Scaling of cotyledon and primary leaf mass versus area in Acer platanoides seedlings under different light conditions.

Cotyledons play an important role in early seedling establishment. However, relative to primary leaves, cotyledons tend to have a different investment-on-return strategy. To detect the potential differences in the mass (M) versus area (A) scaling relationships between cotyledons and primary leaves in different light environments, a total of 75 Acer platanoides seedlings were sampled at an open site (n = 52; light availability: 74 ± 5 %) and a shaded site (n = 23; light availability: 4.2 ± 1.2 %). Reduced major axis regression protocols were used to fit the M versus A scaling relationships of primary leaves and cotyledons. The bootstrap percentile method was used to test the significance of the differences in the scaling exponents of M versus A between the two light environments. The scaling exponents of cotyledons at both two sites, as well as the primary leaves at the shade site, were greater than unity indicating 'diminishing returns', while the scaling exponent of primary leaves at the open site was smaller than unity indicating 'increasing returns'. The data collectively indicated light-dependent shifts in support investments and differences in the function of cotyledons and primary leaves. Average leaf structural traits displayed significant differences between the two light environments in accordance with the premium in enhancing photosynthetic capacity in high light and light interception in low light. Although the trait responses to light availability were similar for primary leaves and cotyledons, primary leaves were more responsive to light availability, indicating lower plasticity of cotyledons in response to light levels. These results advance our understanding of the roles of cotyledons and primary leaves in the life history of seedlings in different forest light environments.