Canopy dynamics and phenology of a boreal black spruce wildfire chronosequence

Abstract

This research quantified the canopy phenology, growing season and inter-annual dynamics of the overstory and understory vegetation in a boreal wildfire chronosequence in northern Manitoba, Canada. Optical measurements of the canopy radiation regime were made in the 2004–2006 growing seasons to evaluate the spatial and temporal variation in the effective leaf area index ( L e) and fraction of intercepted photosynthetic active radiation ( F IPAR) of the canopy. Black spruce dominated the overstory L e ( L e O ) in the oldest stands (74- and 154-year-old), while deciduous shrub and tree species dominated the youngest stands (1- to 23-year-old). L e O varied from a minimum (mean ± S.D.) of 0.26 ± 0.33 to a maximum of 2.33 ± 1.20 and from 0.63 ± 0.56 to 1.20 ± 0.56 for the understory vegetation ( L e U ). F IPAR O was negligible in the youngest stands, and reached a peak in the oldest stands studied. L e U comprised a significant percentage of total L e (>30%) and F IPAR (>20%) for all stands, except the oldest, 154-year-old stand. We observed two key features in the L e and F IPAR seasonal cycles: (i) significant changes ( p < 0.001) in L e U , increasing by 41% across all sites; and (ii) >80% difference in seasonality exhibited by the overstory in the mixed/deciduous and black spruce dominated stands. L e varied by less than 20% among the three years. Canopy green-up was positively correlated to spring temperatures and occurred within 20–28 days for all stands. The pronounced increase in wildfires in the boreal forest in recent years necessitates additional studies to better understand their effects on the species composition and structure of canopies of all vegetation strata.