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

Background/Question/Methods

Management of uneven-age forests will benefit from a better understanding of tree responses to light environment. 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. Competition indices were computed by measuring the size and distribution of neighboring trees. Drop in residual deviance quantified the variation in growth explained by light after accounting for variation in tree structure. Competition indices were compared for model fitness with AIC_c. 

Results/Conclusions

The largest tree had 69 m³ of wood and bark, 3208 m² of leaf area, and grew 1.04 m³ yr^-1. The smallest tree had less than 1 m³ of wood and bark, 134 m² of leaf area, and grew 0.04 m³ yr^-1. 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 in terms of model fitness than the next best competition index. By sampling across wide gradients of tree size and aboveground vigor, we demonstrated the importance of light for Sequoia sempervirens growth. Distance-dependent evaluation of neighbor tree crown volume is a promising ground-based method to account for the effects of light availability on variation in tree-level productivity.