Effects of soil drainage, canopy position, and needle age on leaf area index for a black spruce boreal chronosequence

Leaf area index (LAI) and vegetation cover are primary drivers of ecosystem models that simulate water and carbon exchange. Along with specific leaf area (SLA), LAI is critical for accurate physiological models at the stand, landscape, and biome levels. Wildfire is the primary disturbance in the boreal forest, producing a mosaic of different-aged stands with different LAI structures. The objectives of this study were to (i) compare several experimental methods for determining SLA; (ii) examine the effects of stand age, soil drainage, canopy position, tree species, and leaf age on specific leaf area (SLA); and (iii) characterize overstory and understory SLA, LAI and foliage biomass for a 130-year boreal black spruce chronosequence. The study was conducted on a 130-year boreal black spruce chronosequence near Thompson, Manitoba. The experimental design was a nested factorial design with soil drainage nested inside of stand age; separate well-drained and poorly drained areas were located within each of the seven sites in the chronosequence. The comparison of two experimental methods for determining leaf area (volume displacement vs. flatbed scanner) produced highly correlated results (N = 50, R² = 0.91). Preliminary ANOVA results indicate that significant effects for SLA included needle age, stand age, the age * species interaction (all p < 0.01), and soil drainage (p = 0.01). Canopy position (top, middle, or bottom of canopy) was not significant (p = 0.16). Specific leaf area values for black spruce (Picea mariana (Mill.) BSP) averaged 5.44 and 4.61 m² kg^-1 for current-year and older foliage, respectively, and 6.20 and 4.68 m² kg^-1 for jack pine (Pinus banksiana Lamb.). Values for deciduous species were considerably higher. Overstory hemispheric area index (HSAI) varied significantly (p = 0.02) across the chronosequence, from 0.22 m² m^-2 in the young stands to 5.83 m² m^-2 in the older ones. These LAI figures were in good agreement with previous optically based measurements performed at the older sites on the chronosequence.