In April of 2018, I visited the backfire area in the forest borders west of the old Loran C Tower at Riske Creek.
I was interested in seeing the impact of the back fires started on July 17, 2017 compared to the high-ranking wildfires the day before. As I suspected from looking at the satellite images on Google Earth, there were different intensities, with some areas mostly cleared of understory plants while others were slightly impacted. As indicted by other researchers, the aspen areas seemed the least impacted.
I returned to the same area May 22 of this year to take more pictures and measurements of the cavities left by burned root systems.
As expected, there were some small trees burned completely, leaving shallow soil craters. What I had not expected was the extent of the craters left by a large Douglas Fir stump that burned the entire underground portion of the root system.
As the photo shows, a portion of the main stump remained, but all lateral roots were burned up. What was surprising was the size and extent of the root cavities. These large support roots consisted of six main branches in all directions. Judging from the cavities, which were two to three feet across and extending out 10 or more feet, these roots provided a substantial base for a very large tree.
Three of the largest root systems were still three inches in diameter as far away as 35 feet from the stump. With the nearly complete combustion of the root system and mentally filling in the craters with assumed root biomass, it becomes clear the marvellous buttressed base of the tree.
I was aware of this to some extent when a number of large fir trees blew over in a forest adjacent to my property. A particularly wet season followed by a freakish tornado-like wind storm blew over some healthy trees, creating some huge root balls, some more than six feet across, but many of the larger roots had broken off.
It was not until I was able to take some measurements of the fir tree roots at Riske Creek that I realized the roots of a large tree covered hundreds of square feet.
What I find interesting is the complete combustion of some roots while other stumps in the same area show only a slight charring. No doubt it is due to a much older and dryer root system, which allowed the fire to move throughout a root mass which covered more than 2,500 square feet. It would also be interesting to see how long it took for the roots to burn to the end, as this demonstrates the difficulty of detecting burning roots and the concern about how long the fire lingers underground following the main fire event. Fir trees are noted for their resistance to blow down and I could see why, if most have such an extensive root system.
A quick review of the literature indicates that most of the comments were about the difficulty of removing old stumps and roots, but a few articles did mention the dangers associated with the cavities on hiking trails following a severe wildfire.
Jim Hilton is a professional agrologist and forester who has lived and worked in the Cariboo-Chilcotin for the past 40 years. Now retired, Hilton still volunteers his skills with local community forests organizations.
