Before a major fire engulfed the Fox Lake area in the summer of 1998, much of the forest was made up of white spruce. Three years later, seedlings are starting to poke through the soil again, but the jury is out on what species of tree will dominate this forest in the future.

Jill Johnstone, a graduate student at the University of Alaska in Fairbanks, is studying tree establishment at several sites in the Yukon, including the Fox Lake area. Tree succession has been studied in many places, but this is the first research of its kind to take place in the Yukon.

Johnstone has seeded different tree species on a variety of test plots; in some places the fire burned hot enough that only bare mineral soil remains while on other plots organic materials such as needles, leaves and moss still cover the ground to varying degrees.

Bare mineral soil is usually considered to be the best surface for reseeding as it provides a more stable water supply for the roots of the seedlings. But after completing her second field season, Johnstone is finding some unexpected results.

As expected, more seedlings are establishing themselves on the mineral soils, but they are not all surviving the winters. When Johnstone checked the plots in the spring, she found lots of dead seedlings scattered across the ground, killed by needle ice that had formed over the winter.

"In the spring you can look at the surface of the soil, and see where it is rough, and you can see where bits of soil have been pushed up by needle ice. It pushes up the seedling with the surface of the soil and they are basically lying there dead."

Most previous research has shown that moss and litter prevent reseeding because the tiny roots of the seedlings cannot penetrate the organic layer to reach the soil and they also are more vulnerable to drying out. Reseeding is most successful in the first five years after a fire, as after that time the buildup of organic materials from fast-growing shrubs, herbs and moss will prevent reseeding.

But Johnstone's test plots show that the regrowth of moss in particular presents plusses and minuses for the seedlings. More seeds establish themselves on the bare mineral soils than on soils with a layer of moss, but the moss also insulate the soil and prevent the formation of needle ice, so a higher percentage of the seedlings are surviving in test plots where the moss has regrown.

Johnstone points out that this study will also have implications for forestry in the north. After a severe burn that exposes lots of mineral soil, the regrowth is going to be denser than in areas where fires have not burned the entire organic layer.

"For forestry, you might have a better stand later on when the trees are widely spaced and do not compete for light. A severe burn is more likely to produce a stand that needs to be thinned in 20 to 30 years."

Johnstone is also checking which species of trees are more successful at establishing themselves in the different plots. In general larger seeds have a better chance of reseeding themselves after a fire.

"Lodgepole pine has the biggest seeds, so it seems to be best able to colonize the organic soils. But that leads to the question then of why there are these low-lying areas with thick organic material that always seem to be dominated by spruce."

Johnstone thinks the answer lies in the fact that spruce are better survivors. While they may not establish themselves as rapidly as the pine, they also have a slower growth rate and can tolerate low nutrient availability and wet conditions better than the pine.

Black spruce and lodgepole pine are well adapted to the fire cycle that regularly renews the boreal forest. Both of these species have serotinous cones that are sealed shut by resins. It takes the heat of a fire to melt the resins and release the seeds.

White spruce cones are not serotinous and they release their seed every year, so intense fires will consume the seeds and cones from these trees. The heavy seeds of these trees are also not easily carried long distances by wind, so they rely on live trees from the edges of a burn or unburned islands of trees for regeneration.

As well as tracking survival of the seedlings in the test plots, Johnstone has also transplanted seedlings grown in a greenhouse so that she can more accurately compare growth rates, and determine whether the soils affect the growth of a seedling, as well as its establishment.

On the one hand, soils with an organic layer could encourage faster growth as the seedlings are more protected and the soils may stay moister underneath. On the other hand, bare mineral soils warm up more quickly, and have more ash on them that can provide nutrients.

Johnstone says that even though tree succession has been studied at other places in the boreal forest, the results may not apply to the Yukon as conditions here are unique. Many of the studies have been conducted in Alaska where lodgepole pine does not grow and where the soils are different.

She plans to continue her Yukon work for several more years, For more information Jill Johnstone can be contacted by e-mail at ftjfj@uaf.edu or by phone at (907) 474-7929.