Ecosystems

Snow cover is reducing in the Pacific Northwest, and so too is the niche for evergreen trees in the region

Snow cover can delay the growing season of plants, but reducing it can also have negative consequences.
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Research into how plants grow back after a forest fire has revealed that snow cover may have a crucial role in a successful recovery. The study by Andrew Wilson and colleagues in the Journal of Geophysical Research: Biogeosciences will help scientists understand how forests will change in the coming years as the planet warms.

“As wildfire activity continues to increase and intensify in the Northwest, understanding what shapes revegetation on severely burned forested landscapes is vital for guiding management decisions,” co-author Kevin Bladon said in a press statement.

Postfire vegetation in the Bull of the Woods Wilderness, Oregon. Anne Nolin, UNR.

Understanding how forest fires impact the ecology of the Pacific Northwest is difficult, as the Cascade Mountains of Washington and Oregon are a complicated landscape. There are many factors. Wilson and colleagues list fire severity, pre-fire vegetation, elevation, slope, aspect, rain, and snow accumulation and melt. Analysis showed the significant factors were summer precipitation, snow cover, and elevation. While the elevation of an area isn’t likely to change, except over geological time, precipitation is changing with the climate.

Summer precipitation was the biggest factor in plant growth. However, snow cover, when you wouldn’t expect plants to be growing, also mattered. The reason is the importance of water reserves.

When rain falls in the Cascades, it cascades down the mountains into the river system. When snow falls, it lies there. The snow cover effectively forms a reservoir that is released into the soil over many days as it thaws. Reduced snow cover can matter in two ways. A reduced amount of snow means there is less water released. An earlier melt date means it seeps away earlier and provides less of a buffer to any summer droughts.

Climate change has already increased the percentage of winter precipitation that falls as rain rather than snow, reducing the spring snow-water equivalent – a metric for how much water snow contains – and has caused snowmelt to begin earlier in the spring than it used to, said Anne Nolin – who led the study, in the same press release. Pacific Northwest snowpacks have seen the greatest declines of any seasonal snow region in the West.

“And wildfire season length in the western U.S. overall has increased by roughly 25 days in recent decades, including a massive increase in the Northwest from the mid-1970s, when it was 23 days, to 116 days in the early 2000s,” Bladon said. “That’s attributable mainly to warmer temperatures and drier conditions in the spring and summer.”

The changes in the fire conditions mean that the area, left to its own devices, would evolve. In place of the evergreen trees, fire-adapted deciduous forests would rise.

What is interesting in the paper is what the authors suggest to do with that information. The authors write: “These findings do not imply inherently beneficial or detrimental post-fire vegetation trends. With shifting climate trends in the PNW, migration of vegetation types and accompanying fire regimes may be the most adaptive path forward for forested landscapes. Fires can be viewed as an opportunity for forests to re-organize into ecosystems better structured to survive warmer winters, longer fire seasons, and greater drought stress. One of the major challenges moving forward is how to reconcile ecological forces of a changing climate with goals of ongoing post-fire management practices, where are often oriented towards re-establishing forests as they existed pre-disturbance.”

In this case, replanting evergreens could be like trying to hold back the tide. Foresters aiming to restore a healthier and more stable ecosystem may wish to adapt their restoration to introduce better-adapted plants. Given the cultural importance of some forests, such an approach will be controversial. However, if the region’s hydrology is changing, there will be tough questions to answer about what can be conserved and what cannot. 

RESEARCH ARTICLE

Wilson, A.C., Nolin, A.W., Bladon, K.D., 2021. Assessing the Role of Snow Cover for Post‐Wildfire Revegetation Across the Pacific Northwest. J Geophys Res Biogeoscihttps://doi.org/10.1029/2021jg006465

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