Plant Invasion

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.

Secondary succession in former arable fields (i.e. old fields) might be altered by the colonization of invasive alien species, with possible community-wide impacts, hindering the ability of old fields to become species-rich communities. However, the effects of invasive species on local communities have rarely been addressed in the light of secondary succession. Therefore, we studied the impact of the highly invasive Solidago canadensis on plant and pollinator communities along a gradient of invasion severity in old fields with different ages (1–20 years since last ploughing) in Southern Transylvania, Romania. We asked whether the invasion of S. canadensis causes shifts in (1) the composition and diversity of plant communities, and (2) pollinator communities along the successional gradient. Further, we asked (3) to what extent the presence of S. canadensis affected flower visitation of native plant species by pollinators. According to our results, the invasion reduced the native plant species richness throughout succession, although the most profound negative effect on plant diversity and vegetation naturalness was exerted in older successional communities. The invasion of S. canadensis had a negative effect on the abundance of bees irrespective of the old field age; however, there was no similar negative effect on hoverflies. Native flowers experienced reduced visitation by wild bees, honey bees and hoverflies due to the augmented presence of S. canadensis. Therefore, the invasion of this perennial plant species diverts the trajectory of vegetation succession, alters the mutualistic links between the native elements of these old fields, and causes a non-desired alternative stable states to be installed.