Estimating canopy fuel load with hemispherical photographs: A rapid method for opportunistic fuel documentation with smartphones

Using smartphones and upward facing (hemispherical) photos, the team developed a fast and inexpensive method for assessing canopy fuel load. This methods paper demonstrates proof-of-concept: Cameron, Diaz and Beverly 2021.

The amount of fine fuel suspended in the canopy of a forest will in part determine whether or not a wildfire can sustain itself by spreading horizontally through the tree crowns. Aerial fuels consumed by crown fires consist primarily of fine fuels in the form of live needle foliage. Canopy fuel load (CFL) is the weight of combustible aerial biomass per unit area. CFL is used to calculate canopy bulk density (CBD), which is simply the amount of fuel available to burn per unit volume. 

Crown fuel load can be measured by combining stand density estimated from field measurements with species-specific tree biomass equations. But these field campaigns are time consuming and costly, so we developed a simple, photo-based approach that would be suitable for generating estimates of CFL opportunistically.

Hemispherical (upwards facing) photos have long been used to measure forest canopy openness and leaf area index, but standard methods require cumbersome equipment like a DSLR camera paired with a professional-grade hemispherical lens.

Until recently,  these photos could not be taken in sunny conditions. Fortunately, our collaborator Gastón M. Díaz, at the National Scientific and Technical Research Council of Buenos Aires, Argentina developed an R package that allowed us to utilize hemispherical photos taken under variable light conditions that would be expected in opportunistic settings.

The photos were taken with an iPhone 7 mounted with an inexpensive fisheye lens attachment. The relaxed protocol we developed for photo acquisition eliminated cumbersome equipment with hand-leveling, which worked well in our undulating hummocky terrain.

The team compared canopy openness estimated from hemispherical photographs to CFL derived from field measurements in boreal conifer stands and found a strong linear relationship.