Contribution of competition indices and edaphoclimatic variables to the prediction of undercork diameter growth of adult cork oak trees in the Portuguese Montado

Abstract

Edaphoclimatic variables and inter-tree competition effects may be observed in adult cork stands. Expressing parameters of tree growth models as a function of these drivers seems therefore appropriate. This work aimed to study the impact of this procedure in the model prediction capacity of cork oak tree diameter growth models. The used dataset consisted of individual adult tree measurements obtained between 1995 and 2019 from seventy-five plots installed across most of the cork oak distribution in Portugal. Tree diameter growth was modelled with the Richards growth function, formulated as an age-independent difference equation. Edaphoclimatic variables and stand metrics were added to one or both model parameters. Then, the best performing four edaphoclimatic models were refitted while testing for significance of adding both distance-dependent and -independent competition indices to model parameters. Finally, a mixed-effects modelling approach was used, by including in the best performing models plot-level random effects to account for the hierarchical structure of the data. Precipitation and Lang index combined with Schists class (Lithology variable) originated the best performing edaphoclimatic models (highest modelling efficiency = 0.98077). Of the nine competition indices that originated suitable models and improved the predictive capacity of their respective edaphoclimatic model, eight were distance-dependent and one was distance-independent (highest modelling efficiency = 0.98082). Adding plot-level random effects did not improve the models. Edaphoclimatic variables showed a higher model improvement than the competition indices, which still improved their respective edaphoclimatic models. Most of the suitable competition indices were spatially explicit, but their performance was equalled by a distance-independent one. The proposed models will be incorporated in existing growth and yield models becoming important tools for forest managers, as they consider climate, soil and competition in cork oak diameter growth prediction in Montado stands.

Bibliographic Information