratory has developed a
new network of earlywood,
latewood and total ring width
chronologies in Mexico and
western Texas. The
chronologies include samples
from Douglas-fir (Pseudotsuga
menziesii), Mexican baldcypress
(Taxodium mucronatum) and
one Montezuma pine (Pinus
montezumae) stand. Several
recent collections are not
mapped.
Analysis of these new tree-ring records shows that tree growth can be strongly influenced
by climatic events such as drought, and that several periods of reconstructed drought apparent in
the tree-ring record from Mexico and in some cases across much of North America, are
coincident with well documented episodes of famine and attendant social disruptions in the
colonial period and earlier. As part of the research reported here, we have analyzed several
drought and famine events in the colonial and prehispanic eras using our new tree-ring data and a
variety of historical documents.
Tree-Ring Records in Mexico
Tree-ring chronologies have been used to effectively reconstruct climate history in the
United States (e.g., Blasing and Duvick 1984; Meko et al. 1995; Cook et al. 1999), but much less
dendrochronological research has been conducted in Mexico. Tree-ring chronologies were first
developed for Mexico in the 1940’s by Schulman (1944). Schulman sampled Douglas-fir
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(Pseudotsuga menziesii), pine (Pinus sp.), true fir (Abies sp.), Cupressus, and Mexican
baldcypress in Durango, the Valley of Mexico, and in Oaxaca, but was only able to develop one
exactly dated chronology of Douglas-fir in Durango.
Papers have also been published on dendroarchaeology in northwestern and central
Mexico (Scott 1966), exploratory tree-ring work in Oaxaca (Naylor 1971), and the reconstruction
of Pacific sea surface temperatures using chronologies from Mexico (Douglas 1980). More
recently, new chronologies and climate reconstructions were reported by Villanueva-Diaz and
McPherson (1996) from the Animas Mountains of New Mexico and Sonora, Mexico. Biondi
(2001) reported the development of a 400-year long tree-ring chronology near the high elevation
treeline in Colima, Mexico, using Pinus hartwegii, and Diaz and colleagues (2001) developed
the first reconstruction of precipitation in Baja California Sur using Pinus lagunii.
The TRL has developed strong research collaborations in Mexico. In the past several
years we have reported on several research projects in Mexico including the development of treering reconstruction of ENSO, the reconstruction of precipitation in Durango, and the
development of the first tree-ring chronologies in the tropics of Mexico (Stahle et al., 1998b;
2000a; Cleaveland et al. 2003). We have also examined the relationship between warm season
precipitation and latewood growth in Douglas-fir in Mexico (Therrell et al. 2002) and we have
described the 16th Century megadrought which influenced huge areas of the United States and
Mexico (Stahle et al., 2000b) and has now been implicated in some of the worst epidemics of
disease in Mexican history (Acuña-Soto et. al., 2002).
Objectives
The overall goals of our tree-ring research in Mexico are the development of long treering chronologies, the reconstruction of climate variables for central Mexico, and an analysis of
the role of climate extremes in Mexican history. The specific objectives of this NGS project
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include: the development of 500+ year-long tree-ring chronologies from central and southern
Mexico, with emphasis on the Basin of Mexico, the development of earlywood (EW) latewood
(LW) and total ring width (TRW) chronologies from new collections and previously developed
chronologies; the development of tree-ring reconstructions of climate and other variables such as
crop yield in central Mexico; and the use of these records to examine the effects of severe
drought on agriculture, health, and social order during the prehispanic through late colonial
period.
Tree-Ring Chronologies
All tree-ring chronologies we have developed in Mexico are based on increment cores
from living trees and cross sections cut from fallen logs and other subfossil wood. Each core is
exactly crossdated, using standard dendrochronological techniques (e.g., Douglass 1941; Stokes
and Smiley 1996). Following the crossdating procedure, the EW and LW growth increments are
measured to a precision of .001 mm with a stage micrometer using simple optically based criteria
described by Stahle et al. (2000a). Crossdating and measurement accuracy are statistically
screened using the computer program COFECHA, which uses cross-correlation analysis to
scrutinize the dating of each core measurement series compared to a master time series (Holmes,
1983). We commonly remove some LW series or partial segments from the chronology
development process due to poor cross-correlation with the site chronology. This loss of signal is
generally due to a decline in LW growth variance in older conifers (Meko and Baisan, 2001).
The remaining ring width series are detrended to remove growth trends and to eliminate
differences in growth rate between trees (e.g., Cook, 1985). All tree-ring samples collected by
the TRL are accessioned and permanently archived with the University of Arkansas Museum.
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Tree Rings, Crop Yields and Socioeconomic Impacts in Colonial and Prehispanic Mexico:
We used the LW width chronology from Cuauhtemoc la Fragua, Puebla, to reconstruct
maize yield over central Mexico, from A.D. 1474-2001 (Therrell et al. 2004b). Precipitation
during the early wet season is highly correlated with LW growth and is vital to maize yield in
highland Mexico because it promotes full maturation of the crop before the killing frosts of
autumn (Eakin 2000). The Cuauhtemoc LW chronology is highly correlated with the available
maize yield data and explains 65% of the maize yield variance in the 22-year calibration period
(1980-2001; Figure 2).
Figure 2. The combined annual maize
yield for the states of Puebla, Tlaxcala
and Veracruz (solid red line), and our
latewood width reconstruction (dashed
black line) for the period 1980-2001 (r =
0.79; P≤0.0001). The strong relationship
between Douglas-fir latewood width and
regional maize yield in central Mexico is
principally related to their mutual
response to the timing and amount of
spring-early summer rainfall.
The 528-year long maize yield reconstruction provides the first continuous, exactly dated
estimate of food production during the late prehispanic, colonial, and modern eras in Mexico
(Figure 3). Although the observational record of maize yield is quite short, experimental splitperiod calibration and verification tests are passed (Figure 2). Historical documentation of
drought and poor maize yield also provide compelling validation for the overall accuracy of the
reconstruction (Figure 3).
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Figure 3. The annual (thin black line)
and decadally-smoothed (heavy black
line) reconstruction of maize yield in
central Mexico from 1474-2001. The
seven most severe and prolonged periods
of poor maize yield after 1500 are
indicated, along with highlights of
historical references to drought and
famine in central Mexico (Gibson, 1964;
Florescano, 1980; 1986).
Although much of the historic information that we have studied from Mexico is from the
colonial period, important evidence of climate and its impacts are also available from the
prehispanic period. The Aztec codices were a pictorial record of prehispanic Mexico (Figure 4),
and included references to drought and famine. Most codices were destroyed by Spanish
conquistadores, but a few survived and others were recreated from memory by scribes soon after
conquest (e.g., Quinones Keber 1995). We searched the major surviving codices for climate
information and found 13 unambiguous Aztec references to drought during the prehispanic and
early colonial period. The available tree-ring data from Mexico (Figure 5) confirm drought in
nine of these 13 Aztec drought years, and mean tree growth is significantly below average during
these 13 years (p < 0.05; Figure 6). These tree-ring data provide the first independent “crossvalidation” of the Aztec drought chronology, including the catastrophic drought and ‘Famine of
One Rabbit’ in AD 1454 (Therrell et al. 2004a). The tree-ring data also corroborate a common
belief among the Aztec that the year ‘One Rabbit’ was associated with misfortune. One Rabbit
was the first year of the 52-year Aztec calendar cycle, and Aztec folklore suggested that its
occurrence heralded famine and catastrophe. In reference to the famine in the first ‘One Rabbit’
year of the colonial era, 1558, the annotation in the Codex Telleriano-Remensis states that:
In this year one rabbit [I Rabbit], if one looks carefully at this count, it will always be
seen that in this year [Rabbit] there was famine and death… And thus they consider this
year as a great omen, for it always falls on one rabbit.
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Figure 4. Detail of Folio 32 v(erso) from the Codex
Telleriano-Remensis portraying the ‘Famine of One Rabbit’
in 1454 (year sign for One Rabbit at top right). The image
is thought to represent dust storms and the dead victims of
the famine. The famine apparently resulted from a multiyear drought, possibly coupled with an early autumn frost
event in 1453. A number of other 16th Century Aztec
pictographic codices and Nahua language annals document
this drought and famine, which is corroborated by the treering chronology from Durango.
The tree-ring data indicate that the Aztec’s fear of famine and catastrophe in One Rabbit
years may have been based on long experience. Ten of thirteen ‘One Rabbit’ years that occurred
between A.D. 882 and 1558 were immediately preceded by below normal tree growth in the year
‘13 House,’ and the mean of the preceding ‘13 House’ years is significantly below normal (p <
0.1; Figure 7). These 13 House years include very severe low-growth periods in 1037, 1089,
1297, and 1557. Below normal LW growth in central Mexico is associated with poor maize
harvest. So the Aztec belief in the ‘Curse of One Rabbit’ may have arisen because of drought and
poor maize yields in the year immediately prior to One Rabbit. This amazing coincidence
between drought, famine and the Aztec calendar cycle apparently ended with the Aztec empire.
There is no significant relationship between the eight ‘One Rabbit’ years and tree growth after
the 1558 event [the mean of the ‘13 House’ years after 1557 is slightly above normal (not
shown)].
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Figure 5. The annual values for the total ring width tree-ring chronologies from Casas Grandes,
Chihuahua (blue), Cerro Baraja, Durango (green), and Cuauhtemoc la Fragua, Puebla (red), are shown
from A.D. 850 to 2001 along with a smoothed version that highlights decadal variability (black) values.
The Casas Grandes record was dated with long-range comparisons to El Malpais, NM, and other long
tree-ring chronologies in the southwest United States. The 39-year gap from 1337 to 1375 between the
end of the Casas Grandes archaeological pine chronology and the beginning of the Cerro Baraja
chronology is indicated. For this figure the variance of Casas Grandes (std dev = .338), and Cerro
Baraja (std dev = .331) chronologies have been adjusted to match the variance structure of the
Cuauhtemoc la Fragua (std dev = .220) chronology.
In collaboration with our colleague, Rodolfo Acuña-Soto, we have recently reported on
the apparent link between the 16th century megadrought and epidemics of ‘cocoliztli’ what is
now believed to have been an indigenous hemorrhagic fever (Acuña-Soto et al. 2002). Little
quantitative data exists regarding the 1545 event, but the impact of the 1576 episode was well
documented by nationwide censuses that were taken in 1570 and again in 1580. It is estimated
that nearly two million people died as a result of the 1576 epidemic (Acuña Soto et al. 2004).
The disease appeared again in each of the next three centuries and each time was associated with
drought conditions. In fact, the four most catastrophic epidemics of cocoliztli all occurred in
‘wet’ years immediately preceded by severe drought (Figure 8). This ‘sequence of climate
extremes’ has been implicated in several modern disease outbreaks including hanta virus in the
Colorado Plateau (e.g. Hjelle and Glass, 2000; Epstein 2002).
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Figure 6. Results of a Superposed Epoch
Analysis (e.g., Haurwitz and Brier, 1981)
comparing 13 Aztec drought years with
the tree-ring data available for central
and northern Mexico during the same
years. The mean ring width index for the
13 Aztec drought years (year 0) is 0.86
(star), which is significantly less than the
average of all remaining years (p ≤ 0.05).
The values for each of the 13 drought
events identified by the Aztec codices are
indicated (in year 0 and –4). The mean
for each of the four years prior to and
one year after the event year are also
shown. Significantly above normal
growth (p ≥ 0.05) occurred four years
prior to the Aztec drought years. This is
reminiscent of the periodicity of the El
Niño-Southern Oscillation, which has a
strong influence on modern climate over
portions of Mexico (in winter, El Niño
years tend to be wet while La Niña years
are often dry).
The late 18th Century ‘El Año del Hambre’ (The Year of Hunger), one of the worst
famines in Mexican colonial history probably occurred as a result of an extended drought and an
early autumn frost. The continent-wide network of tree-ring chronologies (Cook et al. 1999) now
indicates that drought extended from southern Mexico northward into the southern Great Plains
for the three year period 1785-1787 (Figure 9). The effects of the drought were compounded by
an early autumn frost on 27 August, 1785, that destroyed the already meager maize crop (e.g.,
Gibson, 1964; Figure 3). The consequent famine and disease epidemic is believed to have killed
as many as 300,000 people, and Florescano (1976) argues that El Año del Hambre contributed to
the social instability that led to the War for Mexican Independence beginning in 1810.
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Figure 7. The Superposed Epoch Analysis of tree growth for the
13 ‘One Rabbit’ years between 882 and 1558, preceding and
during the Aztec Empire. The 13 ‘One Rabbit’ years were 882,
934, 986, 1038, 1090, 1142, 1194, 1246, 1298, 1402, 1454, 1506, and
1558. The mean ring width index was just above the long-term
average during these 13 years. However, the mean value of the
years immediately preceding One Rabbit was significantly below
normal (year-1 = 0.85 p < 0.1), which indicates drought and crop
failure leading into One Rabbit (e.g., Cleaveland et al. 2003;
Therrell 2003). This result suggests that the Aztecs did indeed
suffer famine and misfortune during many One Rabbit years.
After 1558 there is no significant association between drought or
low tree growth during or preceding One Rabbit years (not
shown). So the ‘Curse of One Rabbit’ appears to have been purely
coincidental and ended with the Aztec Era.
Figure 8. Cocoliztli was a highly lethal
hemorrhagic fever (Acuña-Soto et al. 2000). The
four greatest epidemics of cocoliztli occurred in
1545, 1576, 1736, 1813, and appear to have killed
over 20 million people (Acuña-soto et al. 2000). A
superposed epoch analysis (SEA) using tree-ring
data from Mexico indicates that these four great
epidemics occurred in wet years preceded by
drought, the classic sequence of climatic extremes
associated with many modern infectious diseases
(Epstein 2002).
The severe drought conditions shown in Figure 9 over central Texas are described in a
number of letters from Domingo Cabello, the commander of the presidio of San Antonio de
Bexar, New Spain’s northernmost outpost in Texas. He writes that the extreme heat and drought
of 1785 caused crop failure and made it difficult to feed the troops (Kielman 1971). The unusual
conditions reported in Mexico during the mid 1780s may be related to the climatic effects of the
eruption of the Laki fissure in Iceland in 1783-1784. Apparently the eruption impacted climate
conditions in the Northern Hemisphere for several years, with abnormally hot and dry summers
and severely cold winter seasons reported throughout much of North America and Europe during
this period (Thordarson et al. 1993).
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Figure 9. Tree-ring reconstructed Palmer Drought Severity Index
for the three-year period 1785-1787 indicates that the severe ‘El
Año del Hambre’ drought covered virtually all of Mexico and
extended northward into the Great Plains. The spatial
distribution of the drought resembles a pattern commonly seen in
this region and has been associated with cool sea surface
temperatures in the eastern Pacific (e.g. Fye et al. 2003; PDSI
reconstruction data courtesy, Ed Cook, pers comm 2004).
Incomplete mapping in northern Canada and southern Mexico
reflects lack of tree-ring data in these areas.
The funding we received from the National Geographic Committee for Research and
Exploration for the one-year project ‘Prehispanic Dendrochronology’ in Mexico has leveraged
our NSF-funded research in the development of climate sensitive tree-ring chronologies in
Mexico, the reconstruction of regional climate and crop yield, and investigations into the
influence of climate in Mexican history and prehistory. Additional opportunity exists for the
development of Mexican tree-ring chronologies extending into the prehispanic period and a great
deal of information on Mexican history and prehistory has yet to be studied. We have recently
been awarded a new four-year grant from NSF to continue our work in Mexico and would like to
submit another request to the Committee for additional work on the social impacts of past
climate in Mexico.
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