The Diversity of Heath Flowering Phenology– Revealing Fine Scale Patterns of Heterogeneity by High Resolution Drone Cameras

Unmanned aerial vehicles (UAV) open up new perspectives for the repetitive spatial monitoring of vegetation stands and possibly even for fine scale analyses of individual plants. The potentials of recent UAV camera systems for integrating high spatial resolution (< 10 cm per image pixel) and area wide mapping are undergoing research. In particular, robust algorithms are needed for a spatially explicit characterization of individual plants, their structural composition and related functional traits. Such information can be used to examine ecological interdependencies that determine habitat establishment, and in general, to evaluate the habitat quality for nature conservation purposes [1]. In our study we developed a UAV-based methodological procedure to analyse open heathland areas that are protected in the European Natura 2000 network, and therefore, need to be managed to preserve a favourable conservation status [2]. We mapped the flowering phase of regenerated dwarf shrub heath (Calluna vulgaris) stands 2 years after burning and of unmanaged old stands using a standard RGB drone camera. From the UAV imaging point cloud a digital orthophoto and a digital surface model was generated with 2 cm pixel resolution. A method was developed that extracts every individual plant on the basis of RGB-colour value classification, texture filtering and local maxima estimation on a normalized digital surface model. For every plant the maximum plant height as well as the RGB colour distribution could be extracted and related to field surveys of plant traits. On that basis, a colour model was constructed that statistically predicts the phenological heath plant status with regard to fractional cover of flowers, fruits, vegetative shoots and senescent plant components. Our study reveals that the flowering phase of Calluna vulgaris is spatially much more divers than expected. Colour models from simple RGB imagery show that phases of completely vegetative, withered, fruit and full flowering plants spatially coexist in heterogeneous alternations of individual plants, in fact, independent of growth age after burning (height < 40cm). Only unmanaged old stands (height > 40 cm) exhibit significantly lower fractions of fruit and flowers while senescence is increased. However, the high phenological diversity after fire management shows that driving factors of shoot extension and flower productions, thus, are triggered by additional mechanisms that are not affected by habitat management practice. UAV-based imagery can help to observe such effects for a more detailed evaluation of management effects. REFERENCES: 1. Neumann C, Weiss G, Schmidtlein S, Itzerott S, Lausch A, Doktor D, Brell M., 2015. Gradient-based assessment of habitat quality for spectral ecosystem monitoring. Remote Sensing. 7(3), 2871-98. 2. Habitat Directive, 1992. Council Directive 92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna and flora. Official Journal of the European Union. 206, 7-50.