Ricardo Villalba

Disturbance regime and disturbance interactions in a Rocky Mountain subalpine forest

The spatial and temporal patterns of fire, snow avalanches and spruce beetle outbreaks were investigated in Marvine Lakes Valley in the Colorado Rocky Mountains in forests of Picea engelmannii, Abies lasiocarpa, Pseudotsuga menziesii and Populus tremuloides. Dates and locations of disturbances were determined by dendrochronological techniques. A geographic information system (GIS) was used to calculate areas affected by the different disturbance agents and to examine the spatial relationships of the different disturbances. In the Marvine Lakes Valley, major disturbance was caused by fire in the 1470s, the 1630s and the 1870s and by spruce beetle outbreak in c. 1716, 1827 and 1949 (...)

FIRE HISTORY IN NORTHERN PATAGONIA: THE ROLES OF HUMANS AND CLIMATIC VARIATION

The effects of humans and climatic variation on fire history in northern Patagonia, Argentina, were examined by dating fire scars on 458 trees at 21 sites in rain forests of Fitzroya cupressoides and xeric woodlands of Austrocedrus chilensis from 39° to 43° S latitude. Climatic variation associated with fires was analyzed on the basis of 20th-century observational records and tree ring proxy records of climatic variation since approximately AD 1500. In the Austrocedrus woodlands, fire frequency increases after about 1850, coincident with greater use of the area by Native American hunters. Increased burning, particularly in the zone of more mesic forests, is also strongly associated with forest clearing by European settlers from about 1880 to the early 1900s. The marked decline in fire frequency during the 20th century coincides with both the demise of Native American hunters in the 1890s and increasingly effective fire exclusion. Strong synchroneity in the years of widespread fire at sample sites disperse...

A 3620-year temperature record from Fitzroya cupressoides tree rings in southern South America.

A tree-ring width chronology of alerce trees (Fitzroya cupressoides) from southern Chile was used to produce an annually resolved 3622-year reconstruction of departures from mean summer temperatures (December to March) for southern South America. The longest interval with above-average temperatures was from 80 B.C. to A.D. 160. Long intervals with below-average temperatures were recorded from A.D. 300 to 470 and from A.D. 1490 to 1700. Neither this proxy temperature record nor instrumental data for southern South America for latitudes between 35� and 44�S provide evidence of a warming trend during the last decades of this century that could be related to anthropogenic causes. The data also indicate that alerce is the second longest living tree after the bristlecone pine (Pinus Iongaeva).

Climatic fluctuations in northern Patagonia during the last 1000 years as inferred from tree-ring records

Abstract Millennium-old alerce trees ( Fitzroya cupressoides (Mol.) Johnst.) have been used to develop a 1120-year reconstruction of the summer temperature departures for the Andes of northern Patagonia in Argentina. Four main climatic episodes can be distinguished in this proxy paleoclimatic record. The first, a cold and moist interval from A.D. 900 to 1070, was followed by a warm-dry period from A.D. 1080 to 1250 correlative with the Medieval warm epoch of Europe. Afterward, a long, cold-moist period followed from A.D. 1270 to 1670, peaking around A.D. 1340 and 1650. These cold maxima are contemporaneous with two principal Little Ice Age events registered in the Northern Hemisphere. Warmer conditions then resumed between A.D. 1720 and 1790. These episodes are supported by glaciological and palynological data in Patagonia. Following a cold period in the early 1800s, tree-ring indices have oscillated around the long-term mean, except for a warmer period from A.D. 1850 to 1890. Correlations between the Rio Alerce reconstruction and the regional weather stations indicate that the tree-ring variations are correlated with a homogeneous summer weather pattern covering Patagonia east of the Andes from 38° to 50°S.

Tree-Ring and Glacial Evidence for the Medieval Warm Epoch and the Little Ice Age in Southern South America

A tree-ring reconstruction of summer temperatures from northern Patagonia shows distinct episodes of higher and lower temperature during the last 1000 yr. The first cold interval was from A.D. 900 to 1070, which was followed by a warm period A.D. 1080 to 1250 (approximately coincident with theMedieval Warm Epoch). Afterwards a long, cold-moist interval followed from A.D. 1270 to 1660, peaking around 1340 and 1640 (contemporaneously with earlyLittle Ice Age events in the Northern Hemisphere). In central Chile, winter rainfall variations were reconstructed using tree rings back to the year A.D. 1220. From A.D. 1220 to 1280, and from A.D. 1450 to 1550, rainfall was above the long-term mean. Droughts apparently occurred between A.D. 1280 and 1450, from 1570 to 1650, and from 1770 to 1820. In northern Patagonia, radiocarbon dates and tree-ring dates record two major glacial advances in the A.D. 1270–1380 and 1520–1670 intervals. In southern Patagonia, the initiation of theLittle Ice Age appears to have been around A.D. 1300, and the culmination of glacial advances between the late 17th to the early 19th centuries.

Snowpack Variations in the Central Andes of Argentina and Chile, 1951–2005: Large-Scale Atmospheric Influences and Implications for Water Resources in the Region

Abstract The snowpack in the central Andes (30°–37°S) is the primary source for streamflow in central Chile and central-western Argentina, but few published studies are available on snowpack variability in the region. This paper develops the first regional snowpack series (1951–2005) from Chilean and Argentinean snow course records. This series shows a strong regional signal, marked interannual variability, and a positive, though nonsignificant, linear trend. Correlations with local precipitation and temperature records reveal a marked association with conditions in central Chile. High snow accumulation is generally concurrent with El Nino events in the tropical Pacific, but only 5 of the 10 driest years coincided with La Nina events. Evaluation of 500-hPa geopotential height anomaly maps during extreme snow years highlights the crucial significance of tropospheric conditions in the subtropical and southeast Pacific in modulating snowfall. Correlations with gridded SST and SLP data and multiple regression...

INFLUENCES OF LARGE-SCALE CLIMATIC VARIABILITY ON EPISODIC TREE MORTALITY IN NORTHERN PATAGONIA

In the context of potential global warming, it is critical that ecologists bridge the typically local spatial scale of ecology to the regional scale of climatology by linking ecosystem responses to variations in the large-scale synoptic controls of regional climates. In northern Patagonia, Argentina, we related regional-scale tree mortality events over the past -100 years to annual and decadal-scale climatic variations associated with changes in the major synoptic climatic controls of the southeastern Pacific region, including the El Nifio-Southern Oscillation (ENSO). In nine stands of Austrocedrus chilensis, a xeric conifer, we used dendrochronological techniques to date the outermost tree ring on dead-standing and fallen trees to estimate the dates of tree death for 336 trees. To evaluate climatic conditions during periods of high tree mortality, we used regional records of precipitation and temperature from six climate stations and also used a regional set of 24 tree ring chronologies from Austrocedrus. Good preservation of the resinous wood of Austrocedrus allowed relatively precise dating of tree deaths over the past -90 years. Episodes of massive tree mortality coincide with exceptionally dry springs and summers during the 1910s, 1942-1943, and the 1950s. Although there is a general regional synchroneity of tree death associated with drought, intra-regional variations in the intensity of droughts, as interpolated and mapped from the regional network of tree ring chronologies, are also reflected by north-to-south variations in tree mortality patterns. Periods of drought and associated tree mortality during the 20th century in northern Patagonia are strongly associated with above average sea level atmospheric pressure off the coast of Chile at the same latitudes. Temperature and precipitation in northern Patagonia are highly influenced by the intensity and latitudinal position of the southeastern Pacific anticyclone, which, in turn, are greatly affected by ENSO. Tree mortality in northern Patagonia appears to be intensified by extreme events of the Southern Oscillation and is more strongly coincident with El Nifio events along the coast of northern Peru. These results, in combination with previously established climatic influences on fire occurrence and tree seedling establishment, strongly link stand-level and regional-scale forest dynamic processes in northern Patagonia with variations in large-scale atmospheric conditions.

Recent Trends in Tree-Ring Records from High Elevation Sites in the Andes of Northern Patagonia

A new set of tree-ring records from the Andes of northern Patagonia, Argentina (41° S) was used to evaluate recent (i.e., last 250 years) regional trends in tree growth at upper treeline. Fifteen tree-ring chronologies from 1200 to 1750 m elevation were developed for Nothofagus pumilio, the dominant subalpine species. Samples were collected along three elevational transects located along the steep west-to-east precipitation gradient from the main Cordillera (mean annual precipitation >4000 mm) to an eastern outlier of the Andes (mean annual precipitation >2000 mm). Ring-width variation in higher elevation tree-ring records from the main Cordillera is mainly related to changes in temperature and precipitation during spring and summer. However, the response to climatic variation is also influenced by local site factors of elevation and exposure. Based on the relationships between Nothofagus growth and climate, we reconstructed changes in snow cover duration in late spring and variations in mean annual temperature since A.D. 1750. Abrupt interannual changes in the mean annual temperature reconstruction are associated with strong to very strong El Niño-Southern Oscillation events. At upper treeline, tree growth since 1977 has been anomalously high. A sharp rise in global average tropospheric temperatures has been recorded since the mid-1970s in response to an enhanced tropical hydrologic cycle due to an increase in temperature of the tropical Pacific. Temperatures in northern Patagonia have been anomalously high throughout the 1980s, which is consistent with positive temperature anomalies in the tropical Pacific and along the western coast of the Americas at c.a. 40° S latitude. Our 250-year temperature reconstruction indicates that although the persistently high temperatures of the 1980s are uncommon during this period, they are not unprecedented. Tropical climatic episodes similar to that observed during the 1980s may have occurred in the recent past under pre-industrial carbon dioxide levels.

REGIONAL PATTERNS OF TREE POPULATION AGE STRUCTURES IN NORTHERN PATAGONIA : CLIMATIC AND DISTURBANCE INFLUENCES

1 Variations in regional patterns of age structures of the conifer Austrocedrus chilensis near the forest/steppe ecotone of northern Patagonia, Argentina were investigated in relation to climatic variation and changes in disturbance regimes. Climatic variation was derived from the instrumental record beginning in c. 1910 and was also inferred from variations in ring widths of Austrocedrus over the past 200 years. Variations in regional patterns of fire were derived from prior studies based on dendrochronological evidence of fire. 2 Most trees that established since c. 1960 could be dated to an annual resolution. Almost no Austrocedrus establishment has occurred since the warm-dry period began in c. 1980. In contrast, many trees established during the cool-wet conditions between 1963 and 1979 with peaks in numbers of surviving trees corresponding to particularly cool-wet conditions in 1964-66 and 1973-75. 3 Throughout the twentieth century, relatively few Austrocedrus survive from periods of extreme and persistent (> 5 years) drought. In contrast, short droughts (1-2 years) are not reflected in age structure patterns. Periods of reduced radial growth reflect spring-summer moisture deficits and coincide with approximately decade-long periods of fewer surviving trees. 4 Prior to c. 1900, relationships of age structure to variation in radial growth are less clear due to the confounding influences of increasingly inaccurate age determinations of older trees, and the cumulative effects of past changes in disturbance regimes. 5 Variations in both climate and disturbance regimes influence the regional pattern of age structures of Austrocedrus. For example, near the ecotone the frequency of surface fires has declined drastically during the present century as has the severity of browsing by livestock during the past 40 years. The reduction in these disturbances was probably a necessary condition for the increase in Austrocedrus establishment during the 1963-79 cool-wet interval.

Climatic Influences on the Growth of Subalpine Trees in the Colorado Front Range

We examined variations in tree growth responses to climatic variations among different tree species and habitat types in the subalpine zone of the Colorado Front Range. We constructed 25 tree ring site chronologies (11 of Picea engelmannii, 9 of Abies lasiocarpa, 4 of Pinus contorta var. tatifolia, and 1 of Pinusflexilis) from a series of subalpine habitats ranging from xeric to wet. To establish tree growth responses to climatic variation, we used correlation and response function analyses to compare variations in ring widths with monthly temperature and precipitation records. At the driest sites, growth of Picea and Abies tracked climatic variation similarly. At mesic and wet sites, however, these species differed in their responses to climatic variation. The responses of Pinus contorta, sampled over a narrower range of habitat types, differed from those of Picea and Abies but did not differ among sites. Steep environmental gradients in the subalpine zone of the Front Range accounted for most of the observed differences in growth responses to climatic variation. Even at adjacent sites that differ only slightly in topographic position, tree growth responses to climatic variation were distinct. Interspecific differences in response to climatic variations generally were less important than site differences. Intersite differences in tree growth responses to climatic variation can be used as indicators of environmental differ- ences among subalpine habitats.