Recruitment and ontogenic patterns of stunting and growth release of black spruce (Picea mariana) in post-fire Kalmia heaths

Recruitment of seed regenerating plants after disturbance depends on adequate seed supply and favourable microsites that allow germination and seedling establishment (safe sites). Forest disturbance in nutrient stressed conifer-ericaceous communities in boreal and temperate forests become quickly dominated by ericaceous shrubs and restrict tree recruitment. Habitat filter due to limitation of safe sites has been attributed to inadequate seedling regeneration. Furthermore, large proportions of the regenerating seedlings exhibit stunted growth but some over time exhibit growth release. Despite wide occurrence of poor natural regeneration of conifers in ericaceous shrubs, the ontogenic patterns of stunting and growth release of conifer seedlings received little attention. We used post-fire black spruce- Kalmia ecosystems to study this. We hypothesized that (i) post-fire residual OM thickness controls seedling recruitment by limiting safe sites even in presence of sufficient seed supply, (ii) black spruce density of a site depends on its OM depth, which controls safe sites, and (iii) partially safe sites, which permit germination and seedling establishment with stunted growth, can become safe sites overtime allowing growth release. We experimentally tested black spruce seed regeneration on seedbeds with variable OM thickness and studied the ontogenic patterns of seedling/sapling growth in eight post-fire Kalmia heaths along an 8-34 year chronosequence. We found that despite ample seed supply OM depth controls seedling recruitment. In the chronosequence study we found only 17.8% of burnt area accounts for occupied safe sites, recruiting only 317 seedlings/ha and only 13% of them exhibit normal growth, others remain stunted for 5-20 yrs. With time, successive cohorts of stunted seedlings overcome growth check. Black spruce density was negatively associated with OM depth and Kalmia cover. The probability of growth release was associated negatively with OM depth and positively with soil respiration. We conclude that poor black spruce regeneration in Kalmia heath is largely due to safe site limitation controlled by OM depth, which also controls seedling stunting and growth release. The novelty of this work is the discovery of ontogenic patterns of conifer seedlings in nutrient stressed habitats. Our findings have implications for tree regeneration in other environmentally and nutritionally stressed ecosystems such as treelines.