Paleoclimate - Dendrochronology and Dendroecology

What are the main distinctions between dendroecology and dendroclimatology? Use specific examples to support your arguments.

Fiona Fang, Trinity Hall, 11/2024, 998 words

Dendroclimatology and dendroecology are both fields that study the annual growth rings in woody plants. The former uses the information from dated rings to investigate past and present climates, while the latter focuses on changes in local ecology. This essay will evaluate the key distinctions between these disciplines, including (1) their aims and focuses, (2) their sampling and data processing methods, and (3) their spatial and temporal scopes.

Dendroclimatology focuses on using tree rings to reconstruct past climate conditions, such as temperature, precipitation, and drought patterns. In a temperature reconstruction based on an assembly of tree rings from the extratropical Northern Hemisphere, approximately 90% of reconstructed grid points indicate warmer conditions during the Medieval Climate Anomaly compared to the Little Ice Age, with spatially variable magnitudes. The cooling effects of volcanic eruptions is also captured, with 96% of reconstructed grid points showing colder conditions following eruptions between 750 and 1988 CE (Anchukaitis et al., 2017). Similarly, dendroclimatic studies use tree rings to reconstruct precipitation patterns. For instance, tree-ring data from Platycladus orientalis near Bailin Castle suggests that the decline of the Ming Dynasty correlates with abnormally low precipitation and its subsequent ecological impacts (Chen et al., 2022). Juniper samples in the Qinghai-Tibet Plateau (QTP) reveals that precipitation of the last 50 years was historically high for this region, exceeding levels observed in at least 3,500 years (Yang et al., 2014). Furthermore, broad-scale tree-ring networks support the reconstruction of general circulation patterns and climatic oscillations. For example, tree-ring and ice core isotope data reveal that the North Atlantic Oscillation (NAO) was predominantly in positive phase from the late 15th to early 17th centuries, showed lower variability during the 18th and 19th centuries, which then increased again in the 20th century, with low-frequency variability modulated by anthropogenic greenhouse gas forcing (Cook, D’Arrigo and Mann, 2002). Tree-ring studies also enhance our understanding of global circulation systems through correlations with variables like sea surface temperature, as demonstrated in the southern Indian Ocean (Cook et al., 2000) and West Africa (Schöngart et al., 2006).

Dendroecology uses tree rings to analyze ecological disturbances, forest dynamics, and ecosystem changes over time. Fires often damage the cambial layer of trees, leaving scars in woody tissue that persist as records of past fire events (Bradley, 2015). For example, research on dry conifer forests in the Southwestern United States reveals that while contemporary fire frequency is less than 20% of historical levels, fire severity has increased significantly (McClure et al., 2024). Insect outbreaks can also be reconstructed using tree-ring characteristics, as defoliation suppresses radial growth and alters earlywood and latewood widths, wood density, cell structure, and even isotopic composition (Lynch, 2012). In the Swiss Alps, studies of larch bud moth defoliation in European larch (Larix decidua Mill.) show that blue intensity is a reliable proxy for reconstructing defoliation events (Arbellay et al., 2018). Ulrich et al., (2022) argues stable isotope to be another crucial proxy for the reconstruction. In addition to fire events, forest succession, the progressive development of plant communities following stand-replacing disturbances like glacial retreat or volcanic eruptions, can also be examined through tree ring studies (Speer, 2010). For instance, Fastie (1995) documented primary succession in Glacier Bay, Alaska, by analyzing forest development over 250 years after glacial retreat. He found that in this area, primary forest succession was accelerated because of proximity to seed sources along the trimline of the glacier, which is the highest elevation that the glacier ice reaches. Mature trees left above the trimline produced seeds that repopulated the newly exposed ground.

The divergent goals of dendroecology and dendroclimatology—ecological processes versus climatic reconstruction—result in different sampling and data processing methods, as well as varying scales of interest. For sampling, dendroecology studies generally select a variety of tree species within an ecosystem to capture responses to ecological factors. For example, stand-age structures require that all living and dead trees be sampled in a plot to quantify the current forest composition and past conditions (Bergeron, 2000). Dendroclimatology, on the other hand, focuses on climate-sensitive species, such as those at the latitudinal or altitudinal limits of their range. Like dendroecology studies, dendroclimatology studies required multiple trees from a site to be sampled, but the aim is for building a robust sample depth and enhancing the reliability of climate reconstruction (Speer, 2010, p 274). As for data processing, ecological variabilities unrelated to climate need to be removed in dendroclimatology studies (Cook et al., 1990). Conversely, dendroecology studies, climate is often the “noise” and should be controlled or removed so that the effect of the disturbance agent can be isolated (Speer, 2010, p299). These climate subtraction techniques ae well-establised, particularly in studies of the western spruce budworm (eg. Weber and Schweingruber, 1995).

Regarding spatial scale, dendroecology tends to focus on localized reconstructions, providing insights into specific ecological events or conditions within a forest or region. In contrast, dendroclimatology often contributes to larger-scale climate reconstructions, sometimes covering entire hemispheres. Therefore, climate division data is often preferred to individual site data in setting up and calibrating the reconstruction (Speer, 2010). Finally, for temporal scale, dendroecology studies often focus on short-term events. For example, pouglas-fir tussock moth produced a four to five-year signature of sharply reduced growth, and the western spruce budworm generally lasted fore 10 years (Speer, 2010). Dendroclimatology, however, typically focuses on long-term continuous records spanning hundreds or thousands of years, though sudden events like floods or water table changes may also be of interest (e.g., Sigafoos, 1964, reconstruction of Potomac River flood history in Washington, DC).

To summarize, dendroecology and dendroclimatology both rely on tree ring records but differ in their aims, methods, and scales of interest. Both fields have high social-economic significance: dendroecology provides insights that aid forest management, conservation, and understanding ecological responses to environmental stressors, while dendroclimatology is essential for modeling global climate patterns and guiding climate change policy decisions (Speer, 2010). They can also be combined to cross-validate findings, providing a comprehensive understanding of how climate influences ecological changes over time.

Supervisor Feedback: Great essay, well done!! It is very well-written, well-structured, engages with many relevant examples and draws many comparisons. Very well done! 

##Bibliography

Anchukaitis, K.J. et al. (2017) ‘Last millennium Northern Hemisphere summer temperatures from tree rings: Part II, spatially resolved reconstructions’, Quaternary Science Reviews, 163, pp. 1–22. Available at: https://doi.org/10.1016/j.quascirev.2017.02.020.

Arbellay, E. et al. (2018) ‘Tree-ring proxies of larch bud moth defoliation: latewood width and blue intensity are more precise than tree-ring width’, Tree Physiology, 38(8), pp. 1237–1245. Available at: https://doi.org/10.1093/treephys/tpy057.

Bergeron, Y. (2000) ‘Species and Stand Dynamics in the Mixed Woods of Quebec’s Southern Boreal Forest’, Ecology, 81(6), pp. 1500–1516. Available at: https://doi.org/10.1890/0012-9658(2000)081[1500:SASDIT]2.0.CO;2.

Bradley, R.S. (2015) Paleoclimatology: reconstructing climates of the quaternary. Third edition. Amsterdam: Elsevier.

Chen, F. et al. (2022) ‘Abnormally low precipitation-induced ecological imbalance contributed to the fall of the Ming Dynasty: new evidence from tree rings’, Climatic Change, 173(1), p. 13. Available at: https://doi.org/10.1007/s10584-022-03406-y.

Cook, E. et al. (1990) ‘Data Analysis’, in E.R. Cook and L.A. Kairiukstis (eds) Methods of Dendrochronology. Dordrecht: Springer Netherlands, pp. 97–162. Available at: https://doi.org/10.1007/978-94-015-7879-0_3.

Cook, E.R. et al. (2000) ‘Warm-season temperatures since 1600 BC reconstructed from Tasmanian tree rings and their relationship to large-scale sea surface temperature anomalies’, Climate Dynamics, 16(2), pp. 79–91. Available at: https://doi.org/10.1007/s003820050006.

Cook, E.R., D’Arrigo, R.D. and Mann, M.E. (2002) ‘A Well-Verified, Multiproxy Reconstruction of the Winter North Atlantic Oscillation Index since a.d. 1400’. Available at: https://journals.ametsoc.org/view/journals/clim/15/13/1520-0442_2002_015_1754_awvmro_2.0.co_2.xml (Accessed: 16 November 2024).

Fastie, C.L. (1995) ‘Causes and Ecosystem Consequences of Multiple Pathways of Primary Succession at Glacier Bay, Alaska’, Ecology, 76(6), pp. 1899–1916. Available at: https://doi.org/10.2307/1940722.

Lynch, A.M. (2012) ‘What Tree-Ring Reconstruction Tells Us about Conifer Defoliator Outbreaks’, in Insect Outbreaks Revisited. John Wiley & Sons, Ltd, pp. 126–154. Available at: https://doi.org/10.1002/9781118295205.ch7.

McClure, E.J. et al. (2024) ‘Contemporary fires are less frequent but more severe in dry conifer forests of the southwestern United States’, Communications Earth & Environment, 5(1), pp. 1–11. Available at: https://doi.org/10.1038/s43247-024-01686-z.

Schöngart, J. et al. (2006) ‘Climate–growth relationships of tropical tree species in West Africa and their potential for climate reconstruction’, Global Change Biology, 12(7), pp. 1139–1150. Available at: https://doi.org/10.1111/j.1365-2486.2006.01154.x.

Sigafoos, R.S. (1964) Botanical Evidence of Floods and Flood-plain Deposition. U.S. Government Printing Office.

Speer, J.H. (2010) Fundamentals of Tree-ring Research. University of Arizona Press.

Ulrich, D.E.M. et al. (2022) ‘Insect and Pathogen Influences on Tree-Ring Stable Isotopes’, in R.T.W. Siegwolf et al. (eds) Stable Isotopes in Tree Rings: Inferring Physiological, Climatic and Environmental Responses. Cham: Springer International Publishing, pp. 711–736. Available at: https://doi.org/10.1007/978-3-030-92698-4_25.

Weber, U.M. and Schweingruber, F.H. (1995) ‘A dendroecological reconstruction of western spruce budworm outbreaks (Choristoneura occidentalis) in the Front Range, Colorado, from 1720 to 1986’, Trees, 9(4), pp. 204–213. Available at: https://doi.org/10.1007/BF00195274.

Yang, B. et al. (2014) ‘A 3,500-year tree-ring record of annual precipitation on the northeastern Tibetan Plateau’, Proceedings of the National Academy of Sciences, 111(8), pp. 2903–2908. Available at: https://doi.org/10.1073/pnas.1319238111.