When you or I visit any forest after a wildfire, we are clued in to what we see, which can look pretty dramatic from what we’re accustomed to. Oregon has an average of 347 trees per forested acre, which provides the lush, green view we love. After significant wildfires like we’ve seen on the east side of the Willamette Valley this year, that lush, green view has been replaced by brown trees with scorched trunks and needles, or entire hillslopes that are now blackened sticks.
So why do the Forest Service’s severity maps show so little high severity? Most of the fires in Western Oregon only report between 2 and 12% high severity. That’s clearly not what we see when we drive through or what we see in the news – what’s going on?
There are a few things to consider. Forest Service Burned Area Emergency Response (BAER) teams come in and the first thing they do is map the soil burn severity. Over the years, scientists have learned that what happened to the soil – not the above ground vegetation – is a critical indicator of recovery. From the soil burn severity map, geologists can predict debris flow hazards, hydrologists can predict changes to stream flows, and soil scientists can predict erosion potential.
Clearly, above ground vegetation does matter, and can aid in the recovery of a burned forest. For example, in areas where the trees were scorched and killed, those conifer trees will drop their needles, which provides very helpful natural ground cover. Since post-fire soil erosion is a major concern of soil scientists, this natural ground cover plays a crucial role in slowing the interaction between rain drops and soil particles that would otherwise get washed down the hillslope.
The Forest Service also creates a vegetation mortality map that focuses on the wildfire effects to the forests and is reported in percent of basal area loss. Basal area is the average amount of an area (such as an acre) occupied by tree stems This product helps other scientists, such as wildlife biologists, botanists, and silviculturists understand what to expect from this changed landscape for wildlife habitat, invasive weeds, and timber production.
So, what you’re seeing with your eyes is correct – lots of trees and other vegetation did die in these wildfires. But nature is resilient, and we’ve already seen evidence of many plant species sprouting up amongst the newly blackened and open canopy forest.
See the attached PDFs below for the maps and graphics.