Growth, mortality, and morphological response of European beech and downy oak along a light gradient in sub-Mediterranean forest

We examined light as a niche partitioning factor between the late-successional European beech (Fagus sylvatica L.) and the mid-successional downy oak (Quercus pubescens Willd.), two dominant species in the sub-Mediterranean area of southern France. For these species we estimated sapling growth models (in radius and in height) as a function of light availability. Beech had a higher growth in low light and a higher asymptotic growth rate than oak. We estimated species-specific growth-mortality functions. Beech showed a higher tolerance to slow growth than oak. By linking light-growth functions and growth-mortality functions, we found that beech had a lower mortality at low light than oak. Beech saplings had a higher probability of survival than oak at low and at high light levels. Beech exhibited the highest plasticity of morphological traits (i.e., biomass allocation, leaf morphology, and architectural traits) as a function of light. Since beech has higher growth and survival than oak at variable light regimes, we conclude that niche partitioning for light cannot explain the coexistence of these two species. We propose that disturbance and water stress should be explicitly taken into account to understand niche partitioning and succession in the sub-Mediterranean area.