Interactive effects of burn severity and canopy cover on ecophysiology of tree seedlings in boreal forests

Background/Question/Methods

Disturbances in forested ecosystems play important roles in the assembly of forest communities. Wildfires are particularly important in boreal forests for establishing seedlings because they remove competing vegetation, increase the availability of nutrient, water and light resources, and alter microclimate conditions. Yet, forests typically experience a mosaic of mixed-severity fires, creating a range of micro-habitats with dramatically differing characteristics. At the ground-level, high severity burns remove all organic matter and expose the mineral soil, while moderate burns leave a hydrophobic layer and low severity burns have only ephemeral effects on vegetation and soil. At the canopy-level, after a fire the overhead crown cover may be completely eliminated or remain wholly intact. Many forestry practices in the boreal regions create similar disturbances through soil scarification, prescribed burning and clear-cutting. The impacts of ground- and canopy-disturbances on growing conditions typically improve physiological performance of establishing seedlings, although the benefits may differ among species. We tested the hypothesis that seedling performance would improve with soil and canopy disturbances, particularly for early- compared to late-successional species. In a boreal forest in Sweden, we outplanted seedlings of four species (two conifers and two deciduous) in a full factorial study that experimentally manipulated ground-level disturbances (undisturbed control, low, medium and high burn severity, and topsoil removal) in a clear-cut and forested condition.

Results/Conclusions

The ground-level disturbances improved seedling ecophysiological performance, but generally only when coupled with canopy disturbance. Specifically, seedlings growing in the clear-cut on the medium and high burn severity treatments had increased growth rates, leaf N and P concentrations (for all species), increased photosynthetic rates and specific leaf area (for deciduous species) and increased respiration rates (for conifers). The increase in foliar nutrients with disturbance severity was especially strong for the deciduous species. Under forested conditions these plant traits were largely unaffected by the ground treatments. One exception in the forest was an increased foliar P on the medium and heavy burn treatments for the deciduous species, although low phosphorous use efficiency indicated that the increase in P did not confer an equal increase in photosynthetic rates. Our late-successional coniferous species (Picea abies) was the only species to exhibit decreased performance with increasing ground disturbances in the forest, indicating its preference for undisturbed sites. The response of each species to disturbance provides a mechanistic understanding of how different combinations and severities of disturbances impact seedling ecophysiology, which ultimately governs forest community compositions following disturbances.