Timing Is Important: Unmanned Aircraft vs. Satellite Imagery in Plant Invasion Monitoring

Abstract

The rapid spread of invasive plants makes their management increasingly difficult. Remote sensing offers a means of fast and efficient monitoring, but still the optimal methodologies remain to be defined. The seasonal dynamics and spectral characteristics of the target invasive species are important factors, since, at certain time of the vegetation season (e.g., at flowering or senescing), plants are often more distinct (or more visible beneath the canopy). Our aim was to establish fast, repeatable and a cost-efficient, computer-assisted method applicable over larger areas, to reduce the costs of extensive field campaigns. To achieve this goal, we examined how the timing of monitoring affects the detection of noxious plant invaders in Central Europe, using two model herbaceous species with markedly different phenological, structural, and spectral characteristics. They are giant hogweed (Heracleum mantegazzianum), a species with very distinct flowering phase, and the less distinct knotweeds (Fallopia japonica, F. sachalinensis, and their hybrid F. × bohemica). The variety of data generated, such as imagery from purposely-designed, unmanned aircraft vehicle (UAV), and VHR satellite, and aerial color orthophotos enabled us to assess the effects of spectral, spatial, and temporal resolution (i.e., the target species' phenological state) for successful recognition. The demands for both spatial and spectral resolution depended largely on the target plant species. In the case that a species was sampled at the most distinct phenological phase, high accuracy was achieved even with lower spectral resolution of our low-cost UAV. This demonstrates that proper timing can to some extent compensate for the lower spectral resolution. The results of our study could serve as a basis for identifying priorities for management, targeted at localities with the greatest risk of invasive species' spread and, once eradicated, to monitor over time any return. The best mapping strategy should reflect morphological and structural features of the target plant and choose appropriate spatial, spectral, and temporal resolution. The UAV enables flexible data acquisition for required time periods at low cost and is, therefore, well-suited for targeted monitoring; while satellite imagery provides the best solution for larger areas. Nonetheless, users must be aware of their limits.

On the Use of Unmanned Aerial Systems for Environmental Monitoring

“…UAS represent a promising option enabling timely, fast, and precise monitoring that is important for many plant species, particularly those that are invasive [122][123][124]. Flexibility of the data acquisition enabled by the UAS is very important, since plants are often more distinct from the surrounding vegetation in certain times of their growing season [125]. Besides rapid monitoring of newly invaded areas, the UAS methodology enables prediction/modelling of invasion spread that can be driven by a combination of factors, such as habitat and species characteristics, human dispersal, and disturbances [126].…”

Invasive alien plant species: Their impact on environment, ecosystem services and human health

“…The luxuriant growth as extensive monospecific stands, greater amount of biomass and rapid spread through unique functional traits enable the IAPS to dominate the community. These ecological attributes, differentiating the IAPS from natives is exploited through remote sensing and GIS (Mullerova et al, 2017). Therefore, the incorporation of functional/eco-physiological IAPS traits in spectral/spatial mapping tools can explicitly differentiate them from native plants (Niphadkar and Nagendra, 2016).…”

“…Multi-scale remote sensing tools effectively assist in invasion suitability mapping and sustainability of management practices. For example, through remote sensing, we can assess the possible return of IAPS on eradicated sites, thus judge the efficacy of this management practice (Mullerova et al, 2017). Land suitability and ecosystem process models are being used in conjunction with the geospatial tools for managing the IAPS (He et al, 2015;Walsh, 2018).…”

Optimizing the Timing of Unmanned Aerial Vehicle Image Acquisition for Applied Mapping of Woody Vegetation Species Using Feature Selection

“…In the context of cost-effectiveness [1], the presented methodology can be easily implemented within a reasonable overall cost by using accessible off-the-shelf products. For obtaining preliminary phenological information, a consumer-grade digital camera with no NIR capabilities is sufficient (see Figure 8) [61,70,118,119]; drones and other available fixed wing UAVs have been proven to be a suitable tool for collecting repetitive high-quality overhead imagery [6,34,35,120]; while open source free GIS software's (Geographic Information Systems; e.g., QGIS and R) includ sufficient image processing tools.…”