Environmental monitoring plays a central role in diagnosing climate and management impacts on natural and agricultural systems; enhancing the understanding of hydrological processes; optimizing the allocation and distribution of water resources; and assessing, forecasting, and even preventing natural disasters. Nowadays, most monitoring and data collection systems are based upon a combination of ground-based measurements, manned airborne sensors, and satellite observations. These data are utilized in describing both small-and large-scale processes, but have spatiotemporal constraints inherent to each respective collection system. Bridging the unique spatial and temporal divides that limit current monitoring platforms is key to improving our understanding of environmental systems. In this context, Unmanned Aerial Systems (UAS) have considerable potential to radically improve environmental monitoring. UAS-mounted sensors offer an extraordinary opportunity to bridge the existing gap between field observations and traditional air-and space-borne remote sensing, by providing high spatial detail over relatively large areas in a cost-effective way and an entirely new capacity for enhanced temporal retrieval. As well as showcasing recent advances in the field, there is also a need to identify and understand the potential limitations of UAS technology. For these platforms to reach their monitoring potential, a wide spectrum of unresolved issues and application-specific challenges require focused community attention. Indeed, to leverage the full potential of UAS-based approaches, sensing technologies, measurement protocols, postprocessing techniques, retrieval algorithms, and evaluation techniques need to be harmonized. The aim of this paper is to provide an overview of the existing research and applications of UAS in natural and agricultural ecosystem monitoring in order to identify future directions, applications, developments, and challenges.

Robinia pseudoacacia, invaded many countries a long time ago and is now a common part of the Central European landscape. Positive economic but negative environmental impacts of Robinia result in conflicts of interest between nature conservation, forestry, urban landscaping, beekeepers and the public when defining management priorities. Because current legislation will determine the future distribution of Robinia in the landscape, a comprehensive view of this species is necessary. Although this species is well studied, most of the scientific papers deal with the economic aspects. Other information is published in local journals or reports. Therefore we reviewed the ecological and socio-economic impact of Robinia placing particular emphasis on the species’ history, vegetation ecology, invasiveness and management. In Central Europe, Robinia is limited climatically by late spring frost combined with a short vegetation period, soil hypoxia, shade and frequent major disturbances. The long historical tradition of using Robinia for afforestation has resulted in its popularity as a widespread forest tree and it being an important part of the economy in some countries. The main reasons are its fast growth, valuable and resistant wood, suitability for amelioration, reclamation of disturbed sites and erosion control, honey-making and recently dendromass production. On the other hand, a side-effect of planting this nitrogen-fixing pioneer tree, very tolerant of the nature of the substrate, is its propagation and spread, which pose a problem for nature conservation. Robinia is considered invasive, threatening especially dry and semi-dry grasslands, some of the most species-rich and endangered types of habitat in the region, causing extinction of many endangered light-demanding plants and invertebrates due to changes in light regime, microclimate and soil conditions. Other often invaded habitats include open dry forests and shrubland, alluvial habitats, agrarian landscapes, urban and industrial environments and disturbed sites, e.g. post-fire sites, forest clearings or degraded forestry plantations. Without forestry, black locust abundance would decrease during succession in forests with highly competitive and shade tolerant trees and in mature forests it occurs only as admixture of climax trees. The limited pool of native woody species, lack of serious natural enemies and a dense cover of grasses and sedges can suppress forest succession and favour the development of Robinia monodominant stands over 70 years old. A stratified approach, which combines both tolerance in some areas and strict eradication at valuable sites, provides the best option for achieving a sustainable coexistence of Robinia with people and nature.

Many exotic plant invaders pose a serious threat to native communities, but little is known about the dynamics of their impacts over time. In this study, we explored the impact of an invasive plant Heracleum mantegazzianum (giant hogweed) at 24 grassland sites invaded for different periods of time (from 11 to 48 years). Native species' richness and productivity were initially reduced by hogweed invasion but tended to recover after ~30 years of hogweed residence at the sites. Hogweed cover declined over the whole period assessed. A complementary common garden experiment suggested that the dynamics observed in the field were due to a negative plant-soil feedback; hogweed survival and biomass, and its competitive ability were lower when growing in soil inocula collected from earlier-invaded grasslands. Our results provide evidence that the initial dominance of an invasive plant species and its negative impact can later be reversed by stabilising processes.

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.

Summary 1.The initiation of an invasion event is rarely dated in studies of alien plants. Data from aerial photographs documenting the invasion from the outset facilitate the quantification of the rate of spread, allowing researchers to analyse species' population dynamics and providing a basis for management. 2. For 10 sites invaded by Heracleum mantegazzianum in the Slavkovsk y les, Czech Republic, aerial photographs from 11 sampling dates between 1947 (before invasion started) and 2000 were analysed. The area covered by the invader was measured digitally in a 60-ha section of landscape, and information obtained on invaded habitats, year of invasion, flowering intensity and structure of patches. Invaded area was regressed on residence time (time since the beginning of invasion) and regression slopes were used to measure the rate of spread. Data were analysed by  , multiple regression and path analysis. 3. Pastures and fields contributed 84·7% to Heracleum total cover, forest and scrub 13·7% and human settlements 1·6% at the later stage of invasion. The direct effect of the rate of invasion on invaded area (0·82) was greater than that of residence time (0·22), but the total effect (direct and indirect) of residence time was only slightly less (0·79) than that of the rate of invasion (0·82). As invasion proceeded, the populations spread from linear habitats to the surrounding landscape. Mean rate of areal spread was 1261 m 2 year − 1 and that of linear spread 10·8 m year − 1 . Flowering intensity did not exhibit any significant trend over time. 4. Synthesis and applications. The strong effect of the rate of spread on the invaded area indicates that local environmental conditions hardly limit the spread of Heracleum . The species is easily detectable on aerial photographs taken at flowering and early fruiting times, from June to August. Knowledge gained from aerial photographs allows managers to identify dispersal foci and to focus control efforts on linear landscape structures with developing populations. Knowledge of the rate of spread and habitat vulnerability to invasion facilitates the identification of areas at highest risk of immediate invasion.

Coppicing, once a common type of management in European broadleaved forests, was abandoned in many places after WWII. This form of management provided a variety of structural and microclimatic conditions for tree and understorey vegetation. After the abandonment of this intensive management, succession towards mature close forests ensued, and suitable habitats for species ecologically connected to coppicing were reduced. In our study, we chose a region in central Europe where coppicing was the dominant type of forest management until the first half of the 20 th century but was abandoned after WWII. We investigated long-term changes in both woody and herbaceous species composition in the Lower Morava UNESCO Biosphere Reserve using historical sources and vegetation plot resurveys from the 17 th to the 21 st century. The impact of coppice abandonment on vegetation composition and on the conservation value of forests was evaluated. Dominant tree species appeared to be very stable throughout the past four centuries, but changes occurred in their proportions. A shift from species rich oak-hornbeam woodland towards species poorer communities with increasing proportions of lime, ash and maple was observed after the abandonment of coppicing. The observed tendencies partly differed according to site and data source. The conservation value of forests was measured as the occurrence of red-list species, which were considerably reduced after coppice abandonment. To stop the process of biodiversity loss and support the goals of nature conservation, the re-establishment of coppice management is proposed.

Plant invasions represent a threat not only to biodiversity and ecosystem functioning but also to the character of traditional landscapes. Despite the worldwide efforts to control and eradicate invasive species, their menace grows. New techniques enabling fast and precise monitoring and providing information on spatial structure of invasions are needed for efficient management strategies to be implemented. We present remote sensing assessment of a noxious invasive species Heracleum mantegazzianum (giant hogweed) that integrates different data sources, spatial and spectral resolutions, and image processing techniques. Panchromatic, multispectral and color very high spatial resolution (VHR) aerial photography , resolution 0.5 m), and medium spatial resolution satellite data (Rapid Eye 2010, resolution 5 m) were analyzed to assess their potential for hogweed monitoring by using pixel-(both supervised and unsupervised) and object-based image analysis (OBIA, automated hierarchical, iterative, and rule-based). Both point and grid based accuracy assessment was carried out. Described methods of object-based image analysis of VHR data enabled monitoring of hogweed at high classification accuracies measured by various means, regardless of the spectral resolution of the data provided that the data came from the species flowering period. Although the proposed automated processing of VHR data is relatively time-effective and standardized, application over large areas would be rather demanding due to the size of datasets, and multispectral satellite data of medium spatial resolution (lower than the size of individuals) was therefore tested. On such imagery, only larger stands could be identified but still the pixel-based supervised classification achieved moderate accuracy. Depending on the size of the area of interest and the detail needed the very high or medium spatial resolution data (acquired at the species flowering period) are to be used. High accuracies achieved for VHR data indicate the possible application of described methodology for monitoring invasions and their long-term dynamics elsewhere, making management measures comparable precise, fast and efficient.

European broadleaved forests have been influenced by humans for centuries. Historical management practices are related to environmental conditions but the role of socio-economic factors is also important. For the successful restoration of traditional management for conservation purposes, detailed knowledge on management history and on the driving forces of historical forest changes is necessary. In order to reconstruct long-term spatio-temporal dynamics in forest management, we chose the Pálava Protected Landscape Area, Czech Republic and analyzed archival sources spanning the past seven centuries. Forests in the study area comprise two relatively large woods (Děvín and Milovice) with different environmental conditions. Historical forest management in both woods was coppicing. The coppice cycle was lengthened from 7 years (14 century) to more than 30 years (19 century) with a fluctuating density of standards. After WWII, coppicing was completely abandoned. This led to pronounced changes in forest age structure accompanied by stand unification indicated by a sharp decrease in the Shannon index of age diversity. To study local attributes responsible for spatial patterns in coppice abandonment, we constructed a regression model with the date of abandonment as a dependent variable and three groups of explanatory variables: i) remoteness of forest parcels, (ii) morphometric environmental factors and iii) site productivity. In Děvín Wood, coppicing was abandoned gradually with the pattern of abandonment related significantly to slope steepness and forest productivity. Poorly accessible upper slopes and low productive forest sites were abandoned earlier. By contrast, in Milovice Wood, where no clear topographic gradient is present, the abandonment of coppicing was not related to any of the variables we studied. Our study brings insights into the history and consequences of past management practices, and can be used in current attempts to re-establish coppice management for conservation purposes and as a source of sustainable energy.