Fine root dynamics with stand development in the boreal forest

1. Despite greater importance of below ground in influencing terrestrial carbon and nutrient cycling than above ground, how below-ground biomass, production, turnover and mortality change with stand development remains poorly understood. 2. Here, we used a postfire boreal forest chronosequence that spanned over 200 years (3, 11, 29, 94, 142 and 205 years since fire) to study how the dynamics of fine roots (≤2 mm in diameter) vary with stand age. We collected 756 sequential cores and 270 ingrowth cores, each separated to layers and live/dead, resulting in a total of 5076 fine root samples to quantify fine root biomass, production, mortality and turnover rates. 3. With stand development, fine root biomass increased from 3 to 94-year-old, and declined thereafter, whereas necromass increased to 142-year-old and levelled off at 205-year-old. Fine root production and turnover increased from 3 to 11-year-old stands, and declined thereafter. Fine root mortality increased from 3 to 142-year-old stands. 4. Our study, the first to show four simultaneous stand age–dependent below-ground attributes, indicated that fine roots in young stands turn over faster than in old stands based on the estimates from both sequential and ingrowth cores. Despite some similarities among the studied attributes, the peaks they reached were not the same. Our results suggest that fine root dynamics are influenced by changes in species composition and soil properties associated with stand development in the boreal forest, especially the increase in forest floor nitrogen: phosphorus ratios as stand age increases.