Abstract
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 six to twelve 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 3819 planted trees. For three tree species (Archidendron pauciflorum, Durio zibethinus, 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.
Highlights
The area of oil palm (Elaeis guineensis) cultivation has strongly increased over the past decades (FAO, 2016; Vijay et al, 2016), and its cultivation generates high economic returns (Rist et al, 2010; Clough et al, 2016; Euler et al, 2016)
The canopy cover from the oil palms was 56% (SD ±17%), and the canopy cover from the planted trees was 15% (SD ±13%)
Pixels with dense oil palm canopy cover 80–100% were 37% less frequent in non-thinned plots than in thinned plots (Figure 3)
Summary
The area of oil palm (Elaeis guineensis) cultivation has strongly increased over the past decades (FAO, 2016; Vijay et al, 2016), and its cultivation generates high economic returns (Rist et al, 2010; Clough et al, 2016; Euler et al, 2016). In comparison with other land use systems, and in particular with forests, such plantations present severely reduced biodiversity and impaired ecosystem functions (Foster et al, 2011; Barnes et al, 2014; Drescher et al, 2016; Dislich et al, 2017). Agroforestry systems that combine agricultural crops and trees often harbor higher biodiversity and provide more diverse ecosystem functions than mono-agricultural land uses (Bhagwat et al, 2008; Jose, 2012; Tscharntke et al, 2012; Barrios et al, 2017; Ashraf et al, 2018). Including agroforestry zones and forest reserves in oil palm-dominated landscapes has been proposed as a way to better balance economic and ecologicalg needs (Koh et al, 2009). There is very little experience with the management of oil palm agroforestry
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