Neil Pederson

Hydrometeorological Reconstructions for Northeastern Mongolia Derived from Tree Rings: 1651–1995*

Reconstructions of annual (prior August‐current July) precipitation and streamflow, 345 yr in length (1651‐ 1995), are presented for northeastern Mongolia based on tree-ring width data. These precipitation and streamflow reconstructions account for 54% and 48% of the respective variance in instrumental data over the past 50 years. Variations in instrumental precipitation and streamflow are within the range of those reconstructed over the length of tree-ring records. However, there appear to be more frequent extended wet periods during the twentieth century. Multitaper spectral analysis revealed statistically significant peaks at 10.8 and 12.8 yr for the precipitation reconstruction, and at 12.8 and 20.3‐23.8 yr for the streamflow reconstruction. Similarly, singular spectrum analysis identified spectral modes of variation at 12 and 21 yr for both series. These spectra resemble those found for tree-ring-based precipitation reconstructions in central China as well as the western United States, and may reflect solar influences on the climate of Mongolia.

1738 years of Mongolian temperature variability inferred from a tree‐ring width chronology of Siberian pine

1738 years (AD 262–1999) of temperature variability are inferred from tree-ring widths of Siberian pine at Solongotyn Davaa (Sol Dav), a timberline (2420 m) site in Mongolia. This chronology can account for 33% of the temperature variance from 1882–1993. The warmest conditions over the past millennium are during the 20th century. The 1999 ring width has the highest index value over the past millennium. Both warmer and colder intervals are inferred during the “Medieval Warm Epoch”. The most severe cold occurred in the 19th century. Unusual cold and frost in AD 536–545 coincide with extremes in other proxies and historical accounts, confirming a widespread, catastrophic event. Trends resemble those of other Eurasian paleoseries, and hemispheric-scale reconstructions over the past millennium. More chronologies such as Sol Dav are essential to improve coverage in the uncertain earlier centuries of these reconstructions and their estimates of natural variability relative to recent anthropogenic change.

The impacts of increasing drought on forest dynamics, structure, and biodiversity in the United States

We synthesize insights from current understanding of drought impacts at stand-to-biogeographic scales, including management options, and we identify challenges to be addressed with new research. Large stand-level shifts underway in western forests already are showing the importance of interactions involving drought, insects, and fire. Diebacks, changes in composition and structure, and shifting range limits are widely observed. In the eastern US, the effects of increasing drought are becoming better understood at the level of individual trees, but this knowledge cannot yet be confidently translated to predictions of changing structure and diversity of forest stands. While eastern forests have not experienced the types of changes seen in western forests in recent decades, they too are vulnerable to drought and could experience significant changes with increased severity, frequency, or duration in drought. Throughout the continental United States, the combination of projected large climate-induced shifts in suitable habitat from modeling studies and limited potential for the rapid migration of tree populations suggests that changing tree and forest biogeography could substantially lag habitat shifts already underway. Forest management practices can partially ameliorate drought impacts through reductions in stand density, selection of drought-tolerant species and genotypes, artificial regeneration, and the development of multistructured stands. However, silvicultural treatments also could exacerbate drought impacts unless implemented with careful attention to site and stand characteristics. Gaps in our understanding should motivate new research on the effects of interactions involving climate and other species at the stand scale and how interactions and multiple responses are represented in models. This assessment indicates that, without a stronger empirical basis for drought impacts at the stand scale, more complex models may provide limited guidance.

Is the growth of birch at the upper timberline in the Himalayas limited by moisture or by temperature

Birch (Betula) trees and forests are found across much of the temperate and boreal zones of the Northern Hemisphere. Yet, despite being an ecologically significant genus, it is not well studied compared to other genera like Pinus, Picea, Larix, Juniperus, Quercus, or Fagus. In the Himalayas, Himalayan birch (Betula utilis) is a widespread broadleaf timberline species that survives in mountain rain shadows via access to water from snowmelt. Because precipitation in the Nepalese Himalayas decreases with increasing elevation, we hypothesized that the growth of birch at the upper timberlines between 3900 and 4150 m above sea level is primarily limited by moisture availability rather than by low temperature. To examine this assumption, a total of 292 increment cores from 211 birch trees at nine timberline sites were taken for dendroecological analysis. The synchronous occurrence of narrow rings and the high interseries correlations within and among sites evidenced a reliable cross-dating and a common climatic ...

Is an Epic Pluvial Masking the Water Insecurity of the Greater New York City Region

AbstractSix water emergencies have occurred since 1981 for the New York City (NYC) region despite the following: 1) its perhumid climate, 2) substantial conservation of water since 1979, and 3) meteorological data showing little severe or extreme drought since 1970. This study reconstructs 472 years of moisture availability for the NYC watershed to place these emergencies in long-term hydroclimatic context. Using nested reconstruction techniques, 32 tree-ring chronologies comprised of 12 species account for up to 66.2% of the average May–August Palmer drought severity index. Verification statistics indicate good statistical skill from 1531 to 2003. The use of multiple tree species, including rarely used species that can sometimes occur on mesic sites like Liriodendron tulipifera, Betula lenta, and Carya spp., seems to aid reconstruction skill. Importantly, the reconstruction captures pluvial events in the instrumental record nearly as well as drought events and is significantly correlated to precipitation...

Pluvials, droughts, the Mongol Empire, and modern Mongolia

Significance A 1,112-y tree-ring record of moisture shows that in opposition to conventional wisdom, the climate during the rise of the 13th-century Mongol Empire was a period of persistent moisture, unprecedented in the last 1,000 y. This 15-y episode of persistent moisture likely led to a period of high grassland productivity, contributing fuel to the Mongol Empire. We also present evidence that anthropogenic warming exacerbated the 21st-century drought in central Mongolia. These results indicate that ecosystems and societies in semiarid regions can be significantly affected by unusual climatic events at the decadal time scale. Although many studies have associated the demise of complex societies with deteriorating climate, few have investigated the connection between an ameliorating environment, surplus resources, energy, and the rise of empires. The 13th-century Mongol Empire was the largest contiguous land empire in world history. Although drought has been proposed as one factor that spurred these conquests, no high-resolution moisture data are available during the rapid development of the Mongol Empire. Here we present a 1,112-y tree-ring reconstruction of warm-season water balance derived from Siberian pine (Pinus sibirica) trees in central Mongolia. Our reconstruction accounts for 56% of the variability in the regional water balance and is significantly correlated with steppe productivity across central Mongolia. In combination with a gridded temperature reconstruction, our results indicate that the regional climate during the conquests of Chinggis Khan’s (Genghis Khan’s) 13th-century Mongol Empire was warm and persistently wet. This period, characterized by 15 consecutive years of above-average moisture in central Mongolia and coinciding with the rise of Chinggis Khan, is unprecedented over the last 1,112 y. We propose that these climate conditions promoted high grassland productivity and favored the formation of Mongol political and military power. Tree-ring and meteorological data also suggest that the early 21st-century drought in central Mongolia was the hottest drought in the last 1,112 y, consistent with projections of warming over Inner Asia. Future warming may overwhelm increases in precipitation leading to similar heat droughts, with potentially severe consequences for modern Mongolia.

The Legacy of Episodic Climatic Events in Shaping Temperate, Broadleaf Forests

In humid, broadleaf-dominated forests where gap dynamics and partial canopy mortality appears to dominate the disturbance regime at local scales, paleoecological evidence shows alteration at regional-scales associated with climatic change. Yet, little evidence of these broad-scale events exists in extant forests. To evaluate the potential for the occurrence of large-scale disturbance, we used 76 tree-ring collections spanning ∼840 000 km2 and 5327 tree recruitment dates spanning ∼1.4 million km2 across the humid eastern United States. Rotated principal component analysis indicated a common growth pattern of a simultaneous reduction in competition in 22 populations across 61 000 km2. Growth-release analysis of these populations reveals an intense and coherent canopy disturbance from 1775 to 1780, peaking in 1776. The resulting time series of canopy disturbance is so poorly described by a Gaussian distribution that it can be described as “heavy tailed,” with most of the years from 1775 to 1780 comprising th...

Mongolian tree-rings, temperature sensitivity and reconstructions of Northern Hemisphere temperature

Much of northern Asia is lacking in high-resolution palaeoclimatic data coverage. This vast region thus represents a sizeable gap in data sets used to reconstruct hemispheric-scale temperature trends for the past millennium. To improve coverage, we present a regional-scale composite of four tree-ring width records of Siberian pine and Siberian larch from temperature-sensitive alpine timber-line sites in Mongolia. The chrono logies load closely in principal components analysis (PCA) with the first eigenvector accounting for over 53% of the variance from ad 1450 to 1998. The 20-year interval from 1974 to 1993 is the highest such growth period in this composite record, and 17 of the 20 highest growth years have occurred since 1946. Thus these trees, unlike those recently described at some northern sites, do not appear to have lost their temperature sensitivity, and suggest that recent decades have been some of the warmest in the past 500 years for this region. There are, however, comparable periods of inferred, local warmth for individual sites, e.g., in 1520– 1580 and 1760–1790. The percent common variance between chronologies has increased through time and is highest (66.1%) in the present century. Although there are obvious differences among the individual chrono logies, this result suggests a coherent signal which we consider to be related to temperature. The PCA scores show trends which strongly resemble those seen in recent temperature reconstructions for the Northern Hemi sphere, very few of which included representation from Eurasia east of the Ural Mountains. The Mongolia series therefore provides independent corroboration for these reconstructions and their indications of unusual warming during the twentieth century.

A long-term perspective on a modern drought in the American Southeast

The depth of the 2006‐9 drought in the humid, southeastern US left several metropolitan areas with only a 60‐120 day water supply. To put the region’s recent drought variability in a long-term perspective, a dense and diverse tree-ring network—including the first records throughout the Apalachicola‐Chattahoochee‐Flint river basin—is used to reconstruct drought from 1665 to 2010 CE. The network accounts for up to 58.1% of the annual variance in warm-season drought during the 20th century and captures wet eras during the middle to late 20th century. The reconstruction shows that the recent droughts are not unprecedented over the last 346 years. Indeed, droughts of extended duration occurred more frequently between 1696 and 1820. Our results indicate that the era in which local and state water supply decisions were developed and the period of instrumental data upon which it is based are amongst the wettest since at least 1665. Given continued growth and subsequent industrial, agricultural and metropolitan demand throughout the southeast, insights from paleohydroclimate records suggest that the threat of water-related conflict in the region has potential to grow more intense in the decades to come.

Uncertainty, Emergence, and Statistics in Dendrochronology

Some fundamental concepts of dendrochronological analysis are reviewed in the context of statistically modeling the climatically related environmental signals in cross-dated tree-ring series. Significant uncertainty exists due to our incomplete mechanistic understanding of radial growth of most tree species in the natural world, one where environmental effects are unobserved, uncontrolled, and steadily changing over time. This biological uncertainty cascades into the realm of statistical uncertainty in ways that are difficult to quantify even though the latter may be well constrained by theory. Therefore, great care must be taken to apply the many well-developed and tested statistical methods of dendrochronology in ways that reduce the probability of making false inferences. This is especially true in the case of biological emergence. This is a special case of uncertainty that arises from the way in which trees as complex organisms can have properties expressed in their ring widths that are impossible to predict from a basic understanding of lower-level physiological processes. Statistical modeling must be conducted in ways that allow for the discovery of such phenomena and, at the same time, protect from the incorrect acceptance of spurious emergent properties. To reduce the probability of the latter, we argue that model verification be an important part of any dendrochronological inquiry based on statistics. Correlation and response function analysis is used to illustrate some of the concepts discussed here. The value of empirical signal strength statistics as predictors of climatic signal strength in tree rings is also investigated.