Characterizing Canopy Openness in Open Forests: Spherical Densiometer and Canopy Photography Are Equivalent but Less Sensitive than Direct Measurements of Solar Radiation

In forest ecosystems, canopy openness affects understory light availability, plant growth, and tree species recruitment, thus shaping future forest composition, structure, and functional diversity. Foresters must correctly and quickly measure canopy openness to meet their management objectives. To help guide the selection of an appropriate method for measuring canopy openness, we compared three common techniques that vary in cost, complexity, and time required for measurements and data processing: smartphone-based hemispherical photography, spherical densiometer measurements, and direct measurements of solar radiation (using AccuPAR ceptometer). We measured canopy openness using these three methods on 28 permanent forest health monitoring plots in pine-oak forests of the Central Pine Barrens of Long Island in New York State. By analysis of variance and regression analyses, we found the three methods (particularly densiometer and hemispherical photographs) yielded broadly equivalent and strongly positively correlated descriptions of canopy openness. The direct measurements of solar radiation seemed to have a greater potential to detect subtle variation in forest understory light. Forest managers may sufficiently characterize canopy openness using quick and cheap methods (e.g., spherical densiometers) and avoid larger costs of devices for direct light measurements (e.g., ceptometers) and the larger data-processing times of hemispherical photography. Study Implications: We present the results of comparing three common methods for measuring canopy openness that vary in their cost and ease of use (spherical densiometer, smartphone-based hemispherical photography, and ceptometer). The traditional spherical densiometer was equivalent to the two more costly and complex methods in estimating canopy openness in open forests (similar to 20% mean openness), but the ceptometer should be used when canopy openness is low (conservatively <10%). Canopy openness affects forest regeneration and biodiversity; thus, foresters need to measure it accurately. Understanding the tradeoffs between accuracy, complexity, and cost among methods for quantifying canopy openness is essential when managing forest ecosystems or wildlife.

Automated summary

Canopy openness in forest ecosystems has significant impacts on forest regeneration and biodiversity. Foresters need to accurately measure canopy openness, and the study found that using quick and cheap methods, such as spherical densiometers, can sufficiently characterize canopy openness while avoiding the larger costs and complexities of other methods.