# Mapping Wild Leek through the Forest Canopy Using a UAV
## 4 Discussion
Previous research in Gatineau Park revealed that areas with large and continuous patches of vegetation can be detected with 30 m resolution Landsat 8 imagery acquired during the period when wild leek emerges [48], suggesting that this free source of satellite imagery may be suitable as the basis for wild leek monitoring in the park. The ability of Landsat imagery to detect understory species has been demonstrated before, where invasive bamboo species [22] and European buckthorn [21] were mapped under deciduous forests. An advantage of using a time series to capture phenological differences throughout the year, as achieved by these studies, is that prior knowledge or testing on the optimal dates is not needed. Using our method will require adjusting to yearly variability in the phenology of the wild leek. However, there are several shortcomings associated with using moderate-resolution satellite imagery such as that from Landsat 8 or Sentinel-2 for tracking changes in the wild leek distribution. Not all vegetation types present in Gatineau Park can be distinguished with such imagery, regardless of the size of the patch, and even very large wild leek patches, e.g., 10-20 m across, would be difficult to detect if not surrounded by other patches [48]. A study in urban forests demonstrated that high resolution aerial photos performed much better than Landsat imagery in mapping bush honeyusckle, with a large omission error (48%) occurring from the medium resolution imagery classification [29]. More importantly, from a practical monitoring perspective, even the five-day revisit time of Sentinel-2 means that only a few images will be produced during the short period every spring when wild leek is visible though the leafless canopy, and such images will often be partly covered by clouds. The feasibility of monitoring wild leek distributions with moderate-resolution satellite imagery remains to be fully tested. Figure 7: Workflow for processing UAV-acquired imagery into a binary classification map. Boxes indicate processing steps, blue text describes the data produced by each step. There are several shortcomings in using UAV imagery for understory species detection at a large scale. UAV-based monitoring, with the method outlined above, achieves a high accuracy at a reasonable cost, but covers only small areas. UAVs are relatively cheap to acquire and deliver high resolution images, but the limited flight time of most models (~20-30 min), combined with the requirement in Canadian aviation regulations to keep a UAV within the line of sight of the pilot, limits their use to small areas for each flight [67]. This is especially the case in forested areas where wild leek is predominantly found, and in which keeping the UAV within one's line of sight is challenging and there are not always suitable areas for take-off and landing. Despite these limitations, using UAV-based remote sensing data as an alternative to time-consuming field surveys enables the acquisition of valuable geospatial information at a reasonable cost. For monitoring purposes, sampling the entire extent of a population can be worth the time and resources if the given population is relatively small and easy to access. For example, in Mont-Megantic National Park where the majority of the wild leek population is found at one site, a count of all plants is performed with a monitoring frequency of two years [68]. An extensive plant count would produce more accurate information than a remote sensing-based classification, but would be time-consuming and expensive to implement at large scales. More rapid field survey methods may yield data with a comparable or lower accuracy than UAV-based surveys, as shown for the invasive yellow flag iris in British Columbia [41]. In Gatineau Park or other parks where wild leek extends over large areas that are often difficult to access, covering vast areas on foot to record individual plants or patches is not a viable option, nor is acquiring remote sensing data from UAVs for the entirety of the park.