Crown modeling by terrestrial laser scanning as an approach to assess the effect of aboveground intra- and interspecific competition on tree growth

Individual tree growth is controlled by numerous factors, but intertree competition is of special importance as it is the only parameter that can be managed by silvicultural measures. Many indices dealing with aboveground interference use geometric standard crown shapes to quantify the competitive strength of neighbors on the target tree. As such crowns do not realistically represent the spatial heterogeneity in the canopy, a terrestrial laser scanner was used to gain detailed information on crown characteristics which may be used for a more precise modeling of the relationship between competition and growth. We examined groups of 5 trees growing in three major geographic regions of Germany to investigate the effect of intra- and interspecific competition for light on basal area growth of target beech trees (Fagus sylvatica). A competition index based on individual crown shapes derived from TLS-data correlated better with target tree growth than an index using conventional geometric crown shape models. Both measures ascribed the highest competitive strength to intraspecific competition. In contrast to the valuable hardwoods for which a slightly lower crown competition was calculated, the conifer species Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) had considerably weaker competitive effects on beech. The growth response of the investigated target trees reflected the strength in competition (highest relative basal area growth of beech if mixed with Scots pine > Norway spruce > valuable hardwoods > intraspecific competition) even though the increment measurements with self-acting dendrometers covered only one vegetation period. Neighborhood diversity and target tree growth were correlated, indicating stronger effects of intra- compared to interspecific interference. The utilization of a terrestrial laser scanner provided insights into canopy structure of the competitive neighborhood that may enhance our understanding of competitive interactions.