A pair of disturbances common in Western Canada's boreal forests, when combined, may have an unexpected benefit of limiting the spread of non-native plant species, a University of Alberta study shows.
The research gauged the interactive effect that natural wildfires and the presence of seismic lines - narrow clearings cut into forests for oil and gas exploration - had on the establishment and spread of non-native plants growing beside roads.
When the two disturbances co-occurred in the same area, they mitigated the spread of invasive plants, like dandelions and clover, further into the forest.
Scattered by wind and vehicles travelling the roads, the seeds of such plants can take root and choke out species native to forests, like saskatoons and fireweed, upsetting biodiversity and resiliency.
The study helps answer a crucial question of whether overlapping forest disturbances magnify this effect of roads in Alberta's boreal forest - and how to better understand the impact of those dynamics, says Leonardo Viliani, who led the study as part of his PhD in conservation biology under the supervision of professor and co-author Scott Nielsen in the Faculty of Agricultural, Life & Environmental Sciences.
"The interaction of these disturbances can play a critical role in either promoting or limiting the establishment and spread of these invasive plants," he notes.
The researchers looked at 11 forest sites in northern Alberta where seismic lines intersected with seven roads within areas that were either untouched by wildfire or had burned within two years. They measured differences in the ratio of non-native to native plant cover along seismic lines at increasing distances from the roads.
The results showed that for unburned areas, invasive plant cover was highest next to the road but declined as it spread further into the forest. As well, the study confirmed existing knowledge thatseparately, wildfires, and particularly seismic lines, each increase the spread of non-native plant species from roadsides into the forest.
But in areas where wildfire and seismic lines co-occurred, native plants began to hold their ground significantly better, with the ratio between native and non-native species balancing out starting seven metres from the roadside.
The findings suggest a "buffering effect" from the interaction between the two disturbances, where fire reduced the expansion - and the impact - of non-native plants along the seismic lines, Viliani notes.
"It's possible that because the boreal forest is very adapted to wildfires through thousands of years of evolution, native plants just grow back better and thrive after a fire."
The findings show how wildfire, when interacting with human-caused disturbances, can help counteract that impact, Viliani adds.
"Since fire is a natural dynamic in the boreal forest, it can also have a positive effect."
As well, 68 per cent of all the plant species studied showed significant shifts in their abundance, based on how these various disturbances overlapped. "This could have a big effect on plant communities there," Viliani notes. "It could be that either native plant communities survive better or that there is a better colonization of the burned sites."
The findings can helpresource companies prioritize restoration efforts for their seismic lines to those located in unburned forest first, he suggests.
"If the lines have burned, it seems that the native plants do come back."
The findings also point to the importance of considering the complexity of increasingly common overlapping disturbances in forests, Viliani suggests.
"While we used to study these invasions by looking at a single cause, like a specific plant's traits or one weather event, there's a bigger picture. In our rapidly changing world, it is the combination of many different threats working together that changes the landscape."
