Abstract
Wildfire disturbance is important for tree regeneration in boreal ecosystems. A considerable amount of literature has been published on how wildfires affect boreal forest regeneration. However, we lack understanding about how soil-mediated effects of fire disturbance on seedlings occur via soil abiotic properties versus soil biota. We collected soil from stands with three different severities of burning (high, low and unburned) and conducted two greenhouse experiments to explore how seedlings of tree species (Betula pendula, Pinus sylvestris and Picea abies) performed in live soils and in sterilized soil inoculated by live soil from each of the three burning severities. Seedlings grown in live soil grew best in unburned soil. When sterilized soils were reinoculated with live soil, seedlings of P. abies and P. sylvestris grew better in soil from low burn severity stands than soil from either high severity or unburned stands, demonstrating that fire disturbance may favor post-fire regeneration of conifers in part due to the presence of soil biota that persists when fire severity is low or recovers quickly post-fire. Betula pendula did not respond to soil biota and was instead driven by changes in abiotic soil properties following fire. Our study provides strong evidence that high fire severity creates soil conditions that are adverse for seedling regeneration, but that low burn severity promotes soil biota that stimulates growth and potential regeneration of conifers. It also shows that species-specific responses to abiotic and biotic soil characteristics are altered by variation in fire severity. This has important implications for tree regeneration because it points to the role of plant–soil–microbial feedbacks in promoting successful establishment, and potentially successional trajectories and species dominance in boreal forests in the future as fire regimes become increasingly severe through climate change.
Highlights
Fire is a recurrent natural disturbance in northern coniferous forests that controls vegetation dynamics, structure, diversity and function
We found that effects of burn severity on both soil biotic and abiotic properties were important for seedling growth, but that effects of soil abiotic properties were overall more important, seedlings of different tree species varied in their responses to only soil biotic properties
Our results have several implications. They highlight that regeneration of B. pendula is unresponsive to changes in soil biota and is instead mostly driven by changes in abiotic soil properties
Summary
Fire is a recurrent natural disturbance in northern coniferous forests that controls vegetation dynamics, structure, diversity and function. For many parts of the boreal region, burn probability, fire area and fire intensity are projected to increase and become more common as the climate warms (de Groot and others 2013) This is in part due to increasing duration of extreme fire-promoting conditions, such as warmer temperatures (Gillett and others 2004). These changes in fire regime are predicted to alter plant and soil community structure and cause elevated tree mortality (Stephens and others 2013), alter soil organic matter content (Czimczik and others 2005) and alter functioning of soil microbial communities due to increased loss of organic material (Allison and Treseder 2011). More frequent and higher intensity fires are likely to induce shifts in how boreal forests will regenerate and recover from fire disturbances (Johnstone and others 2010; Alexander and others 2018), in part as a consequence of fire-induced effects on loss of soil organic matter and survival of mycorrhizal fungi needed for tree seedling regeneration (Dahlberg and others 2001; Dahlberg 2002)
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