ARTICLE IN PRESS

Dendrochronologia 22 (2005) 169–174

1125-7865/$ - se

doi:10.1016/j.de

�CorrespondE-mail addr

www.elsevier.de/dendro

ORIGINAL ARTICLE

Climate, land-use and Prosopis ferox recruitment in the Quebrada deHumahuaca, Jujuy, Argentina

Mariano S. Morales�, Ricardo Villalba, Jose A. Boninsegna

Departamento de Dendrocronologıa e Historia Ambiental, IANIGLA-CRICYT, Mendoza, Argentina

Received 7 December 2004; accepted 28 April 2005

Abstract

Prosopis ferox grows in upper-elevation, inter-montane valleys between 2600 and 3800m in Northwestern Argentinaand Southern Bolivia. Throughout its range, P. ferox has largely been used as local fuel, building material and food forlivestock. In order to evaluate the causes of the recent increase in P. ferox density and range distribution, we comparetree recruitment of P. ferox at 15 stands in the Quebrada de Humahuaca, Jujuy, Argentina, with the environmentaland socioeconomic changes in the past 150–200 years. Based on tree recruitment, a general pattern of greaterestablishment from the mid-1970s to 1990 was identified. This regional pattern is consistent with precipitationvariations during the 20th century showing abundant rainfall in the 1970s and 1980s. In addition, historical recordsindicate that the density of domestic grazers decreased dramatically in the second half of the 20th century. Thereduction of browsing by livestock might have also favored the P. ferox recruitment over this interval. However,during the 1990s reduced establishment has occurred at a time of low stock densities suggesting significantprecipitation is necessary for the expansion and establishment of these forests. Simply reducing stocking levels will notlead to the long-term recovery of these forests.r 2005 Published by Elsevier GmbH.

Keywords: Prosopis ferox; Land-use; Rural migration; Prepuna; Forest recovery

Introduction

Prosopis ferox (Griseb) woodlands form an importantvegetation community in the high dry valleys of North-western Argentina (NOA) and Southern Bolivia andhave been widely used as a local source of fuel, timberand food for livestock. During the 19th and early 20thcentury, P. ferox wood was the most important sourceof energy (cooking and heating) in the Humahuacaregion of Jujuy province, Argentina. Even now, P. ferox

woodlands provide 70% of the energy used by the local

e front matter r 2005 Published by Elsevier GmbH.

ndro.2005.05.004

ing author.

ess: mmorales@lab.cricyt.edu.ar (M.S. Morales).

population. Although the use of fossil fuel (gas) isgradually increasing in the region, it only represents26.7% of the total energy used by local populations(Mirada, 2003).During the 18th and 19th centuries most of the

population in Jujuy was concentrated in the Quebradade Humahuaca and Puna, strategic regions thatlinked the economies of the present territories ofArgentina with their historic markets in the city ofPotosı. This pattern of population distribution wasmodified toward the end of the 19th century due tothe increasing predominance of other regions, mostlythe central and subtropical lowland valleys. After the1930s, the increasing demand for laborers by sugar

ARTICLE IN PRESSM.S. Morales et al. / Dendrochronologia 22 (2005) 169–174170

mills in the subtropical lowlands and the construction ofrailroads facilitated the migration of people fromrural to urban centers and the reduction in land-useintensity (Preston, 1996; Arzeno, 2003; Reboratti et al.,2003).Concurrently with these social processes, there have

been significant climatic changes in this region since themid-19th century. Instrumental records reveal a steadyincrease in the regional precipitation since the mid 1950sthat surpassed all historic levels (Minetti and Vargas,1997). Variations in ring width of Juglans australis,indicate that the regional precipitation reached itshistorical minimum in the 1860s and 1870s, oscillatedaround the long-term mean during the first half of the20th century and has increased steadily since mid-1950s(Villalba et al., 1998).Due to the important role of P. ferox as a source of

energy for the local populations in the Quebrada deHumahuaca, we reconstructed the dynamics of P. ferox

woodlands to identify the dominant factors controllingwood production during the 20th century. Documentaryarchives and dendrochronological methods were used todetermine the relationships between human distur-bances (changes in land-use and grazing), the recentincrease in precipitation, and P. ferox recruitment. Wealso outline the economic implications of wood avail-ability for the local communities in the Quebrada deHumahuaca.

Material and methods

Study area

P. ferox grows between 2600 and 3800m elevation inthe arid inter-montane valleys (total annual precipita-tion o300mm) located between Camargo (�201S) inBolivia and the northern part of the Calchaquı valley(�251S) in Argentina (Fig. 1; Legname, 1982; Saldıas-Paz, 1993; Beck et al., 2003). Floristically, the regionbelongs to the Prepunena phytogeographical Provincewith numerous elements from the Monte Province(Cabrera, 1976). Gochnatia glutinosa (Don), Trichocer-

eus pasacana (Weber) Britton et Rose, Cassia crassir-

amea (Benth.) and several Opuntia spp. (Ruthsats andMovia, 1975; Cabrera, 1976) are common componentsin the P. ferox woodlands. P. ferox forms openwoodlands with individuals grouped in patches, com-monly associated with Cactaceae species over a shrubbymatrix. The longevity observed in some individuals(approximately 500 years) in the southern latitudinalrange of their distribution (Seclantas, Salta, Argentina),indicate that P. ferox is a very promising species fordendroclimatological studies in subtropical montaneecosystems (Morales et al., 2001).

Climate

In NOA, temperature decreases with elevation accord-ing to local environmental lapse rates while rainfall variesdue to the combined effects of elevation and exposure tothe northeasterly winds, which bring humidity to theregion (Bianchi and Yanez, 1992). Major mountainranges are oriented in a north–south direction, generatingabundant orographic rainfall on the eastern slopes anddry inter-montane valleys on the western slopes. Thewhole region is characterized by a monsoonal rainfallpattern in which summer precipitation, depending ongeographical location and elevation, comprises 80–100%of the annual total (Prohaska, 1976).Climatic data from Humahuaca (231 100S, 651 200W;

2940m ) and La Quiaca (221 060S, 651 360W; 3460m),are representative of the climatic conditions at the studysites. Humahuaca and La Quiaca have mean annualtemperatures of 10 and 9.5 1C and annual precipitationof 175 and 322mm, respectively. The moisture deficit ismost marked between April and December and there isrecharge of the soil during the summer months(January–February). For La Quiaca, the range ofmonthly mean temperatures between December andJune (8.7 1C) is smaller than the mean diurnal range of14.3 1C in January and 23.5 1C in July.

Collection and processing of samples

Differences in P. ferox habitats were determined usinggeomorphologic criteria. These habitats were classified asalluvial-fan, piedmont alluvial plain, river-terrace andtemporary-creek woodlands. Fifteen plots were locatedacross the Quebrada de Humahuaca (Fig. 1). The size ofthe plots varied with tree density. A minimum of 40individuals per plot were sampled. Cross-sections at theroot collar were obtained for individuals with diametero10 cm. Wedges from the main stem or lateral brancheswere collected from larger trees. Growth rings werecorrectly dated to the year of formation following standarddendrochronological procedures (Stokes and Smiley, 1968).Individual ages were determined by counting the

number of growth rings in each cross-section. Mostcounting was conducted using a binocular microscope.For samples with low rate of growth, supplementalcounting was conducted on histological slides. In somecases (15% of the total samples), when the rings weretoo thin to count precisely, we estimated dates leading toan error of 71–3 years in the determination ofestablishment.

Climatic data

In order to analyze the precipitation fluctuationsacross NOA during the last 100 years, we collected

ARTICLE IN PRESS

23°7

'30'

'S

65°22'30''W

-70-20

-22

-24

-26

-68 -66 -64 -62

Bolivia

Chile

Argentina

Prosopis ferox

0 50

mm100

65˚15'W

23°1

5'S

23°2

2'30

"S

Fig. 1. Study area showing the distribution of P. ferox in Argentina and Bolivia (left); and the location of the 15 plots in the

Quebrada de Humahuaca, Jujuy, Argentina (right).

M.S. Morales et al. / Dendrochronologia 22 (2005) 169–174 171

precipitation data from 82 sites located between21.5–291S and 62.5–691W. These include datafrom Bianchi and Yanez (1992), the ArgentineanWeather Service, the Oak Ridge National Laboratory(ORNL, USA) and the International ResearchInstitute (IRI, USA). From this original dataset, only32 records complied with the quality, reliability andminimum length (56 years) required for the studyof long-term temporal and spatial analyses of precipi-tation variations in NOA (De Membiela, Personalcommunication).

Documentary data

Through the analysis of historical documents werecovered data on regional demography (rural–urbanpopulation) and land-use changes (number of livestock).This information was compiled from the Population and

the Agronomic National Surveys starting in 1869 and1908, respectively. Surveys were not consistently con-ducted on regular intervals. This information is avail-able at the Argentinean Institute for Statistics andSurveys (INDEC), Buenos Aires, Argentina.

Results

A regional picture of the timing of tree recruitmentwas derived by summing the data from all 15 plots tominimize the effects of local events on individual P.

ferox recruitment across the Quebrada de Humahuaca.This analysis revealed a general pattern of greaterestablishment from the mid-1970s to 1990 (Fig. 2b).During this period of abundant recruitment, a signifi-cant decrease is observed in 1983 (Fig. 2b). Afterapproximately 1992 recruitment decreases considerably(Fig. 2b).

ARTICLE IN PRESS

Fig. 2. Relationships between: (a) precipitation variations

from 32 weather stations from Northwest Argentina (from De

Membiela, in preparation), (b) regional recruitment of

Prosopis ferox (b) and livestock changes (c) in the Quebrada

de Humahuaca. The gray rectangle represents the period

1973–1992.

Fig. 3. Changes in the population of Jujuy province during the

last 220 years separated by geographical regions. (a) total

number and (b) expressed as percentage of the total popula-

tion.

M.S. Morales et al. / Dendrochronologia 22 (2005) 169–174172

The instrumental precipitation data from the NOAregion show no major trend between 1930 and 1972, buta significant increase after 1973 (Fig. 2a). Mean annualprecipitation during this 1973–1992 humid periodaveraged almost double the precipitation of the previous40 years. For example, at Jujuy mean annual rainfallwas 744mm between 1934–1972 and 1002mm in1973–1990. Except for 1996, the most recent 10 yearsin the record seem to have returned to the early1930–1972 regime (Fig. 2a).During the first half of the 20th century, the

subtropical regions in NOA recorded an abruptpopulation increase (Fig. 3a) associated with theestablishment and growth of sugar mills. Duringthe past three decades, the principal cities recorded thelargest population growth in the region (Fig. 3b). Theexponential growth recorded in the subtropical andcentral valley regions during the 20th century waslargely due to emigration of the population from thePuna and the Quebrada. In consequence, population inthe highlands grew very slowly (see Gil Montero, thisissue) (Fig. 3a). Additionally, the local urbanization orconcentration of the rural population in the small townswas also observed along the Quebrada de Humahuaca.Associated with these migratory processes, an im-

portant reduction in the number of livestock occurredacross the Quebrada de Humahuaca. Although no dataare available, the large area of abandoned fields

observed in the region today are testimony to a moreextensive agriculture early in the 20th century. For theDepartment of Humahuaca, the number of livestockgradually decreased from the beginning of the 20thcentury to mid-1970s, when an abrupt reduction wasrecorded, mainly for sheep and goats (Fig. 2c).

Discussion

Our data on P. ferox dynamics suggest a recovery ofthis species in the Quebrada de Humahuaca over thepast century. The increase in precipitation together witha decrease in the intensity of land-use by humansappears to be the dominant factors driving the recoveryof P. ferox woodlands. However, it is important, forfuture management of this region, to determine therelative importance of these two factors. The radialgrowth of P. ferox is highly favored by abundantrainfall during the growing season (Morales et al., 2004).The present study also suggests that the recovery of P.

ferox in the region has been facilitated by changes inprecipitation. The abrupt increase in tree recruitmentduring the period 1973–1992 is associated with abun-dant precipitation during the same period. The reduc-tion in P. ferox recruitment registered during the 1990s

ARTICLE IN PRESSM.S. Morales et al. / Dendrochronologia 22 (2005) 169–174 173

coincides with decreased precipitation during the sameperiod.On the other hand, population and agronomic

surveys indicate emigration of the rural populationand a reduction in the density of domestic animalsduring the 20th century (Fig. 2c). It is highly possiblethat the decreases in domestic animals and theconsequent reduction of grazing pressure also favor P.

ferox recruitment. According to the livestock surveys, asignificant reduction in the number of domestic animalsoccurred between the 1974 and 1988 surveys. Thisdecrease in the grazing pressure could have facilitatedthe P. ferox recruitment between approximately 1975and 1990.Although no data on numbers of domestic animals

are available since the last survey in 1988, we believethat numbers have been stable or continued to declinesince that year. However, although reduced grazingpressure has continued through the 1990s few trees havebeen established during this decade. This suggests thatthe precipitation reduction in the past 10 years hasplayed a more important role than livestock in regulat-ing tree recruitment. Another possibility is that thecohort of P. ferox that established in 1970–1990 is nowin equilibrium with the present grazing pressure and alarge reduction in livestock would be necessary tomaintain the rate of abundant recruitment recorded in1973–1993 interval.Due to its excellent properties (and the lack of

alternative species in the region), the wood of P. ferox

has been used as the main source of energy in theQuebrada de Humahuaca (Mirada, 2003). Althoughthe extraction of wood for fuel shows a slight decreasein the last decades, it still provides 70% of energyin the region (Mirada, 2003). Our study suggeststhat intensive grazing to the levels recorded early inthe 20th century and sustainable wood production in theQuebrada de Humahuaca are incompatible over a long-term perspective. Grazing pressure reduces standingbiomass, perhaps below sustainable levels and alsoreduces tree establishment and survival. However,reducing livestock alone will not necessarily producean increase in tree establishment and wood availability.The lack of recruitment in the 1990s suggests thatabundant precipitation (in combination with reducedlevel of grazing pressure) is needed to secure P. ferox

tree establishment. Therefore sustaining future viablewood production from this region requires both themaintenance of reduced livestock levels and the con-tinued occurrence of decadal scale wetter periodsthat favor establishment and survival of new forest.Although the substitution of wood by fossil fuelfor cooking and heating could reduce the pressureon wood resources, we predict a long-term use of theP. ferox woodlands due to the low income of localpopulations.

Acknowledgements

This research was supported by the Inter-AmericanInstitute for Global Change Research (IAI) CRN3:Climate from Treelines, the National Research Councilof Argentina (CONICET), and Argentinean Agency forPromotion of Science and Technology (PICTR02-123).We are grateful to Alberto Ripalta, Sergio Londero,Guillermo Garcıa-Zamora and Susana Monge for thefield assistance.

References

Arzeno M. Cambio y permanencia en el campesinado. In:

Reboratti C, editor. La Quebrada. Buenos Aires: La

Colmena; 2003. p. 123–38.

Beck S, Paniagua Z, Yevara Garate M. Flora y vegetacion: las

areas de Rodero, Tilcara y Volcan. In: Reboratti C, editor.

La Quebrada. Buenos Aires: La Colmena; 2003. p. 47–70.

Bianchi AR, Yanez C. Las precipitaciones del noroeste

argentino. Salta, Argentina: Instituto Nacional de Tecno-

logıa Agropecuaria; 1992. 388pp.

Cabrera A, editor. Regiones fitogeograficas argentinas. En-

ciclopedia argentina de agronomıa y jardinerıa. Tomo 2,

fasc. 1. Buenos Aires: ACME; 1976 85pp.

Gil Montero R, Villalba R, Tree rings as a surrogate for

economic stress—an example from the Puna of Jujuy,

Argentina in the 19th century. Dendrochronologia this

issue.

Legname PR. Arboles indıgenas del Noroeste Argentino, vol.

32. Opera Lilloana, Argentina: Ministerio de cultura y

educacion; 1982. p. 1–123.

Minetti JL, Vargas WM. Trends and jumps in the annual

precipitation in South America, south of the 15S. Atmos-

fera 1997;11:205–21.

Mirada A. Uso de la lena en Humahuaca. In: Reboratti C,

editor. La Quebrada. Buenos Aires: La Colmena; 2003.

p. 177–92.

Morales MS, Villalba R, Grau HR, Villagra PE, Boninsegna

JA, Ripalta A, Paolini L. Potencialidad de Prosopis ferox

Griseb (Leguminosae, subfamilia: Mimosoideae) para

estudios dendrocronologicos en desiertos subtropicales de

alta montana. Revista Chilena de Historia Natural

2001;74:889–96.

Morales MS, Villalba R, Grau HR, Paolini L. Rainfall

controlled tree growth in high elevation subtropical

treelines. Ecology 2004;85:3080–9.

Preston D. People on the move: migrations past and present.

In: Preston D, editor. Latin America development:

geographical perspectives. Harlow: Longman Scientific

and Technical; 1996. p. 165–87.

Prohaska F. The climate of Argentina, Paraguay and

Uruguay. In: Schwerdtfeger W, editor. World survey of

climatology: climates of Central and South America.

Amsterdam: Elsevier; 1976. p. 13–112.

Reboratti C, Garcıa Codron JC, Albeck M, Castro H, Arzeno

M. Una vision general de la Quebrada. In: Reboratti C,

editor. La Quebrada. Buenos Aires: La Colmena; 2003.

p. 17–46.

ARTICLE IN PRESSM.S. Morales et al. / Dendrochronologia 22 (2005) 169–174174

Ruthsats B, Movia CP, editors. Relevamiento de las estepas

andinas del noreste de la provincia de Jujuy, Argentina.

Buenos Aires: Fundacion para la educacion, la ciencia y la

cultura; 1975 126pp.

Saldıas-Paz M. Mimosoideae. In: Killeen TJ, Garcia E, Beck

SG, editors. Guıa de arboles de Bolivia: Herbario Nacional

de Bolivia y Missouri Botanical Garden; 1993. p. 420–56.

Stokes MA, Smiley TL, editors. An introduction to

tree-ring dating. Chicago: University of Chicago Press;

1968 73pp.

Villalba R, Grau HR, Boninsegna JA, Jacoby GC, Ripalta A.

Tree-ring evidence for long-term precipitation changes in

subtropical South America. International Journal of

Climatology 1998;18:1463–78.