Papers on Cassiope tetragona as climate proxy

This is a list of papers on Cassiope tetragona as climate proxy. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

UPDATE (February 5, 2013): Weijers et al. (2013) added.
UPDATE (April 4, 2012): Raybeck et al. (2012) added.

Reconstructing High Arctic growing season intensity from shoot length growth of a dwarf shrub – Weijers et al. (2013) “Annual shoot length of the circumarctic dwarf shrub Cassiope tetragona has proved to be a reliable proxy for past and ongoing climate change in the Arctic. This is based on its strong linear relationship with monthly climate parameters. Monthly means are, however, coarse units for prediction of growth in marginal regions with short growing seasons. An alternative to monthly averages are parameters that quantify the growing season length (GSL) and its intensity (growing degree-days; GDD₅). GDD₅ is defined as the cumulative daily mean temperature above 5°C. GSL is defined as the number of days on which the average temperature exceeds 5°C. The aims of this study were to test whether these parameters are a better predictor of growth than monthly means and to reconstruct past High Arctic growing season climate. Correlative analysis shows that GDD₅ is a better predictor of annual shoot length growth than mean monthly temperatures and GSL, both at C. tetragona’s European northern and southern distribution limit, as well as at its assumed climatic optimum. Svalbard Airport GDD₅ was reconstructed back to 1857. The reconstruction shares 61% of variance with the instrumental record. This opens the possibility to obtain an Arctic network of climate reconstructions with high temporal and spatial resolution through construction of C. tetragona shoot length chronologies.” Stef Weijers, Friederike Wagner-Cremer, Ute Sass-Klaassen, Rob Broekman, Jelte Rozema, The Holocene January 30, 2013 0959683612470178, doi: 10.1177/0959683612470178.

Multiproxy reconstructions of climate for three sites in the Canadian High Arctic using Cassiope tetragona – Raybeck et al. (2012) “We developed calibration models and reconstructed climate for sites in the central and eastern Canadian High Arctic using dendroclimatological and stable isotope analysis techniques on the dwarf-shrub, Cassiope tetragona. Our results may suggest complex temporal and spatial patterns of climate change in the region over the past century. For sites on Bathurst and Devon Islands, we reconstructed fall mean and June–July mean temperature using multiple linear regression analysis that explained 54 % and 40 % of the variance, respectively. The predictor variables included annual growth, annual production of leaves, flower buds and annual δ¹³C values for the Bathurst Island model, and annual growth and δ¹³C values for the Devon Island model. Both models revealed warmer than average temperatures throughout the mid-20th century, followed by a cooling trend from the early 1960s and mid-1970s at the Devon and Bathurst Island sites, respectively. Temperatures remained cool until the early 1980s and then increased until 1998/1999 at both sites. Our models are supported by other paleoclimate proxies and the instrumental record from the Canadian Arctic. For sites on Axel Heiberg and Bathurst Islands, we developed models using multivariate regresssion for February and March total precipitation that explained 44 % and 42 % of the variance, respectively. The Axel Heiberg Island model included annual production of flowers and flower buds, as well as annual δ¹³C values as predictor variables, while the Bathurst Island model only included the annual production of flower buds as a predictor. Both models showed lower than average precipitation from the early to mid-1900s, followed by increasing precipitation from the late 1980s to 1998/1999. Our precipitation models, supported by instrumental and proxy data, suggest a trend of increasing late-winter/early spring precipitation in the late 20th century. The lack of a single detectable climate signal across the study sites suggests local climate, topography, genetic variation and/or ecological conditions may dictate, in part, site responses and result in a heterogeneous climatescape over space and time. Yet, like other arctic paleoclimate proxies, chronology error and temporal discrepancies may complicate our interpretations. However, comparisons with other arctic proxies and the meteorological record suggest our models have also registered a regional climate signal.” Shelly A. Rayback, Gregory H. R. Henry and Andrea Lini, Climatic Change, DOI: 10.1007/s10584-012-0431-7.

Dendrochronology in the High Arctic: July air temperatures reconstructed from annual shoot length growth of the circumarctic dwarf shrub Cassiope tetragona – Weijers et al. (2010) “The instrumental Arctic climate record is both temporally and spatially limited. Therefore, there is a need for reliable climate proxies to increase knowledge of past and future Arctic climate change. Annual shoot length increase of the circumarctic dwarf shrub species Cassiope tetragona represents such a new climate proxy. We measured annual shoot length increase of 32 plant samples of the circumarctic dwarf shrub species C. tetragona using the presence of wintermarksepta within the stems, resulting in a 169 year growth chronology (1840–2008) for a High Arctic site on Svalbard. This is the longest growth chronology for dwarf shrub species produced up to now. Relationships between climate and Cassiope growth were investigated through correlative, response function and forward stepwise multiple regression analysis. July average air temperature was found to be the most important factor determining growth, by itself capable of explaining 41% of the variance in shoot growth between 1912 and 2008. The second best predictors were previous year September precipitation sums and average air temperatures, along with several previous growth parameters. A multiple regression model explaining growth with current July and previous year September temperature, combined with previous growth of lag 1, 2 and 5 years as predictors explains 70% of the observed variance in growth. July temperatures and previous year September precipitation sums alone explain 59% of the variance in standardized growth. Mean July air temperature was reconstructed for the period between 1876 and 2007 by a growth-temperature transfer model, using current and following year’s growth. The estimated temperatures correlated well with measured temperatures over the calibration (1912–1959) and verification (1960–2007) period: R2 = 0.34 and R2 = 0.47, respectively. The instrumental record (1912–2008) extended with these reliable mean July temperature estimates (1876–1911) reveals a significant warming trend on Svalbard since 1876 of 0.07 °C decade−1 on average. This study shows that the climate–growth relationships in C. tetragona, its longevity, its annual resolution, the availability of (sub)fossil fragments in tundra soil cores and its circumartic distribution make it a very valuable tool for climate reconstructions beyond the instrumental record and in areas lacking meteorological data, throughout the Arctic.” Stef Weijers, Rob Broekmana and Jelte Rozema, Quaternary Science Reviews, doi:10.1016/j.quascirev.2010.09.003.

Annual growth of Cassiope tetragona as a proxy for Arctic climate: developing correlative and experimental transfer functions to reconstruct past summer temperature on a millennial time scale – Rozema et al. (2009) “Annual growth of the polar evergreen shrub Cassiope tetragona on Svalbard was evaluated as a proxy for Arctic summer temperatures. Transfer functions were derived from temperature-growth correlations of shoots and from a temperature-growth response, obtained from experimental warming using open top chambers (OTC) in high Arctic tundra vegetation at Isdammen approximately 1.5 km southeast of Longyearbyen, Svalbard (78°N, 15 E) and in Longyeardalen, 3 km west of Isdammen from 2004 to 2006. Air temperatures, monitored throughout the summer months, were 1.3 °C higher inside the OTCs than in the control plots. Annual stem growth was measured by tagging stems and leaves, and in the lab with shoots harvested from OTCs and control plots. Annual growth parameters assessed were leaf production, sum of length and weight of individual leaves, and stem length increment derived from leaf scar distances and the distances between wintermarksepta in the stem. Wintermarksepta are formed at the end of the summer growth period when the pith is narrowing and consist of dense and dark tissue (Fig. 1b). The variation of annual growth in a 34-year site chronology (based on Cassiope shoots from the surroundings of the OTCs and control plots) correlated strongly with the mean summer temperature on Svalbard. The number of leaf pairs, leaf length and stem length also increased in the OTC warmed plots in the second and third year of warming. Transfer functions were derived from the temperature-annual growth correlations from a single shoot from Longyeardalen, from the cross-dated Isdammen site chronology and from the growth response to experimental warming. Based on leaf scar distances and distances between wintermarksepta of well-preserved subfossil shoots in arctic tundra soil, annual stem length increase was assessed for the layers of a soil core collected at the Isdammen site. Based on the derived transfer functions summer temperature of the period relating to the 15 cm deep tundra soil core layer, radiocarbon dated at 4230±40 bp, may have been 3.0 °C lower than the present-day 6.2 °C value. These results indicate that the transfer functions can be used to reconstruct past temperatures, beyond the time range of instrumental temperature and ice core records of Svalbard.” Jelte Rozema, Stef Weijers, Rob Broekman, Peter Blokker, Bert Buizer, Chantal Werleman, Hassan El Yaqine, Hanneke Hoogedoorn, Miguel Mayoral Fuertes, Elisabeth Cooper, Global Change Biology, Volume 15, Issue 7, pages 1703–1715, July 2009. [Full text]

Reconstruction of Summer Temperature for a Canadian High Arctic Site from Retrospective Analysis of the Dwarf Shrub, Cassiope tetragona – Rayback & Henry (2006) “We used retrospective analysis of the widespread evergreen dwarf-shrub, Cassiope tetragona, to reconstruct average summer air temperature for Alexandra Fiord, Ellesmere Island, Canada. Retrospective analysis is a technique based on dendrochronological methods. In this study, chronologies are based on the morphological characteristics of the plant stems. Two growth and two reproduction chronologies, ranging from 80 to 118 years long, were developed from each of two populations at the High Arctic site. We used multiple regression models to develop a 100-year-long (1895-1994) reconstruction of July-September average air temperature that explained 45% of the climatic variance in the instrumental record. The reconstruction revealed an increase in summer temperature from ~1905 to the early 1960s, a cooling trend from the mid-1960 to the 1970s, and an increase in temperature after 1980. These historical temperature patterns correspond well with those from other climate proxies from sites on Ellesmere and Devon Islands. As well, the similarity between our model and an arctic-wide proxy temperature time series suggests that the Cassiope-based reconstruction contains a large-scale temperature signal. There is great potential for the development of proxy climate data using Cassiope tetragona from sites throughout the Arctic.” Shelly A. Rayback, Gregory H. R. Henry, Arctic, Antarctic, and Alpine Research, Volume 38, Number 2 / May 2006, 228-238.

Dendrochronological Potential of the Arctic Dwarf-Shrub Cassiope tetragona – Rayback & Henry (2005) “In this report, we describe the use of dendrochronological techniques on the circumpolar, evergreen dwarf-shrub, Cassiope tetragona. Using techniques such as crossdating and standardization, and the software programs COFECHA and ARSTAN, we developed C. tetragona growth and reproduction chronologies for sites in the Canadian High Arctic. High-resolution chronologies may be used to reconstruct past climate and phase changes in large-scale modes of atmospheric circulation (e.g. Arctic Oscillation, North Atlantic Oscillation), to investigate the growth and reproductive responses of the plant to ambient and manipulated environmental variables, and to reconstruct the plant’s past ecohydrology (δ¹⁸O, δD, δ¹³C), gas exchange (δ¹³C) and mineral nutrition (δ¹⁵N). As C. tetragona is a circumpolar species, chronologies may be developed throughout the Arctic at sites where no trees exist, and thus provide new information on the past climate and environmental history of sites and regions previously unstudied.” Shelly A. Rayback and Gregory H. R. Henry, Tree-Ring Research 61, 1):43-53. 2005, doi: 10.3959/1536-1098-61.1.43. [Full text]

Arctic and North Atlantic Oscillation phase changes are recorded in the isotopes (δ¹⁸O and δ¹³C) of Cassiope tetragona plants – Welker et al. (2005) “The Arctic and North Atlantic Oscillations (AO/NAO) are large-scale annual modes of atmospheric circulation that have shifted in the last 30 years. Recent changes in arctic climate, including increasing surface air temperature, declining sea ice extent, and shifts in the amounts seasonality of precipitation are linked to the strong positive phase of the AO/NAO. Here, we show that phase changes in the AO/NAO are recorded in the isotopic (δ18O and Δ-carbon isotope discrimination) characteristics of the long-lived circum-arctic plant, Cassiope tetragona, as summer rain has become a more important water source than snowmelt water which in turn has lead to decreases in Δ and reductions in plant stem growth. These isotopic records in C. tetragona may facilitate reconstructions of climate, plant–soil water relations, plant gas exchange attributes and a mechanistic understanding of growth responses to shifts in atmospheric circulation. If plant specimens were available for populations across the arctic as part of the International Polar Year, these archives could provide a circum-arctic record of historical climate change and associated shifts in physiological plant performance and growth.” Jeffrey M. Welker, Shelly Rayback, Greg H. R. Henry, Global Change Biology, Volume 11, Issue 7, pages 997–1002, July 2005.

Responses to natural climatic variation and experimental warming in two tundra plant species with contrasting life forms: Cassiope tetragona and Ranunculus nivalis – Molau (1997) “Two circumpolar tundra plant species, the evergreen dwarfshrub Cassiope tetragona and the perennial herb Ranunculus nivalis, were studied at Latnjajaure in northern Swedish Lapland during three consecutive growing seasons (1993–95) as a contribution to the ITEX programme. Open-top chambers (OTCs) were used in a passive heating experiment, and the performance of the plants in unmanipulated controls was correlated with climatic fluctuations among the years. Phenological, vegetative, and reproductive variables were measured. In both species phenological responses were controlled mainly by ambient air temperature. In the evergreen C. tetragona vegetative growth was controlled mainly by the influx of global solar radiation and was not temperature-dependent, whereas the opposite applied in the herbaceous R. nivalis. Vegetative growth in C. tetragona was rather stable among years as well as between treatments, whereas it was strongly influenced by annual climate in R. nivalis. Both species increased their reproductive success with increasing temperature, but R. nivalis was also radiation-dependent in this case, probably because of its green, photosynthetic nutlets. Ovule number in R. nivalis increased steadily in the experimentally heated plots during the study in response to the constant temperature amelioration above the ambient. At the community level, evergreen C. tetragona seems to have low competitive ability under warmer conditions. The situation for vernal low-growing herbs like R. nivalis is more complex; despite a strong positive response to increased temperature, they may exhibit decreased reproductive success if overgrown by a vigorous graminoid canopy.” U. Molau, Global Change Biology, Volume 3, Issue S1, pages 97–107, December 1997.

Retrospective Analysis of Growth and Reproduction in Cassiope tetragona and Relations to Climate in the Canadian High Arctic – Johnstone & Henry (1997) “Techniques of retrospective growth analysis, adapted from dendrochronology, were applied to Cassiope tetragona, an evergreen dwarf-shrub, sampled at Alexandra Fiord. Ellesmere Island, Canada. A new method of delimiting annual growth increments through patterns in leaf node placement along a stem was utilized. Chronologies of mean annual stem elongation, leaf production, and flower production were developed, and estimates of these parameters agree with those obtained for other arctic populations of C. tetragona. Stem elongation and leaf production were positively correlated in the same year. Flower production was positively correlated with growth in the previous year, but negatively correlated with growth in the same year. This pattern was interpreted as the effects of resource allocation strategies, namely, the preemption of within-plant resources by flower production once flowering is initiated. All chronologies were significantly correlated with climate records from Alexandra Fiord and Eureka, Ellesmere Island, with the majority of significant correlations occurring with June and July temperatures. Flower production appeared to be most sensitive to variations in summer temperatures, and climate response functions which included previous growth explained up to 84% of the variation in the flowering chronology. Unstandardized leaf and flower number chronologies were used to provide an independent test of the climate transfer function presented in Havström et al. (1995). The results indicate that C. tetragona may be used successfully to generate proxy climate data, although use of standardized chronologies is recommended. Two predictive models for July temperatures at Alexandra Fiord, based on standardized chronologies, are presented to provide future opportunities for verification and application of this technique.” Jill F. Johnstone and Greg H. R. Henry, Arctic and Alpine Research, Vol. 29, No. 4 (Nov., 1997), pp. 459-469.

Little Ice Age Temperature Estimated by Growth and Flowering Differences between Subfossil and Extant Shoots of Cassiope tetragona, an Arctic Heather – Havström et al. (1995) “1. A unique opportunity to study conditions for plant growth at the onset of glaciation was offered as a retreating glacier at Ellesmere Island, Canada, revealed well-preserved, subfossil plants (411±70 radio-carbon years old) of Cassiope tetragona, an arctic dwarf-shrub previously used to study climate-related growth of modern plants. 2. Growth and flowering of the ancient and modern shoots of C. tetragona from the same locality were examined retrospectively. The ancient shoots produced leaves in each, and flowers in each except one, of the last 26 years before they died, although this production was significantly lower and less variable among years than in the modern shoots. 3. Predictions based on regression between modern plant performance and climatic data from the study site imply that the mean July temperature of the period immediately preceding the glaciation of the area was about 0.7⚬C lower than today. This estimate is independently supported by the correlation between growth and mean July temperature seen today among different sites. 4. The results support the idea that the pre-Little Ice Age plants were killed suddenly by permanent snow embedment and not by glacial movements or temperature limitations as such.” M. Havstrom, T. V. Callaghan, S. Jonasson and J. Svoboda, Functional Ecology, Vol. 9, No. 4 (Aug., 1995), pp. 650-654.

Differential Growth Responses of Cassiope tetragona, an Arctic Dwarf-Shrub, to Environmental Perturbations among Three Contrasting High- and Subarctic Sites – Havström et al. (1993) “Three populations of Cassiope tetragona (Ericaceae) were subjected to in situ environmental perturbations simulating predictions of global warming. The populations were selected to represent different parts of the range of the species, one growing in a high arctic coastal heath at Ny-Ålesund (Svalbard, northern part of the species’ range), one at a subarctic fellfield at 1150 m a.s.l. at Abisko, Swedish Lapland, and one in a subarctic tree-line heath at 450 m a.s.l. at Abisko, southern part of the species’ range. The manipulations included nutrient addition, shading and two levels of temperature enhancement using passive greenhouses. The micrometeorological effects of the shading treatment was similar to that of a mountain birch canopy and the temperature enhancement treatments had the desired effect to increase the average air temperature by 2-4°C. Greenhouses which had a gap between the soil and the greenhouse plastic were particularly successful in creating the desired climatic perturbation without causing extreme maximum temperatures or other unwanted side-effects. The environmental manipulations caused strikingly different responses in the vegetative growth pattern of main shoots of C. tetragona among the three populations: at the subarctic tree-line heath, nutrient addition caused a substantial increase in growth, whereas it was the temperature enhancement treatments that caused increases, although smaller, at the subarctic fellfield and the high arctic heath sites. At the high arctic site, we also found growth reduced in response to shading, but at the subarctic sites, and particularly at the tree-line heath site, shading caused a marked etiolation of the shoots. Hence, different factors seem to produce very different responses in the vegetative growth of C. tetragona in different parts of its geographical range. We conclude that competition for nutrients and light are the main limiting factors for the growth of Cassiope tetragona near the lower distributional limit (LODIL) of the species, but that temperature is the main limiting factor in the northern parts of its range, and at high altitudes in the southern parts of its range. We also suggest that the direct effect of predicted future climatic warming on the growth of Cassiope tetragona will increase towards the north, whereas a possible indirect effect of increasing nutrient availability following a temperature increase will be the main effect in the southern and lower parts of its range. These responses could, however, be modified by shading from other species responding to environmental change by increased growth.” Mats Havström, Terry V. Callaghan and Sven Jonasson, Oikos, Vol. 66, No. 3 (Apr., 1993), pp. 389-402.

Historical Records of Climate-Related Growth in Cassiope Tetragona from the Arctic – Callaghan et al. (1989) “(1)Shoots of the circumpolar species Cassiope tetragona were collected on brief visits to three remote arctic and subarctic sites, two in Svalbard and one in Swedish Lapland. The shoots were subsequently analysed by measuring leaf lengths in strict sequence along individual shoots. (2) This evergreen species retained up to 232 leaves per shoot. Leaf lengths, plotted against leaf position on the shoots, revealed two trends: (i) more or less regular waves caused by the alternation of short spring and autumn leaves with long summer leaves, and (ii)an ontogenetic trend represented by a general increase in leaf length with increasing distance between the point of origin of the leaf and the crigin of the shoot. (3) The seasonal trend of leaf length was used to delimit annual complements of leaves, of which up to twenty persisted. The number of leaves was counted for each year and the ontogenetic trend of leaf length was removed by statistical methods so that leaf length indices could be calculated and relative lengths compared, both between years within populations and between populations. Three indices of leaf length were derived: maximum, minimum and the total of all leaf length indices for each year. (4)Correlation analysis between the four measures of annual leaf performance showed several similarities between the two Svalbard populations, a few between the low altitude population from Svalbard and that from Swedish Lapland and none between the higher altitude Svalbard population and that from Swedish Lapland. (5) Correlation analysis between annual leaf performance and mean monthly temperature and monthly total precipitation showed that July temperatures and precipitation during May were particularly important for leaf development in the Svalbard populations. July temperatures represent mid-summer conditions during a very short growing season in Svalbard, whereas May is normally the driest month in this region of generally low precipitation. Ambient temperature is usually sub-zero for most of May and precipitation as snow is probably important in protecting the sensitive shoot apices of C. tetragona which lack true buds. (6) In Swedish Lapland, the number of leaves per year was correlated with summer temperatures but only negatively with precipitation which was greater at the Swedish site than in Svalbard. At the Swedish site, therefore, the protection of leaf primordia from frost is probably greater than in Svalbard because of a more persistent snow cover. (7) Correlations between the number of leaves per year and leaf length indices in the previous year, together with correlations between leaf performance and weather conditions in the previous year, were often significant. In general, the same weather variables were correlated with leaf performance as in the within-year comparisons. (8) The correlations between the number of leaves per year and the other measures of leaf performance and weather in the previous year were particularly strong in the Svalbard populations. This demonstrates the preformation of an annual leaf complement by the High Arctic Svalbard populations. This may be an important mechanism to buffer production against particularly adverse weather conditions during the growing season. Leaf preformation was apparent but not so clearly demonstrated at the subarctic-alpine Swedish site. (9) Significant multiple regression models of leaf performance were obtained in ten out of the twelve cases. Five models accounted for more than 50% of the variation in leaf performance and two of these accounted for more than 65%. The most significant relationships were found for total leaf length index at the two Svalbard sites and number of leaves per year at the Swedish site. Weather variables in the preceding year, particularly precipitation in May, were usually represented in the models. (10) Retrospective analysis of the historical records of growth preserved in ungrazed herbaceous material from the Arctic can lead to the dating of specific events and the construction of models of long-term climate-related growth even though the period spent in the field is brief.” T. V. Callaghan, B. A. Carlsson and N. J. C. Tyler, Journal of Ecology, Vol. 77, No. 3 (Sep., 1989), pp. 823-837. [Full text]