Aug 22, 2017
Science Tuesday: Frost on Fire
Type: Research Synthesis
Climate change can sometimes feel like an abstraction, especially in places with moderate weather, like where I live on the northern California coast. Even during periods of extreme drought or severe fire weather across the West, my coastal climate keeps delivering a cool breeze and thick fog, and it’s hard to imagine that it was ever very different from this.
It was with this coastal complacency that I visited my sister in Alaska earlier this month. She lives in Anchorage, and though this was my third visit in the last few years, it was my first time ever visiting during the summer. And surprisingly, it was also the first time that I ever experienced a glacier up close. On one of my last days in Alaska, we headed out to Portage Lake, which is about an hour from Anchorage. We took a tour boat up to the top of the lake, where Portage Glacier is perched dramatically above turquoise water, and chunks of glacier float ominously on the lake’s surface. It was here, watching my three-year-old son peeking over the rail at the glacier — young observing old — that I felt the power of time. Only 100 years ago, Portage Glacier covered the entire lake; when my son is 100 years old, the glacier may be gone — no abstraction about it.
Later, reading through some fire science literature for Alaska, I learned that Portage Lake had been the site of a fire history study that was published in 2003 (Lynch et al.). I’ve been curious about interactions between fire and climate in boreal forests since my visit, especially because of all the recent fires in Greenland and Canada, so it was neat to connect my glacier visit directly to the fire history of that region. In the study, which included charcoal analyses of sediment cores from five different lakes across the Kenai Peninsula and Alaska’s interior, the authors found that major shifts in fire regimes were likely tied to large-scale shifts in vegetation over the last 9,000 years. Periods where deciduous forests dominated were associated with lower fire occurrence, and fires became much more frequent with the establishment of coniferous (spruce) forests, which have more bountiful and flammable fuels than their deciduous counterparts.
A more recent paper by Dash et al. (2016) explored the concept of top-down (climate) versus bottom-up (land cover/vegetation) effects on fire regimes in Alaskan boreal forests. In that paper, they confirm that vegetation can act as the dominant influence on fire regimes, but only during weather conditions that are less favorable for burning. Under more severe fire weather scenarios, climate can override differences in vegetation, and both deciduous (less flammable) and coniferous (more flammable) forests are prone to increased burning. Dash et al. show that in recent decades, increasing summer temperatures and aridity have led to increases in the number of fires and area burned in boreal forests, and those patterns extend across the different vegetation cover classes.
From these studies, we can better understand the ways that vegetation and climate act as drivers of fire regimes, but what about the reverse? How are these shifting fire regimes influencing future patterns of vegetation and climate?
In Alaskan boreal regions, the differences between deciduous forests and coniferous forests extend well beyond flammability. Coniferous forests, particularly black spruce (Picea mariana), tend to grow on thick organic soils. These soils insulate the sub-surface from warm summer temperatures, allow for development and maintenance of permafrost, and thereby represent one of the most important carbon sinks in North America. Black spruce has a competitive edge in these sites because of its large, robust seeds, which can tolerate and become established on the well-drained (i.e., often dry and inhospitable) organic soils. Deciduous species, including birch and aspen, require mineral soil to establish, and are therefore limited to areas with thin organic soil layers.
And this is where fire comes in: increased fire frequencies and severities in boreal forests are causing increased consumption of the soil organic layer and degradation of permafrost, and some researchers are predicting that this could cause landscape-scale shifts in vegetation from coniferous forest to deciduous forest, as birch and aspen take advantage of newly exposed mineral soils. These changes would have implications not only for black spruce as a species, but also for the viability of permafrost and for the carbon storage capacity of the entire region (Hoy et al. 2016). Interestingly, some experts predict that a shift toward deciduous dominance would also mean a shift toward a less frequent fire regime, which could act as a negative feedback on the larger context of increased fire activity in a warming climate. However, as we know from the fire history studies discussed earlier in this blog and from recent modeling efforts (Johnstone et al. 2011), even widespread changes in vegetation likely won’t be able to mitigate the powerful effects of increasing temperatures and aridity.
As I’m writing this, I’m thinking back on Portage Lake and wishing I’d looked a little more closely — not at the glacier, but at the surrounding forests. If I’d read some of these papers ahead of time, I might have noticed the footprints of fires that burned around Portage Lake in 1947 and 1969, or I might have sought out a black spruce forest to feel the springy soil underfoot. The glacier is a glaring sign of change, even to my untrained eyes, but the forest dynamics are even more intriguing.
Dash, C. B., Fraterrigo, J. M., and Hu, F. S. (2016). Land Cover Influences Boreal-Forest Fire Responses to Climate Change: Geospatial Analysis of Historical Records from Alaska. Landscape Ecology, 31(8), 1781-1793.
Hoy, E. E., Turetsky, M. R., and Kasischke, E. S. (2016). More Frequent Burning Increases Vulnerability of Alaskan Boreal Black Spruce Forests. Environmental Research Letters, 11: 1-11.
Johnstone, J. F., Rupp, T. S., Olson, M., and Verbyla, D. (2011). Modeling Impacts of Fire Severity on Successional Trajectories and Future Fire Behavior in Alaskan Boreal Forests. Landscape Ecology, 26(4), 487-500.
Lynch, J. A., Clark, J. S., Bigelow, N. H., Edwards, M. E., and Finney, B. P. (2002). Geographic and Temporal Variations in Fire History in Boreal Ecosystems of Alaska. Journal of Geophysical Research: Atmospheres, 107(D1).
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