Variant allometric scaling relationships between bud size and secondary shoot growth in Quercus faginea: implications for the climatic modulation of canopy growth

The relationships between primary and secondary growth in tree populations of contrasting climates are poorly understood. We tested the hypotheses that bud size and stem cross-sectional area are related through allometric relationship in shoots and that their scaling slopes change in response to climatic stress. We sampled three Quercus faginea populations subjected to contrasting climates and elevations. The main components of the current-year shoots (length, cross-sectional area, apical bud mass, and number of buds) were measured in ten trees per site, and the relationships among them were analyzed using structural equation models. Cross-sectional area and apical bud mass were allometrically related and they were higher in the mid-elevation site than elsewhere. In the mid-elevation site, the relationship between cross-sectional area and apical bud mass was the strongest and its scaling slope was the highest. Hence, for a given increase in shoot cross-sectional area, trees from the mid-elevation site produced bigger buds than trees from the other sites. Trees from the mid-elevation site showed a greater potential for primary growth since mild temperatures and low-drought stress improve secondary shoot growth leading to an increased bud size. Therefore, secondary growth affects to a great extent bud size through allometric scaling which is modulated by climatic stress.