Hemlock Legacy Project (HeLP): A paleoecological requiem for eastern hemlock

Eastern North American forests have effectively lost two major tree species (American chestnut and American elm) in the last 100 years and two more, eastern and Carolina hemlock, will be functionally extinct over much of their ranges within a couple of decades. The loss of eastern hemlock is of particular concern because hemlock is: (1) a foundation species; (2) one of the longest-lived tree species over much of temperate eastern North America; and (3) sensitive to climatic variation and ecosystem disturbance, making it an ideal species for the reconstruction of environmental history. Unlike American chestnut, we have a small window of opportunity to salvage environmental histories from hemlock before they are lost. In this progress report, we review the extensive body of science derived from this paleoenvironmental archive and urge scientists from eastern North America to sample and archive old-growth hemlock while living and dead material remain. Here we describe a community-based approach to salvaging paleoenvironmental archives that could serve as a model for collections from other foundation species currently threatened by exotic forests pests and pathogens (e.g. whitebark pine, ash). The approach supports Schlesinger’s (2010) call for ‘translational ecology’ by building connections between scientists, students, environmental NGOs, and land managers focused on old-growth forests

Temperature and precipitation in Mongolia based on dendroclimatic investigations

Recent tree-ring studies in Mongolia provide evidence of unusual warming that is in agreement with large-scale reconstructed and recorded temperatures for the Northern Hemisphere and the Arctic. One Mongolian proxy record for temperature extends back over 1000 years and several others are over 350 years in length. Precipitation reconstructions based on tree rings reflect recent increases but also indicate that the increases are within the long-term range of variations. Spectral analyses of recorded precipitation data and the reconstructions support the hypotheses of quasi-solar periodicity in precipitation variation, previously suggested by others

Land snail diversity can reflect degrees of anthropogenic disturbance

Faunal indicators of old-growth forests in heavily-disturbed regions are virtually non-existent. However, land snails, in particular micro-snails, could reflect the impact of land-use on ecosystem integrity. Because of their size, limited mobility, and propensity to spend their entire lives at one locality due to lack of migratory behavior, micro-snails are susceptible to changes in land-use within forested ecosystems. Therefore, we proposed the hypothesis that micro-snails would reflect land-use in forested ecosystems. We sampled snail communities in paired old-growth and second-growth forests in three distinct ecoregions. Species diversity, richness, and abundance were greater in two of the three old-growth sites compared to paired second-growth sites. Across all of the ecoregions, 21 out of 70 species had an affinity for old-growth. Eighteen of which were statistically significant. These results suggest that anthropogenic disturbance plays a key role in shaping species diversity and community structure of land snail fauna. However, site specific factors also appear to be important moderators of the response, and the mechanisms of the process remain to be studied. Snails appear to be a promising group of organisms to use as indicators of historic forest disturbance. In order to maintain ecological integrity, forest managers should consider management strategies that are low impact and protect existing patches of old-growth forests

Privatization, Drought, and Fire Exclusion in the Tuul River Watershed, Mongolia

Global wildfire frequency and extent are expected to increase under projected climate change in the twenty-first century, yet little is known about how human activities might affect this trend. In central Mongolia, there has been a 2.5°C rise in spring and summer temperatures during the last 40 years and a decrease in moisture availability during the latter half of the twentieth century. Concurrently, Mongolia has experienced multiple shifts in socioeconomic systems during the twentieth century, most notably the establishment of a Soviet-backed communist economy in the 1920s and a rapid transition to privatization in the 1990s. Observed records of fire in the late twentieth century suggested that fire activity had increased, but no long-term data existed to place these trends in a historical context. Our objective was to identify spatial and temporal patterns in fire occurrence in the forest-steppe ecotone of the Tuul River watershed in the context of changing climatic and social conditions since 1875. We used fire-scarred trees to reconstruct past fire occurrence during the period 1875–2009. Our results indicate a significant association between human activity and fire occurrence independent of climatic variables. The greatest evidence for an anthropogenic fire regime exists following the transition to a free market economy during the early 1990s when land-use intensification near the capital city of Ulaanbaatar resulted in fire exclusion. We emphasize the importance of including socio-political variables in global models of wildfire potential, particularly where fuels limit fire activity

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 signal in the tree-ring width variations. From March to May, all nine tree-ring-width site chronologies showed a strong positive response to total precipitation and a less-strong negative response to temperature. During the instrumental meteorological record (from 1960 to the present), years with a high percentage of locally missing rings coincided with dry and warm pre-monsoon seasons. Moreover, periods of below-average growth are in phase with well-known drought events all over monsoon Asia, showing additional evidence that Himalayan birch growth at the upper timberlines is persistently limited by moisture availability. Our study describes the rare case of a drought-induced alpine timberline that is comprised of a broadleaf tree species

A Tree-Ring-Based Reconstruction of Delaware River Basin Streamflow Using Hierarchical Bayesian Regression

A hierarchical Bayesian regression model is presented for reconstructing the average summer streamflow at five gauges in the Delaware River basin using eight regional tree-ring chronologies. The model provides estimates of the posterior probability distribution of each reconstructed streamflow series considering parameter uncertainty. The vectors of regression coefficients are modeled as draws from a common multivariate normal distribution with unknown parameters estimated as part of the analysis. This leads to a multilevel structure. The covariance structure of the streamflow residuals across sites is explicitly modeled. The resulting partial pooling of information across multiple stations leads to a reduction in parameter uncertainty. The effect of no pooling and full pooling of station information, as end points of the method, is explored. The no-pooling model considers independent estimation of the regression coefficients for each streamflow gauge with respect to each tree-ring chronology. The full-pooling model considers that the same regression coefficients apply across all streamflow sites for a particular tree-ring chronology. The cross-site correlation of residuals is modeled in all cases. Performance on metrics typically used by tree-ring reconstruction experts, such as reduction of error, coefficient of efficiency, and coverage rates under credible intervals is comparable to, or better, for the partial-pooling model relative to the no-pooling model, and streamflow estimation uncertainty is reduced. Long record simulations from reconstructions are used to develop estimates of the probability of duration and severity of droughts in the region. Analysis of monotonic trends in the reconstructed drought events do not reject the null hypothesis of no trend at the 90% significance over 1754–2000

A comparison of times series approaches for dendroecological reconstructions of past canopy disturbance events

Time series analysis can identify outliers in tree-ring widths that may not only indicate past disturbances, but may also estimate the subsequent effects of these disturbances on tree growth. Finding a way to isolate these disturbance signals from tree-ring time series could have broad applications in forest ecology and management. Time series outliers may be expressed as pulse, step, or trend interventions, but few dendroecological studies have explored how well these different types of interventions express the response of tree-ring widths to a canopy disturbance resulting in a release event. This study addresses that question by comparing two different time series approaches for detecting release events: a pulse intervention approach and a new combined step and trend (CST) intervention approach. These methods are tested against tree-ring collections with known historical canopy disturbance events: northern red oaks (Quercus rubra) in New York released by the chestnut blight during the early 1920s, eastern hemlocks (Tsuga canadensis) in Pennsylvania released by adjacent selective logging in 1910, and northern red oaks and chestnut oaks (Quercus montana) in West Virginia released through an experimental thinning in 1982. Clusters of CST interventions, but not pulse interventions, were detected for all three collections during and immediately after the known disturbance events, showing that a CST intervention approach consistently reconstructs these release events. In addition, a CST intervention approach isolated canopy disturbance signals from tree-ring widths as disturbance-growth indices. Detrending disturbances from tree-ring widths provides an alternative approach to reconstruct climate in closed-canopy forests; however, just as importantly, disturbance-growth indices created through this method can also reconstruct changes in tree growth rates, biomass, or carbon resulting from a past disturbance event or forest thinning

Influence of wood harvest on tree-ring time-series of Picea abies in a temperate forest

Tree-ring width data are the prime source of high-resolution climate reconstructions covering recent millennia. Their variations, from year-to-year, are calibrated against regional instrumental data to evaluate the strength of associations with temperature and precipitation records, though the level of variance explained by climatic variables is frequently less than 50%. Among the remaining factors affecting tree growth, the influence of forest management for tree-ring width time-series used to resolve annual climate reconstructions remains relatively unexplored. We here evaluate the impact of conventional single-tree harvesting on tree-ring data using a compilation of circumstantially mapped Picea abies sites in western Germany. Climate signals are explored by calibration against regional temperature and precipitation data, and the influences of forest management analyzed using long-term logging data spanning the past 40–80 years and mapped stumps (and neighboring trees) spanning the past 20 years. Our results indicate a weak but statistically significant control of total May–July precipitation of P. abies growth. This association is only marginally, if at all, affected by forest management, i.e. wider tree-rings due to improved access to light and nutrients in years after logging events are either not found or in line with increased precipitation sums following single-tree harvest. These findings suggest only minor influences of selective harvesting on Picea tree-ring growth in western Germany, and overall limited biases of annually resolved climate reconstructions from lower elevation central European sites due to historical logging events

Pluvials, droughts, the Mongol Empire, and modern Mongolia

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

Comparing Tree‐Ring and Permanent Plot Estimates of Aboveground Net Primary Production in Three Eastern U.S. Forests

Forests account for a large portion of sequestered carbon, much of which is stored as wood in trees. The rate of carbon accumulation in aboveground plant material, or aboveground net primary productivity (aNPP), quantifies annual to decadal variations in forest carbon sequestration. Permanent plots are often used to estimate aNPP but are usually not annually resolved and take many years to develop a long data set. Tree rings are a unique and infrequently used source for measuring aNPP, and benefit from fine spatial (individual trees) and temporal (annual) resolution. Because of this precision, tree rings are complementary to permanent plots and the suite of tools used to study forest productivity. Here we evaluate whether annual estimates of aNPP developed from tree rings approximate estimates derived from colocated permanent plots. We studied a lowland evergreen (Howland, Maine), mixed deciduous (Harvard Forest, Massachusetts), and mixed mesophytic (Fernow, West Virginia) forest in the eastern United States. Permanent plots at the sites cover an area of 2–3 ha, and we use these areas as benchmarks indicative of the forest stand. We simulate random draws of permanent plot subsets to describe the distribution of aNPP estimates given a sampling area size equivalent to the tree-ring plots. Though mean tree-ring aNPP underestimates permanent plot aNPP slightly at Howland and Fernow and overestimates at Harvard Forest when compared with the entire permanent plot, it is within the 95% confidence interval of the random draws of equal-sized sampling area at all sites. To investigate whether tree-ring aNPP can be upscaled to the stand, we conducted a second random draw of permanent plot subsets simulating a twofold increase in sampling area. aNPP estimates from this distribution were not significantly different from results of the initial sampling area, though variance decreased as sampling area approaches stand area. Despite several concerns to consider when using tree rings to reconstruct aNPP (e.g., upscaling, allometric, and sampling uncertainties), the benefits are apparent, and we call for the continued application of tree rings in carbon cycle studies across a broader range of species diversity, productivity, and disturbance histories to fully develop this potential