Q&A with Dr. Brett Eaton
Assistant Professor in Physical Geography, University of British Columbia

On a walk through a forest, you may see mounds created by uprooted trees and characterized by stilt-like roots. These trees have been toppled by wind and other factors. Enough such trees of similar age in an area can give a minimum time since the windstorm that toppled the original stand. The technique, drawn from an older natural history tradition, was integrated into the “historical reconstructive technique” developed by Earl Stephens for his 1955 PhD dissertation at Harvard University. Looking at the age, growth rates, position, shape, and species of trees, the micro topography, and the mixing of soil types, Stephens was able to construct a detailed forest and soil history in a small area of the Harvard Forest.

You can see an example of these trees in our “Across Canada” photos section.

Foster, D., G. Motzkin, J. O’Keefe, E. Boose, D. Orwig, J. Fuller, and B. Hall. “The Environmental and Human History of New England.” In Forests in Time: the environmental consequences of 1,000 years of change in New England, edited by David Foster and John Aber, 43-100. New Haven and London: Yale UP, 2004.

1. A common feature in some forested landscapes is an uprooted tree mound with a younger tree growing on top of it. What causes these trees to fall, and how rapidly do new trees establish themselves in this context?

In most cases, trees fall due to disease or old age, combined with a wind storm. In some locations, fire first compromises the strength of the trees, and then they blow over. Also, exceptionally strong windstorms are capable of blowing over perfectly healthy, young trees. Most colonization of exposed ground occurs in the first decade after disturbance, though in some cases, a seedling may have been established prior to the disturbance.

2. How do researchers date these tree mounds?

These features are typically dated by counting the tree rings of colonizing trees. It involves either extracting a small core from the tree using an increment borer or by cutting the colonizing tree down and taking a slice of the trunk back to the lab.

3. How do researchers use these findings? Can you give any examples of projects that have used this method?

These sorts of features can be used to date the disturbance history. Therefore, they can be used to investigate a wide range of processes, including debris flows, forest fires, and the lateral migration of stream channels. A good example of this is the PhD work done by Matthias Jacob, who has used tree rings and other disturbance indicators to reconstruct the debris flow frequency characteristics of the Coast Mountains.

4. What are some other methods that might be used alongside this to develop a sense of the natural history of a forest (e.g. soil analysis, microtopographies)?

Other methods that can be used to date the natural history of a forest include Carbon 14 dating, for which dramatic improvements in precision and accuracy have been achieved in recent years; tree-ring analysis of various fallen and dead tree;, and a range of recent dating techniques, including optical luminescence dating and cosmogenic radionuclide.

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Feature image: Forested Landscape (circa 1700), Cornelis Huysmans. Accessed via Wikimedia Commons.