on theology & ecology

Now Available:

Letters from the Ecotone:
Ecology, Theology, and Climate Change
by Andrew Nagy-Benson & Andrea Lloyd

Letters from the Ecotone invites readers into an open-hearted dialogue between friends—a scientist and a pastor. In a series of letters written during the pandemic, Lloyd and Nagy-Benson explore the realities of climate change from the perspectives of ecology and Christian theology. The authors seek common ground, where science and religion meet and share a vision of flourishing life on earth. At a time when the climate crisis is quickly emerging as an existential threat, this book charts a journey imbued with the insights of ecological science and the wisdom of the Christian tradition.

Published by Resource Publications, an imprint of Wipf & Stock. Available from the publisher, at the links below, or at your local bookstore!

The Land Mourns: Hosea testifies to an earth that laments with its people
The Christian Century (September 1, 2022)

“To take seriously the land’s mourning is to acknowledge that the grief that we feel—solastalgia, ecological grief, climate grief—is wider than our own lives. In that acknowledgment is an invitation to become even greater participants. What might happen if we used creation’s lament as inspiration for what to do with our own too rarely acknowledged grief?”


on forest ecology & climate change

Eagle Summit, Alaska
A severe fire — the extent of which is still visible in the bright pink of fireweed growing in the most severely burned areas — led to rapid expansion of aspen trees into the tundra areas beyond the previous forest limit at this site.

Trees have fascinated me for as long as I can remember. Their stillness, their endurance of hard seasons: as a child, these mysteries called to me. I answered that call by becoming a forest ecologist. I was trained as a dendrochronologist, which means that I studied tree rings – the annual growth layers, visible on the surface of cut stumps, that record each year of a tree’s life – that tell its life story. What motivated my science was a version of the question that drew me as a child: how is it that a large, upright being – one that is rooted in place and can’t flee when the going gets tough – responds to and survives changes in the environment?

Tree Cores, Siberia
These cores, taken from two different trees, show the variability in annual growth rings within any tree. Wide rings reflect years when growing conditions were good; narrow rings reflect years when growing conditions were poor. The ring produced in the year 2000 is marked with four dots in each core – notice how the similarities in the patterns of wide and narrow rings between the two trees. That similarity indicates that the trees are responding to similar changes in their environment.

That question took me into the world of climate change, and to the Arctic, where temperature has a profound impact on life and where climate was warming faster than just about anywhere on earth. What would rapid warming mean, I wondered, for trees in the Arctic? The publications below record the answer to that question in much detail. In a nutshell, I found that trees did respond to warming in the Arctic. In the coldest forests – at treeline, that place where forests give way to more cold-adapted tundra vegetation – trees were colonizing places that had previously been too cold. Wherever I looked, I found the same pattern: young spruce trees, growing out in the tundra, beyond the limit of the older, taller trees. Forests were moving north.

Treeline, the Alaska Range.
Smaller, younger white spruce trees – almost all less than about 75 years old – are visible scattered throughout the shrubby tundra in the foreground. The older forests, in the lower stretches of this broad valley, can be seen in the background. Trees there are up to a few hundred years old.

But the story doesn’t end there. When we studied how fast trees were growing, my colleagues and I found that warming temperatures were leading to a decline in tree growth in many parts of the boreal forest, especially warmer locations. That might be due to drought stress, which can be exacerbated in warm years. Or, it might be that optimum temperatures for white spruce are already being exceeded. That change hints at bigger changes to come, if climate warming continues: even as they expand into colder tundra areas, cold-adapted species like white spruce are likely to decline in the warmer parts of their range – favoring species like aspen that are more tolerant of warmer temperatures.

Why does all this matter? As an ecologist, I’d say that it matters for the sake of these forests – and for the species in them. There’s a strong local impact of the kinds of changes we were seeing – one that affects spruce trees, boreal animals, and the people who live in the boreal forest zone. But it matters for the rest of us, too. Changes in the boreal forest are caused by climate – and they affect climate. As forests replace tundra, for example, the albedo of the earth changes: snow-covered tundra landscapes become dark, tree-covered forest landscapes that absorb more solar radiation and warm the local environment. And if forest growth declines, trees absorb and store less carbon dioxide – a change that would exacerbate human-caused warming. These climate feedbacks, as they’re known, are significant. And they mean that what happens in the boreal forest will have ripple affects far beyond the boreal zone.

That’s how I’d the answer the question of why this matters as an ecologist. But climate change matters to me as a Christian, too – for the same reasons, and for some different reasons. That’s the subject of the book 24 Letters, an excerpt of which is provided above. It’s also a subject that I reflect on in the sermons that I preached on 11/17/2021, 11/22/2020, and 9/2/2018 – the text of which can be found on the Preaching page of this website.

Treeline, interior Alaska
This photograph gives a visual sense of the difference in albedo – surface brightness – between tundra, in the foreground, and boreal forest, in the background. An expansion of the boreal forest dramatically reduces the surface albedo, which can amplify anthropogenic warming.

Links to and summaries of key findings from my published works are provided below, where available. Most of these articles are behind paywalls, but I am happy to provide PDF reprints on request (lloyd.andi@gmail.com).

Book chapters

Lloyd, A.H., P.F. Sullivan, and A.G. Bunn (2017)  Upper and Latitudinal Treelines.  In: M. Amoroso, P. Baker, J. J. Camaeror-Martinez, and L. Daniels, Editors.  Dendroecology: Tree-ring analyses applied to ecological studies.  Springer-Verlag, New York. DOI: 10.1007/978-3-319-61669-8

Despite a consistent global relationship between the position of alpine and arctic treeline and temperature, fine-scale variability in treeline response to climate is widespread. In this chapter, we describe two advances in the application of dendroecology to treeline environments. … Together, these approaches allow us to understand the causes of fine-scale variation in treeline response to warming, reconcile that variation with global-scale correlations between treeline and temperature, and better predict future responses of treeline ecosystems to warming.

(from the Abstract)

Lloyd, A. H., M. E. Edwards, B. P. Finney, J. Lynch, V. A. Barber, and N. Bigelow. (2006) Holocene Development of the boreal forest. Pages 62-78 in F. S. Chapin, III, M. Oswood, K. Van Cleve, L. Viereck, and D. Verbyla, editors. Alaska’s Changing Boreal Forest. Oxford University Press, Oxford.

Paleoecological data provide insight into patterns of change in vegetation and in the factors, such as climate and disturbance, that cause vegetation change. Disturbance by fire, insect, and mammalian herbivores and, in floodplains, flooding are the primary drivers of changes in population structure, community composition, and species distribution in the boreal forest on time scales of years to decades (Chapter 7). On longer time scales, such as centuries to millennia, the role of variation in regional climate in determining compositional changes in the boreal forest is also clearly visible. Variability in regional climate may act directly on boreal species (e.g., causing changes in species distributions) or indirectly, by altering disturbance regimes.

(from the Abstract)

Lloyd, A.H., P.A. Duffy, D.H. Mann. 2013. Nonlinear responses of white spruce growth to climate variability in interior Alaska. Canadian Journal of Forest Research43: 331-343, doi: 10.1139/cjfr-2012-0372.

Optimum climate conditions for white spruce are projected to become increasingly rare. This is likely to cause short-term declines in tree growth and forest productivity and to lead, over the longer term, to a contraction of white spruce to the cooler, moister parts of its range in Alaska.

Lloyd, A.H., A. Bunn, and L. Berner.  2010.  A latitudinal gradient in tree growth response to climate warming in the Siberian taiga.  Global Change Biology.  doi:10.1111/j.1365-2486.2010.02360.x.

We investigated how the growth of three Siberian taiga species, Larix cajanderi, Picea obovata, and Pinus sylvestris, responded to climate across a latitudinal gradient in central Siberia. We concluded that warming is likely to lead to increases in tree growth only at the very northernmost sites studied. Furthermore, evergreen conifers (Picea and Pinus) are likely to increase in prevalence in areas currently dominated by deciduous larch (Larix).

Chapin, F.S., A.D. McGuire, R.W. Ruess, T.N. Hollingsworth, M.C. Mac, J.F. Johnstone, E.S. Kasischke, E.S. Euskirchen, J. Jones, M. T. Jorgenson, K. Kielland, G.P. Kofinas, M.R. Turetsky, J. Yarie, A.H. Lloyd, D.L. Taylor.  2010.  Resilience of Alaska’s boreal forest to climatic change.  Canadian Journal of Forest Research.  40(7):1360-1370.

Projections of continued warming suggest that Alaska’s boreal forest will undergo significant functional and structural changes within the next few decades that are unprecedented in the last 6000 years. The impact of these social–ecological changes will depend in part on the extent of landscape reorganization between uplands and lowlands and on policies regulating subsistence opportunities for rural communities.

(From the Abstract)

Lloyd, A.H. and A.G. Bunn.  2007. Response of the circumpolar boreal forest to 20th century climate variability. Environmental Research Letters. 2 045013 (13pp)   doi:10.1088/1748-9326/2/4/045013.

In summary, our analysis of a circumpolar network of
tree-ring chronologies indicates that tree growth at many
boreal sites has declined since the mid-1900s, despite rising
temperatures. This phenomenon has occurred in all ten of
the species studied, although it has occurred with significantly
greater-than-expected frequency in five species: all members
of the genus Picea except P. sitchensis, and Pinus banksiana.
Our results suggest three likely explanations for the increased
prevalence of inverse responses to warming: direct temperature
stress, temperature-mediated drought stress, and, in some
locations, pollution

(from the article, page 11)

Lloyd, A.H., C.L. Fastie, and H. Eisen.  2007.  Fire and substrate interact to control the northern range limit of black spruce (Picea mariana) in Alaska.  Canadian Journal of Forest Research.  37:2480-2493.

Our data thus suggest that the distribution of black spruce ecosystems is unlikely to change in response to future climate warming unless fire, and with it the conditions that promote sexual reproduction, becomes more common; both models and historical studies seem to indicate that this is likely in Alaska. 

(from the Discussion)

F. S. Chapin, III*, M. Sturm, M. C. Serreze, J. P. McFadden, J. R. Key, A. H. Lloyd, A.D. McGuire,  T. S. Rupp, A. H. Lynch, J. P. Schimel, J. Beringer, W. L. Chapman, H.E. Epstein, E. S. Euskirchen, L. D. Hinzman, G. Jia, C.-L. Ping, K. D. Tape, C. D. C.Thompson , D. A. Walker, and J. M. Welker.  2005. Role of Land-Surface Changes in Arctic Summer Warming.  Science. 310(5748):657-660.

Pronounced terrestrial summer warming in arctic Alaska correlates with a lengthening of the snow-free season that has increased atmospheric heating locally by about 3 watts per square meter per decade (similar in magnitude to the regional heating expected over multiple decades from a doubling of atmospheric CO2). The continuation of current trends in shrub and tree expansion could further amplify this atmospheric heating by two to seven times.

(from the Abstract)

Lloyd, A.H., A.E. Wilson, C. L. Fastie, R. M. Landis.  2005.  Population dynamics of black and white spruce in the southern Brooks Range, Alaska.  Canadian Journal of Forest Research. 35 (9): 2073-2081

Lloyd, A.H. 2005.  Ecological histories from Alaskan treelines provide insight into future change.  Ecology.  86(7):1687-1695. https://doi.org/10.1890/03-0786

Tree line advance is ubiquitous, but asynchronous in time, occurring significantly earlier in the White Mountains in interior Alaska than in western Alaska or the Alaska Range. … Although continued advance of white spruce forests is the most likely scenario of future change, variability in the rate of forest response to warming may be likely due to limitation of spruce establishment in highly permafrost-affected sites, changes in seed dispersal and early establishment, and recent changes in the growth responses of individual trees to temperature. 

(From the Abstract)

Hinzman, L., N. Bettez, F.S. Chapin, M. Dyurgerov, C. Fastie, B. Griffith, A. Hope, H.P. Huntington, A. Jensen, D. Kane, D.R. Klein, A. Lynch, A. Lloyd, A.D. McGuire, F. Nelson, W.C. Oechel, T. Osterkamp, C. Racine, V. Romanovsky, D. Stow, M. Sturm, C.E. Tweedie, G. Vourlitis, M. Walker, D. Walker, P.J. Webber, J. Welker, K. Winker and K. Yoshikawa. Evidence and implications of recent climate change in terrestrial regions of the Arctic. (2005). Climate Change. 72 (3): 251-298

Our holistic review presents a broad array of evidence that illustrates convincingly; the Arctic is undergoing a system-wide response to an altered climatic state. New extreme and seasonal surface climatic conditions are being experienced, a range of biophysical states and processes influenced by the threshold and phase change of freezing point are being altered, hydrological and biogeochemical cycles are shifting, and more regularly human sub-systems are being affected. 

(From the Abstract)

Overpeck, J. et al.  2005.  Arctic system on Trajectory to New, Seasonally Ice-Free State.  EOS.  86: 309-316.

The Arctic system is moving toward a new state that falls outside the envelope of glacial-interglacial fluctuations that prevailed during recent Earth history. This future Arctic is likely to have dramatically less permanent ice than exists at present. At the present rate of change, a summer ice-free Arctic Ocean within a century is a real possibility, a state not witnessed for at least a million years. The change appears to be driven largely by feedback-enhanced global climate warming, and there seem to be few, if any processes or feedbacks within the Arctic system that are capable of altering the trajectory toward this “super interglacial” state.

(From the Abstract)

Epstein, H.E.,  J. Beringer, W.A. Gould, A.H. Lloyd, C.D. Thompson, F.S. Chapin III, G.J. Michaelson, C.L. Ping, T.S. Rupp, D.A. Walker. 2004.  The nature of spatial transitions in the Arctic.  Journal of Biogeography. 31:1917-1933.

Lloyd, A.H., K. Yoshikawa, C. L. Fastie, L. Hinzman, and M. Fraver.  (2003).  Effects of permafrost degradation on woody vegetation at arctic treeline on the Seward Peninsula, Alaska.  Permafrost and Periglacial Processes. vol 14. doi: 10.1002/ppp.446.

Lloyd, A.H. and C.L. Fastie (2003).  Recent changes in treeline forest distribution and structure in interior Alaska.  Ecoscience. 10(2):176-185.

Treeline advance was ubiquitous: cone-bearing spruce are present beyond the current forest limit at all but one site, and tree density has increased at and beyond the forest limit in recent decades at all sites. 

(from the Abstract)

Fastie, C.L., A.H. Lloyd, and P. Doak (2003).  Fire history and postfire development in an upland watershed of interior Alaska.  Journal of Geophysical Research.  Vol. 18, No. D1, 8150.  Doi:10.1029/2001JD0000570.

Lloyd, A.H., T. S. Rupp, C.L. Fastie, and A. M. Starfield.  (2002).  Patterns and dynamics of treeline advance on the Seward Peninsula, Alaska.  Journal of Geophysical Research-Atmospheres. 107(D2): Article no. 8161.

Our reconstructions of forest response to past warming indicate that in upland tundra spruce have successfully established progressively farther from the forest limit since the 1880s. Shrub tundra has thus been converted to low-density forest–tundra within a band extending approximately 10 km from the forest limit. 

(from the Abstract)

Sveinbjörnsson, B., A. Hofgaard, and A.H. Lloyd.  2002.  Natural causes of the tundra-taiga boundary.  Dynamics of the Tundra-Taiga InterfaceAmbio. Special Report No. 12: 23-29.

Lloyd, A.H. and C.L. Fastie.  2002.  Spatial and temporal variability in tree growth and climate response of treeline trees in Alaska.  Climatic Change.  52:481-509.

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. 

(from the Abstract)

Lloyd, A.H.  1998.  Elevational controls over foxtail pine seedling growth at treeline in the Sierra Nevada.  EcoScience. 5(3): 250-257.

Lloyd, A.H.  1997.  Response of treeline populations of foxtail pine (Pinus balfouriana) to climate variation over the last 1,000 years.  Canadian Journal of Forest Research. 27:936-942.

The inverse correlation between recruitment and climate suggests
that water balance may mediate the effects of temperature on tree-line forests, a hypothesis that is supported by a significant
positive correlation between seedling establishment and winter snowpack during the last 50 years. Despite large changes in
the elevation of tree line at these sites during the time period of the reconstruction, populations near tree line were largely
unaffected by climate variation, suggesting that steep gradients in vulnerability to climate change may exist at tree line in the
Sierra Nevada

(from the Abstract)

Lloyd, A.H. and L.J. Graumlich.  1997.  A 3,500 year record of changes in the structure and distribution of forests at treeline in the Sierra Nevada, California, U.S.A.  Ecology. 78 (4): 1199-1210.

We reconstructed a 3500-yr history of fluctuations in treeline elevation and tree abundance in the southern Sierra Nevada. Treeline elevation was higher than at present throughout most of the last 3500 yr. Declines in the abundance of live trees and treeline elevation occurred twice during the last 1000 yr: from 950 to 550 yr BP and from 450 to 50 yr BP. The earlier decline coincided with a period of warm temperatures (relative to present) in which at least two severe, multidecadal droughts occurred. This decline was apparently triggered by an increase in the rate of adult mortality in treeline forests. The more recent decline occurred during a period of low temperatures lasting for up to 400 yr and was apparently caused by a sustained failure of regeneration in combination with an increased rate of adult mortality. The apparent past importance of precipitation in controlling the position and structure of the treeline ecotone suggests that climatic controls over treeline may be more complex than previously thought. In the Sierra Nevada, responses of high-elevation forests to future warming may depend strongly on water supply.

(from the Abstract)

Graumlich, L.J. and A.H. Lloyd.  1996.  Dendroclimatic, ecological and geomorphological evidence for long-term climatic change in the Sierra Nevada, U.S.A.  in Dean, J.S. , D.M. Meko, T.W. Swetnam, Eds.  Tree-Rings, Humanity, and the Environment.  Radiocarbon, Tucson.  Pp. 51-59.

Conkey, L.E., M. Keifer, and A.H. Lloyd.  1995.  Disjunct jack pine (Pinus banksiana Lamb.) structure and dynamics, Acadia National Park, Maine.  Ecoscience.  2(2):168-176.

Lloyd, A.H., W.S. Armbruster, and M.E. Edwards.  1994.  Ecology of a steppe-tundra gradient in interior Alaska.  Journal of Vegetation Science.  5:897-912.

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