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  • Publication
    Accès libre
    Scattered trees in an oil palm landscape: Density, size and distribution
    (2021)
    Yevgeniya Korol
    ;
    Watit Khokthong
    ;
    Zemp, Clara 
    ;
    Bambang Irawan
    ;
    Holger Kreft
    ;
    Dirk Hölscher
    In tropical landscapes dominated by oil palm monocultures, scattered trees can contribute to biodiversity, regulate diverse ecosystem functions and deliver goods and services. However, basic quantitative information about such trees is often lacking. The objectives of our study were to identify the landscape-wide density and distribution patterns of scattered trees in an oil-palm-dominated area of Sumatra (Indonesia), and to estimate their size. The study area with the total of 1120 ha was situated in Jambi province in the lowlands of Sumatra. In 2016, the year of our assessment, 83% of the area was covered by oil palm monocultures including industrial and smallholder plantations; other land-use types included rubber plantations and secondary forests. An earlier land-use classification suggests that oil palm cultivation began before 1990 on 41% of the area. The study area was mapped in 2016 using a fixed-wing drone equipped with red-green-blue and near-infrared cameras. We counted all visible trees in the aerial image. In the entire study region, we detected 10.1 scattered trees/ha. In areas where oil palm plantations were established before 1990, the tree density was 67% lower than in the area where oil palm was introduced later. The median tree crown diameter was 4.5 m, which corresponds to an estimated diameter at breast height of 12 cm; thus, most trees were small-statured. The trees were spatially clustered and often aligned along roads and rivers. In conclusion, we found a considerable number of scattered, mostly small-statured trees. This suggests that most trees were young and disappeared before reaching larger dimensions. To ensure the survival of trees and further provision of related ecosystem services, scattered trees in the oil palm landscape need to be conserved and/or restored.
  • Publication
    Accès libre
    Drone-Based Assessment of Canopy Cover for Analyzing Tree Mortality in an Oil Palm Agroforest
    (2019)
    Watit Khokthong
    ;
    Zemp, Clara 
    ;
    Bambang Irawan
    ;
    Leti Sundawati
    ;
    Holger Kreft
    ;
    Dirk Hölscher
    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.