Individual-based approach as a useful tool to disentangle the relative importance of tree age, size and inter-tree competition in dendroclimatic studies

In this work, an individual-based approach was used to assess the relative importance of tree age, size, and competition in modulating the individual dendroclimatic response of Quercus robur L. This was performed in a multi-aged forest in northwestern Spain under a wet Atlantic climate. All trees in five replicated forest stands with homogeneous soil conditions were mapped and inter-tree competition was quantified with a distance-dependent competition index. Tree rings of cored trees were crossdated and total age was estimated on individuals where the pith was missed. The climatic response was evaluated by bootstrapped correlations of individual tree-ring chronologies with climatic records. Inter-annual growth variation, i.e., mean sensitivity, was independent of tree age and bole diameter, but modulated by competition. Water excess in previous summer-autumn and spring negatively affected growth, while warmer September conditions favored growth. Individual response to climate was independent of tree age, but related to the joint effect of tree bole diameter and competition. Larger oaks in less competitive environments responded more plastically to climatic stress, while smaller trees under high competition levels were less responsive to climate. Strong inter-tree competition reduced growth plasticity but amplified the vulnerability of smaller oaks to the particularly rainy conditions of the study area. These findings suggest that inter-tree competition is a relevant size-mediated extrinsic factor that can potentially modulate individual radial growth variation and its response to limiting climatic conditions in temperate deciduous forests. This study highlights the value of individual-based approach as a useful tool that informs about the relative contribution of factors modulating the climatic response of tree-ring growth.