Diel patterns of leaf carbohydrate concentrations differ between seedlings and mature trees of two sympatric oak species

Abstract

A fundamental aspect of the carbon cycle is the exchange of carbon between plants and the atmosphere. It is, therefore, important to understand factors that affect differences in gas exchange and carbon balance within and among species. Concentrations of nonstructural carbohydrates are often used as a proxy for carbon balance. We determined diel patterns of leaf carbohydrate concentrations in relation to irradiance (sun vs. shade) in seedlings and mature trees of two sympatric oak species (Quercus virginiana Mill. and Quercus hemisphaerica Bartram ex Willd.). For seedlings, we also measured leaf gas exchange. Higher sun exposure significantly increased photosynthesis and carbohydrate concentrations in both species. Carbohydrate concentrations of seedling leaves showed strong diel fluctuations, whereas concentrations in mature tree leaves did not. This contrast might be attributed to faster carbohydrate export from leaves of mature trees. The difference in sink strength between seedlings and adults may be related to the decreasing ratio of leaf mass to plant mass with ontogeny, increasing the demand for carbohydrates per unit leaf mass. Seedlings and mature trees are clearly functionally different and care must be taken when extrapolating results from seedling experiments to mature trees.