This is a list of papers on the divergence problem relating to tree-ring studies. 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 (April 3, 2013): Zhang & Wilmking (2010), Loader et al. (2010), Martín-Benito et al. (2010), Porter & Pisaric (2011), Daux et al. (2011), Weijers et al. (2012), and Fang et al. (2012) added. Thanks to Barry for pointing them out.
Tree growth and its association with climate between individual tree-ring series at three mountain ranges in north central China – Fang et al. (2012) “Individual tree-ring series may show changed growth trends and divergent climate–growth associations even within a site, highlighting the need to examine tree growth and its climate association before building a chronology. We provided a case study for the stratification and temporal variability of tree growth and its climate associations of individual cores for three mountain ranges in north central China. Tree growth is mainly limited by moisture conditions in previous July–September and current June–August. Repeated sampling and field investigations of Picea wilsonii at Xinglong Mountain over a growth year of 2004 suggested that the growing season is from about the end of April to the end of September. It appears that the moisture conditions in previous and current growing seasons are crucial for tree growth in this region. However, a decrease in drought limitation was observed for a few tree-ring series. We thereby built the pooled chronology and sub-site chronologies with only drought-sensitive tree rings similar climate–growth relationships from the three mountain slopes. Growth disturbances of tree-ring series are detected by checking the occurrence of successively low values of the biweight series, which are treated by fitting a flexible curve.” Keyan Fang, Xiaohua Gou, Fahu Chen, Yingjun Li, Fen Zhang, Miklos Kazmer, Dendrochronologia, Volume 30, Issue 2, 2012, Pages 113–119, http://dx.doi.org/10.1016/j.dendro.2011.04.003.
No divergence in Cassiope tetragona: persistence of growth response along a latitudinal temperature gradient and under multi-year experimental warming – Weijers et al. (2012) “Background and Aims The dwarf shrub Cassiope tetragona (Arctic bell-heather) is increasingly used for arctic climate reconstructions, the reliability of which depends on the existence of a linear climate–growth relationship. This relationship was examined over a high-arctic to sub-arctic temperature gradient and under multi-year artificial warming at a high-arctic site. Methods Growth chronologies of annual shoot length, as well as total leaf length, number of leaves and average leaf length per year, were constructed for three sites. Cassiope tetragona was sampled near its cold tolerance limit at Ny-Ålesund, Svalbard, at its assumed climatic optimum in Endalen, Svalbard, and near its European southern limit at Abisko, Sweden. Together these sites represent the entire temperature gradient of this species. Leaf life span was also determined. Each growing season from 2004 to 2010, 17 open top chambers (OTCs) were placed near Ny-Ålesund, thus increasing the daily mean temperatures by 1·23°C. At the end of the 2010 growing season, shoots were harvested from OTCs and control plots, and growth parameters were measured. Key Results All growth parameters, except average leaf length, exhibited a linear positive response (R2 between 0·63 and 0·91) to mean July temperature over the temperature gradient. Average leaf life span was 1·4 years shorter in sub-arctic Sweden compared with arctic Svalbard. All growth parameters increased in response to the experimental warming; the leaf life span was, however, not significantly affected by OTC warming. Conclusions The linear July temperature–growth relationships, as well as the 7 year effect of experimental warming, confirm that the growth parameters annual shoot length, total leaf length and number of leaves per year can reliably be used for monitoring and reconstructing temperature changes. Furthermore, reconstructing July temperature from these parameters is not hampered by divergence.” Stef Weijers, Inger Greve Alsos, Pernille Bronken Eidesen, Rob Broekman, Maarten J.J.E. Loonen, and Jelte Rozema, Ann Bot (2012) 110 (3): 653-665. doi: 10.1093/aob/mcs123. [Full text]
Can climate variations be inferred from tree-ring parameters and stable isotopes from Larix decidua? Juvenile effects, budmoth outbreaks, and divergence issue – Daux et al. (2011) “Larch wood has been used for centuries as a building material. Hence, the study of the tree-ring width, the latewood maximal density, and the oxygen and carbon isotope composition of the cellulose of this tree provides potential and valuable insights when reconstructing past climate variability. We explore the qualities and limitations of these proxies, focusing on a forest standing the Névache valley (French Alps). The analysis of 15 trees demonstrates a small intra-tree variability in comparison with the inter-tree variability of δ13C and δ18O, and shows that 6 trees, at least, must be pooled to make a population-representative sample. Our results show no juvenile effect for δ13C. Unlike tree-ring width and density, δ13C and δ18O are not altered by budmouth outbreaks. These two parameters therefore appear well suited for climate reconstructions, and depict a strong correlation with July–August temperature and relative humidity. The δ18O of larch cellulose is also strongly linked with the previous winter (December–March) oxygen isotopic composition of the precipitation. This is consistent with a winter water recharge of soil and ground. The past variations of July–August maximum temperature and relative humidity were established using two different combinations of the isotopic ratios. Uncertainties on the reconstructions are estimated respectively at ± 1.4 °C and 3.6%. Inter-annual variations of temperature and relative humidity are well reproduced. However, the reconstructed July–August temperature series diverges from the instrumental one, being lower, after ca. 1990. The effects of the variation through time of the depth of source water and of the ecophysiological response of trees to rising CO2 on the isotope composition are discussed as possible causes of divergence. This ‘divergence problem’ strongly questions the possibility of producing appropriate isotope-based temperature calibration.The relationships between isotopes and the July–August relative humidity are more stationary than those with temperature. This may reflect the first order control of the relative humidity on δ13C via the stomatal conductance and its influence on the evaporative enrichment of the oxygen of the leaf-water. Our study suggests that past variations of relative humidity in the French Alps can be accurately estimated using the stable isotope composition of larch cellulose.” V. Daux, J.L. Edouard, V. Masson-Delmotte, M. Stievenard, G. Hoffmann, M. Pierre, O. Mestre, P.A. Danis, F. Guibal, Earth and Planetary Science Letters, Volume 309, Issues 3–4, 15 September 2011, Pages 221–233, http://dx.doi.org/10.1016/j.epsl.2011.07.003. [Full text]
Temperature-growth divergence in white spruce forests of Old Crow Flats, Yukon Territory, and adjacent regions of northwestern North America – Porter & Pisaric (2011) “We present a new 23-site network of white spruce ring-width chronologies near boreal treeline in Old Crow Flats, Yukon Territory, Canada. Most chronologies span the last 300 years and some reach the mid-16th century. The chronologies exhibit coherent growth patterns before the 1930s. However, since the 1930s, they diverge in trend and exhibit one of two contrasting, but well-replicated patterns we call Group 1 and Group 2. Over the instrumental period (1930–2007) Group 1 sites were inversely correlated with previous-year July temperatures while Group 2 sites were positively correlated with growth-year June temperatures. At the broader northwestern North America (NWNA) scale, we find that the Group 1 and Group 2 patterns are common to a number of white spruce chronologies, which we call NWNA 1 and NWNA 2 chronologies. The NWNA 1 and NWNA 2 chronologies also share a single coherent growth pattern prior to their divergence (ca. 1950s). Comparison of the NWNA 1/NWNA 2 chronologies against gridded 20th-century temperatures for NWNA and reconstructed northern hemisphere summer temperatures (ad 1300–2000) indicates that all sites responded positively to temperature prior to the mid-20th century (at least back to ad 1300), but that some changed to a negative response (NWNA 1) while others maintained a positive response (NWNA 2). The spatial extent of divergence implies a large-scale forcing. As the divergence appears to be restricted to the 20th century, we suggest that the temperature response shift represents a moisture stress caused by an anomalously warm, dry 20th-century climate in NWNA, as indicated by paleoclimatic records. However, because some sites do not diverge and are located within a few kilometres of divergent sites, we speculate that site-level factors have been important in determining the susceptibility of sites to the large-scale drivers of divergence.” Trevor J. Porter, Michael F. J. Pisaric, Global Change Biology, Volume 17, Issue 11, pages 3418–3430, November 2011, DOI: 10.1111/j.1365-2486.2011.02507.x.
Twentieth-century summer temperature variability in the southern Altai Mountains: A carbon and oxygen isotope study of tree-rings – Loader et al. (2010) “The Altai mountains, southern Siberia, represent an area of significant scientific interest and exceptional palaeoecological potential. To assess the influence of climate on the stable isotopic composition of tree-ring cellulose and the potential of this record as a palaeoclimate proxy, replicated stable oxygen and carbon isotope time-series were developed for the twentieth century from four Siberian Pine (Pinus sibirica Du Tour) trees growing near the settlement of Aktash, Russian Altai mountains, southern Siberia. Bootstrapped calibrations for the local instrumental period (AD 1954—2000) reveal strong correlations between summer (July—August) growing season temperatures and tree-ring oxygen (r2=0.55) and carbon (r2=0.30) isotopes. Covariance observed between both carbon and oxygen isotope data suggest a common (stomatal) control. The resulting empirical model was used to reconstruct regional summer temperatures for the twentieth century. No divergence is observed between the non-detrended tree-ring isotope series and instrumental data nor is there any twentieth-century summer warming trend. The strong isotopic signal preserved in the tree-ring series supports the wider application of this approach to explore climatic variability and environmental trends during past millennia through analysis of new and existing long tree-ring chronologies from this region.” N.J. Loader, G. Helle, S.O. Los, F. Lehmkuhl, G.H. Schleser, The Holocene November 2010 vol. 20 no. 7 1149-1156, doi: 10.1177/0959683610369507.
Divergent growth responses and increasing temperature limitation of Qinghai spruce growth along an elevation gradient at the northeast Tibet Plateau – Zhang & Wilmking (2010) “Divergent responses between tree growth and climate factors have been widely reported at high latitudes in the northern hemisphere. Here we show variable climate-growth relationships and divergent growth responses of Qinghai spruce (Picea crassfolia) along an elevation gradient at a mid-latitude site at the northeastern Tibetan Plateau, China. Trees from higher elevations, limited mainly by temperature, show divergent growth trends over time and two responses to climate. Some trees show increasing positive and some increasing negative responses to growing season temperature during the last decades. Trees from lower treeline show a strengthening drought stress signal over time and no divergent growth trends within sites. Our results indicate that single tree analysis might be a worthwhile tool to (1) uncover spatial–temporal changes in climate-growth relationships of trees, (2) better understand future growth performance and (3) help overcome current limitations of tree ring based climatic reconstructions.” Yongxiang Zhang, Martin Wilmking, Forest Ecology and Management, Volume 260, Issue 6, 15 August 2010, Pages 1076–1082, http://dx.doi.org/10.1016/j.foreco.2010.06.034. [Full text]
Black pine (Pinus nigra Arn.) growth divergence along a latitudinal gradient in Western Mediterranean mountains – Martín-Benito et al. (2010) “Most studies of tree-growth and climate report positive responses to global warming in high latitudes and negative responses at lower ones. We analyzed tree-ring width of Pinus nigra Arn. along a 500 km latitudinal transect in the Iberian Peninsula to study the temporal trend and climate forcing in tree radial growth during the last century. Tree growth was enhanced by cool summers and moist cold seasons. Increased moisture stress has decreased tree growth rates. However, we present evidence of growth increases in some trees in all sampled populations after 1980’s. Climate change negatively (positively) affected between 72% (5%) of trees in the southern populations and 40% (25%) in the north Trees with positive growth trends were favored by winter temperatures and their abundance was inversely correlated with forest productivity. Our findings add evidences of tree growth divergence in the Mediterranean basin and show the gradual transition between forests where positive (temperate and boreal) and negative (Mediterranean) growth trends dominate.” Darío Martín-Benito, Miren del Río and Isabel Cañellas, Ann. For. Sci. Volume 67, Number 4, June 2010, DOI: http://dx.doi.org/10.1051/forest/2009121. [Full text]
Divergence pitfalls in tree-ring research – Esper & Frank (2009) No abstract, but here’s a quote from the text: “Rather than re-iterating these arguments, or adding on non-linear statistics or other methodologies to ‘handle’ DP, we wish to take a step back to the basics and describe a number of pitfalls that may be encountered when processing and analyzing tree-ring and temperature data, and that can lead to an accidental detection of DP.” Jan Esper and David Frank, Climatic Change, Volume 94, Numbers 3-4, 261-266, DOI: 10.1007/s10584-009-9594-2. [Full text]
Changing relationships between tree growth and climate in Northwest China – Zhang et al. (2009) “Recently, several studies have shown changing relationships between tree growth and climate factors, mostly in the circumpolar north. There, changing relationships with climate seem to be linked to emergent subpopulation behavior. Here, we test for these phenomena in Northwest China using three tree species (Pinus tabulaeformis, Picea crassifolia, and Sabina przewalskii) that had been collected from six sites at Qilian Mts. and Helan Mts. in Northwest China. We first checked for growth divergence of individual sites and then investigated the relationship between tree growth and climate factors using moving correlation functions (CF). Two species, Pinus and Sabina, from two sites clearly showed growth divergence, not only in the late twentieth century as reported in other studies, but also over nearly the whole record. In divergent sites, one chronology shows more stable relationships with climate factors (usually precipitation). In non-divergent sites, nearly all relationships either vary in strength or become non-significant at one point. While this might possibly be related to increased stress on some trees due to increasing temperature, the exact causes for this shift in sensitivity remain unclear. We would like to highlight the necessity for additional studies investigating possible non-stationary growth responses of trees with climate, especially at sites that are used for climate reconstruction as our sites in Northwest China.” Yongxiang Zhang, Martin Wilmking and Xiaohua Gou, Plant Ecology, Volume 201, Number 1, 39-50, DOI: 10.1007/s11258-008-9478-y. [Full text]
Testing for tree-ring divergence in the European Alps – Büntgen et al. (2008) “Evidence for reduced sensitivity of tree growth to temperature has been reported from multiple forests along the high northern latitudes. This alleged circumpolar phenomenon described the apparent inability of temperature-sensitive tree-ring width and density chronologies to parallel increasing instrumental temperature measurements since the mid-20th century. In addition to such low-frequency trend offset, the inability of formerly temperature-sensitive tree growth to reflect high-frequency temperature signals in a warming world is indicated at some boreal sites, mainly in Alaska, the Yukon and Siberia. Here, we refer to both of these findings as the ‘divergence problem’ (DP), with their causes and scale being debated. If DP is widespread and the result of climatic forcing, the overall reliability of tree-ring-based temperature reconstructions should be questioned. Testing for DP benefits from well-replicated tree-ring and instrumental data spanning from the 19th to the 21st century. Here, we present a network of 124 larch and spruce sites across the European Alpine arc. Tree-ring width chronologies from 40 larch and 24 spruce sites were selected based on their correlation with early (1864–1933) instrumental temperatures to assess their ability of tracking recent (1934–2003) temperature variations. After the tree-ring series of both species were detrended in a manner that allows low-frequency variations to be preserved and scaled against summer temperatures, no unusual late 20th century DP is found. Independent tree-ring width and density evidence for unprecedented late 20th century temperatures with respect to the past millennium further reinforces our results.” Ulf Büntgen, David Frank, Rob Wilson, Marco Carrer, Carlo Urbinati, Jan Esper, Global Change Biology, Volume 14, Issue 10, pages 2443–2453, October 2008, DOI: 10.1111/j.1365-2486.2008.01640.x. [Full text]
On the ‘Divergence Problem’ in Northern Forests: A review of the tree-ring evidence and possible causes – D’Arrigo et al. (2007) “An anomalous reduction in forest growth indices and temperature sensitivity has been detected in tree-ring width and density records from many circumpolar northern latitude sites since around the middle 20th century. This phenomenon, also known as the “divergence problem”, is expressed as an offset between warmer instrumental temperatures and their underestimation in reconstruction models based on tree rings. The divergence problem has potentially significant implications for large-scale patterns of forest growth, the development of paleoclimatic reconstructions based on tree-ring records from northern forests, and the global carbon cycle. Herein we review the current literature published on the divergence problem to date, and assess its possible causes and implications. The causes, however, are not well understood and are difficult to test due to the existence of a number of covarying environmental factors that may potentially impact recent tree growth. … Although limited evidence suggests that the divergence may be anthropogenic in nature and restricted to the recent decades of the 20th century, more research is needed to confirm these observations.” [Full text]
A matter of divergence: Tracking recent warming at hemispheric scales using tree ring data – Wilson et al. (2007) “In this study, we compiled TR data and published local/regional reconstructions that show no divergence against local temperatures. These data have not been included in other large-scale temperature reconstructions. Utilizing this data set, we developed a new, completely independent reconstruction of ENH annual temperatures (1750–2000). This record is not meant to replace existing reconstructions but allows some degree of independent validation of these earlier studies as well as demonstrating that TR data can better model recent warming at large scales when careful selection of constituent chronologies is made at the local scale. Although the new series tracks the increase in ENH annual temperatures over the last few decades better than any existing reconstruction, it still slightly under predicts values in the post-1988 period.” [Full text]
Growth/climate response shift in a long subalpine spruce chronology – Büntgen et al. (2006) “Decreasing (increasing) moving correlations with monthly mean temperatures (precipitation) indicate instable growth/climate response during the 1760–2002 period. Crucial June–August temperatures before ∼1900 shift towards May-July temperature plus August precipitation sensitivity after ∼1900. Numerous of comparable subalpine spruce chronologies confirm increased late-summer drought stress, coincidently with the recent warming trend.”
Divergent tree growth response to recent climatic warming, Lake Clark National Park and Preserve, Alaska – Driscoll et al. (2005) “One subpopulation diverges from historical temperature data after 1950 and one shows increased growth consistent with warming or exceeds expected growth increases. The growth decline may be due to temperature‐induced drought stress that acts on some trees. Unprecedented climatic changes are triggering diverse growth responses between and within study sites that may greatly complicate dendroclimatic reconstructions of past climate conditions.”
Recent climate warming forces contrasting growth responses of white spruce at treeline in Alaska through temperature thresholds – Wilmking et al. (2004) “Our findings of both positive and negative growth responses to climate warming at treeline challenge the widespread assumption that arctic treeline trees grow better with warming climate. High mean temperatures in July decreased the growth of 40% of white spruce at treeline areas in Alaska, whereas warm springs enhance growth of additional 36% of trees and 24% show no significant correlation with climate. Even though these opposing growth responses are present in all sampled sites, their relative proportion varies between sites and there is no overall clear relationship between growth response and landscape position within a site. Growth increases and decreases appear in our sample above specific temperature index values (temperature thresholds), which occurred more frequently in the late 20th century.” [Full text]
Thresholds for warming-induced growth decline at elevational tree line in the Yukon Territory, Canada – D’Arrigo et al. (2004) “Here we identify a temperature threshold for decline in a tree ring record from a well-established temperature-sensitive site at elevational tree line in northwestern Canada. The positive ring width/temperature relationship has weakened such that a pre-1965 linear model systematically overpredicts tree ring widths from 1965 to 1999. A nonlinear model shows an inverted U-shaped relationship between this chronology and summer temperatures, with an optimal July–August average temperature of 11.3°C based on a nearby station. This optimal value has been consistently exceeded since the 1960s, and the concurrent decline demonstrates that even at tree line, trees can be negatively affected when temperatures warm beyond a physiological threshold.” [Full text]
Large-scale temperature inferences from tree rings: a review – Briffa et al. (2004) “However, in many tree-ring chronologies, we do not observe the expected rate of ring density increases that would be compatible with observed late 20th century warming. This changing climate sensitivity may be the result of other environmental factors that have, since the 1950s, increasingly acted to reduce tree-ring density below the level expected on the basis of summer temperature changes. This prevents us from claiming unprecedented hemispheric warming during recent decades on the basis of these tree-ring density data alone. Here we show very preliminary results of an investigation of the links between recent changes in MXD and ozone (the latter assumed to be associated with the incidence of UV radiation at the ground).”
Dendroclimatic reconstruction of maximum summer temperatures from upper treeline sites in Interior British Columbia, Canada – Wilson & Luckman (2003) “Significant changes are also noted in the relationships between summer mean, maximum and minimum temperatures in this region in the last few decades with a greater absolute rate of increase in mean and minimum temperatures. These changing relationships suggest it is prudent to model tree-ring response to a variety of temperature parameters rather than using mean-temperature values.” [Full text]
Boreal temperature variability inferred from maximum latewood density and tree-ring width data, Wrangell Mountain region, Alaska – Davi et al. (2003) “After around 1970 the RW series show a decrease in growth, while station data show continued warming, which may be related to increasing moisture stress or other factors.”
Spatial and Temporal Variability in the Growth and Climate Response of Treeline Trees in Alaska – Lloyd & Fastie (2002) “We found that there was substantial regional variability in response to climate variation. Contrary to our expectations, we found that after 1950 warmer temperatures were associated with decreased tree growth in all but the wettest region, the Alaska Range. Although tree growth increased from 1900–1950 at almost all sites, significant declines in tree growth were common after 1950 in all but the Alaska Range sites. We also found that there was substantial variability in response to climate variation according to distance to treeline. Inverse growth responses to temperature were more common at sites below the forest margin than at sites at the forest margin. Together, these results suggest that inverse responses to temperature are widespread, affecting even the coldest parts of the boreal forest. Even in such close proximity to treeline, warm temperatures after 1950 have been associated with reduced tree growth. Growth declines were most common in the warmer and drier sites, and thus support the hypothesis that drought-stress may accompany increased warming in the boreal forest.” [Full text]
Long-Term Temperature Trends and Tree Growth in the Taymir Region of Northern Siberia – Jacoby et al. (2000) “These warm-season temperatures correlate with annual temperatures and indicate unusual warming in the 20th century. However, there is a loss of thermal response in ring widths since about 1970. Previously the warmer temperatures induced wider rings.” [Full text]
Reduced growth of Alaskan white spruce in the twentieth century from temperature-induced drought stress – Barber et al. (1999) “Here we present multi-proxy tree-ring data (ring width, maximum late-wood density and carbon-isotope composition) from 20 productive stands of white spruce in the interior of Alaska. The tree-ring records show a strong and consistent relationship over the past 90 years and indicate that, in contrast with earlier predictions, radial growth has decreased with increasing temperature. Our data show that temperature-induced drought stress has disproportionately affected the most rapidly growing white spruce, suggesting that, under recent climate warming, drought may have been an important factor limiting carbon uptake in a large portion of the North American boreal forest. If this limitation in growth due to drought stress is sustained, the future capacity of northern latitudes to sequester carbon may be less than currently expected.”
Influence of snowfall and melt timing on tree growth in subarctic Eurasia – Vaganov et al. (1999) “Here we report an analysis of tree-ring and climate data from the forest–tundra zone, in combination with a mechanistic model of tree-ring growth, to argue that an increasing trend of winter precipitation over the past century in many subarctic regions led to delayed snow melt in these permafrost environments. As a result, the initiation of cambial activity (necessary for the formation of wood cells) has been delayed relative to the pre-1960 period in the Siberian subarctic. Since the early 1960s, less of the growth season has been during what had previously been the period of maximal growth sensitivity to temperature. This shift results not only in slower growth, but also in a reduced correlation between growth and temperature.” [Full text]
Trees tell of past climates: but are they speaking less clearly today? – Briffa et al. (1998) “However, a dramatic change in the sensitivity of hemispheric tree–growth to temperature forcing has become apparent during recent decades, and there is additional evidence of major tree–growth (and hence, probably, ecosystem biomass) increases in the northern boreal forests, most clearly over the last century. These possibly anthropogenically related changes in the ecology of tree growth have important implications for modelling future atmospheric CO2 concentrations. Also, where dendroclimatology is concerned to reconstruct longer (increasingly above centennial) temperature histories, such alterations of ‘normal’ (pre–industrial) tree–growth rates and climate–growth relationships must be accounted for in our attempts to translate the evidence of past tree growth changes.” [Full text]
Reduced sensitivity of recent tree-growth to temperature at high northern latitudes – Briffa et al. (1998) “During the second half of the twentieth century, the decadal-scale trends in wood density and summer temperatures have increasingly diverged as wood density has progressively fallen. The cause of this increasing insensitivity of wood density to temperature changes is not known, but if it is not taken into account in dendroclimatic reconstructions, past temperatures could be overestimated. Moreover, the recent reduction in the response of trees to air-temperature changes would mean that estimates of future atmospheric CO2 concentrations, based on carbon-cycle models that are uniformly sensitive to high-latitude warming, could be too low.”
Tree Ring Width and Density Evidence of Climatic and Potential Forest Change in Alaska – Jacoby & D’Arrigo (1995) “Ring width and density measurements from the same trees can produce distinctly different climatic information. … The recent increase in temperatures combined with drier years may be changing the tree response to climate and raising the potential for some forest changes in Alaskan and other boreal forests.” [Full text]
Delayed.oscillator – Yamal Emulation II: Divergence
John Cook – The hockey stick divergence problem