Land Use Change and Mountain Biodiversity - Chapter 20 ppsx

275 20 Climatic and Anthropogenic Influences on the Dynamics of Prosopis ferox Forests in the 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 Copyright © 2006 Taylor & Francis Group, LLC 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 single 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. 3523_book.fm Page 276 Tuesday, November 22, 2005 11:23 AM Copyright © 2006 Taylor & Francis Group, LLC 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 3523_book.fm Page 277 Tuesday, November 22, 2005 11:23 AM Copyright © 2006 Taylor & Francis Group, LLC 278 Land Use Change and Mountain Biodiversity 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 2 4 6 2 4 6 No. trees 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) 3523_book.fm Page 278 Tuesday, November 22, 2005 11:23 AM Copyright © 2006 Taylor & Francis Group, LLC Climatic and Anthropogenic Influences on the Dynamics of Prosopis ferox Forests 279 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 migration processes. In the Humahuaca and Tilcara departments, the stock- ing rate decreased steadily from the beginning 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 Pr ec ip it at io n (a mp lit udes PC 1) -1 0 1 2 3 Pr ec ip it at io n 1 930 194 01 95 01 96 01 970 1 980 1990 2000 No. of trees 0 5 10 15 20 25 Estimated dates Pith dates Minimum ages 3523_book.fm Page 279 Tuesday, November 22, 2005 11:23 AM Copyright © 2006 Taylor & Francis Group, LLC 280 Land Use Change and Mountain Biodiversity 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 grazing 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 01 86 01 890 1920 1950 1980 2010 No. of inhabitants (in thousands 0 50 10 0 15 0 20 0 25 0 30 0 Hum ahuac a Ye ar s 1880 1 900 1920 1 940 1960 1 980 2000 0 5 10 15 20 25 Ju ju y (a) 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 (a) (b) 2000 No. of livestock (in thousands) 3523_book.fm Page 280 Tuesday, November 22, 2005 11:23 AM Copyright © 2006 Taylor & Francis Group, LLC Climatic and Anthropogenic Influences on the Dynamics of Prosopis ferox Forests 281 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. 3523_book.fm Page 281 Tuesday, November 22, 2005 11:23 AM Copyright © 2006 Taylor & Francis Group, LLC 282 Land Use Change and Mountain Biodiversity REFERENCES Aggawarl, R.K., Gupta, J.P., Saxena, S.K., and Muth- ana, K.D. (1976). Studies on soil physico- chemical and ecological changes under twelve year old desert tree species of West- ern Rajasthan. Indian For , 102: 863–872. Aide, T.M. and Grau, H.R. (2004). Globalization, migration and Latin American ecosystems. Science , 305: 1915–1916. Archer, S., Bassham, C.R., and Maggio, R. (1988). Autogenic succession in a subtropical savanna: conversion of grassland to thorn woodland. 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International Journal of Climatology, 18: 1463–1478. 3523_book.fm Page 282 Tuesday, November 22, 2005 11:23 AM Copyright © 2006 Taylor & Francis Group, LLC . 22, 200 5 11:23 AM Copyright © 200 6 Taylor & Francis Group, LLC 278 Land Use Change and Mountain Biodiversity FIGURE 20. 1 Age structure of Prosopis ferox in each plot (a–l) and. 22, 200 5 11:23 AM Copyright © 200 6 Taylor & Francis Group, LLC 280 Land Use Change and Mountain Biodiversity low establishment between 1960 and 1970, a marked increase between 1974 and. November 22, 200 5 11:23 AM Copyright © 200 6 Taylor & Francis Group, LLC 282 Land Use Change and Mountain Biodiversity REFERENCES Aggawarl, R.K., Gupta, J.P., Saxena, S.K., and Muth- ana,