Estimation of holm oak flowering intensity in dehesa farms using high-resolution aerial images
The intensity of flowering of the holm oak trees is important for the annual phenological monitoring and as a predictive index of final acorn production. Their male flowers present in long catkins of intense yellow color and the estimation of their abundance in the field is a time-consuming task that becomes unfeasible at a large scale. In this work, a methodology to estimate the intensity of flowering of oak trees using RGB (Red Green Blue) images, provided by an unmanned aerial vehicle, was tested. During the spring of 2019, three aerial zenith images of 3 cm spatial resolution were taken in three selected dehesa sites, together with simultaneous ground digital photographs per tree (50 at each site). The intensity of flowering was visually estimated using the ground digital photographs in three categories, ranging from 1 (little or no flowering) to 3 (high flowering). A simple flowering intensity index, based on the closeness to pure yellow within a Cartesian RGB space, was developed to check the relationship between the drone images and the visually analyzed photographs. The results showed that those trees with lower flowering intensity were grouped in higher yellow distances and the high flowering intensity trees in the lower ones. As a result, it can be concluded that this index was able to identify qualitatively the flowering intensity of holm oaks at the farm level and could be useful for future phenological or productivity applications.
Instituto de Investigación y Formación Agraria y Pesquera (Spain)
Research scientist at the Institute of Agricultural and Fisheries Research and Training (IFAPA), part of the Department of Agriculture and Rural Development of Andalusia. Her work addresses as a general objective the improvement of knowledge and monitoring of the exchanges of water, energy and carbon in the soil-plant-atmosphere system, through the combination of remote sensors, models and measurements. This line of work is applied on agroforestry systems, at a local and regional scale, with a broad focus on applications that support management decisions and special attention to water resources; and with a more specific focus on the use of remote sensors in complex ecosystems such as Mediterranean oak savanna, known as dehesa.
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