Tamm review: Tree interactions between myth and reality

For a long time, interaction between plants in both general plant and forest science has been somewhat limited to competition, although an intuitive, non-scientific understanding of facilitation has influenced the development of a wide range of forest management techniques including silvicultural systems. Despite this, competition has dominated many ecological theories and the interpretation of density effects. Ongoing research in tree mechanics and in verifying the stress-gradient hypothesis reveal that our understanding of tree interactions is still incomplete. Promising research is currently underway attempting to uncover the physiological receptors and processes related to interaction whilst in the past largely plant performance, e.g. plant size and growth rates, was used for verifying competition. We give an overview of quantification methods and suggest a standardisation by common construction principles. Plant performance has been much used for providing empirical evidence of interactions, but when used in isolation it can be a confounding criterion leading to misinterpretations despite promising new developments in quantifying tree interaction. We therefore suggest using pattern-oriented modelling in analyses of tree interactions, where several plant traits are applied simultaneously.