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

Abstract

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.

Review and assessment of smartphone apps for forest restoration monitoring

“…We found several apps for estimating canopy openness in our review. Canopy openness is a proxy for understory light environment and productivity and is an important factor for plant growth and tree species recruitment (Holl et al 2018; Russavage et al 2021). Prior to the advent of apps, canopy openness was measured either manually with a densiometer, or via a two‐step process of taking a hemispherical photo of the canopy and extracting metrics with postprocessing software.…”

“…However, when multiple field crews are collecting data over long periods of time, smartphone apps may diminish human‐caused discrepancies in overall estimates (Beeles et al 2021). Ultimately, practitioners need to evaluate the trade‐offs among complexity, accuracy, cost, and postprocessing times when deciding which tool to use for the estimation of canopy openness (Russavage et al 2021).…”

Practical Activities Promoting Engagement in Forest Ecology Research

“…A similar approach was tested before, and successfully proved to be a simple and reliable method for the estimate of canopy cover (Brown et al 2000). While several alternative techniques are available, the canopy disk is considerably cheaper, more easily replicable, and more straightforward to use than most of these (Černý et al 2019;Russavage et al 2021). While a certain subjectivity is required and could produce a bias, the high number of replications obtained through the involvement of citizen scientists would compensate for individual user subjectivity when all are pooled.…”

“…Using a mobile phone app designed to create a hemispherical image from fixed permanent locations under several forest types has been shown to give good reproducibility (Arietta 2022). This is a functional approach, but lacks the pedagogical aspect of the canopy disk and is subject to the same difficulties in standardisation as traditional hemispherical photography (Ribas Costa et al 2022;Russavage et al 2021;Torresan et al 2021). In comparison, our canopy observation activity provides a cheap, simple, and novel way to estimate the penetration of light to the understorey.…”

“…In comparison, our canopy observation activity provides a cheap, simple, and novel way to estimate the penetration of light to the understorey. A densitoimeter could be provided to use as an alternative approach (e.g., YouTube video, how to use a densitoimeter), although obtaining accurate estimates using densitoimeters is known to be challenging (Applegate 2000;Russavage et al 2021).…”