The chronosequence approach


Change is the only constant; time marches forward whether we want it to or not. Some of us would like time to slow down so we could enjoy our summer weekends for a little while longer. Many of us wish we could slow it down and finally make time to smell the proverbial roses.

Cliffs of Varna. This outcrop gives us an idea of what the land looked like without trees.

But a being a scientists, I would like to see things in fast forward, or even reverse. Not just to see how things were in the 1800s, but how have things changed over the decades and centuries. Seeing what the landscape and forests of the northeastern US looked like before European settlers, or even before Native Americans tribes moved in.
Since we lack time travel (at least at the moment), we have to find other ways to time travel and witness how forests and landscapes have changed. One of the prime examples of doing this, at least for soils, is the use of the chronosequence approach, which substitutes space for time. Thus, using forests clear-cut at different ages, we might be able to ‘watch’ a change in nutrients through time.

In a study I conducted with Dr. Andy Friedland and fellow Friedland lab alumnus Dr. Chelsea Petrenko, we used a chronosequence to look at changes in nutrients in forest stand age after clear-cutting. We sampled soils in different aged forests (1 – 120+ years) to look at how nutrients have changed through time. This approach relies on some key assumptions, that each forest soils was similar before the clear-cutting event. That changes are due to forest regeneration not other processes like fires or pollution. Lastly, it assumes that we can see differences due to clear-cutting above the natural noise of spatial variation in soils.
Our study found nutrients to be significantly impacted due to clear-cutting. Roughly, 50 – 60% of Calcium and Magnesium were lost over 50 years following clear-cutting. A similar loss of 40 – 70% was observed for micronutrients, Cu, Zn, and Mn. Thus, if you were to sample the soils for nutrients after clear-cutting every year over fifty years, you’d see a big loss in nutrients. While this isn’t as cool as a time-machine, this approach has let us zip through 100+ years of forest aging in a matter of a few years of collecting data and analyzing it.

The loss of Calcium and Magnesium could be bad for forests but the trees may be okay if they are able to make up the difference in nutrients by obtaining their nutrients directly from rocks. Their roots secrete organic acids and promote bacteria that help ‘eat’ rocks for nutrients.
Hopefully we use a similar approach to determine if the forests will be okay in the long run. Its just a matter of sitting down and designing a nice experiment to ‘time-travel’ again.

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