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This article is part of the Spring 2022 issue of the Earth Science Matters Newsletter.

Douglas-fir forest in northwestern California
Dense stands of Douglas-fir surround South Twin Lake in the Klamath bioregion of northwestern California.

A team of federal scientists, academics, and Tribal members recently collaborated on a study that demonstrated the strong influence of Indigenous stewardship on forest conditions in northern California for at least a millennium. Indigenous burning practices coupled with lightning-induced fires kept forest carbon low, at approximately half of what it is today, and kept forests more open and less dense. Forest management and intentional ignitions also resulted in low forest fuel levels that allowed local Indigenous people to produce food and basketry materials, clear trails, reduce pests, and support ceremonial practices for generations.

These stable forest conditions appear to have enhanced the resiliency and health of the fire-prone forests of northern California. However, colonization and twentieth century fire suppression policies have transformed California forests. Forests today are denser and more prone to catastrophic large wildfire than in the past. As restoration ecologists attempt to improve the health of California forests, a key question becomes – restoration to what?

The research team merged multiple lines of evidence from the Klamath Mountains in northern California to help answer this question. They integrated Karuk and Yurok oral histories, Indigenous Traditional Ecological Knowledge (ITEK), a pollen-based vegetation abundance reconstruction, fire scars from tree stumps, a paleofire (past fires that occurred before instrumental record keeping) reconstruction based on sedimentary charcoal, and historic forest inventory data. The evidence was consistent with human management actions on the forest, particularly Indigenous ignitions that kept forest fuels low. Data also show that the current landscape – a dense Douglas fir–dominant forest – is unlike any seen in the preceding 3,000 years.

diagram of idealized vegetation response to climate vs. human activity
Figure 1. Idealized vegetation response to climate vs. human activity.  Top panel shows climatically-driven vegetation change without the influence of people. Bottom panel shows human-caused vegetation change where increases in fire use create more open forest conditions despite cooler/wetter conditions, such as during the Little Ice Age. Credit: Clarke Knight, USGS.

Climate is often presumed to be the most important control on vegetation dynamics during the pre-colonial period, not people. Periods of wetter and colder conditions often lead to less fire on a landscape, the promotion of more shade-tolerant taxa, and more forest closure (Figure 1, top panel). The authors tested the expected effects of climate on northern California forest conditions and found that climate alone could not account for the trends in their data. For example, during the Little Ice Age – a period of cooler and wetter conditions between 1300-1850AD (600-100 years before present) – the authors found a signal of increased fire and vegetation openness (Figure 1, bottom panel), which they corroborated statistically (Figure 2), indicating human involvement in controlling and shaping the forest environment.

graphs of charcoal, drought, and vegetation through time
A) Trends in charcoal accumulation (CHAR, a measure of paleofire), Palmer Drought Severity Index (PDSI, a measure of climatic conditions), and vegetation response index (VRI, a measure of forest openness) were plotted through time at Fish Lake, one of two study sites. The Little Ice Age (LIA, blue panel) and Medieval Climate Anomaly (MCA, yellow panel) indicate two time periods of known climatic changes. For example, during the MCA when climate was relatively warmer and drier, CHAR (orange line) increased, in part because forest fuels were drier and easier to burn. B) Trends in correlations between CHAR, PDSI, and VRI over time. There is a significant positive correlation between CHAR and VRI (red line) during the climatically cooler period of the Little Ice Age (600-100 years before present) as predicted by the author’s conceptual model that accounts for human-caused vegetation change through burning practices. Other correlations were found at various times throughout the record. For example, there was a significant positive correlation between CHAR and PDSI around 800 years before present. Modified from Figure 4 in (Knight et al. 2022)

This research quantifies what stable, historic forests in California looked like and shows that frequent fire, in part ignited by people, limited forest fuels and shaped the forest for millennia. This finding is important because California is planning to use forest ecosystems to store carbon as part of climate mitigation efforts. The results of this study suggest a large-scale intervention could be required to achieve the historical conditions that supported forest resiliency and reflected Indigenous influence.

The paper, “Land management explains major trends in forest structure and composition over the last millennium in California’s Klamath Mountains” was recently published in the journal PNAS.

<< Back to Spring 2022 Newsletter


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