How do crown structure, light availability, and competition affect trunk growth of (Sequoia sempervirens)?

While tree-level productivity is largely determined by crown size and aboveground vigor (i.e., independent dimensions of tree structure), light availability ultimately controls growth rate. Competition indices are typically used in modeling instead of actual measurements of light. Our goals were to determine which measure of light best predicts trunk growth of Sequoia sempervirens, quantify the amount of growth variation explained by light after accounting for effects of tree structure, and to compare model fitness of competition indices. Twenty-four trees spanning a wide range of light environments were randomly selected from stands of different ages, including trees 24–71 m tall and 20–560 years old. Tree structure and growth were quantified via intensive measurements of each tree's main trunk and appendages. Light availability was quantified via hemispherical photographs taken throughout the crown of each tree. Competition indices were computed by measuring the size and distribution of neighboring trees. After accounting for effects of tree structure, light explained an additional 10% of variation in trunk wood volume increment. Light alone explained 49% of the variation in trunk wood volume increment, while size alone explained 41%. The growth model with average mid-crown openness as a measure of light was 19 times more likely than the model without a light variable. The best competition index was 1200 times more likely than the next best competition index. Distance-dependent evaluation of neighbor tree crown volumes is a promising ground-based method to account for the effect of light availability has on trunk growth.