Polylepis Tarapacana Research Articles

First dendroarchaeological dates of prehistoric contexts in South America: chullpas in the Central Andes

Despite the existence of long-lived tree species with excellent tree-ring characteristics, the use of dendrochronological techniques for dating archaeological contexts has been very limited in South America. Recent research in the Bolivian Altiplano of the Central Andes has yielded a network of Polylepis tarapacana chronologies that goes back to the 13th century. This species was regularly used by local populations since pre-Columbian times as raw material for beams, lintels, hooks and other architectural features in houses, storage chambers, and sepulchers. The aim of this study is to argue in favor of the potential of P. tarapacana for providing high resolution dates of significant archaeological events during the pre-Columbian era. The chronology of chullpas (burial towers and storage chambers) in the southern Andean Altiplano is used as a case study. Growth rings from P. tarapacana have provided several centennial-length dendrochronological records including a reference regional chronology covering the last 786 years in the Central Andean highlands. Based on this long reference chronology calendar years were assigned to woody pieces from pre-Hispanic chullpas. Dendroarchaeological results are consistent with contextual evidence and most radiocarbon dates from woody samples and related archaeological items. Our results indicate that P. tarapacana has a high potential for providing annually-resolved tree-ring dates for archaeological contexts in the Altiplano since the early 13th century.

Precipitation changes in the South American Altiplano since 1300 AD reconstructed by tree-rings

Abstract. Throughout the second half of the 20th century, the Central Andes has experienced significant climatic and environmental changes characterized by a persistent warming trend, an increase in elevation of the 0 °C isotherm, and sustained glacier shrinkage. These changes have occurred in conjunction with a steadily growing demand for water resources. Given the short span of instrumental hydroclimatic records in this region, longer time span records are needed to understand the nature of climate variability and to improve the predictability of precipitation, a key factor modulating the socio-economic development in the South American Altiplano and adjacent arid lowlands. In this study we present the first quasi-millennial, tree-ring based precipitation reconstruction for the South American Altiplano. This annual (November–October) precipitation reconstruction is based on the Polylepis tarapacana tree-ring width series and represents the closest dendroclimatological record to the Equator in South America. This high-resolution reconstruction covers the past 707 yr and provides a unique record characterizing the occurrence of extreme events and consistent oscillations in precipitation. It also allows an assessment of the spatial and temporal stabilities of the teleconnections between rainfall in the Altiplano and hemispheric forcings such as El Niño-Southern Oscillation. Since the 1930s to present, a persistent negative trend in precipitation has been recorded in the reconstruction, with the three driest years since 1300 AD occurring in the last 70 yr. Throughout the 707 yr, the reconstruction contains a clear ENSO-like pattern at interannual to multidecadal time scales, which determines inter-hemispheric linkages between our reconstruction and other precipitation sensitive records modulated by ENSO in North America. Our reconstruction points out that century-scale dry periods are a recurrent feature in the Altiplano climate, and that the future potential coupling of natural and anthropogenic-induced droughts may have a severe impact on socio-economic activities in the region. Water resource managers must anticipate these changes in order to adapt to future climate change, reduce vulnerability and provide water equitably to all users.

Regional Differences in South American Monsoon Precipitation Inferred from the Growth and Isotopic Composition of Tropical Trees*

Abstract The authors present results on the relationship between tree-ring proxies and regional precipitation for several sites in tropical South America. The responsiveness of oxygen isotopes (δ18O) and seasonal growth as precipitation proxies was first validated by high-resolution sampling of a Tachigali myrmecophila from Manaus, Brazil (3.1°S, 60.0°W). Monthly growth of Tachigali spp. was significantly correlated with monthly precipitation. Intra-annual measurements of cellulose δ18O in Tachigali spp. were also significantly correlated with monthly precipitation at a lag of approximately one month. The annual ring widths of two tropical tree taxa, Cedrela odorata growing in the Amazon (12.6°S, 69.2°W) and Polylepis tarapacana growing in the Altiplano (22.0°S, 66.0°W), were validated using bomb-derived radiocarbon 14C. Estimated dates were within two to three years of bomb-inferred 14C dates, indicating that these species exhibit annual rings but uncertainties in our chronologies remain. A multiproxy record spanning 180 years from Cedrela spp. showed a significant negative relationship between cellulose δ18O and January precipitation. A 150-yr record obtained from Polylepis spp. also showed a significant negative relationship between δ18O and March precipitation, whereas annual ring width showed a significant positive correlation with December precipitation. These proxies were combined in a multivariate framework to reconstruct past precipitation, revealing a significant increase in monsoon precipitation at the Amazon site since 1890 and a significant decrease in monsoon precipitation at the Altiplano since 1880. Proxy time series also showed spatial and temporal coherence with precipitation variability due to El Niño forcing, suggesting that oxygen isotopes and ring widths in tropical trees may be important diagnostics for identifying regional differences in the response of the tropical hydrologic cycle to anthropogenic warming.

Morphological and ecophysiological traits shaping altitudinal distribution of three Polylepis treeline species in the dry tropical Andes

Numerous species of the genus Polylepis form the highest treeline in the world, with striking dissimilarities in their upper altitudinal limits. The commonly accepted hypothesis is that growth at a treeline is limited by temperature. Here, using in situ records of various morphological and ecophysiological traits, we aimed to identify other factors influencing altitudinal distribution of three congeneric species from the dry tropical Andes: Polylepis rugulosa, Polylepis tarapacana and Polylepis tomentella. While P. tarapacana and P. tomentella reach their altitudinal limit at around 5000 m asl, P. rugulosa does not thrive above 4300 m, but precipitation is markedly lower in its distribution area. The three species responded to altitude by a change of morphological (e.g. decreased tree height and leaf size) and ecophysiological (e.g. decrease of transpiration rate, nutrient concentration or enrichment in the 13C isotope) traits, and this response was generally more pronounced in P. rugulosa. In comparison with P. tarapacana and P. tomentella, P. rugulosa displayed higher transpiration rates. Waxes from the abaxial (stomatous) leaf side of P. rugulosa were most strongly enriched in 13C. Furthermore, leaves of all species studied here had exceptionally low N and P concentrations. Trade-offs linked to changes in leaf area (e.g. bigger leaves, higher photosynthetic capacity but elevated transpiration) seem to drive differentiation and adaptations to altitude among these three congeneric species. We hypothesize that, while the upper distribution limit of P. tarapacana and P. tomentella is largely driven by low temperature, water is an important additional factor controlling the altitudinal distribution of P. rugulosa. Our results suggest that water stress needs to be taken into account among the factors shaping the altitudinal distribution of tropical treeline species.

Dendroclimatological reconstructions in South America: A review

Recent years have seen a consolidation and expansion of tree-ring sample collection across South America. Most collections are concentrated in the temperate forests along the eastern and western slopes of the Southern Andes (32°S to 55°S). However, important advances in the reconnaissance and collection of new woody species useful for dendrochronology have recently been documented in new regions. The development of chronologies in tropical and subtropical arid regions of the Cordillera, and in particular the Bolivian Altiplano, is probably one of the most important recent advances in South American dendrochronology. Polylepis tarapacana, growing at 4000–4500 m elevation on the Altiplano, has yielded more than ten chronologies spanning the past 700 years. These records are highly correlated with summer variations in climate. The development of chronologies in the humid subtropics and tropics remains a major challenge. The number of tree-ring chronologies built up using species from these regions (ca. 40) is comparatively low in relation to the extent of tropical forests. The recognition of strong climate signals in tree rings from Cedrela species provides a unique opportunity to develop a tree-ring network in subtropical and tropical South America. The future of dendroclimatology in South American tropical regions is perceived as extremely promising. Reconstructions of temperature, rainfall, streamflow, snow and regional atmospheric circulation based on ring width, density and stable isotopes, have been conducted using tree-ring chronologies from subtropical and temperate forests. These chronologies have also been used in studies relating South American tree rings to high-resolution proxies from other continents, and studies analyzing past changes in atmospheric circulation. The comparison of climatic reconstructions based on tree rings with projected atmospheric circulation patterns provides a useful bridge between past and future trends in global climate change, and its implications for human welfare and socio-economic development. Some examples of this bridging are presented in this review. Future research should continue the development of long tree-ring chronologies to improve detection of decadal to centennial climatic variations and to distinguish between natural and human-induced climatic changes in South America. Efforts to develop new tree-ring chronologies in the tropical lowlands should also be encouraged. Collaboration among South American countries in training young scholars is crucial to maintain and increase the progress of dendroclimatology in the region. Initiatives facilitating the interaction between scientists from the Americas and overseas, such as done by the IAI and PAGES projects, should be broadened and their long-term continuation assured.

Allometry and effects of extreme elevation on growth velocity of the Andean tree Polylepis tarapacana Philippi (Rosaceae)

Increasing elevation can restrict the expansion of tree species by negatively affecting growth. In this study, we estimated growth velocity and evaluated the effects of extreme elevation on radial growth of the Andean tree Polylepis tarapacana Philippi (Rosaceae). We also developed different models to predict the relationship between age (number of tree-rings) and allometric characters (total height and diameter). Samples of trees and shrubs were collected between 4,200 and 4,600 m asl and analyzed using standard dendrochronological methods. Results evidenced that P. tarapacana is a slow-growing species since it grows less than 5 mm per year. Furthermore, elevation has a negative effect on radial growth. Morphometric models showed that a positive relationship between total height and diameter exists but not a significant relation between number of tree-rings and morphometry. Elevation had a major role in the radial growth of P. tarapacana which interacts with microclimatic conditions. Careful considerations should be made at the moment of using allometry as a surrogate for age in ecological studies.

El Niño-Southern Oscillation signal in the world's highest-elevation tree-ring chronologies from the Altiplano, Central Andes

El Niño-Southern Oscillation (ENSO) is the largest source of inter-annual variability operating in the earth's climate system with enormous ecological, social, and economic impacts. As the instrumental record of ENSO variability is limited to the past 150 yr, the search for proxies sensitive to ENSO has become a priority task in recent years. It is vital to expand the sparse network of annually resolved and exactly dated ENSO-sensitive proxies to entirely capture the spatial variability of the ENSO coupled ocean-atmospheric phenomenon. Several tree-ring records have been used to reconstruct past ENSO variability, however, none of them are from tropical South America. The Polylepis tarapacana woodlands form the world's highest elevation treeline. Trees grow on the Altiplano plateau between 4000–5200 m elevation (16°–23° S). The climate variability in this climatically transitional tropical–subtropical region is strongly modulated by ENSO. Two chronologies of P. tarapacana along the Western Cordillera in the tropical central Andes were analyzed to determine the strength of the ENSO signal present in these tree-ring records. The growth of P. tarapacana has a strong common signal amongst trees in a single site and between sites across the Altiplano. Ring formation is induced by climatic conditions during the previous and current growing season (December–February), at the time of the strongest ENSO influences on the Peruvian Coast. Tree growth variations show opposite relationships with climate between consecutive growing seasons. During the current growing season, ring-width variations show positive and negative relationships with temperature and precipitation, respectively. Both tree-ring chronologies are significantly correlated with austral spring–summer (August–February) SST in the Niño3.4 region and show oscillatory modes within the classical ENSO bandwidth. Our results support the idea that P. tarapacana chronologies from the south-central tropical Andes provide high-resolution records extremely sensitive to ENSO in the tropical Pacific, and represent an important component to be considered in future multiproxy ENSO reconstructions.

Spatio-temporal variations in Polylepis tarapacana radial growth across the Bolivian Altiplano during the 20th century

We document the dendroclimatological potential of Polylepis tarapacana to estimate past temporal and spatial variations in precipitation across the Bolivian Altiplano (17–23°S). The P. tarapacana chronologies, which presently range between 110 and 705 years in length, represent the highest altitude tree-ring records worldwide. Interannual variations in fourteen tree-ring chronologies located between 3900 and 4850 m elevation were compared using correlation and principal component analyses. There is a strong common signal among the chronologies. For the common interval AD 1890–1999, three dominant patterns of tree growth across the Bolivian Altiplano were identified. The first pattern which explains 52% of the total variance in tree growth is associated with years of above- or below-average tree growth across the whole region. The second and third patterns, which are related to significantly lower common variance (13 and 8%, respectively), are associated with contrasting positive and negative anomalies of growth between the northern and southern sectors, and between the central-western sector and the rest of the region, respectively. In order to determine the climatic significance of the spatial variations of P. tarapacana growth, the interannual patterns in tree growth were compared with spatial anomalies in precipitation across the region. Uniform patterns of above (below) average tree growth are associated with positive (negative) precipitation anomalies in the preceding year that are uniformly distributed across the whole Altiplano. In the westernmost areas of the Altiplano, summer rainfall accounts for more than 80% of total annual precipitation. Therefore, relationships between tree growth and summer or annual precipitation are, in most cases, similar. Contrasting patterns of tree growth are related to opposite anomalies in precipitation during the preceding year in the different sub-regions in the Altiplano (north versus south, central versus north and south). Climatic preconditioning of tree growth was also identified for individual years. For instance, extremely reduced growth in the year 1967 reflects dry conditions in the two preceding years (1965–1966). Long-term intervals (3 or more years) with reduced tree growth resulted from several consecutive years with low precipitation. Our results indicate that the network of tree-ring chronologies of P. tarapacana in the Bolivian Altiplano has the potential to provide annually-resolved precipitation reconstructions in this region for the past 5–7 centuries. This is a valuable source to document decadal-to-century-scale precipitation variability in the Altiplano and its relation to large-scale ocean–atmosphere circulation features.

Altitudinal and seasonal variation of protective and photosynthetic pigments in leaves of the world's highest elevation trees Polylepis tarapacana (Rosaceae)

Abstract In the present study we analyzed both photosynthetic and UV-B absorbing pigment variations in Polylepis tarapacana (Rosaceae) during two contrasting seasons (winter and summer) at two altitudes (4300 and 5000 m a.s.l.) in the Sajama volcano (Oruro, Bolivia). UV-B absorbing pigments, chlorophyll a, b and carotenoids contents as well as the chlorophyll a to chlorophyll b ratio showed clear differences in both seasons. UV-B absorbing pigments, total chlorophyll and chlorophyll a contents as well as chlorophyll a/b values were higher in winter than in summer at both altitudes, while chlorophyll b content and the total chlorophyll to carotenoids ratio showed an opposite pattern. Carotenoids content exhibited not only an increase with altitude but also a significant variation between seasons, the highest value being recorded in winter. We discuss the results in relation to the radiation regime of the Sajama region rather than to the water status of the plant studied, since the latter was similar for both seasons and altitudes.

The Sajama National Park in Bolivia

Sajama National Park in Bolivia's Cordillera Occidental is not only the country's oldest national park; it also contains Bolivia's highest mountain, ice-capped Nevado Sajama, at 6542 m. Created in 1939 by presidential decree with the primary objective of protecting native keñua (Polylepis tarapacana) forests, the Sajama National Park, later ratified by national law, had neither formal administration nor park rangers until 1995.Towering over the barren lands of the Altiplano—Bolivia's highland plateau, at an altitude of about 4000 m—Mount Sajama plays a key role in traditional indigenous mythology, as well as in the formation of the present-day identity of local communities. The latter factor is largely due to a new policy and to multiple activities launched by the Bolivian national parks authority (Servicio Nacional de Áreas Protegidas, SERNAP) during the past decade. As an initial result, local perception of Sajama National Park as an “imposition from above” has given way to referring to the park as “our protected area.”

Growth, demography and carbon relations of Polylepis trees at the world's highest treeline

Summary Growth, reproductive success and non‐structural carbon pools in Polylepis tarapacana Philippi trees were examined across a transect between 4360 and 4810 m altitude on Nevado Sajama, Bolivia. The mean −10‐cm soil temperature of 5·4 °C under trees at the treeline during the 265‐day growing season matched the threshold temperature found at other subtropical and tropical treelines. Beyond 4400 m Polylepis is restricted to the warmer and drier equator‐facing slopes, suggesting a direct thermal limitation of tree growth. Maximum tree height, annual shoot increment and mean tree‐ring width decreased with altitude. Trees near the upper range limit reached a maximum tree height of 3·3 m and a maximum stem diameter of 34 cm. The smallest tree‐height classes dominated populations at all altitudes, and the uppermost site revealed the highest proportion of seedlings. Tree‐size demography indicates a critical phase for tree establishment during the sapling stage, when trees emerge from sheltered niches near the ground. No evidence of a depletion of mobile C stores (sugars, starch and lipids) was found in any tissue type with increasing elevation, suggesting a limitation of C investment (growth) rather than C acquisition (photosynthesis) at treeline.

The Importance of Nurse Associations for Three Tropical Alpine Life Forms

We investigated biotic and abiotic associations for four growth forms in Chile’s Parque Nacional Lauca, a tropical alpine puna ecosystem. We determined the biotic associations between Parastrephia lucida (Meyen) Cabr. [Asteraceae] and Festuca orthophylla Pilger. [Poaceae]. To determine if F. orthophylla was acting as a nurse plant for P. lucida, we used chi-square analysis to test for nurse plant effects. Our results indicated that F. orthophylla roots more often on bare ground and that P. lucida grows more often in association with F. orthophylla than would be expected. In testing for abiotic associations, we observed that both a tree, Polylepis tarapacana [Rosaceae], and a cactus, Tephrocactus ignescens [Cactaceae], showed positive abiotic associations with large boulders. These studies indicate that in an extreme environment, such as the South American puna, abiotic and biotic associations are important for plant survival.

Tree rings as a surrogate for economic stress – an example from the Puna of Jujuy, Argentina in the 19th century

This paper illustrates the use of tree-ring indices as an indicator of economic stress. In the 19th century the rural economy in the Puna of Jujuy, Northwestern Argentina was based on livestock (llamas and sheep), and was heavily dependant on the availability of pasture and water in this arid landscape. In the absence of traditional indicators of socioeconomic stress, we used tree-ring indices from moisture sensitive trees ( Juglans australis Griseb. and Polylepis tarapacana Phil.) as a proxy for droughts and their related economic stresses. Examining the relationship between population mortality and ring-width indices allows inferences to be drawn about the relative importance of environmental and sociopolitical factors in determining mortality patterns that contributed to the depopulation of this area in the late 19th century.

RAINFALL-CONTROLLED TREE GROWTH IN HIGH-ELEVATION SUBTROPICAL TREELINES

It is generally assumed that tree growth in the upper limit of a forest is mainly controlled by summer temperature. This general statement is mostly based on studies from extra-tropical mountains and has been rarely evaluated in subtropical latitudes frequently characterized by drier climates. In the subtropical mountains from Northwestern Argentina (∼23° S), annual precipitation decreases with elevation from >1500 mm at 1200– 1500 m, to <200 mm above 4000 m. In consequence, tree growth at high elevations in the region may be seriously limited by water supply. In order to assess the influence of precipitation on tree growth, we evaluated the relationships between climatic variations and radial growth in four species growing at different altitudinal zones: Juglans australis from the montane cloud forest at 1800 m; Alnus acuminata from the montane savanna-like woodland at 2700 m; Prosopis ferox from the subalpine dryland at 3500 m; and Polylepis tarapacana from the high-elevation alpine dryland at 4750 m. D...

Leaf Gas Exchange and Water Relations in Polylepis tarapacana at Extreme Altitudes in the Bolivian Andes

Stress-induced restrictions to carbon balance, growth, and reproduction are the causes of tree-line formation at a global scale. We studied gas exchange and water relations of Polylepis tarapacana in the field, considering the possible effects of water stress limitations imposed on net photosynthetic rate (PN). Daily courses of microclimatic variables, gas exchange, and leaf water potential were measured in both dry-cold and wet-warm seasons at an altitude of 4 300 m. Marked differences in environmental conditions between seasons resulted in differences for the dry-cold and wet-warm seasons in mean leaf water potentials (-1.67 and -1.02 MPa, respectively) and mean leaf conductances (33.5 and 58.9 mmol m-2 s-1, respectively), while differences in mean PN (2.5 and 2.8 μmol m-2 s-1, respectively) were not as evident. This may be related to limitations imposed by water deficit and lower photon flux densities during dry and wet seasons, respectively. Hence P. tarapacana has coupled its gas exchange characteristics to the extreme daily and seasonal variations in temperature and water availability of high elevations.

Low‐temperature resistance in Polylepis tarapacana, a tree growing at the highest altitudes in the world

ABSTRACTThe Polylepis tarapacana forests found in Bolivia are unique with respect to their altitudinal distribution (4200–5200 m). Given the extreme environmental conditions that characterize these altitudes, this species has to rely on distinct mechanisms to survive stressful temperatures. The purpose of this study was to determine low‐temperature resistance mechanisms in P. tarapacana. Tissue was sampled for carbohydrate and proline contents and micro‐climatic measurements were made at two altitudes, 4300 and 4850 m, during both the dry cold and wet warm seasons. Supercooling capacity (−3 to −6 °C for the cold dry and −7 to −9 °C for the wet warm season) and injury temperatures (−18 to −23 °C for both seasons), determined in the laboratory, indicate that P. tarapacana is a frost‐tolerant species. On the other hand, an increase in supercooling capacity, as the result of significant increase in total soluble sugar and proline contents, occurs during the wet warm season as a consequence of higher metabolic activity. Hence, P. tarapacana, a frost‐tolerant species during the colder unfavourable season, is able to avoid freezing during the more favourable season when minimum night‐time temperatures are not as extreme.

Silvopastoral systems in arid and semiarid zones of northern Chile

The main silvopastoral systems carried out in arid and semiarid zones of northern Chile are presented. Plantations with quenoa (Polylepis besseri) in the Precordillera (3,000–4,000 meters above sea level) and with ‘quenoa de altura’ (Polylepis tarapacana) in the High Plateau (over 4,000 m.a.s.l.) are described. For extremely arid zones (2 mm/yr), tamarugo (Prosopis tamarugo) and algarrobo (Prosopis chilensis) plantations in Pampa del Tamarugal are reviewed. In this zone 20,483 ha have been planted, 88% with P. tamarugo. These plantations feed a variable herd of 7,000–9,000 sheep and goats per yr. The forage produced per tree varies from 20 to 70 kg/yr for 14 to 22 yr old trees, planted in densities of 100/ha. For the semiarid zone, plantations with forage shrubs of the genus Atriplex are described. Between 1975 and 1989, about 37,000 ha were planted in the coastal zone of Coquimbo region, 60% of them with A. nummularia and the rest with A. repanda. Productivity varies widely in both species. It depends on management techniques and environmental variables. Afforestation with such species offers green forage during periods of scarcity of natural grasses, especially during long droughts. The rate of afforestation is declining because the government has stopped planting, and the private sector is planting less than expected. This is because often yields have been low, due to either natural causes or deficient management.