Separating the climatic signal from tree-ring width and maximum latewood density records

We propose a technique for separating the climatic signal which is contained in two tree-ring parameters widely used in dendroclimatology. The method is based on the removal of the relationship between tree-ring width (TRW) and maximum latewood density (MXD) observed for narrow tree rings from high latitudes. The new technique is tested on data from three larch stands located along the northern timberline in Eurasia. Correlations were calculated between the temperatures of pentads (five consecutive days), TRW chronologies and MXD chronologies calculated according to the standard and proposed methods. The analysis confirms the great importance of summer temperature for tree radial growth and tree-ring formation. TRW is positively correlated with the temperature of four to eight pentads (depending on the region) at the beginning of the growth season, but MXD as obtained by the standard technique is correlated with temperature over a much longer period. For maximum density series from which the relationship between MXD and TRW has been removed (MXD′), there is a clear correlation with temperatures in the second part of the growing season. These results are consistent with the known dynamics of tree-ring growth in high latitudes and mechanisms of tree-ring formation.     

Yearly maps of summer temperatures in Western Europe from A.D. 1750 to 1975 and Western North America from 1600 to 1982:

The primary purpose of this publication is to make available two series of historical maps showing summer (April–September mean) temperatures over western Europe from A.D. 1750 and western North America from A.D. 1600. These maps have been reconstructed using networks of temperature-sensitive tree-ring chronologies as part of a continuing collaboration between the Swiss Federal Institute for Forest, Snow and Landscape Research (FSL) at Birmensdorf, Switzerland and the Climatic Research Unit (CRU) at the University of East Anglia, Norwich, U.K. The maps were reconstructed using relatively long maximum latewood density chronologies selected from the extensive networks of tree-ring data produced at the FSL. The reconstructions were produced using a spatial regression technique developed at CRU.The basis for the reconstructions are wood samples from various conifers growing at cool-moist sites close to the alpine or boreal timberlines. This material was analysed by x-ray densitometry and the resulting maximum latewood density chronologies were calibrated against meteorological data over a 100-year calibration period.These results emphasize the value of dendroclimatological research which can provide continentalscale reconstructions of past climate which represent a potential resource for climatologists, biologists, and historians with an interest in the spatial variability of summer temperatures over the middle to high latitudes.     

A Pinus cembra L. tree-ring record for late spring to late summer temperature in the Rhaetian Alps,Italy

Ongoing climate change strongly affects high-elevation environments in the European Alps, influencing the cryosphere and the biosphere and causing widespread retreat of glaciers and changes in biomes. Nevertheless, high-elevation areas often lack long meteorological series, and global datasets cannot represent local variations well. Thus, proxy data, such as tree rings, provide information on past climatic variations from these remote sites. Although maximum latewood density (MXD) chronologies provide better temperature information than those based on tree-ring width (TRW), MXD series from the European Alps are lacking. To derive high-quality temperature information for the Rhaetian Alps, Pinus cembra L. trees sampled at approximately 2000 m a.s.l. were used to build one MXD chronology spanning from 1647 to 2015. The MXD data were significantly and highly correlated with seasonal May-September mean temperatures. The MXD chronology showed a generally positive trend since the middle of the 19^th century, interrupted by short phases of climatic deterioration in the beginning of the 20^th century and in the 1970s, conforming with the temperature trends. Our results underline the potential for using Pinus cembra L. MXD to reconstruct mean temperature variations, especially during the onset and latter part of the growing season, providing additional information on parts of the growing season not inferred from TRW. Future studies on MXD for this species will increase the availability of temporal and spatial data, allowing detailed climate reconstructions.     

Seasonal and synoptic climatic drivers of tree growth in the Bighorn Mountains,WY, USA (1654–1983 CE)

In the United States’ (US) Northern Rockies, synoptic pressure systems and atmospheric circulation drive interannual variation in seasonal temperature and precipitation. The radial growth of high-elevation trees in this semi-arid region captures this temperature and precipitation variability and provides long time series to contextualize instrumental-era variability in synoptic-scale climate patterns. Such variability in climate patterns can trigger extreme climate events, such as droughts, floods, and forest fires, which have a damaging impact on human and natural systems. We developed 11 tree-ring width (TRW) chronologies from multiple species and sites to investigate the seasonal climatic drivers of tree growth in the Bighorn Mountains, WY. A principal component analysis of the chronologies identified 54% of shared common variance (1894–2014). Tree growth (expressed by PC1) was driven by multiple seasonal climate variables: previous October and current July temperatures, as well as previous December and current April precipitation, had a positive influence on growth, whereas growth was limited by July precipitation. These seasonal growth-climate relationships corresponded to circulation patterns at higher atmospheric levels over the Bighorn Mountains. Tree growth was enhanced when the winter jet stream was in a northward position, which led to warmer winters, and when the spring jet stream was further south, which led to wetter springs. The second principal component, explaining 19% of the variance, clustered sites by elevation and was strongly related to summer temperature. We leverage this summer temperature signal in our TRW chronologies by combining it with an existing maximum latewood density (MXD) chronology in a nested approach. This allowed us to reconstruct Bighorn Mountains summer (June, July, and August) temperature (BMST) back to 1654, thus extending the instrumental temperature record by 250 years. Our BMST reconstruction explains 39–53% of the variance in regional summer temperature variability. The 1830s were the relatively coolest decade and the 1930s were the warmest decade over the reconstructed period (1654–1983 CE) – which excludes the most recent 3 decades. Our results contextualize recent drivers and trends of climate variability in the US Northern Rockies, which contributes to the information that managers of human and natural systems need in order to prepare for potential future variability.     

Maximum June–July temperatures in the southwest Yukon over the last 300 years reconstructed from tree rings

A network of seven high-elevation white spruce tree-ring chronologies from the southwest Yukon is used to reconstruct June–July maximum temperatures (T_max) back to 1684 AD, explaining 46.6% of the climatic variation over the 1946–1995 calibration period. The chronologies are characterised by low interannual ring-width variability and display similar patterns of ring-width variation across the sample area over the last 300 years. The driving force of this common signal appears to be a tree growth response to summer temperatures across the region. The reconstruction compares well with regional records of temperature variability derived from annual ring-width and maximum density data. Periods of cooler temperatures correspond with solar minima and glacier advances, particularly during the early 18th and the early 19th centuries. The maximum reconstructed temperatures are in the 1940s with 20th century values averaging 0.46 °C higher than the 1684–1899 period. In contrast to several regional studies, there does not appear to be a reduction in the sensitivity of tree growth to temperature at these sites during the late 20th century. Instead, a slight increase in the strength of the temperature–tree growth relationship is observed during recent decades. A possible explanation for this difference is the absence of significant summer season warming in the southwest Yukon region in comparison to other areas that have demonstrated this response change. This research is part of an ongoing project aimed at assessing the spatial and temporal potential of dendroclimatic reconstructions from the Yukon Territory.     

Climate sensitivity of high- and low-elevation Larix decidua MXD chronologies from the Tatra Mountains

Maximum latewood density (MXD) measurements from high-elevation/-latitude sites are an important proxy for summer temperature reconstructions. Here, we present 201 MXD series from living larch (Larix decidua Mill.) trees that were growing at around 850 and 1450 m a.s.l. in the Slovakian Tatra Mountains, together with 56 MXD series from historical timbers of the same species and region. We explore the climate signal at the high- and low-elevation sites and assess the effects of varying temperature and precipitation regimes on MXD formation. Ranging from spring temperature to summer precipitation, the elevation-specific climate sensitivity suggests that the MXD measurements from living and relict sources should not be merged for paleoclimatic studies. This finding emphasizes the challenge of attributing a predominant climate factor that controls wood formation across a wide range of historical constructions. A better understanding of the ‘true’ climate signal requires more samples during the period of overlap between the living and historical trees.     

Stationarity of climate-growth response is only marginally influenced by the soil moisture regime in Western Siberia

Stationary (time-stable) relationships between a tree-ring proxy and climatic drivers are a prerequisite for using tree rings as paleo-climatological archives, but non-stationarity has been detected worldwide. Here we use a classical, temperature-sensitive treeline site in Western Siberia to specifically test the influence of micro-site conditions (wet versus dry) on the stationarity of climate-growth relationships in three co-existing conifer species: Larix sibirica Ledeb., Picea obovata Ledeb., and Pinus sibirica DuTour. We test two commonly used tree-ring proxies, annual tree-ring width (TRW) and maximum latewood density (MXD), using moving windows and the bootstrapped transfer function stability test (BTFS). Summer temperature is the main driver of tree growth in all three species, but non-stationarities exist in all species and both tree-ring proxies. For TRW, we found stationarity only for larch from both micro-sites, while for MXD, we found stationarity for spruce from both micro-sites, and for pine from the wet micro-site. Micro-site variability did not seem to affect stationarity in any systematic way. We highlight the necessity to systematically test the influence of different methods of stationarity tests, since BTFS was more sensitive than moving-window analysis. Taken together, our results underscore the importance of testing the assumption of stationarity for diverse micro-sites, different species and proxies at all sites prior to any tree-ring based temperature reconstruction, since even within one site results can be drastically different.     

Large-scale,millennial-length temperature reconstructions from tree-rings

Over the past two decades, the dendroclimate community has produced various annually resolved, warm season temperature reconstructions for the extratropical Northern Hemisphere. Here we compare these tree-ring based reconstructions back to 831 CE and present a set of basic metrics to provide guidance for non-specialists on their interpretation and use. We specifically draw attention to (i) the imbalance between (numerous) short and (few) long site chronologies incorporated into the hemispheric means, (ii) the beneficial effects of including maximum latewood density chronologies in the recently published reconstructions, (iii) a decrease in reconstruction covariance prior to 1400 CE, and (iv) the varying amplitudes and trends of reconstructed temperatures over the past 1100 years. Whereas the reconstructions agree on several important features, such as warmth during medieval times and cooler temperatures in the 17th and 19th centuries, they still exhibit substantial differences during 13th and 14th centuries. We caution users who might consider combining the reconstructions through simple averaging that all reconstructions share some of the same underlying tree-ring data, and provide four recommendations to guide future efforts to better understand past millennium temperature variability.     

A lonely dot on the map: Exploring the climate signal in tree-ring density and stable isotopes of clanwilliam cedar,South Africa

Clanwilliam cedar (Widdringtonia cedarbergensis; WICE), a long-lived conifer with distinct tree rings in Cape Province, South Africa, has potential to provide a unique high-resolution climate proxy for southern Africa. However, the climate signal in WICE tree-ring width (TRW) is weak and the dendroclimatic potential of other WICE tree-ring parameters therefore needs to be explored. Here, we investigate the climatic signal in various tree-ring parameters, including TRW, Minimum Density (MND), Maximum Latewood Density (MXD), Maximum Latewood Blue Intensity (MXBI), and stable carbon and oxygen isotopes (δ¹⁸O and δ¹³C) measured in WICE samples collected in 1978. MND was negatively influenced by early spring (October-November) precipitation whereas TRW was positively influenced by spring November-December precipitation. MXD was negatively influenced by autumn (April-May) temperature whereas MXBI was not influenced by temperature. Both MXD and MXBI were negatively influenced by January-March and January-May precipitation respectively. We did not find a significant climate signal in either of the stable isotope time series, which were measured on a limited number of samples. WICE can live to be at least 356 years old and the current TRW chronology extends back to 1564 CE. The development of full-length chronologies of alternative tree-ring parameters, particularly MND, would allow for an annually resolved, multi-century spring precipitation reconstruction for this region in southern Africa, where vulnerability to future climate change is high.     

A Norway spruce tree-ring width chronology for the Common Era from the Central Scandinavian Mountains

Fennoscandia is one of the most prominent regions in the world for dendroclimatological research. Yet, millennium-long tree-ring chronologies in this region have mainly been developed from Scots pine (Pinus sylvestris L.). To explore the possibility of building long-term chronologies using other dominating tree species in the region, this paper presents the first two millennia-long Norway spruce (Picea abies (L.) Karst.) ring-width chronology from Northern Europe. The chronology is composed of living trees and subfossil wood and covers the period from BCE 115 to 2012 CE. A sufficiently replicated and robust chronology is built for the past 360 years back to 1649 CE. Further back in time, the common growth signal is reduced, and hence the reliability of the earlier section of the chronology is lower. The climate calibration results show that the spruce ring-width correlation with June-July mean temperatures over the period 1901–2012 is positive and significant (r = 0.6, p < 0.01) and representing the temperature variability of a spatial domain covering west-central Scandinavia. These results show the ability of Norway spruce to serve as a proxy for paleoclimatic research and the possibility of extending the chronology far back in time in the region, and therefore present an opportunity for carrying out new inter-and intraregional proxy analyses.     

Blue intensity as a temperature proxy in the eastern United States: A pilot study from a southern disjunct population of Picea rubens (Sarg.)

Annual surface air temperatures across the eastern United States (US) have increased by more than 1 °C within the last century, with the recent decades marked by an unprecedented warming trend. Tree-rings have long been used as a proxy for climate reconstruction, but few truly temperature-sensitive trees have been documented for the eastern US, much less the Appalachian Mountains in the Southeast. Here, we measure blue intensity (BI) and ring width (RWI) in red spruce growing at the southernmost latitudinal range margin of the species on the North Carolina-Tennessee border to test the efficacy of using either metric as a temperature proxy in the eastern US. The BI and RWI chronologies spanned 1883–2008 and had an interseries correlations of 0.42 and 0.54, respectively, but time series were trimmed to the period 1950–2008 due to low sample depth. We discovered strong, positive, and stable correlations between both current-year early fall (September–October) T_max (r = 0.62; p < 0.001) and T_mean (r = 0.51; p < 0.001) and ΔBI during the period 1950–2008, but found no significant relationships between temperature and RWI. We show BI metrics measured in red spruce to be a promising temperature proxy for the southern Appalachian Mountain region. Future research should focus on testing [1] the efficacy of using BI on red spruce collected from across the species range, and [2] the potential for using BI as a temperature proxy in other conifers distributed in the eastern US.     

Vegetation formations and associations of the zonobiomes along the North American Pacific coast: from northern California to Alaska

This phytosociological study, carried out according to the Braun–Blanquet method and supported by cluster analysis, describes Walter's zonobiomes along the North American Pacific coast between the California–Oregon state border and Alaska (USA), including some interior zones of British Columbia and the Yukon Territory (Canada). Twenty two floristic associations are identified and each is characterized by a unique floristic combination, a distinctive geographical range and particular bioclimatic or edaphic conditions.     

Responses of ring widths and maximum densities of Larix gmelinii to climate on contrasting north- and south-facing slopes in central Siberia

An analysis was performed of the climatic responses of the radial growth of Larix gmelinii (Rupr.) Rupr. at two sites—both of which included contrasting north- and south-facing slopes—in Tura, central Siberia, with the development of ring width and maximum-density chronologies for each slope. Both residual and standard chronologies of ring widths were positively correlated with temperature from late May until mid June on all four slopes. By contrast, standard chronologies of ring widths were negatively correlated with precipitation during the winter (from October to April) and in May on the north-facing slope at site 1 and on the south-facing slope at site 2 respectively. The negative correlations with precipitation during the winter and in May on some of the slopes suggested that delayed snowmelt in early spring might inhibit the radial growth of L. gmelinii, and the effects of snow are likely to vary with topography. Both residual and standard chronologies of maximum densities were positively correlated with temperature in early July on all four slopes. Maximum densities were also positively correlated with precipitation during summer of the previous year on all the slopes. These suggest that no major differences exist in terms of responses of maximum density to climatic factors between the north- and south-facing slopes.     

Predicting woodrat (Neotoma) responses to anthropogenic warming from studies of the palaeomidden record

Aim The influence of anthropogenic climate change on organisms is an area of great scientific concern. Increasingly there is recognition that abrupt climate transitions have occurred over the late Quaternary; studies of these shifts may yield insights into likely biotic responses to contemporary warming. Here, we review research undertaken over the past decade investigating the response of Neotoma (woodrats) body size and distribution to climate change over the late Quaternary (the last 40,000 years). By integrating information from woodrat palaeomiddens, historical museum specimens and field studies of modern populations, we identify potential evolutionary responses to climate change occurring over a variety of temporal and spatial scales. Specifically, we characterize climatic thresholds in the past that led to local species extirpation and/or range alterations rather than in situ adaptation, and apply them to anticipate potential biotic responses to anthropogenic climate change. Location Middens were collected at about 55 sites scattered across the western United States, ranging from about 34 to 46° N and about 104 to 116° W, respectively. Data for modern populations were drawn from studies conducted in Death Valley, California, Missoula, Montana and the Sevilleta LTER site in central New Mexico. Methods We analysed faecal pellets from midden series collected at numerous cave sites across the western United States. From these we estimated body mass using techniques validated in earlier studies. We compared body size fluctuations at different elevations in different regions and integrated these results with studies investigating temperature–body size tradeoffs in modern animals. We also quantify the rapidity of the size changes over the late Quaternary to estimate the evolutionary capacity of woodrats to deal with predicted rates of anthropogenic climate change over the next century. Results We find remarkable similarities across the geographical range to late Quaternary climate change. In the middle of the geographical range woodrats respond in accordance to Bergmann's rule: colder climatic conditions select for larger body size and warmer conditions select for smaller body size. Patterns are more complicated at range boundaries, and local environmental conditions influence the observed response. In general, woodrat body size fluctuates with approximately the same amplitude and frequency as climate; there is a significant and positive correlation between woodrat body size and generalized climate proxies (such as ice core records). Woodrats have achieved evolutionary rates of change equal to or greater than those needed to adapt in situ to anthropogenic climate change. Main conclusions In situ body size evolution is a likely outcome of climate change, and such shifts are part of a normal spectrum of adaptation. Woodrats appear to be subject to ongoing body size selection in response to fluctuating environmental conditions. Allometric considerations suggest that these shifts in body size lead to substantial changes in the physiology, life history and ecology of woodrats, and on their direct and indirect interactions with other organisms in the ecosystem. Our work highlights the importance of a finely resolved and long‐term record in understanding biotic responses to climatic shifts.     

A likelihood approach to estimating animal density from binary acoustic transects

We propose an approximate maximum likelihood method for estimating animal density and abundance from binary passive acoustic transects, when both the probability of detection and the range of detection are unknown. The transect survey is purposely designed so that successive data points are dependent, and this dependence is exploited to simultaneously estimate density, range of detection, and probability of detection. The data are assumed to follow a homogeneous Poisson process in space, and a second-order Markov approximation to the likelihood is used. Simulations show that this method has small bias under the assumptions used to derive the likelihood, although it performs better when the probability of detection is close to 1. The effects of violations of these assumptions are also investigated, and the approach is found to be sensitive to spatial trends in density and clustering. The method is illustrated using real acoustic data from a survey of sperm and humpback whales.