Growth, biomass allocation, and adventitious roots of balsam fir seedlings growing in closed-canopy stands

Well-documented shade adaptations include physiological and morphological traits of shoots and leaves. Belowground shade adaptations (roots or other buried structures) are rarely mentioned in the literature and have never been studied in tree seedlings. This study evaluates the functional role of adventitious roots developed by most balsam fir (Abies balsamea) seedlings under a closed canopy. Our goal was to describe the relationships between growth, biomass allocation, and adventitious root development in balsam fir seedlings (5-26 cm tall, 9 to 33 y old) excavated under a closed canopy (< 8% full sunlight). Seedling height, branch number and crown width increased linearly with stem diameter measured at ground level. The dry mass of stems and roots increased exponentially and tree vigour (photosynthetic/non-photosynthetic biomass ratio) decreased exponentially as stem diameter increased. Results indicate that shade tolerance is limited by size and concurrent radial increment at the stem base. Results also demonstrate that the proportion of adventitious roots and the number of annual growth units buried in humus (age of the belowground section) increased as seedlings aged. Therefore, the reverse taper phenomenon caused by the formation of adventitious roots simultaneously limits the radial expansion of the stem at ground level and the maximum size attainable by seedlings. Balsam fir's shade tolerance is attributable to root as well as shoot adaptations. The gradual burial of the stem in humus probably increases persistence in shade by limiting the plant's respiration costs.