Beyond the cones: How crown shape plasticity alters aboveground competition for space and light-Evidence from terrestrial laser scanning

There are many indications that for a true understanding of aboveground canopy competition, the concept of symmetric trees is oversimplified and unsatisfactory; in spite of that, this concept is still commonly used in forest ecology research. In this study we analyzed and quantified the effect of tree/crown asymmetry on crown-to-crown interactions and canopy light availability with respect to tree size and species. Geometric crown models were used to represent the concept of symmetric trees, while data from terrestrial laser scanning were employed to constitute real crown shapes, positions and mutual crown-to-crown interactions. We developed an original approach for measuring three-dimensional crown asymmetry, separating the effect of positional crown shift and local crown plasticity, and analyzed their effect in aboveground competition for space and light. In comparison with reality, the models neglecting tree asymmetry were only poor predictors of trees mutually competing for space. Geometric models taking the positional crown shift into account were good predictors of 'space competitors' for Norway spruce, but were still insufficient for European beech. This is because for spruce crown shifting seems to be the major neighbor avoidance strategy, while beech in addition exhibited high local crown shape plasticity. Additionally, of the two species beech showed overall greater crown plasticity, which (in contrast to spruce) decreased only slowly with increasing tree size. Importantly, the concept of symmetric trees significantly underestimates the potential canopy light availability (and thus overestimates canopy competition for light), because asymmetric and the plastic 'puzzle-like' arrangement of real tree crowns is more effective than assumed symmetric organization. This most likely inserts a systematic bias into stand growth simulators that are based on the concept of symmetric trees.