Understanding Long-Term Ecological Change with Tree Rings

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Categories: GSI Online LibraryTeaching Effectiveness Award Essays

by Kevin Krasnow, Environmental Science, Policy, and Management

Teaching Effectiveness Award Essay, 2012

ESPM 11, Americans and the Global Forest, explores how humans have managed and impacted forest ecosystems in the USA and abroad. One persistent problem we encounter when teaching about the history of forests is that it is difficult for students to grasp the time-scale of a forest. The long-lived trees and slow rate of ecological change in a forest make it difficult for students to appreciate the scale and magnitude of historic changes that have had a profound impact on the forest we see today. In the class, we explored local forests and showed photos and videos, but we didn’t have any meaningful way for students to interact with long-term forest change. I decided to leverage my own research to devise an inquiry-based experience for students to explore the history of our own Sierra Nevada forests.

A major pillar of my own research is to reconstruct forest fire frequency and spatial patterns using fire-scar dendrochronology. Simply put, we can use tree ring samples from trees that have been injured, but not killed, by a wildfire to date fire events in the past. Single samples can record tens of fire events going back hundreds of years. The major goal was for students to understand how the fire regime of the Sierra Nevadas was dramatically altered with the advent of Euro-American management in the early twentieth century, and what impact this has had on the forests of today. At this time, frequent, low-severity fires (often purposely set by Native Americans) were halted and replaced with a strict policy of fire suppression, which has greatly altered the structure and composition of the Sierran Forests over the past century. Without frequent fires, the forests have become much more dense with young trees, are more prone to large damaging wildfires, and are increasingly dominated by one or two individual species.

For the lesson, I selected high quality fire-scar samples that clearly showed frequent fires in the 1700–1800s and a long fire-free interval from the 1900s to the present (physical evidence of a century of fire suppression in a forest that once burned frequently). I instructed the student in the basics of tree growth and fire-scar formation and then showed them how to date fire events in the samples. Student pairs were given an individual fire-scar sample and instructed to date the fires. Groups enjoyed working with the samples, and it was personally satisfying to watch them engage with a physical piece of forest history. Once students had dated their fires, I had them add their data to a fire composite chart showing each sample from the class. The visual display of historic fires began to tell the important history of fire in the Sierra Nevada. I posted a copy of the composite chart for students to analyze and asked them to identify patterns and formulate hypotheses about the history of this particular forest for our next class.

The next class was a lively one. Students had many ideas about the patterns in the data and shared some creative hypotheses about the causes. This led to a lively discussion of Native American forest management, sheep grazing, climate change, mining, logging, and Euro-American settlement. The students were actively participating in their own learning process by asking keen questions, sharing ideas, disagreeing with one another, and digging deep into what really happened in this forest and how it impacted the forest we see today. They were engaged in a way that they never would have been if I had merely told them the history of fire in the Sierras.

Toward the end of the semester we asked students to write three ways their view of the forest has changed through this class. Almost all students commented that their ideas of forest fire have changed drastically, and that they now appreciate the benefits of wildfires and look at today’s dense forests with different eyes. Many of them also listed this activity as a meaningful one because they were asked to “solve a puzzle” and enjoyed the “tree ring window to the past.”