Disentangling Effects of Soil Abiotic and Biotic Factors on Tree Seedling Regeneration Following Boreal Forest Wildfire

Changes in fire regime of boreal forests are predicted to alter plant and soil community structure and cause elevated tree mortality, increased loss of soil organic matter and reduced survival and functioning of soil microbial communities. While the impact of wildfire disturbance on plant mortality and post-fire successions in boreal forests has been studied extensively, little is known about how changes in soil properties after fire, including biotic and abiotic properties, individually and interactively impact tree seedling regeneration. The aim of this study was therefore to disentangle how tree seedling performance is independently and interactively controlled by soil biotic versus abiotic properties following wildfire.

We performed two greenhouse experiments in which seedlings of Betula pendula, Pinus sylvestris and Picea abies were grown in soils collected from forest stands in east-central Sweden that had been subjected to three burn severities (high, low and unburnt) following a large-scale wildfire. The first experiment consisted of live soil originating from every stand in each burn severity class crossed with each tree species. The second experiment was similar, except that all soil was first sterilized, and then was crossed with live soil inoculum originating from each of the burn severity classes and grown with each tree species. The results showed that tree seedlings subjected to live soil grew best in soil from unburnt stands (experiment 1), and that P. abies and P. sylvestris seedlings increased growth when planted in soil inoculated with biota from low burn severity stands compared to high burn severity or unburnt stands (experiment 2). In contrast, B. pendula was not responsive to soil inoculum treatments, but instead was driven by post-fire abiotic properties of soils. These results indicate that fire disturbances may lead to reduced regeneration of conifers, unless soil biota is maintained or has recovered, and further suggests that soil from high burn severity stands may constrain seedling regeneration, whereas soil biota from low burn severity stands promotes growth and regeneration of conifers. Our study also shows that different seedling species respond differently to abiotic and biotic soil properties altered by different burn severities, which is relevant because burn probability and fire intensity are projected to increase and become more common in many parts of the boreal region as the climate warms.