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Tree-rings and climate relationships for Douglas-firchronologies from the Sierra Madre Occidental,

Mexico: A 1681-2001 rain reconstruction

Martha González-Elizondo a,*, Enrique Jurado b,~, José Návar b,l,- .

Abstract

Five Pseudotsuga memiesii chronologies from the Sierra Madre Occidental were used for comparative climate-growthassociation purposes. Two new site chronologies were constructed for the first time; dated total tree-ring raw data of threeadditional site chronologies, previously documented, were used to compute comparable chronologies. Correlation analysisamong the five chronologies reveals significant statistical association between most of them (r = 0.33-0.72; p < 0001). AlI

chronologies were used individually to examine climate-growth associations considering both, local and regional climate dataobtained from four meteorological stations in the regían. Total regional precipitation correlated extremely well withstandardized growth índices (up to r = 0.84; p < 0.000 1); maximum temperature had an inverse association with growth,correlating lowly but significantly (up to r = -0.62;p < 0.0001), while mínimum and mean temperatures did not corre late at all.

The Las Bayas, a new chronology, was the highest correlated with regional November-May and November-August totalprecipitation (r = 0.80; p < 0.0001 and r = 0.84; p < 0.0001, respectively). Using Las Bayas chronology total November-May

regional precipitation was reconstructed from 1681 to 200 l and used to identify the main dry and wet periods during the lastthree centuries. Most of the drought and wetepisodes identified were consistent with those reported earlier for northem Mexico;an important exception was the 1950s interval that has been recognized widely as one of the most severe droughts in this region,and that based on reconstruction presented in this study was identified as a rather wet episode. The presence of wintertime ElNiño events and chronologies were statistically related indicating that wet and cold winters in the Sierra Madre mountain range

* Corresponding author. Tel.: +52618 814 2091; fax: +526188144540.

E-mail addresses: martha~onzel@yahoo.com.mx (M. González-Elizondo), villanueva.jose@inifap.gob.mx (J. Villanueva).1 Tel.: +52821 2124895; fax: +52821 21242 51x251.2 Tel.: +52 871 719 11 34.

0378-1127/$ - see front matter ({;) 2005 Elsevier B.V. Al] rights reserved.

doi: 10.10 16/j.foreco.2005.03.012

M. Socorro González-Elizondo a, José Vill~~ueva C,L, Oscar Aguirre b.lJavier Jiménez 0,1

a CI/DIR Unidad Durango, Instituto Politécnico Nacional, Sigma sin. Fracco 20 de Noviembre, Durango,

Durango CoP. 34220, Méxicob Facultad de Ciencias Forestales, Universidad Autónoma de Nuevo León, Postal Box 41, Linares,

Nuevo León CoP. 67700, Méxicoc INIFAP. CEN1D-RASPA, Km 605 Margen derecha del Canal Sacramento, Gómez Palacio,

Durango, CoP. 35140. México

Received 3 September 2004; received in revised form 23 March 2005; accepted 25 March 2005

40 M. González-Elizondo et al. / Forest Ecology and Management 213 (2005) 39-53

enhance diameter growth of fir trees. Further studies are recommended to update and extend the present chronologies as well asthe exploration of new sites and species to extend the network of climate sensitive chronologies in this regio!}@ 2005 Elsevier B. V. AII rights reserved.

Keywords: Dendrochronology; Mexico; Reconstructed precipitation; Climate; Tree-ring chronologies; Douglas-fir

Resumen

Se usaron cinco cronologías de Pseudotsuga menziesii de la Sierra Madre Occidental con el fin de analizar comparativamentela asociación entre el crecimiento y el clima. Dos de estas cronologías se construyeron por primera vez y tres cronologíasadicionales se desarrollaron utilizando los datos sin procesar, de ancho total de anillo de cronologías documentadas previamente.Los análisis de correlación entre las cinco cronologías revelan una buena asociación entre ellas (r = 0.33--0.72; p < 000 1). Cada

una de las cinco cronologías se usó por separado para analizar la asociación clima-crecimiento. Se utilizaron datos deprecipitación total, temperatura media, mínima y máxima, tanto mensuales como estacionales, de cada una de cuatro estacionesmeteorológicas de la región, así como los promedios regionales de las mismas. La precipitación total se correlaciona bien con losíndices decrecimiento estandarizados (hasta r = 0.84; p < 0.0001). La temperatura máxima se correlaciona de manera inversa,débil pero significativamente (hasta r = -0.62; p < 0.000 1), las temperaturas mínimas y promedio no muestran ninguna

correlación con el crecimiento. La cronología de Las Bayas, una de las nuevas, fue la más altamente correlacionada con laprecipitación total regional de noviembre a mayo y de noviembre a agosto (r = 0.80; p < 0.000 l Y r = 0.84; p < 0.000 l

respectivamente). Utilizando la cronología de Las Bayas se reconstruyó la precipitación total de noviembre a mayo de 1681-2001 con base en la cual se identificaron los principales períodos de sequía y de lluvias abundantes en la región durante losúltimos tres siglos. La mayoría de estos intervalos con lluvias por debajo o por arriba de la media coinciden con los registradosanteriormente por otros autores para el Norte de México. Una excepción importante fue el período de los 1950s que ha sidoampliamente reconocido como una de las sequías más severas en la región y que aquí se reconstruye como un episodio más bienhúmedo. Se encontró una relación estadística positiva entre la presencia de eventos El Niño y los índices de crecimiento,indicando que los inviernos húmedos y fríos en la Sierra Madre Occidental favorecen el crecimiento en diámetro dePseudotsuga. Se recomiendan estudios adicionales para actualizar y extender las presentes cronologías, así como la exploraciónde nuevos sitios y nuevas especies para ampliar la red de cronologías de la región sensibles a factores climáticos.(9 2005 Elsevier B. V. AII rights reserved.

Palabras clave: Dendrocronología; México; Reconstrucción de precipitación; Clima; Cronologías de anillos de crecimiento; Pseudotsuga

1. Introduction rivers and reservoirs located in the States of Sinaloa,Nayarit and Coahuila that are used for agricultural andpublic supplies. On the other side, studies of climatevariability are rare in Mexico due in part to limitationsin instrumental meteorological data. Most meteor-ological records in Durango started in the 1960s andautomatic collection not until the 1990s. Only the ElSalto and Durango city meteorological stations beganin the 1940s. Tree-ring records are good proxy dataresource for periods before the instrumental data

(Fritts, 1976). Douglas-fir (Pseudotsuga menziesii)has pro ven to be a species sensitive to climate innorthem Mexico (Villanueva-Díaz and McPherson,1996, 1999; Stahle et al., 1999; Díaz-Castro et al.,2002). The construction of a chronology network ofPseudotsuRa menziesii and other potentiallv climate

Study of climate variability in northern Mexico isimportant due to the influence of El Niño/SouthernOscillation (ENSO) (Cavazos-Pérez, 1998; Stahleet al., 1998), a phenomenon of natural climatevariability, which affects the global climate but hasa particularly strong impact on the moisture regime ofthe subtropics (Walker and Steffen, 1999). In addition,rainfall on the Sierra Madre Occidental mountainrange plays an important role in the regional andnational economy both in forest productivity andwater regulation. More than 25% of the nationaltimber harvested comes from the uplands of the stateof Durango (INEGI, 1999), and forests in this afea arethe main sources of sprin~s and streams that feed

M. González-Elizondo el al.! Foresl Ecology and Managemenl 213 (2005) 39-53

sensitive species is thus an important way to studyclimate variability in this regían.

A few dendrochronological studies have beencarried out in Mexico since the 1940s (Schulman,1944; Scott, 1963, 1966; Naylor, 1971; Stokes et al.,1976; Rueda, 1983). However, it was until the lastdecade that dendrochronology is becoming a commonresearch tool used in ecology and climatology inMexico (Huante et al., 1991; Alvarado et al., 1993;Fulé and Covington, 1994, 1996, 1997, 1999;Villanueva-Díaz, 1995; Villanueva-Díaz and McPher-son, 1995; Villanueva-Díaz et al., 2000; Díaz-Castro,1995; Díaz-Castro et al., 2001, 2002; Stahle and

Cleaveland, 1993; Stahle et al., 1999; Biondi andFessenden, 1999; Biondi, 2000, 2001; Therrell et al.,2002; Pohl et al., 2003; Cleaveland et al., 2003).

In an effort to increase the knowledge of climatevariability in northem Mexico and its potential effectson natural ecosystems, as well as to expand the tree-ring chronologies network for this region, we used fivetree-ring chronologies to explore climate-growthrelationships. We used both local and regionaltemperature and precipitation instrumental data,tree-ring samples were collected for this study (twosites) and we algo used data collected previously byothers (three sites). This is part of a larger study in

27°

25°

23°

1040 10201080 1060

Fig. l. Geographical distribution of tree-ring chronologies (black circles) and meteorological stations (Iower case letters) used to analyze tree-rings and climate association. Chronologies: (1) Las Bayas (BAY); (2) Cerro Banderas (BAN); (3) Jiménez del Teúl (TEU); (4) El Salto (SAL);(5) Cerro Barajas (BAR). Climatic stations: (a) Las Bayas; (b) El Salto; (c) Navíos; (d) Otinapa. Additional geographical data are included in

Tables 1 and 2.

~

42 M. González-Elizondo et al. / Forest Ecology and Management 213 (2005) 39-53

which we are seeking field evidence of possible treegrowth response to changes in temperature, precipita-tion and the increase in atmospheric COz recordedin the last several decades. The objectives of thisparticular study were: (1) to analyze correlationbetween five total ring-width site chronologies fromthe study region; (2) to determine the associationbetween each chronology and climate data; (3) tocompare the dendroclimatic potential of all chron-ologies; (4) to use the best growth-climate relation-ship to reconstruct regional climate; (5) to describethe main features of dendroclimatic variability in theregion and to explore the ENSO effect on thechronologies.

America (Da vis et al., 1997) where 2900 phaner-ogamous species are estimated to be presento A cooland wet climate prevails in most of the Douglas-firstands ofthe region, with an average annual rainfall of950 mm, precipitating mostly from June to September.Mean January and July temperatures are 7 and15.5 °C, respectively. Altitude ranges from 2600 to3100 m. Natural vegetation consists of mixed coniferforests dominated by Pseudotsuga menziesii andusually one or two species of Pinus (P. durangensis,P. leiophylla, P. ayacahuite and/or P. pseudostrobus)often associated with Quercus and Arbutus. Importantunderstory shrubs include Garrya wrightii, Ribesceriferum and Vaccinum geminifiorum. The groundcover consists of different species of grasses, herbsand ferns.1.1. Study oreo

Tree-ring data from five high Sierra MadreOccidental sites were used: sampling was carriedout at two sites in southem Durango state and ring-width raw data were obtained from three additionalsites (Fig. 1, Table 1). Given the transitionallocationbetween Holartic and Neotropical realms plus thecomplexity of its physiography and climates, thisregían presents a great variety of ecosystems wherevine and/or oak forests domínate (González-Elizondo,1983). Fir and vine-tic forests occupy humid ravinesand hillsides with northem exposure at altitudesmainly above 2700 m. Douglas-fir occurs in the mostextensive of these type of mixed coniferous forestswhile Abies durangensis and Picea chihuahuana arerestricted to smaller areas just in a few sites in theSierra Madre Occidental mountain range (González-Elizondo, 1997). The main sampling site, Las Bayas,is located at the Upper Mezquital River Region, onecentre of plant diversity and endemism from Middle

2. Methods

2.1. Clima te data

Climate records consisting of monthly precipita-tion and mean, minimum and maximum monthlytemperature, from four meteorological stations weremade available through the National Water Commis-sion (CNA, 2002) (Fig. 1, Table 2). The four weatherstations considered had at least 30 years of continuousrecords and are located close to, and at comparablealtitudes, to both of our tree-ring new sampling sites(Las Bayas and Cerro Banderas).

2.2. Tree-ring data

Ring-width raw data from five tree-ring localitieswere used to develop site chronologies. Field work

Table 1Geographical distribution oftree-ring sites: Las Bayas (BAY); Cerro Banderas (BAN); Jiménez del Teúl (TEU); El Salto (SAL); Cerro Barajas(BAR)

Las BayasCerro BanderasJiménez del TeúlEl SaltoCerro Barajas5

a Numbers corresponds to Fig. I keys.b Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias.

c Intemational Tree-rin~ Data Bank.

M. González-Elizondo et al./Forest Ecology and Management 213 (2005) 39-53 43

Table 2Meteorological stations with at least 30 years of continuous records nearby the study area

a Letters corresponds to Fig. I keys.b Keys corresponds to Koppen climate classification adapted by García (1973) to Mexico conditions. 80th C(E)(w2) and C(E)(wl) are

semicold, C(E), subhumid with summer precipitation regime (w). The difference among them consists in the degree of humidity: w2 is morehumid than wl.

(Villanueva-Díaz, unpublished data) and raw datafrom the El Salto and from the Cerro Barajas sites(Stahle and Cleaveland, 1993) were obtained from theInternational Tree-ring Data Bank, ITRDB (Fig. 1,Table 1). Raw data from Jiménez del Teúl site include35 time series from 34 trees which were sampled in2001; data from El Salto and from Cerro Barajasinclude 30 and 38 time series respectively coming inturn from 17 and 18 trees that were sampled during1993 by Stahle and Cleaveland from the University ofArkansas (Table 3). Available data from these threeadditional tree-ring sites were included in this study tocompare the regional climate signal of the newchronologies. The El Salto site is the closest to both,the Las Bayas and the Cerro Banderas, 98 and 108 kmapart, respectively. The Jiménez del Teúl si te is 95 kmsouth of Las Bayas and 194 km from The CerroBanderas. The Cerro Barajas site was used in thisstudy in spite of being almost 350 km north of LasBayas because it is the longest Pseudotsuga menziesii

was carried out in two sites and available raw data ofthree previously documented site chronologies wereprocured. Increment coces from 48 old-lookingDouglas-fir trees in Las Bayas and 29 in CerroBanderas were obtained in 2001. Two coces wereremoved from cross-slope sides of each tree at breastheight (ca. 1.3 m) using either 4,3 or 5.15 mmdiameter increment borers. Once dried, coces weremounted on prefabricated wooden mounts andpolished with progressively finer grades of sandpaperto enhance tree-ring boundaries. To ensure accuratedating of each ring, coces were cross-dated (Fritts,1976) by the skeleton plot technique (Stokes andSmiley, 1968). Total ring-width was measured to thenearest 0.01 mm on a Velmex sliding stage andverified using the software program COFECHA

(Holmes, 1996).Additionally, dated ring-width raw data from recent

field work carried out by the staff of INIFAP Tree-Ring Laboratory from Jiménez del Teúl, Zacatecas

Table 3Comparative statistics for five Douglas-fir chronologies from Durango and Zacatecas. México: Las Bayas (BAY); Cerro Banderas (BAN);Jiménez del Teúl (TEU); El Salto (SAL); and Cerro Barajas (BAR)

0.25610.24470.24580.06710.4210.4190.62920.17444.961655-2001347/48/701808-1987180/28/36

0.32290.32770.34270.10190.3940.3860.76914.45243.751675-2001327/29/481913-200088/23/35

0.29960.27560.160.15290.4210.4210.60322.52644.821863-2001139/34/351911-2001

91/31/32

0.18690.25820.59680.20740.3880.3780.6719.13144.631702-1993292/17/301847-1993147/15/24

Mean sensitivityStandard deviationFirst-order autocorrelationSecond-order autocorrelationCorrelation among all radiiCorrelation between treesCorrelation within treesSignal-to-noise ratioVaro in first eigenvector (%)PenadNumber of years/trees/radiiCommon penadNumber of years/trees/radii

44 M. González-Elizondo et al./Forest Ecology and Management 213 (2005) 39-53

chronology ever developed in Mexico with over 600years, therefore, it is scientifically important toanalyze its correlation with the rest of availablechronologies for this region.

2.3. Chronologies development

The computer program ARSTAN (Cook andHolmes, 1996) was used to develop chronologiesfrom the five tree-ring sites involved. Althoughchronologies for Jiménez del Teúl (Villanueva-Díaz,unpublished data), El Salto and Cerro Barajas (Stahleand Cleaveland, 1993) were available, we used insteadthese authors dated tree-ring raw data to computecomparable chronologies. Each series was firstdetrended to remove trends in ring-width due tonon-climatic factors such as increasing age and treesize. In arder to preserve the low frequency signal,conservative simple deterministic equations such asnegative exponential curves or straight lines (eithernegatively decaying or steady state horizontal) werefitted. The individual growth index value wascalculated by dividing the actual ring-width valueby the value predicted from the regression equation(Fritts, 1976). Chronologies using the growth indexvalue were combined into a robust mean growth indexseries for each tree-ring site. Statistical features of thefive chronologies were compared and correlated forthe 1863-1993 common periodo

precipitation) to forrn seasonal climate variables,which were in turn used to compute correlation withgrowth data. The chosen period for correlationpurposes was 1964-1993 which is the common periodshared by the five chronologies and the four weatherstations. Additionally, the correlation between growthchronologies and the complete climate data availablefrom El Salto meteorological station (1940-1993) wasalso computed.

One of the climatic variables that showed highercorrelationwith growth índices was used to develop anequation to reconstruct climate using tree-ring growthíndices as predictors. In addition, all five chronologiesand monthly ENSO index were statistically related byestimating the common Pearson correlation coeffi-cient and the probability of the t value.

3. Results and discussion

3.1. Chronologies comparison

2.4. Growth-climate relationships and precipitationreconstruction

Two new Pseudotsuga menziesii total ring-widthsite chronologies from southem Durango weredeveloped, Las Bayas and Cerro Banderas, runningfrom 1655 to 2001 and from 1675 to 2001,respectively. Three additional chronologies werecomputed using available dated tree-ring raw data:Jiménez del Teúl, Zacatecas (Villanueva-Díaz, unpub-lished data) dating from 1863 to 2001; Cerro Barajasand El Salto (Stahle and Cleaveland data alreadyavailable from ITRDB) dating from 1376 to 1993 andfrom 1702 to 1993 respectively. The construction forthe first time in this study of chronologies from twonew sites expands the present network of regionalchronologies both, spatially and temporally.

Comparative analysis of descriptive statistics(Table 3) suggested that the five chronologies couldhave potential for long-term climate reconstruction.The main variables that are recognized to reflect strongresponse of a chronology to climate are: high meansensitivity, large standard deviation, low first-orderautocorrelation, high correlation between trees and

high signal-to-noise-ratio (Villanueva-Díaz, 1995).Mean sensitivity measures the relative difference inwidth from one ring to the next, its values range fromzero when there is no difference between adjacentrin~s to two when a zero value occurs next to a non-

Simple product-moment Pearson correlation coeffi-cients were computed to identify relationships betweenstandardized growth chronologies and meteorologicalrecords, from each of the four individual meteorolo-gical stations as well as from a regional simple averageof them.

Correlation coefficients between growth chronol-ogies and monthly climate variables (total precipita-tion and mean, minimum and maximum temperature)for a dendroclimatic "year" of 18 months, beginningin July prior to the growth year and ending inDecember of the current year of growth werecalculated. Consecutive monthly climate data thatshowed strong correlation with growth were combined(by simple avera~in~ temperature and seasonal total

M. González-Elizondo et al. 1 Forest Ecology andManagement 213 (2005) 39-53

Cerro Barajas. Similarly, the two new chronologiesdeveloped in this study, Las Bayas and CerroBanderas, also exhibited statistical parameters valuesthat reflect a likely strong response to climate. On theother side, El Salto's chronology showed the lowestvalue of mean sensitivity (0.19) and the highest valueof first-order autocorrelation (0.6), which are twounsuitable characteristics for climatic applications.Nevertheless, it exhibits percent of variance explainedfor the first eigenvector (44.63) and signal-to-noise-ratio (9.13) values close to those of Jiménez del Teúland Cerro Barajas, respectively, the two chronologiesthat according to their statistical features seemed tohave the highest dendroclimatic attributes.

With regard to chronologies association, correla-tion analysis oftheir common 131 years period (1863-1993) indicated that they were in general highly andsignificantly correlated (Table 4). The Bayas chron-ology correlated fairly well with the rest of them(r= 0.48-0.69; p < 0.001); the Cerro Banderaschronology correlated even better than Las Bayaswith three out of four remaining chronologies with rvalues greater than 0.67 (p < 0.001). In contrast, ElSalto's chronology exhibited r values smaller than 0.4(p < 0.001) in three out of four chronologies.Correlation computed for shorter periods (not shown)indicated that El Salto's chronology differed a greatdeal from the others, especially after the mid-1950s.We hypothesized that this drop in correlation amongchronologies mar be due either to localized climaticanomalies in the El Salto afea or mar be influenced bythe proper statistical features of this site chronology,mainly its high autocorrelation that, in turn could berelated with anthropogenic disturbances as loggingand grazing. The relatively high correlation coeffi-cients among all the five chronologies but El Salto's,

zero one in a time series (Fritts, 1976). Large values ofmean sensitivity and standard deviation are desirablefor climate reconstruction purposes since they indicateconsiderable high-frequency variance (Fritts, 1976).Autocorrelation refers to the correlation betweensuccessive values in a time series, autocorrelationcoefficient describes serial dependence or associationin a time series with previous conditions or states(Fritts, 1976), its values proximate to zero the lesserthe lag effect of previous conditions. The signal-to-noise ratio in tum is a measure of the varianceproduced exclusively by climatic conditions. There-fore, trends in growth pattems controlled mainly byclimate will have a higher signal-to-noise ratio. On theother hand, the signal-to-noise ratio will be smaller ifthe growth trends are influenced by non-climaticfactors affecting individual trees such as competitionor disturbances (e.g. tires, logging or insect out-

breaks).According to the statistical parameters that reflect

response to climate, the Cerro Barajas and Jiménez delTeúl chronologies showed the highest dendroclima-tological potential, while the statistical characteristicsof the El Salto chronology suggested a rather weakresponse to climate (Table 3).

The Cerro Barajas chronology had the highestmean sensitivity (0.38) and a relative low first-orderautocorrelation value (0.2), both characteristicsindicate a high dendroclimatic potential for thischronology; nevertheless, it algo showed a rather lowsignal-to-noise ratio value (10.8), which indicate arelative high influence of non-climatic factors ongrowth. Additionally, the higher percent of varianceexplained for the first eigenvector (47) and the relativehigh percent of correlation between trees (0.419)suggest that the Cerro Barajas chronology have astrong common signal, and thus great potential forclimatic reconstruction purposes. According to itsstatistical characteristics values, al so the Jiménezdel Teúl chronology showed a high potential fordendroclimatic purposes. It showed the highest signal-to-noise ratio (22.5) and correlation between trees(0.421), and the lower first-order autocorrelation(0.16); three of the statistics related with highdendroclimatic potential. Further, its mean sensitivity¡0.30) and standard deviation values (0.27), goodindicators of dendroclimatic potential too, are lower inthe Jiménez del Teúl chronology relative to that of The

Table 4Correlation rnatrix between chronologies for their cornrnon 1863-1993 period (p < 0.001)

BAYBANTEUSALBAR

10.720.350.67

10.380.64 0.33

Las Bayas (BA Y); Cerro Banderas (BAN); Jiménez del Teúl (TEU);El Salto (SAL); Cerro Barajas (BAR).

M. González-Elizondo et al./Forest Ecology and Management 213 (2005) 39-53

Dec Jan Feb Mar Jan-Mar Dec-Feb Jan-Febmonths

Fig.2. Correlation between each offive tree-ring chronologies and maximum regional temperature (1964-1993) ofboth single and seasonalizedwinter time months (December-March). Three out of five chronologies exhibited significant correlation with temperature during winter timemonths. Horizontalline represents the threshold level of significance for p ::; 0.05. *AIl values are negative.

suggests similar responses to environmental factorsaffecting the whole region.

the Cerro Barajas site. The relatively high r valuesobtained for The Cerro Barajas chronology wereunexpectedly high, considering the separation (ca.300 km) between the tree-ring site and the weatherstations involved in the correlation analysis. For thisparticular tree-ring site, the correlation coefficientcomputed between the standard chronology andmaximum winter temperature (up to r = -0.62) was

almost as high as those obtained from growth-winterprecipitation correlation analysis with an r value of0.66. In general, the relatively weak growth-tempera-luce association reported in this study might beexplained by the latitude of the study afea, whereprecipitation rather than temperature is recognized tobe the main growth limiting factor (Stahle et al., 1999;DÍaz-Castro et al., 2002; Cleaveland et al., 2003).

In contrast to the rather weak temperature-growthrelationships observed, it was found strong and highlysignificant correlation values among growth indicesand precipitation data (Table 5). In agreement withprior studies in the region (Stahle et al., 1999; DÍaz-Castro et al., 2002) our results suggested thatchronologies correlated better with winter-springrainfall. The continuous period of highest correlationwas the previous November to March of the currentyear, current May and current August rainfall was alsosignificantly correlated (Fig. 3). It was observed abetter correlation when the precipitation data were

3.2. Climate-growth relationship

The correlation analysis between standardized treegrowth indices and climatic variables showed a bettercorrelation with precipitation rather than temperature.In fact, only the maximum temperature of wintermonths (December-March) correlated significantlywith tree growth indices, while minimum and mediumtemperatures were not statistically significant at all.

A negative correlation was found between monthlywinter regional temperature and tree growth indicesfor three out of five chronologies (Fig. 2). Thisnegative correlation could be linked to the borealorigin of Pseudotsuga and its mesic requirements,which in tum determines the species distributionmainly in the cool-humid north facing aspects in thestudy afea where temperature is usually low. Alter-natively, a greater winter temperature increasesevaporation rate of the available soil water, thusdepleting this resource and diminishing its availabilityduring the growing season.

The growth-temperature association was ratherweak but statistically significant (p :::; 0.05), explain-ing up to 31% of the total growth variability in LasBayas's and El Salto's tree-ring sites, and up to 38% in

M. González-Elizondo et al./Forest Ecology and Management 213 (2005) 39-53

Table 5Higher significant correlations (p < 0.001) between each ofthe fivechronologies and seasonal precipitation (both regional mean andeach of four individual climatic stations)

BAY BAN TEU SAL BARClimatic stations'

Regional (1966-1993) 0.84 a 0.72 a 0.76 d 0.77 a 0.66 fLas Bayas (1964-1993) 0.80 a 0.71 a 0.74 d 0.65 a 0.60 dEl Salto (1940-1993) 0.54 b 0.54 b 0.53 b 0.63 b 0.52 bEl Salto (1964-1993) 0.76 b 0.61 b 0.63 e 0.74 e 0.63 fOtinapa (1964-1993) 0.71 b 0.67 c 0.61 c 0.83 a 0.66 fNavios (1964-1993) 0.70 b 0.69 b 0.65 c 0.70 b 0.57 f

Chronologies differed regarding the seasonal rain periods bestassociated with growth índices: (a) November-August; (b) Novem-ber-May; (c) February-May; (d) January-August; (e) November-

February; (f) January-May.a Climatic records were made available through the National

Water Commission (CNA, 2002). Chronologies: Las Bayas(BAY), Cerro Banderas (BAN), Jiménez del Teúl (TEU), El Salto(SAL), and Cerro Barajas (BAR).

correlation coefficients ranged from 0.66 for the CerroBarajas to 0.84 for the Las Bayas chronology (Table 5)explaining from 43% to 71 % of the total growthvariability respectively. The relative low correlationcoefficients showed by the Cerro Barajas chronologywere inconsistent with its statistical characteristics(Table 3), which indicated a likely strong dendrocli-matic potential. This discrepancy could be related to thelong distance between the tree-ring site and themeteorological stations considered in the analysis. Incontrast, the stronger regional climate signal showed byLas Bayas chronology, besides the potential suggestedby its statistical features, may be related to theproximity of the meteorological stations consideredin the analysis.

Comparative analysis ofthe correlation coefficientsbetween chronologies and precipitation (Table 3)suggested inferiority of climate records from the ElSalto whether station (1940-1993) to explain treegrowth variability in the whole region relatively to thatexplained for more recent climate data (1964-1993)from each ofthe whether stations, including El Salto's.This outcome suggests possible errors in early climaterecords from this station. Nevertheless, this apparentinconsistence al so could be related to other situationsas could be climate local scale anomalies or intenselogging during the 1940 and 1950s that could hadmask the climate influence on tree growth. Prelimin-

averaged for the whole region instead of using singleweather stations data (Table 5). The El Saltochronology, however, correlated better with therainfall recorded at Otinapa meteorological station,which is located at a drier site than the Test of them(Table 2). This outcome reinforces OUT hypothesisabout the probable presence of local scale climateanomalies in the El Salto chronology afea.

All chronologies correlated fairly well with regionalprecipitation (1966-1993), significant (p < 0.001)

J A

Fig. 3. Correlation between each of five tree-ring chronologies and regional averaged monthly precipitation (1964-1993). Except Augustrainfall, precipitation during the rainy season (June-September) did not show significant correlation with the radial growth of Douglas-fir in thestudy area. Horizontalline represents the threshold level of significance for p ~ 0.05.

48 M. González-Elizondo et al. / Forest Ecology and Management 213 (2005) 39-53

ary results of a study about growth trends, consideringPseudotsuga mensiezii and Pinus lumholtzii chron-ologies from Durango (in process), suggest that spatialvariability in tree-ring pattems had increased after the1950s decade, and that during the same decade thegrowth trends of most of the chronologies studiedexperienced a c1ear shift of direction, that coincidedwith the climate local scale anomalies possibilitysuggested above and also with changes in tire regimesreported for the region by Fulé and Covington (1996,

1999).The best climate-growth correlation was found

comparing the regional seasonal precipitation (pre-vious November to current August) and Las Bayaschronology (r = 0.84; p < 0.001). However, we choseto document here the reconstruction of regional totalNovember to current May precipitation instead(r = 0.80; p < 0.001). The total caín of the Novem-

ber-May period is best associated with natural rainfallcycles in the study afea since it incorporates only theprecipitation over the dry season, meanwhile theNovember to August precipitation includes the caínover the whole dry season plus a rather undeterminedproportion of summer faino Further, when correlatedseparately, dry season caín is much better associatedwith tree growth than wet season precipitation in thestudy afea.

The reconstructed 321-year (1681-2001) regional

winter-spring (November-May) precipitation usingLas Bayas chronology (Fig. 4) indicates the presenceof three main extended, more than 10 years long,droughts in the periods: 1696-1709, 1857-1876, and1969 to the end of the data series in 2001. Othershorter dry episodes reconstructed were during theintervals: 1739-1743,1749-1755,1772-1779,1797-1805, 1907-1910, 1916-1922, and 1932-1939.

During the three main dry episodes the totalizedNovember-May precipitation was between 24% and27% below the reconstructed mean value (234.3 mm).The severe mid-19th Century drought (1857-1876) hasbeen well documented earlier for northem Mexico(Stahle and Cleaveland, 1993; Díaz-Castro et al., 2002;Cleaveland et al., 2003; Pohl et al., 2003). According totree-ring data available at the ITRDB the 1690s droughthas been reconstructed earlier for Durango andChihuahua (Cerro Barajas and Tabacote chronologies,respectively) but except for an unpublished paper ofTherrell et al. about maíz yield reconstruction, it seems~

rarely mentioned in published papers reviewed. The lastdry spell reconstructed (1969-2001) had a short wetepisode in between (1983-1985) and several singlerainy years (1973,1981,1987, 1990, 1992 and 1997). Ithas been previously documented for northem Mexico(Pohl et al., 2003) as two dry episodes: the 1970s and the1990s. Taking into account the whole 1969-2001interval, reconstructed precipitation indicate 26% lessrain (174 mm) rel!ltive to the mean value (234 mm)reconstructed; but considering only the last decade ofthe 20th Century (1991-2000) this winter rain deficitreached almost 40% (142 mm). Further, 1999 wasreconstructed as the driest winter-spring season in threecenturies.

On the other hand, this reconstruction alsoindicated at least two important wet periods: 1831-1847 and 1953-1968 when the reconstruction suggest39% (325 mm) and 33% (311 mm) rain (November-May) above the reconstructed mean value, respec-tively. According to ITRDB data, the 1830s wetinterval has been reconstructed earlier for several tree-ring chronologies from northem Mexico and SouthemUSA, and also has been documented earlier forDurango (Cleaveland et al., 2003). In contrast,Florescano (1980) mentían the same period as asevere drought in the Valley of Mexico. Remarkably,at least part of the wet interval beginning in 1953reconstructed here for Southem Durango has also beendocumented earlier, but as asevere and very extendeddrought. The 1950-1957 period has been widelyrecognized as one of the 20th Century worst droughtsin Mexico (Therrell et al., 2002; Cleaveland et al.,2003; Pohl et al., 2003). On the contrary, based on LasBayas chronology, the same interval is reconstructedas an average to rather wet episode with almost 8%(252 mm) rain above the reconstructed mean. In fact,only the reconstructed rain for the years 1950, 1951and 1956 were well below the mean whereas 1952-1955 rain were above the average. Consistent withprevious reports the reconstructed rain for thefollowing 1958-1968 period was 40% (329 mm)above the mean. Moreover, 1959 and 1958 wererespectively the second and the seventh wettest yearsof the whole reconstruction (1821-2001).

Statistical association was found between Novem-ber-May reconstructed precipitation and wintertime(December-March) ENSO oscillation (p ~ 0.05).The same association were observed between ENSO~

M. González-Elizondo et al./Forest Ecology and Management 213 (2005) 39-53 49

-.- .Lt)Lt)

~

: , , , , , , , , , , , ,Il) Il) Il) Il) Il) Il)CX) ... "1" ,.. o C')<O ,.. ,.. ,.. CX) CX)... ... ... ... ... ...

years

, , , . , , , , ,In In In In InCO O> N In (X)(X) (X) O> O> O>~ ~ ~ ~ ~

(b)

Fig. 4. (a) Reconstructed total November-May precipitation for Southern Durango. Mexico using Las Bayas standardized growth índices aspredictors. The IO-year mobile mean emphasizes drought and wet episodes. (b) The Bayas (above) and the Cerro Barajas (below) chronologies.The fornler from a site at the middle of the mountain range, the latter from a dry east side site. Both responded to the l850s drought, but differedin their response to the 1950s drought as well as to its ENSO correlation.

with El Niño phenomenon. These sites are located atthe very eastem side of the Sierra Madre Occidentaland appear that El Niño affect mostly the highestelevations and probably the westem ranges of theSierra Madre. That results may be related with thedifference among the association between tireoccurrence and climatic factors founded by Fuléand Covington (1997, 1999): weak association in theLa Michilía biosphere reserve (located at the dry eastslope of the Sierra Madre, nearby the Jiménez del Teúl

oscillation and the chronologies of Las Bayas, ElSalto, and Cerro Barajas. El Niño causes wet and coldwinters in northem Mexico and these climatic condi-tions are suitable for diameter growth of conifers. Apositive water balance, the number of cold hours, or acombination of both may be responsible for higherdiameter growth of tir trees during the presence of ElNiño events.

In contrast, the chronologies of Cerro Banderas andJiménez del Teúl did not show statistical relationships

)( 2CI)"C 1.E.c 1i5

~

g>O~

O

: 11 -

6-2-

~

50 M. González-Elizondo et al./Forest Ecology and Management 213 (2005) 39-53

4. Conclusionschronology) and correspondence between tire andclimate in several northwestem Durango sites locatedat inner Sierra Madre (likewise Las Bayas and El Saltochronologies ).

Visual analysis of plotted chronologies indicatedthat those from sites located at the eastem side of theSierra Madre Occidental (Cerro Banderas, Jiménezdel Teúl and Cerro Barajas) reflected more or lessclearly the 1950s drought, whereas Las Bayas and ElSaltt> chronologies, located inner at the mountainrange reflects just the driest years of such drought(Fig. 4b). The same pattem was found regardingcorrelation between chronologies and ENSO; thecloser to dry eastem side of the Sierra MadreOccidental, the lower the association with ENSO.

Some ecological changes observed in Durangoduring the last two decades seems to be related withthe last long drought reconstructed. The 1990s severedrought along with extreme temperatures have beenlinked with the declination of Quercus and Pinuspopulations in several parts of Mexico (González-Elizondo et al., 2001, 2003; Márquez-Linares, 2004;Vázquez-Silva et al., 2004). An intense freeze duringDecember of 1997 and the extreme high temperaturesduring 1998 and 2000 presumably are the primarycausal agents of tree deaths both directly andindirectly, through increasing tree susceptibility tobark beetle attacks and to other pathogens anddiseases. Droughts also worsen the severity ofwildtires, one of the main forces that are drivingincreasing changes in vegetation cover in the studyregion. For instance, the conversion of extended forestafeas to chaparral has been reported in the eastem si deof the Sierra Madre Occidental in Durango, wherespecies lile Arctostaphylos pungens and Quercusdepressipes are becoming dominant (Casas et al.,1995; González-Elizondo et al., 2001; Márquez-Linares, 2004).

The vegetation cover changes observed in Durangoduring the last two decades of the 20th Century havebeen poorly documented. They have been usuallylinked with livestock overgrazing, logging, bark beetleattacks, tire regimens, water use and managementactivities. The results of this study indicate thatclimate variability, particularly the long time drought .started since the 1970s and intensitied during the1990s may be the underlying force driving changes inland cover.

Two new tree-ring chronologies over 300 years fromsouthem Durango were added to the growing networkof climate sensitive chronologies from the Sierra MadreOccidental: Las Bayas and Cerro Banderas. Bothcorrelated significantly with each other and with threeadditional Douglas-fir chronologies previously devel-oped along the Sierra Madre Occidental: Jiménez delTeúl, El Salto and Cerro Barajas. The El Saltochronology was the less correlated with the fourremaining, its statistical characteristics (mainly its highautocorrelation) suggest that its low correlation with theremaining may be associated partialIy with anthro-pogenic disturbances such as logging and gfazing.Nevertheless, the relative high correlation foundedbetween this chronology and precipitation, tempera-luce, and ENSO, suggest that it is al so refIecting adifferent pattem of climate.

Regarding climate association, in agreement withearlier reports, totalized seasonal precipitation(mainly winter-spring) explain partialIy the growthindex variance of the five chronologies. We al so founda significant association among the maximum winter(January-March) temperature and each of the chron-ologies. Winter-spring precipitation exhibited asignificant positive correlation, but maximum wintertemperature was negatively correlated which may becaused by a depletion of soil water by increasingevaporation. Both temperature and precipitationassociations stress the relative importance of wetand cool winter-spring periods on radial growth ofDouglas-fir in the study afea.

AlI five chronologies showed good dendroclimaticpotential. Las Bayas prevail over the rest due to itshighest correlation with climate variables and its highsignal-to-noise ratio value, which in tum may berelated to its highest sample deep. Nevertheless, eventhe Cerro Barajas chronology, in spite of the longdistance between tree-ring site and meteorologicalstations involved in the analysis, as welI as the El Saltochronology, in spite of its weak statistics (low signal tonoise ratio and high autocorrelation values) showedhighly significant climate correlation.

Regional winter-spring (November-May) precipi-tation was reconstructed based in the Las Bayaschronology. Unlike previous precipitation reconstruc-tionsreported earlier for the region, the reconstruction~

M. González-Elizondo et al. / Forest Ecology and Management 213 (2005) 39-53

logistic support provided by the Universidad Juárezdel Estado de Durango (ISIMA, ECF) during the fieldwork in Las Bayas. We also thank Roseanne D' Arrigoand Sara DÍaz for providing important literature. Wethank Abraham Torres, Julián Cerano, Bibiana Rivas,Miguel A. Botello, Manuel Estrada, Marcos Pineda,and José Hemández for their assistance during field andlaboratory activities. We are also grateful to Pete Fuléand an anonymous reviewer for providing helpfulcomments on the manuscript.

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documented here did not register the 1950s droughtrecognized frequently as the worst drought ofthe 20thCentury. Instead, the period running from 1953 to1969 was reconstructed as an average to rather wet

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Results of this study suggest that chronologiesdiffer in their response to climate following ageographical pattem presumably linked with theirdistribution within the Sierra Madre Occidental.Although the highly significant correlation foundbetween chronologies suggest a common response toregional climate, the drop in correlation around the1950s decade, along with differences found amongchronologies, such as the response to ENSO and to the1950s drought, suggest the existence of geographicalpattems, which should be t!lken as a note of caution in

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Acknowledgements

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