HTRL Research

Researchers in the lab work across many aspects of dendrochronology including work in dendroecology, dendroclimatology, ecosystem ecology. We are not especially diligent about keeping up this website unfortunately. Please get in touch to find out what we are up to now. 



Drought, fire, and insect outbreaks can affect tree growth and demographics. In the lab, there are ongoing projects to build multi-century dendrochronological reconstructions of insect outbreaks like those of the western spruce budworm which can show impressive synchrony ove time and space. See these publications for more information:

Flower, A. 2016. Three Centuries of Synchronous Forest Defoliator Outbreaks in Western North America. PLoS ONE, 11(10), e0164737. PDF and HTML.

Gavin, D.G., Flower, A., Heyerdahl, E.K., Parsons, R., Cohn, G. 2016. Western spruce budworm and wildfire: is there a connection? Fire Management Today.

Flower, A., Gavin, D. G., Heyerdahl, E. K., Parsons, R. A., & Cohn, G. M. (2014). Western spruce budworm outbreaks did not increase fire risk over the last three centuries: a dendrochronological analysis of inter-disturbance synergism. PLoS ONE, 9(12), e114282.

Flower, A., Gavin, D. G., Heyerdahl, E. K., Parsons, R. A., & Cohn, G. M. (2014). Drought-triggered western spruce budworm outbreaks in the interior Pacific Northwest: A multi-century dendrochronological record. Forest Ecology and Management, 324, 16. 

Marine-Derived Nitrogen

When Pacific Salmon migrate from the ocean to freshwater they bring with them nutrients (e.g., nitrogen, carbon) consumed in the marine environment that differ isotopically from the same elements in the riparian and terrestrial landscape. The tissues of those fish are enriched with those heavier marine-derived nutrients and when those fish spawn and die, their carcasses fertilize the surrounding forests. A small fraction of those nutrients get bound up in the xylem of long-lived riparian trees. By doing isotopic analysis on the rings of those trees it might be possible to learn about the nutrient fluxes and biogeochemistry of the ecosystem going back decades to centuries. Work in the lab has focused on determining how mobile this heavy nitrogen is in the cambium.

Bunn A G, Helfield J M, Gerdts J R, Gleaves D A, Drake D C, Sheppard P R (2017) Solvent-based extraction fails to remove mobile nitrogen from sapwood of western red cedar (Thuja plicata). Dendrochronologia, doi:10.1016/j.dendro.2017.03.001


Bristlecone Pine

Bristlecone Pine (Pinus longaeva) are the oldest living non-clonal trees in the world – reaching ages over 5000 years and many these trees are excellent paleoclimate proxies. It can be difficult to separate the dual limiting factors of temperature and precipitation on growth but work in the lab is helping to unravel these climate|growth relationships by using tools from landscape ecology. Our work has shown that individual trees can be subject to different limiting factors depending on small variations in their topographic setting. We are currently working to model the growth response of individual trees growing at the alpine treeline by mapping cold air pooling and soil moisture variability using GIS. By doing so we hope to improve our understanding of climate variability over the late Holocene.

For more information on this project check out a popular science article that highlights our work integrating GIS and tree rings here. You can also look at these peer-reviewed publications involving the lab:

Bruening J M, Tran T J,  Bunn A G, Salzer M W, Weiss S B (2017) Fine-scale modeling of bristlecone pine treeline position in the Great Basin, USA. Environmental Research Letters, doi:10.1088/1748. Environmental Research Letters, doi:10.1088/1748.W., Larson E., Bunn A.G., Hughes M.K.  (2014) Climate response in near-treeline bristlecone pine. Environmental Research Letters, doi:10.1088/1748.W., Bunn, A.G., Graham, N.E., and M.K. Hughes. 2013. Five millennia of paleotemperature from tree-rings and treeline change in the Great Basin USA. Climate Dynamics. doi: 10.1007/s00382.

Bunn, A.G., M.K. Hughes, and M.W. Salzer. 2011. Topographically modified tree-ring chronologies as a potential means to improve paleoclimate inference. Climatic Change. doi: 10.1007/s10584.

Salzer, M.W., M.K. Hughes, A.G. Bunn, and K.F. Kipfmueller. 2009. Recent unprecedented tree-ring growth in bristlecone pine at the highest elevations and possible causes. Proceedings of the National Academy of Sciences. doi: 10.1073pnas.0903029106.

Alaskan Yellow Cedar

Dendroclimatology relies on exploiting climate-limited tree growth as a proxy for past climates. The temperate, mesic environments of Western Washington and Oregon do not typically provide a wealth of species or sites for climate reconstructions that extend multiple centuries. Pacific Northwest Alaskan Yellow Cedar (Xanthocyparis nootkatensis) was previously thought to be a poor paleoclimate resource due to growth asymmetries caused by buttressing and a propensity for false and missing rings. However, recent work has shown that remnant high elevation patches of Yellow-cedar are a viable and unexploited paleoclimate resource. They are also considered the oldest species in the region, with some individual living over 1800 years.

In our preliminary work, we've developed a highly sensitive ring-width chronology that dates back to 1186 AD where growth is significantly correlated to temperature, and distinct climatic episodes (e.g., the Little Ice Age) are well represented. With continued expansion of this project, there is very high likelihood of developing a millennial-length regional temperature reconstruction along the western Cascades where none now exists.

Ecosystem Ecology

Boreal Forest Greening

The expansion of forest vegetation within and into the Arctic is one of the profound transformations that the Arctic land surface is likely to undergo in the coming decades. The spread of forest vegetation has significant ramifications for the Arctic System, as it is likely to cause both positive and negative feedbacks on climate, and to alter the availability of crucial natural resources. Although forest expansion within and into the Arctic has been widespread in recent decades, there is growing evidence that non-linear responses to warming may prevail within areas of expanding forest. In particular, large areas of 'browning' (declining in NDVI) have recently been identified in the southern Arctic.

We are working to integrate measures of tree growth from the ground with space-based measures of canopy reflectance across the boreal. We’ve shown that it is possible to build statistical models that relate tree growth to spectral vegetation indices and that the population-level response trumps strong long-term trends in productivity.

For more information check out a popular-press article in Scientific American that mentions this work as well these peer-reviewed papers:

Bunn, A.G., Hughes, M.K., Kirdyanov, A.V., Losleben, M., Shishov, V.V., Berner L.T., Oltchev, A., and E.A. Vaganov. 2013. Comparing forest measurements from tree rings and a space-based index of vegetation activity in Siberia. Environmental Research Letters. doi:10.1088/1748.

Bond-Lamberty, B., Bunn, A.G., and A.M. Thomson. 2012. Forest browning and large-scale lag effects drive recent trends in high-latitude soil respiration. PLoS ONE. doi:10.1371/journal.pone.0050441.

Berner, L., Beck P., Bunn A.G., Lloyd A.H., and S.J. Goetz. 2011. High-latitude tree growth and satellite vegetation indices: Correlations and trends in Russia and Canada (1982. Journal of Geophysical Research – Biogeosciences. doi:10.1029/2010JG001475.

Lloyd A.H., Bunn A.G., and L. Berner. 2011. A latitudinal gradient in tree growth response to climate warming in the Siberian taiga. Global Change Biology. doi:10.1111/j.1365.2010.02360.x

The Polaris Project

 The Polaris Project is an innovative international collaboration among students, teachers, and scientists seeking to address how carbon is transported and transformed as is moves from the terrestrial uplands to the Arctic Ocean in the Kolyma River watershed. Funded by the National Science Foundation since 2008, the Polaris Project trains future leaders in arctic research and informs the public about the Arctic and global climate change. During the annual month-long field expedition to the Siberian Arctic, undergraduate students conduct cutting-edge investigations that advance scientific understanding of the changing Arctic.

For more information on how to become a part of the Polaris Project visit our website. Here are a few peer-reviewed papers stemming from the lab's involvement with Polaris:

Webb E, Heard K, Natali S, Bunn A G,  Alexander H, Berner L, Kholodov A, Loranty M, Schade J, Spektor V, Zimov N (2017) Variability in above and belowground carbon stocks  in a Siberian Larch watershed. Biogeosciences, 10.5194/bg., P Beck, A.G. Bunn, and S.J. Goetz. 2013. Plant growth and climate variability along the forest-tundra ecotone in northeastern Siberia. Global Change Biology. doi: 10.1111/gcb.12304.

Xu, L. Myneni, R.B., Chapin III, F.S., Callaghan, T.V., Pinzon, J.E., Tucker, C.J., Zhu Z., Bi, J., Ciais, P., Tømmervik, H., Euskirchen, E.S., Forbes, B.C., Piao, S.L., Anderson, B.T. Ganguly, Nemani, S., Goetz, S.J., Beck, P., Bunn, A.G., Cao, C., and J.C. Stroeve. 2013. Temperature and vegetation seasonality diminishment over northern lands. Nature Climate Change. doi:10.1038/nclimate1836.


Species Distribution Modeling

Climate-induced range shifts are being documented worldwide, across ecosystems and taxa. The exceptional velocity of anthropogenic climate change is challenging species responses through dispersal, migration and adaptation.  Will species be able to migrate quickly enough to keep track of their climatic niche? Will dispersal limitations impede migratory progress and result in local extinctions? We are working on these and other questions using species distribution modeling, a correlative approach that relates species occurrence to climate and environmental variables. By integrating independent data sets (such as genetics) we are improving the predictive capacity of these models.   Calibrated predictive models can then be used to generate conservation and management recommendations that prioritize the maintenance of genetic diversity, the mitigating of extinction risks, and ecosystem stability in the face of climate change.

Here is a peer-reviewed paper on SDMs:

Forester, B., DeChaine, E., and A.G. Bunn. Integrating ensemble species distribution modeling and statistical phylogeography to inform projections of climate change impacts on species distributions. Diversity and Distributions. doi:10.1111/ddi.12098.