A Method to Count Olive Trees in Heterogenous Plantations from Aerial Photographs

Olive trees have been of economic and cultural value since pre-Roman times, and continue 1 to dominate landscapes and agriculture in many mediterranean regions. Recent mass losses of olive 2 trees in Southern Italy due to an exotic plant pathogen highlight the need for methods that to monitor 3 the olive trees in a landscape or region operationally. Here, we develop a method for counting olive 4 trees from aerial photographs and test it in areas with a high diversity of olive tree ages, sizes, and 5 shapes. This heterogeneity complicates tree counting as centennial trees often have crowns that 6 are split into multiple segments, resembling multiple crowns, while nearby crowns often form a 7 semi-closed canopy comprising multiple trees. Comparisons with reference counts in two 20 ha sites 8 and over three different years indicate the automated counts tend to be reasonably accurate (median 9 error 13%, n = 6), but heavily influenced by a few olive orchards with particularly high planting 10 densities and a relatively closed canopy in which distinguishing individual trees is challenging. 11 Overall, the algorithm estimated tree densities well (counting 82 to 109 trees/ha versus 87 to 104 12 trees/ha in the reference counts), indicating the method is suitable to track the number of olive trees 13 over large areas. 14