Drone-Based Assessment of Canopy Cover for Analyzing Tree Mortality in an Oil Palm Agroforest

Oil palm monocultures are highly productive, but there are widespread negative impacts
on biodiversity and ecosystem functions. Some of these negative impacts might be
mitigated by mixed-species tree interplanting to create agroforestry systems, but there is
little experience with the performance of trees planted in oil palm plantations. We studied
a biodiversity enrichment experiment in the lowlands of Sumatra that was established in
a 6- to 12-year-old oil palm plantation by planting six tree species in different mixtures on
48 plots. Three years after tree planting, canopy cover was assessed by drone-based
photogrammetry using the structure-from-motion technique. Drone-derived canopy
cover estimates were highly correlated with traditional ground-based hemispherical
photography along the equality line, indicating the usefulness and comparability of the
approach. Canopy cover was further partitioned between oil palm and tree canopies.
Thinning of oil palms before tree planting created a more open and heterogeneous
canopy cover. Oil palm canopy cover was then extracted at the level of oil palms
and individual trees and combined with ground-based mortality assessment for all
3,819 planted trees. For three tree species (Archidendron pauciflorum, Durio zibethinus,
and Shorea leprosula), the probability of mortality during the year of the study was
dependent on the amount of oil palm canopy cover. We regard the drone-based method
for deriving and partitioning spatially explicit information as a promising way for many
questions addressing canopy cover in ecological applications and the management of
agroforestry systems.