Land Use Change and Mountain Biodiversity - Chapter 20 ppsx

Influences on the Dynamics of Quebrada de Humahuaca, Jujuy, Argentina Mariano Morales and Ricardo Villalba INTRODUCTION Prosopis ferox forests constitute an important floristic communit

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Influences on the Dynamics of

Quebrada de Humahuaca, Jujuy, Argentina

Mariano Morales and Ricardo Villalba

INTRODUCTION

Prosopis ferox forests constitute an important floristic community in the intermontane arid valleys of northwestern rural Argentina (NOA)

In the Quebrada de Humahuaca, P ferox forests have been widely used as local sources of fuel and wood for rural construction The fruits and leaves of P ferox represent a major source of fodder for caprine and ovine cattle The impacts

of anthropogenic activity in the region are therefore reflected in the morphology and struc-ture of the remnant forest

With the Spanish conquest, important changes occurred in the resource-use systems

in the Quebrada de Humahuaca Among these changes, the replacement of native plants and indigenous animals by European species pro-duced a distinct and increasing change in the landscape The overexploitation by cattle farm-ing increased the process of desertification, favored by the neglect of agricultural soil pres-ervation techniques and the predation of for-ested communities (Lorandi 1997) The short-age of trees in most of the region led to the overuse of native species, which were suitable for use as fuel to cook food In this manner, the

P ferox woods were reduced to small patches (León 1997)

The impoverishment of the environment, along with the process of proletarianization occurring in the Puna at the beginning of the

20th century, brought about the migration of the rural population to urban centers and the sub-sequent abandonment of fields (Reboratti 1994) A major process of regional migration started in 1930; the rural population left to work

in mills and in other agriculture-related activi-ties in the subtropics, and in railway and road construction In this manner, the dependency on

a monetary income gradually increased, weak-ening the access to the diverse alternative pro-duction methods that had so far sustained the traditional economy of subsistence in the region (Campi and Lagos 1994) The strong rural migration that took place in the Quebrada de Humahuaca caused the abandonment of land intended for pasture and crops

Furthermore, important climatic changes have been registered in the NOA region since the middle of the 19th century A steady increase in precipitation in the region has been registered since around 1950 (Minetti and Var-gas 1997) This increase in precipitation, which does not seem to have historic precedence, has also been documented in dendrochronological data The variations in the thickness of growth rings in two species of the region, Juglans aus-tralis (nogal criollo) and Cedrella lilloi (cedro tucumano), indicate that regional rainfall had reached a historic minimum in the 1860s and 1870s, oscillated around the mean from the beginning of the 20th century until the end of

3523_book.fm Page 275 Tuesday, November 22, 2005 11:23 AM

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276 Land Use Change and Mountain Biodiversity

the 1940s, and has increased steadily over the

last few decades (Villalba et al 1998)

During previous explorations in the region,

we observed an apparent increase in the cover

area of P ferox in the surroundings of the town

of Humahuaca To identify the environmental

changes related to this dynamic process of

for-est expansion, we evaluated the past and present

roles of climatic and anthropogenic factors on

the structure and dynamics of the P ferox

for-ests in the Quebrada de Humahuaca during the

20th century Previous studies of species of the

genus Prosopis indicate that the trees

them-selves create environmental conditions that

facilitate the development of other associated

species (Aggawarl et al 1976; Simpson and

Solbrig 1977; Archer et al 1988) It has been

shown that the woody species of Prosopis in

the arid zones of western Argentina modify the

microclimatic conditions under their crowns,

thereby generating different environmental

con-ditions in the nearby open areas are, then,

ade-quate habitats for companion species (Rossi and

Villagra, 2003) Our study, although

concentrat-ing on the dynamics of a sconcentrat-ingle species, has,

therefore, important implications with respect

to biodiversity

MATERIALS AND METHODS

S TUDY A REA

In Argentina, P ferox grows between 2600 and

3800 masl in arid environments (annual mean

precipitation ~300 mm) from the northern

sec-tor of the Calchaquí Valley, Salta, the Quebrada

de Humahuaca, Jujuy, and the Río Grande de

San Juan Valley on the border between Jujuy

and Bolivia (Legname 1982) In Bolivia, it can

be found in the arid inter-Andean valleys

between 2600 and 3800 masl in the

depart-ments of Potosí, Tarija, and Chuquisaca

(Saldías-Paz, 1993; Lopez, 2000) Floristically,

the region belongs to the phytogeographic

pre-Puna province, with numerous elements of the

Monte province (Cabrera 1976) Among the

common companion species of P ferox are

Tri-chocereus pasacana (cardón), Opuntia

sul-phurea, Opuntia soehrendsii, Opuntia

tilcaren-sis, Parodia spp., Baccharis boliviensis,

Baccharis salicifolia, Gochnatia glutinosa,

Maihueniopsis spp., Schinus latifolius, Senna crassiramea, Lycium venturii, Proustia cunei-folia, and Aphylloclados spartioides (Beck et

al 2003) P ferox forms an open forest with individuals grouped in patches,which are usu-ally associated with Cactacea species and sev-eral Asteraceae shrubs

The temperature in the study area markedly declines with altitude, whereas precipitation depends largely on the topographic location in relation to the bearing of the mountain chains, which intercept the humid air masses Climatic data are taken from sites in Humahuaca (23°10 S, 65°20 W) and La Quiaca (22°06 S, 65°36 W), which are situated at 2940 and 3460

m elevation, respectively, and can be considered representative of the climatic conditions of the study sites Whereas annual temperatures in Humahuaca and La Quiaca lie between 10°C and 9.5°C, the total precipitation on average is

175 and 322 mm, respectively The water deficit

is more distinct between April and December; soil water is partially replenished during sum-mer months (January–February) For La Qui-aca, the annual temperature difference between the hottest month (December) and the coldest month (June) is 8.7°C; this is less than the mean daily temperature range for any month of the year, which ranges from 14.3°C in January to 23.5°C in July

C OLLECTION AND P ROCESSING OF

S AMPLES

The different habitats of P ferox were deter-mined based on geomorphological features of the landscape We used aerial photographs for the delimination of the vegetation units and subsequent verification in the field The com-munities of P ferox were grouped according

to their location on plateaus, alluvial cones, hillsides, or in riparian environments The den-sity of individuals determined the size of the plots, but every plot included at least 40 indi-viduals Sampling was carried out following conventional dendrochronological methods (Stokes and Smiley, 1968) Growth rings were correctly dated by year of formation

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Climatic and Anthropogenic Influences on the Dynamics of Prosopis ferox Forests 277

D ETERMINATION OF P OPULATION

S TRUCTURE

To establish the age structure of the plots, the

age of each individual was determined by

counting growth rings, starting at the outermost

ring next to the bark and ending with the

inner-most ring around the pith For those individuals

in whom the rings could not be unequivocally

recognized with a dissecting microscope, the

growth ring count was carried out using

histo-logical cuts The same procedures were used to

determine the age of individuals displaying

groups of very thin growth rings For these

sam-ples, errors in their ages range from 2 to 5 years

C OLLECTION AND A NALYSIS OF C LIMATIC

D ATA

To analyze annual fluctuations in precipitation

along the Quebrada de Humahuaca over the past

100 years, instrumental data were collected

from meteorological stations in the NOA; the

data were taken from Bianchi and Yañez (1992),

the publications of the National Meteorological

Service, and other international databases from

institutions such as the Oak Ridge National

Lab-oratory (ORNL) and the International Research

Institute (IRI) From these data, information

from weather stations located between 21.5° and

29° S and between 62.5° and 69° W was

com-piled From the total of 82 initially selected

pre-cipitation records, only 32 fulfilled the criteria

of quality, reliability, and minimum time span

(56 years) required for our study The dominant

patterns of variability in the precipitation of

NOA were determined using principal

compo-nent analysis (Cooley and Lohnes, 1971) of the

32 selected records

C OLLECTION AND A NALYSIS OF

A NTHROPOGENIC D ATA

Regional demographic changes (rural migration

process) and land use changes (changes in

stocking rates) were reconstructed using

histor-ical documents The information on

demo-graphic changes and stocking rates was

com-piled from the National Population Censuses

beginning in 1869 (first record) and the

National Agropecuarian Censuses beginning in

1908 This information is available at the National Institute for Statistics and Censuses (INDEC), Buenos Aires, Argentina

RESULTS

P OPULATION S TRUCTURES

Common patterns of tree establishment were recorded in 12 sampling plots of Prosopis ferox

located in different environments aross the Que-brada de Humahuaca The most distinct com-mon feature is the period of increased estab-lishment from around the mid-1970s until the year 1990 This pattern was observed clearly in plots 5, 9, 10, 13, 14, 15, 16, and 42 (Figure 20.1) In plot 12, there was a greater rate of establishment during the 1960s and at the beginning of the 1970s (Figure 20.1) In plot

11, establishment took place from the end of the 1960s until the mid-1980s (Figure 20.1) In plot 41, new recruits had established between

1933 and 1970, with a decrease in establish-ment in the following decades (Figure 20.1) Apart from plot 13, establishment in the 1990s was either zero or greatly reduced

An idea of the regional temporal evolution

of establishment can be obtained from the sum

of population structures of all the plots In this way, the regional signal emerges clearer, as events affecting individual plots are minimized

in the regional mean (Villalba and Veblen, 1997) The total sum of age structures over all plots reflects important temporal changes in the establishment process Individuals that estab-lished between the beginning of the 1930s and mid-1950s were scarce The number of estab-lished individuals increased gradually between the mid-1950s and mid-1970s From then on, there was a marked increase in establishment, which ended in the beginning of the 1990s, when it decreased considerably (Figure 20.1m)

V ARIATION IN P RECIPITATION

The dominant precipitation pattern in the NOA region shows a positive trend during the period from 1930 to 1998 (Figure 20.2) A significant increase in rainfall, beginning in 1973, was observed in all records During this humid period, which in general lasted until 1992, the

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FIGURE 20.1 Age structure of Prosopis ferox in each plot (a–l) and for the entire study region (m) from

1930 onward.

2 4 6

1940 1950 1960 1970 1980 1990 2000 1940 1950 1960 1970 1980 1990 2000

Plot 5

Plot 11 Plot 10 Plot 9 Plot 6

Parcela 42 Plot 41 Plot 16 Plot 15 Plot 14

Plot 13 estimated date of establishment

date of establishment

minimum age

Plot 12

Years

1930 1940 1950 1960 1970 1980 1990 2000

No trees 7 14 21

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

(m)

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mean annual precipitation values were much

higher than in previous decades and in later

years, with the exception of 1996 (Figure 20.2)

D EMOGRAPHIC AND L AND U SE C HANGES

During the colonial period until the end of the

19th century, the majority of the population of

Jujuy was concentrated in the Quebrada and the

Puna, which were strategic regions in linking

the economy of Argentina with its principal

market, Potosí (Larrouy 1927) This pattern was

modified toward the end of the 19th century as,

among other reasons, other regions increased in

importance, in particular the central and

sub-tropical valleys, where the new agroindustries

formed the principal regional economic activity

of the emerging national market

The steady increase of the economy in

sub-tropical regions, largely allocated with sugar

mill activities, speeded up the process of

pop-ulation migration from the Puna to the

low-lands This process, which started at the end of

the 19th century, persisted until the mid-20th

century During the last few decades, a strong

urbanization process led to a concentration of

the population in the principal cities Figure

20.3a clearly shows this exponential increase in

population in the central and subtropical valley

regions since the end of the 19th century In

contrast, the population in the Puna and the

Quebrada regions grew very slowly (Figure

20.3a) Nevertheless, a process of urbanization

concentrating the population in the towns

located along the Quebrada de Humahuaca was recorded In the Humahuaca Department, the

1947 census indicated that the majority of the population was rural (82%), and the urban pop-ulation was very small

From the middle of the 20th century onward, the migration of the rural population

to small towns was observed In the 1960 cen-sus, the rural proportion of the population had decreased to 43% (Figure 20.3b) Unfortu-nately, there exist no recent data about the rela-tionship between rural and urban populations, but we believe that the trends have remained stable, which has led to a further decline in rural population levels over the last decades

A very important reduction in regional cat-tle stocking rates has been observed in associ-ation with these migrassoci-ation processes In the Humahuaca and Tilcara departments, the stock-ing rate decreased steadily from the beginnstock-ing

of the 20th century until the 1970s From then

on, there was a very distinct decline in cattle numbers, particularly in ovine and caprine cat-tle (Figure 20.4a and Figure 20.4b)

DISCUSSION

The dendrochronological studies of Prosopis ferox based on the population structures of 12 plots located in different environments of the Quebrada de Humahuaca show four periods with distinct characteristics: a stage of scant establishment up until the 1960s, a period of

FIGURE 20.2 Comparison between the pattern of establishment of Prosopis ferox and the first principal component of precipitation from 32 meteorological stations in northwestern Argentina.

Ye ar s

-1

0

1

2

3

Pr ec ip it at io n

1 930 194 0 195 0 196 0 1970 1 980 1990 2000

0

5

10

15

20

25

Estimated dates Pith dates Minimum ages 3523_book.fm Page 279 Tuesday, November 22, 2005 11:23 AM

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low establishment between 1960 and 1970, a

marked increase between 1974 and 1990, and

another period of reduced establishment during

the last 10 years This pattern is consistent with

the observed recovery of the forests and their

expansion into open areas

Some of the observed changes in this

regional pattern of establishment coincide with

the recorded changes in precipitation Although

there is a positive trend in rainfall for the

inter-val 1930 to 1998, a distinct leap was recorded

in the year 1973, which coincides with a marked

increase in the number of established

individu-als of P ferox in the region This interval of

higher precipitation represents the most humid

period in the 20th century The decreased

estab-lishment observed during the 1990s coincides

with lower precipitation during the same period

Furthermore, the demographic changes

recorded at a regional level, the subsequent

decrease in cattle stocking rates, and the result-ing decline in grazresult-ing pressure are also factors associated with the observed changes in the pattern of establishment of P ferox The Popu-lation and Agrarian Censuses provide evidence

of an important change in the relationship between the rural and the urban population in the middle of the 20th century, and a marked reduction in cattle stocking rates between 1974 and 1988 This reduction in cattle stocking facilitated the sudden increase in the establish-ment of P ferox observed between the mid-1970s and 1990

Based on these observations, reduced estab-lishment before 1960 can be attributed to the pressure on the vegetation caused by high den-sities of ovine and caprine livestocks In addi-tion to the impact of browsing, there occurred

a relatively dry period, which hindered the establishment of new recruits As cattle

FIGURE 20.3 Demographic variation in (a) the four principal regions of the Jujuy Province, and (b) the rural and urban population in the Department of Humahuaca.

FIGURE 20.4 Temporal variation in cattle stocking rates in the departments of (a) Humahuaca and (b) Tilcara.

Ye ar s

1770 1800 183 0 1 86 0 1 890 1920 1950 1980 2010

0

50

10 0

15 0

20 0

25 0

Ye ar s

1880 1 900 1920 1 940 1960 1 980 2000

0

5

10

15

20

25

Centra l va lle ys Subtropi ca l va lle ys

Pu na Quebrada

Total populatio n

Rur al

Ur ba n

(b)

Humahuaca

Years

1900 1920 1940 1960 1980 2000 0

30 60 90 120 150

Years

1920 1940 1960 1980

Tilcara

Total Sheep Goats

Total Goats Sheep

2000

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stocking decreased and precipitation increased,

the number of established individuals

increased From the mid-1970s until the

begin-ning of the 1990s, the number of individuals

recruited increased markedly, which was a

con-sequence of reduced grazing pressure and the

sudden increase in precipitation The lower

establishment rates recorded in the last 10 years

can be attributed mainly to the decrease in

rain-fall, as grazing pressure in the 1990s remained

stable or even decreased

The observed pattern of establishment may

indicate that both climatic variation and land

use changes regulate the dynamics of P ferox

forests in the Quebrada de Humahuaca The

reduced rates of establishment during the

1990s, despite cattle stocking similar to or

lower than in the 1980s, suggest that variations

in rainfall play an important role in the

estab-lishment of new individuals

The observed recovery and expansion of

P ferox forests may create new habitats for

other species beneath the tree crowns, thereby

increasing the spatial heterogeneity of the

eco-systems and the local biodiversity In agreement

with Aide and Grau (2004), our study indicates

that a reduction of human-induced disturbances

on the landscape facilitates conservation efforts,

as the decline in human pressure and the impact

of associated activities often allows ecosystem

recovery Nevertheless, the recovery of P ferox

forests and the increase in local biodiversity

cannot be solely attributed to the decrease in

human impact Particular climatic conditions,

such as the humid events recorded during the

1970s and 1980s, are necessary for the

estab-lishment and survival of new recruits The

inter-action between social and natural factors

largely determines the future development of

ecosystems and their species richness For this

reason, it is important that we quantify the

rel-ative importance of these environmental forcing

factors to establish management guidelines for

the intermontane and subtropical valleys of the

Andes

SUMMARY

The recent increase of Prosopis ferox, both in

population density and distribution range in the

Quebrada de Huamahuaca, Jujuy, Argentina, appears to be related to major changes in land use and regional precipitation P ferox grows between 2600 and 3800 m elevation in the upper-elevation intermontane valleys in north-western Argentina and southern Bolivia Across its range of distribution, P ferox has been largely used as a local fuel source, as construc-tion material, and as fodder for livestock To determine the factors affecting the recent changes in the population structure of P ferox,

we compared the age structure of the wood-lands with human demographic and land use changes, and with regional variations in precip-itation Based on age structures from 12 stands, four periods of recruitment were identified

Reduced recruitment was recorded until 1960 followed by a gradual increase between 1960 and 1970 An abrupt increase was observed around 1974, which persisted to 1990 Finally, low recruitment was recorded during the past

10 years These regional patterns of recruitment are consistent with the recent expansion of the woodlands Variations in recruitment reflect regional variations in precipitation since the middle of the 20th century Furthermore, demo-graphic and agrarian censuses show significant changes in the relationship between rural and urban populations, and a substantial reduction

of livestock density between 1974 and 1988

Reduced grazing by livestock during this inter-val might have also favored the recruitment of

P ferox trees Similar or lower grazing pressure during the 1990s suggests a greater influence

of precipitation than livestock on the reduced tree establishment during this decade

ACKNOWLEDGMENTS

These studies were financed by the Agencia Nacional de Promoción Científica y Tec-nológica (PICTR 2002-123) and the Instituto Interamericano para el Estudio del Cambio Glo-bal (IAI) The authors wish to thank Alberto Ripalta, Susana Monge, Sergio Londero, and Tromen Villalba for their collaboration in the field

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