Morphological and ecophysiological responses of sheep laurel (Kalmia angustifolia L.) to shade

Dominant understorey shrubs can interfere with tree recruitment and affect forest succession. In boreal forests of eastern Canada, Kalmia angustifolia grows vigorously dominating post-harvest black spruce (Picea mariana (Mill.) BSP) forests and interferes with tree regeneration. Kalmia is a dominant understorey in all stages of forest development, but its response to canopy light level is unknown. We studied the morphological and physiological responses of Kalmia in open sun (control) and experimentally low, medium and high shades allowing 100, 60, 32 and 19 per cent of full sunlight representing 0, 40, 68, and 81 per cent shade, respectively. We tested the hypothesis that Kalmia responds positively to moderate shade by adapting morphologically with larger leaf area and fewer flowering shoots and physiologically by increased stomatal conductance, photosynthesis and foliar nitrogen than low or high shade. We found significantly increased leaf area, specific leaf area and decreased flowering shoots with increasing shade. Kalmia aboveground biomass was 31 per cent higher in low shade and 22 per cent lower in high shade compared with control. The highest maximum photosynthetic capacity (A max) of Kalmia (9 µmol CO2 m−2 s−1) was found in control (full sunlight) and the lowest (5 µmol CO2 m−2 s−1) in the medium-shade treatment. We concluded that Kalmia adapts to moderate shade by morphological plasticity, reproductive trade-off (avoiding sexual reproduction) and physiological acclimation to low light.