Less is more: Effects of competition reduction and facilitation on intra-annual (basal area) growth of mature European beech

Key Message: Species-specific neighborhood identity effects such as competition reduction or facilitation can positively influence growth patterns of Fagus sylvatica at a given site, but are not strong enough to overcome fundamental growth–environment interactions of European beech. Abstract: Competition and growth dynamics operating within multi-species tree stands are more complex than interactions within evenly aged monocultures. In three major geographic regions of Germany, we used electronic dendrometers to analyze the impact of neighborhood identity effects such as competition reduction or facilitation on the intra-annual stem growth patterns of Fagus sylvatica (European beech) on various time scales (day to year). Although not consistently significant, within the same study site average basal area growth relative to initial values was always higher at the ends of the growing seasons if subject tree beeches were exposed exclusively to interspecific competition. Weibull growth curves were fitted to rescaled dendrometer profiles. Analysis of the resulting Weibull scale (T) and shape (m) parameter estimates indicated the following: within the same growing area, initial growth trends of beeches in either con- or allo-specific competitive neighborhoods did not differ significantly, but subject trees exclusively surrounded by Pinus sylvestris benefited through extended duration of growth. These findings are reflected in the results for onset, cessation and total duration of wood formation calculated from inverse Weibull functions. Results of this study tend to confirm the assumption that interspecific interference induces higher daily stem growth rates of beech throughout the entire growing season. However, although particular species-specific neighborhood identity or mixing effects can indeed positively influence growth patterns of Fagus sylvatica at a given site, they are not strong enough to overcome fundamental growth–environment interactions of European beech.