All posts by Greg Wiles

Tree Rings and Climate, Ear Mountain Hoonah, Alaska

We had the good fortune this summer to work remotely with the TRAYLS group out of Hoonah, Alaska.

Figure 1. Google Earth map showing the location of the town of Hoonah and the coring sites. Two tree ring sites were sampled by the group the HN site in the town and the EAR site on Ear Mountain.

Arianna Lapke lead a group of four participants through an ambitious set of projects over much of the summer. Our collaboration with the group centered on meeting virtually with the group to describe the utility and sampling of trees for dendroclimate information. Below are the results of their sampling on EAR Mountain, Chicagof Island and our lab  work at the Wooster Tree Ring Lab.

The group shown coring a Sika spruce just outside of town.
More coring – this time in the rain.

The steep climb up the flank of Ear Mountain to find the old Mountain Hemlocks.

Comparisons of the fast growing Sitka Spruce and the slow growth of the higher elevation Mountain Hemlock.

The cores from the hemlock some over 400 years old show lots of stress , clinging to the mountain side and battered by storms. They are also showing a possible drop in ring-width over time 

So we measured the ring-widths (Nick Wiesenberg and Melita Wiles did) and then we compiled the ring-width data into a chronology aboveThis chronology is the full record going back into the 16th century

This chronology is truncated at 1720 or so when we had at least 4 samples. The most narrow rings follow the 1808 unknown eruption that cooled much of the region – it is unknown as no one knows where the volcano that erupted is located – it is recognized in ice cores.  The other intriguing feature is the relatively recent (last 50 year) drop in ring widths.  It may be due to increased evapotranspiration demands with increasing summer minimum temperatures.  There is a correlation of -0.39 (p<0.04) between tree growth and average April-August minimum temperatures. Other studies have shown that warming night time temperatures lead to increased respiration at night and along with possible greater ET demand or increased cloudiness during the day there may be a decrease in photosynthesis leading to  decreased carbon uptake (Sullivan et al., 2015). This is a promising line of research to further investigate the health of Mt. Hemlock in the region and it is something we plan to pursue with more samples in the future. 

View from Ear Mountain looking north.

Reference cited:
Sullivan, P. F., Mulvey, R. L., Brownlee, A. H., Barrett, T. M., & Pattinson, R. R. (2015). Warm summer nights and the growth decline of shore pine in Southeast Alaska. Environmental Research Letters.

Acknowledgements: We thank Arianna Lapke and the TRAYLS group and look forward to future work with them. This work was supported by the Sherman-Fairchild Foundation, The Luce Funds and the National Science Foundation. NSF Grant AGS 8001184 supported Julia Pearson, Claire Cerne, Ben Gaglioti and Greg Wiles. We also acknowledge the contributions of AMRE participants – Mazvita Chikomo, Srushti Chaudhari and Fred Whenshuo.

Climate Analysis of the White Oaks (Quercus alba) At Kinney Field, Wooster, Ohio

By: Mazvita Chikomo, Srushti Chaudhari, Fred Zhao (as part of the AMRE 2020; The College of Wooster, Tree Ring Lab)

The aim of this study was to analyze White oak trees, to see how old they are and, how they are responding to the wetter and warming climate in Wooster, OH.

Kinney Field, Wooster, OH.

Kinney Field, located in Wooster, OH has long served as a recreational location for various sports and a nice place for public entertainment. On its southwest corner are several old white oaks making it an ideal destination for tree-ring research. The geological setting is an Ice Age kame (hill) left by the retreating Laurentide Ice Sheet approximately 15,000 years ago.  The hill on which the trees grow is thus a well-drained feature built of permeable sediments, likely sand and gravel. We set out to determine the age of the trees, build a tree-ring chronology from the ring-widths, and compare the ring-width series with the monthly meteorological observations recorded at Wooster’s OARDC since C.E. 1888. This helps us better understand how this important tree species is reacting to a changing climate.

Bottom line: Nineteen cores were taken from 11 trees and processed at The College of Wooster Tree Ring Lab. We found that the White Oaks (Quercus alba) growing in the Kinney Field are positively correlated with precipitation in the April, May, June, and July months and have a strong negative correlation with June temperature.

Methods: Nineteen 5-mm diameter tree cores from were collected from 11 trees (Table 1) and, combined to produce a ring-width tree-ring chronology (Figure 2) at the Kinney Fields site, in Wooster, OH (Figure 1). The samples were cross dated in The College of Wooster Tree Ring Laboratory (WTRL) and were measured to the nearest 0.001 mm. This was then statistically cross-dated using the COFECHA (Holmes, 1983) software, and the chronology was then standardized using the ARSTAN software (Cook et al. 1985). The final chronology is made up of 19 cores from 11 trees with a mean series intercorrelation of 0.66 and an average mean sensitivity of 0.24 (Table 2). We used the raw data for the final chronology and point out the upward increasing trend in the series (Figure 2).

Fig. 1: Map showing study site at Kinney Field, Wooster, OH. 

The monthly temperature (1894 to 2019) and precipitation (1888 to 2019) for Wooster Ohio taken at the OARDC data was acquired from the Global Historical Climatology Network (GHCN). The mean annual temperature was 9.8ºC and the average annual precipitation was 947 mm during this time period. The months with the highest precipitation for 1888 to 2019 were June and July, and the highest temperature during the years 1894 to 2019 were June, July, and August. The months with the lowest temperatures were January and February (Figure 3).

The team coring a White Oak.

Measuring tree cores from Kinney Field in the Lab.

Fig. 2: The raw ring width series for the Oaks at Kinney Field, Wooster, OH.

Fig. 3: Climograph showing the annual distribution of precipitation (1888 to 2019) and mean monthly temperature (1894 to 2019) for Wooster.

Fig. 4: Raw ring-widths correlated with monthly temperature records. Only the month of June is significant at the 0.05 level (the common interval is 1895-2019).

Fig. 5: Raw ring-width series correlated with monthly precipitation (1888-2019). The months of April-July are significant (p<0.05) and positive correlations.

Fig. 6: April-July total precipitation correlated with the raw ring width series with a correlation 0.57.

Discussion: The final ring width chronology is 200 years long from 1820 to 2019. The series intercorrelation is 0.66 whereas the mean sensitivity (measure of year to year variability) is 0.24. The series autocorrelation is 0.60 and is a measure of the persistence as it is when the chronology is correlated with itself. The mean ring width measurement is 4.25 mm, which is significantly high relative to other sites and implies that the trees have proper access to nutrients and there is little competition.  There is a strong positive correlation between the ring width and the precipitation in the months of April through July, and the trendlines for both closely follow each other throughout the chronology ( Figures 5, 6). Therefore, we can gather that the trees are tracking the low frequency increase (last ~100 year rise) in precipitation of the region and are also a good indicator of year to year April – July precipitation records in Wooster, OH. The correlation with monthly temperatures is only significantly negatively correlated with June temperature (Figure 4).  The correlation with June is -0.35 (Figure 5) is likely attributed to the high rates of evapotranspiration in the summer months, which can have negative impact on tree growth.

1. The Kinney oaks are all less than about 200 years old and are therefore likely second growth;
2. The raw ring-width data from 19 tree cores has an upward trend strongly correlated with total April through July precipitation measured at the OARDC since 1888;
3. The negative correlation of -0.35 of raw ring width and June temperature is due to increased evapotranspiration demand during warm Junes.

Co-directors Nick Wiesenberg and Dr. Greg Wiles.

This work is supported by the Sherman-Fairchild Foundation and the Luce Foundation. We also thank the organizers of AMRE 2020.


New Publication on the Climate Response of the Dawn Redwoods

The Wooster Tree Ring Lab has just published a paper with Franklin and Marshall College and The Ohio State University on the climate response of the Dawn Redwood tree. The study site is the beautiful Secrest Arboretum at the OARDC – OSU Wooster Campus. The upshot is that these remarkable and fast-growing trees are clearly doing well and as long as the increase in precipitation that Northeast Ohio is receiving keeps pace with the summer warming they should continue to do well. Why not plant more of these beautiful trees as they thrive, sequester carbon, and provide many other ecosystem services.

The senior author is Lauren Vargo, who is now a glaciologist research scientist at the Antarctic Research Centre in Wellington, Australia. Lauren did much of this work while an undergraduate at Wooster. Lauren is also the recent lead author on this Nature Climate Change contribution.

Here is the technical abstract of the Dawn Redwood paper:


Metasequoia glyptostroboides,a deciduous gymnosperm, also known as dawn redwood, was thought to be extinct until living members of the species were found in China in 1943. Analyzing the climate response of a transplanted stand of the trees can give insights into their physiological plasticity, into their use in restoration and reforestation, as well as into interpreting the environmental conditions of the geologic past from fossil Metasequoia. An annual ring-width chronology—spanning 1955 to 2010 and based on a stand of 19 M. glyptostroboides trees planted in Secrest Arboretum in northeast Ohio, USA—shows negative correlations with maximum monthly temperatures: with the strongest relationship with February and the warm months of June and July, all significant at the 99% confidence levels. A positive May to June precipitation correlation is the strongest moisture signal (p < 0.05) and the narrowest rings in the chronology occurred during the drought of 1987 to 1988, consistent with one of the warmest and driest Junes on record. These results have implications for the future as climate change affects the native and transplanted range of this species. Future response of this species to a changing climate will depend on the relative rates of warming maximum temperatures in the winter and summer, as well as changing moisture conditions during the summer months.

Figure above shows the tree-ring record from a stand of 19 Dawn Redwood trees (upper panel A). The lower blue and red graph (B) is the climate response of the trees – temperature (red bars) is strongly negatively correlated with summer (June and July) temperatures and with February temperature. The summer relationship makes sense as hot summers require higher evapotranspiration demands. The negative correlation with winter is hypothesized to be linked to warming winters leading to less snow cover leaving roots exposed and vulnerable to damaging frosts. This negative relationship may go away as warming continues and frosts become less frequent.

Figure above shows three photos of cores from Dawn Redwood – note the narrow 1988 drought ring (white dots). 

The College of Wooster Paleoclimate class mulls around the Dawn Redwood stand. 

Another great photo of  Dawn Redwoods – they are deciduous conifers so this photo in the early spring before growth.

Many thanks to the Secrest Arboretum for permission to core these impressive trees. We greatly appreciate the support of Jason Veil the Curator of the Arboretum.

New publication on Alaska Yellow Cedar

The fate of Alaska yellow-cedar (Callitropsis nootkatensis D. Don; Oerst. ex D.P. Little) in a changing climate is a fascinating tale. Our collaborator, Lauren Oakes published a book last year called the Canary Tree which is a story of yellow-cedar (YC) and her research. The crux of the story is that increased temperatures and loss of snow pack make fine roots of the YC more vulnerable to frost damage and, in some instances, this freezing is killing extensive stands of the tree.

Three of the students who helped sample the trees in Juneau during the summer of 2017 (photo credit: Jesse Wiles).

Brian Buma, also a recent collaborator and author on our contribution, has written several articles describing the biogeography of YC decline. His work includes (Buma, 2018) the thought that with rapid enough rates of warming in Southeast Alaska, the population of YC may not be vulnerable if the warming also decreases the frost threat. This work is informed by mapping of the southern limit of the species, which is thriving in  Oregon and Washington where winter temperatures are well above 0 degrees C. The leading edge of the decline is most vulnerable as the mean winter temperatures are in the range of between -2 and +2 degrees C. Our study sites in Juneau are entering this temperature range and thus are/will be impacted by warming winters and the transition from snow to rain.

Location of study sites.

Our new paper titled:Yellow-Cedar Blue Intensity Tree Ring Chronologies as Records of Climate, Juneau, Alaska, USA appeared  in the Canadian Journal of Forestry Research and describes well-replicated tree-ring chronologies made up of ring-width and latewood blue intensity measurements. Where previously YC has not been considered to be a strong candidate for reconstructing past climate, we have found that blue intensity measurements have a strong signal much stronger than ring-widths.

The Abstract of our paper is here: This is the first study to generate and analyze the climate signal in Blue Intensity (BI) tree-ring chronologies from Alaskan yellow-cedar (Callitropsis nootkatensis D. Don; Oerst. ex D.P. Little). The latewood BI chronology shows a much stronger temperature sensitivity than ring-widths (RW), and thus can provide information on past climate. The well-replicated BI chronology exhibits a positive January-August average maximum temperature signal for 1900-1975, after which it loses temperature sensitivity following the 1976/77 shift in northeast Pacific climate. This is a temporary loss of temperature sensitivity from about 1976 to 1999 that is not evident in RW or in a change in forest health but is consistent with prior work linking cedar decline to warming. The positive temperature response of BI after 1999 suggests a recovery, which remains strong for the most recent decades, although the coming years will continue to test this observation. A confounding factor is the uncertain influence of a shift in color variation from the heartwood/sapwood boundary. Future expansion of the yellow-cedar BI network and further investigation of the influence of the heartwood/sapwood transitions in the BI signal will lead to a better understanding of the utility of this species as a climate proxy.

Buma, B. 2018. Transitional climate mortality: slower warming may result in increased climate-induced mortality in some systems. Ecosphere 9: e02170.

Wiles G, Charlton J, Wilson R, D’Arrigo R, Buma B, Krapek J, Gaglioti B, Wiesenberg N, Oelkers R., 2019, Yellow-cedar blue intensity tree ring chronologies as records of climate and forest-climate response, Juneau, Alaska, USA. Canadian Journal of Forest Research,


Four New Publications from the Wooster Tree Ring Lab

Four new studies from the Wooster Tree Ring Lab have recently appeared in Ecology, Journal of Geophysical Research – Biosciences, The Holoceneand Chemosphere.

Brian Buma lead the study published in Ecologythat described the results of revisiting a classic ecological succession site in Glacier Bay National Park and Preserve. The article is titled100 years of primary succession highlights stochasticity and competition driving community establishment and stabilityWe blogged about some of the fieldwork for this study a few years ago here.

Abstract: The study of community succession is one of the oldest pursuits in ecology. Challenges remain in terms of evaluating the predictability of succession and the reliability of the chronosequence methods typically used to study community development. The research of William S. Cooper in Glacier Bay National Park is an early and well‐known example of successional ecology that provides a long‐term observational dataset to test hypotheses derived from space‐for‐time substitutions. It also provides a unique opportunity to explore the importance of historical contingencies and as an example of a revitalized historical study system. We test the textbook successional trajectory in Glacier Bay and evaluate long‐term plant community development via primary succession through extensive fieldwork, remote sensing, dendrochronological methods, and newly discovered data that fills in data gaps (1940’s to late 1980’s) in continuous measurement over 100+ years. To date, Cooper’s quadrats do not support the classic facilitation model of succession in which a sequence of species interacts to form predictable successional trajectories. Rather, stochastic early community assembly and subsequent inhibition have dominated; most species arrived shortly after deglaciation and have remained stable for 50+ years. Chronosequence studies assuming prior composition are thus questionable, as no predictable species sequence or timeline was observed. This underscores the significance of assumptions about early conditions in chronosequences and the need to defend such assumptions. Furthermore, this work brings a classic study system in ecology up to date via a plot size expansion, new baseline biogeochemical data, and spatial mapping for future researchers for its second century of observation.

Photo taken in the West Arm of Glacier Bay close to where the Cooper plots were “rediscovered” by Buma and others.


Dr. Ben Gaglioti (University of Alaska – Fairbanks) and collaborators just published another innovative study. This time Ben has assembled a time series of traumatic resin ducts (TRDs) in mountain hemlock that  is a record of past winter conditions and the strength of the Aleutian Low. The article appeared in the Journal of Geophysical Research: Biogeosciences.

The study included tree-ring records from four wild outer coast sites along the the Gulf of Alaska. This is the first work to use these TRD features in tree-rings as a proxy for winter storminess.

a – The clearly-stressed trees used in this study. b – Careful observations and measurements from increment cores were taken to work out the timing of maximum wind stress and storminess. c, d –  Examples of Traumatic Resin Ducts in the tree rings.

Once assembled, the decadal variability of the winter time record was clearly related to the Pacific Decadal Oscillation (see below). This new record is the first of its kind and gives us a new record of wintertime variability from the North Pacific.

Figure above shows a frequency diagram TRDs compared with indices of winter Pacific decadal variability. The records compare favorably giving us confidence in this new proxy technique and Ben’s interpretations.
Ben used some of the archives of the Wooster Tree Ring Lab as part of this study and his new technique is one that the Wooster lab can adopt and learn as we continue to analyze new collections and re-analyze our past collections.


Rob Wilson (University of St. Andrews) lead a study from the Yukon. He used blue intensity tree-ring records from white spruce to improve dendroclimatic temperature reconstructions from the southern Yukon.

The study is titled: Improved dendroclimatic calibration using blue intensity in the southern Yukon. and the abstract reads like this: In north-western North America, the so-called divergence problem (DP) is expressed in tree ring width (RW) as an unstable temperature signal in recent decades. Maximum latewood density (MXD), from the same region, shows minimal evidence of DP. While MXD is a superior proxy for summer temperatures, there are very few long MXD records from North America. Latewood blue intensity (LWB) measures similar wood properties as MXD, expresses a similar climate response, is much cheaper to generate and thereby could provide the means to profoundly expand the extant network of temperature sensitive tree-ring (TR) chronologies in North America. In this study, LWB is measured from 17 white spruce sites (Picea glauca) in south-western Yukon to test whether LWB is immune to the temporal calibration instabilities observed in RW. A number of detrending methodologies are examined. The strongest calibration results for both RW and LWB are consistently returned using age-dependent spline (ADS) detrending within the signal-free (SF) framework. RW data calibrate best with June–July maximum temperatures (Tmax), explaining up to 28% variance, but all models fail validation and residual analysis. In comparison, LWB calibrates strongly (explaining 43–51% of May–August Tmax) and validates well. The reconstruction extends to 1337 CE, but uncertainties increase substantially before the early 17th century because of low replication. RW-, MXD- and LWB-based summer temperature reconstructions from the Gulf of Alaska, the Wrangell Mountains and Northern Alaska display good agreement at multi-decadal and higher frequencies, but the Yukon LWB reconstruction appears potentially limited in its expression of centennial-scale variation. While LWB improves dendroclimatic calibration, future work must focus on suitably preserved sub-fossil material to increase replication prior to 1650 CE.

The Figure above shows the location of the Yukon study site and includes various other sites the Wooster lab has worked on in Alaska. Rob has been a great help in the efforts at the Wooster Tree Ring Lab facilitating our lab’s ability to perform these analyses.


Mary Garvin (Biology, Oberlin College) lead the study using tree-rings and chemical analyses entitled: A survey of trace metal burdens in increment cores from eastern cottonwood (Populus deltoides) across a childhood cancer cluster, Sandusky County, OH, USA.  

Abstract: A dendrochemical study of cottonwood trees (Populus deltoides) was conducted across a childhood cancer cluster in eastern Sandusky County (Ohio, USA). The justification for this study was that no satisfactory explanation has yet been put forward, despite extensive local surveys of aerosols, groundwater, and soil. Concentrations of eight trace metals were measured by ICP-MS in microwave-digested 5-year sections of increment cores, collected during 2012 and 2013. To determine whether the onset of the first cancer cases could be connected to an emergence of any of these contaminants, cores spanning the period 1970–2009 were taken from 51 trees of similar age, inside the cluster and in a control area to the west. The abundance of metals in cottonwood tree annual rings served as a proxy for their long-term, low-level accumulation from the same sources whereby exposure of the children may have occurred. A spatial analysis of cumulative metal burdens (lifetime accumulation in the tree) was performed to search for significant ‘hotspots’, employing a scan statistic with a mask of variable radius and center. For Cd, Cr, and Ni, circular hotspots were found that nearly coincide with the cancer cluster and are similar in size. No hotspots were found for Co, Cu, and Pb, while As and V were largely below method detection limits. Whereas our results do not implicate exposure to metals as a causative factor, we conclude that, after 1970, cottonwood trees have accumulated more Cd, Cr, and Ni, inside the childhood cancer cluster than elsewhere in Sandusky County.

Figure that shows the extent of the cancer cluster that coincides with more accumulated Cd, Cr and Ni in the tree-rings.