Plasticity in growth, biomass allocation and root morphology in beech seedlings as induced by irradiance and herbaceous competition

Biomass increment, biomass allocation and fine-root morphology were compared on four-year old Fagus sylvatica seedlings growing under low (11% relative irradiance), medium (14-19%) or high (46%) irradiance under natural Pinus sylvestris canopies, and under full light in a weeded meadow in the French Massif Central. Significant differences in biomass increment were found among plots in relation to light regime and interspecific competition. Light regime had little effect on shoot-to-root ratio and biomass allocation, but had a clear impact on above- and belowground morphological variables. Beech seedlings displayed a lower specific root length (SRL) and a higher specific leaf area (SLA) under shade, thus indicating morphological adjustment to shade. Similarly, competition from herbaceous vegetation had a negligible effect on seedling growth and biomass allocation, but significant impact on fine-root morphology. Low SLA and high SRL values at high irradiance coincided with high growth increments.