Unexpected responses to wildfire by seedlings in the Yukon

As the planet heats up, life in the north is changing rapidly. Fire-adapted trees are predicted to take advantage of wildfires in the tundra, as they open up new sites for colonisation. But is this what is happening? Lucas Brehaut and Carissa Brown studied wildfires at the treeline in subarctic Yukon, Canada. They wanted to know how fires were changing the environment and if seedlings were appearing after the burns. Their research, published in Plant Ecology, indicates that the effects of wildfires are complicated and might not always benefit fire-adapted trees.

Brehaut and Brown are interested in wildfires at the treeline as it marks a boundary between two ecosystems with dramatically different responses to fire. The boreal forest is used to fire, and trees are ready to take advantage of new openings in the landscape caused by burning. The plant life in the tundra, in contrast, is not fire-adapted because natural fires rarely happen there – or at least they used to be rare.

The treeline has been reasonably static, Brehaut and Brown use the word inertia for a few reasons. The cold can reduce the number of viable seeds trees can produce. Soil conditions can prevent tree see germination in the tundra. Even if a seed germinates, tundra vegetation can help prevent seedlings from establishing.

That was fine in the past but now, with the climate warming, a lot of those variables are going to change at the same time. Throw in wildfires clearing patches of land for pioneer species to colonise, and the trees might march north.

Brehaut and Brown examined three sites where fires had burned in the past couple of decades on the boreal forest – tundra boundary in northern Yukon. Two sites were near Eagle Plains, and the third a little further south, west of Dawson. All of the places were cold, with an average maximum temperature of 14 °C in July, plummeting to  -27 °C in January.

Surprisingly, despite the warmer weather and the freshly cleared land, the botanists did not find more seedlings in the burned soil compared to unburned locations. Clearly, something complicated was happening. In the discussion, Brehaut and Brown discuss the conditions that seedlings face and suggest that instead of providing a comfortable bed, the cleared patches create more extreme conditions.

Summer might be difficult for seeds. The burned patches are darker, and so are heat traps, meaning that in the summer, the soil warms up more in the burned areas. This should be good news as it’s more energy for the plants to use, but it could also be drying out the soil, leading to desiccation. What the seeds gain from heat, they lose from thirst.

That’s not the only problem the seeds face. The scientists look beyond the growing season to understand why the seedlings don’t grow by looking at conditions in the other end of the year.

In winter, things flip, and the burned patches are colder than their unburned neighbours. This seems odd. Why would warmer soils turn colder? The answer is snow. At the unburned sites there is plenty of vegetation to trap the snow as it falls and drifts. This creates an insulating blanket that protects the soil underneath from the worst of the cold.

For the burned patches, snow still falls. However, as the ground is comparatively bare, it can drift away, leaving the soil relatively exposed to the cold. Brehaut and Brown found that in burned areas, the soil had more days below freezing, and so any seeds in the soil had more harsh conditions to deal with.

“From the perspective of a seed, the combined effect of low-severity fires (i.e., patchy and partial combustion of vegetation and ground surface), and the change in seasonal soil temperatures and winter conditions lead us to conclude that in the study region, post-wildfire microsites do not promote greater seedling emergence and survival when compared to the unburned treeline,” write Brehaut and Brown. “While conditions will change as time-since-fire increases, the window of opportunity for successful germination and establishment post-wildfire remains short for many boreal tree species… Studies have shown that ideal conditions for successful spruce germination consist of low interspecies competition, and substrates that are warm, moist, and high in nutrients…. This is especially true for black spruce, which disperses most of its seed within the first few years after wildfire…”

“Our results have also revealed that the prediction that wildfire at treeline will create ideal environmental conditions and promote range expansion of fire-adapted species may be overly reductive, as it does not consider the impact of extreme temperature changes on seedlings within the first two decades after wildfire. The prediction also does not consider the interaction between wildfire severity and within-site variability in microsite characteristics… When compared to unburned treelines, our study indicates that the potential seedling establishment at the range edge remains challenging regardless of wildfire.”

READ THE ARTICLE

Brehaut, L. and Brown, C.D. (2022) “Wildfires did not ignite boreal forest range expansion into tundra ecosystems in subarctic Yukon, Canada,” Plant Ecology. https://doi.org/10.1007/s11258-022-01242-9