On a brutally hot day in late August, TLC and Habitat Acquisition Trust (HAT) staff climbed a mountain in search of clues on how a forest naturally regenerates after a fire.
Unfortunately, a fire broke out in July in the Sooke Hills Wilderness Regional Park, owned and managed by the CRD and co-covenanted by TLC and HAT. The fire was approximately 1 hectare in size, thankfully kept small by quick action on the fire department’s part.
After the area was deemed safe, HAT and TLC ventured forth to investigate how much vegetation growth had occurred after one month post-fire.
The scene was quite dramatic – hiking through a green forest, we suddenly came upon the burn site. You really couldn’t miss it. Charred trees, scorched earth and lots of rock… it was barren of all undergrowth except for some crispy looking Scotch broom (that stuff just doesn’t seem to die!). On the car ride headed to the trailhead, we were making bets on how much regrowth we would see and which species. Turns out there was a lot more than we had expected, which is excellent news for a rebounding forest!
The only broad-leaved evergreen tree native to Canada, Arbutus menziesii, a.k.a. pacific madrone, fared quite well in the fire; young arbutus shoots from the base of burned trees were prolific at the site. Arbutus are able to reproduce both sexually and asexually, but primarily reproduces vegetatively by sprouting, especially from their woody, underground regenerative organ known as a burl. Burls are typically known as strange growths that occur on tree trunks or branches, and are reminiscent of a cancerous tumour. However, in the case of the arbutus tree, burls appears to serve as their back-up reproduction option – perfect for when a fire has ripped through a forest, opened up the area to more light, and those pesky shrubs aren’t competing for light or nutrients! The burls also serve as a source of stored carbohydrates for the sprouts. Repeated top-kill by fire encourages burl development, which increases arbutus survival overall.
As arbutus trees age, they become less shade-tolerant and require top light for survival. As such, they are considered a fire-dependent seral species because fire helps clear out competitive conifers, giving them the light they desperately need. Strangely though, this is balanced by the fact that they do not tolerate fire well due to their very thin bark. Arbutus is a bit like Goldilocks; it needs just the right amount of fire. Additionally, the exposed mineral soil and opened canopy associated with recent burns are conducive to arbutus seedling establishment. Unfortunately, any seeds that were in the soil before the fire were likely destroyed due to their sensitivity to heat. However, seeds are dispersed back into the burn site by birds and mammals. Overall, arbutus has rapid, initial post-fire recovery and will likely have benefited from this small fire in the Sooke Hills. It will be interesting to record the quantitative difference in arbutus seedlings and sprouts in the burn site compared to the adjacent unburned forest. It is fascinating to see the rapid recovery of arbutus firsthand in the Sooke Hills, with burls bursting with new green shoots, eagerly enjoying the sunlight.
For those of you who have noticed what appears to be burned branches on otherwise healthy arbutus trees in the forest, where no other visible signs of fire are observed, the culprit is (as it most often is in the forest!) a fungus. Well, two fungi actually. Arbutus are susceptible to a canker disease caused by the fungus Nattrassia mangiferae which infects the phloem (the tissue responsible for transporting nutrients in vascular plants) and vascular cambium (the tissue found between the phloem and the xylem in trees, with xylem responsible for transporting water). Further weakening of the infected arbutus tree then comes through a secondary opportunistic fungus, Fusicoccum aesculi, which causes dieback and gives the limbs and twigs the burned appearance.
Other species observed at the burn site included oceanspray (Holodiscus discolor). Many oceanspray shrubs managed to not burn away entirely, and these will likely regenerate from their root crowns. We observed a few of the mature oceanspray shrubs with new shoots bursting forth from their bases! Interestingly, oceanspray are known to colonize areas that have frequent wildfires, and are among the first plants to send out fresh shoots after a burn. Looks like we can corroborate that! Sprouting allows oceanspray to survive large, intense fires as most of the seedbank is destroyed. Similar to arbutus, fire assists oceanspray by opening up the forest canopy and allowing more light to reach the forest floor, which is supremely helpful for a species that is described as “light-demanding, early successional”! Oceanspray is also drought-tolerant as it is able to slow its rate of transpiration (the movement of water through a plant), which is helpful on dry, rocky sites such as the burn site. As time moves on, we expect to see a proliferation of oceanspray but this will reduce as other species begin to grow back. Competition is constant and fierce in the natural world!
Dull Oregon grape, salal and trailing blackberry were also observed within the burn site. Salal (Gaultheria shallon) is described as a “woody survivor” and generally sprouts from the roots, rhizomes, or stem base after aboveground vegetation is damaged by fire. Birds and mammals may disperse some seed from off-site, but the main form of regeneration is vegetative sprout. Depending on the severity of the fire, salal will continue growing and because the fire in Sooke Hills wasn’t very large or long-burning, we expect salal should rebound quite easily. Trailing blackberry (Rubus ursinus) is quite fire-tolerant, with underground portions of the plant generally surviving most fires. Seeds also appear to germinate in abundance following a fire, so are not as susceptible to heat as the arbutus seeds are. Dull Oregon grape (Mahonia nervosa) commonly sprouts and grows vigorously after fire. Regeneration through seed does occur but vegetative regeneration is the main type of post-fire growth, as is the case for most of the species observed.
TLC and HAT staff established three transects at the site. Transects are paths along which species presence is recorded. We ran one transect in the forest in the middle of the burn site, another transect along the open rocky hillside in the burn site, and the last transect in the forest in an unburned area to serve as a control. Each transect is 30 meters long with known start and end points (thanks to our handy GPS units) so we can find them again. Data were collected at the 10, 20 and 30 meter marks and included both the number of trees still standing and the number of stems and species type of new growth, within a 1 meter square. We will return to these exact spots and record the same data types to determine which species grow most vigorously after a fire and to what degree.
TLC and HAT plan to continue data collection at the site to better understand forest regeneration after a fire, so stay tuned to find out what grows next!
Photo credit Torrey Archer and Andrew MacKinnon.