REMOTE SENSING GREGORY P. ASNER CHO-YING HUANG REMOTE SENSING TECHNOLOGIES AND APPROACHES

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1 Cohen, A. N., and B. Foster The regulation of biological pollution: Preventing exotic species invasions from ballast water discharges into California coastal water. Golden Gate University Law Review 30: Fowler, A. J., D. M. Lodge, and J. F. Hsia Failure of the Lacey Act to protect US ecosystems against animal invasion. Frontiers in Ecology and the Environment 5: Jenkins, P. T., K. Genovese, and H. Ruffler Broken Screens: The Regulation of Live Animal Imports in the United States. Washington, DC: Defenders of Wildlife. Kerwin, C. M Rulemaking: How Government Agencies Write Law and Make Policy, 3rd ed. Washington, DC: CG Press. Lodge, D. M., S. Williams, H. J. MacIsaac, et al Biological invasions: Recommendations for U.S. policy and management. Ecological Applications 16: Skrzycki, C The Regulators: Anonymous Power Brokers in American Politics. Lanham, MD: Rowman and Littlefield Publishers. Sumner, D. A., ed Exotic Pests and Diseases: Biology and Economics for Biosecurity. Ames: Iowa State Press and Wiley-Blackwell. REMOTE SENSING GREGORY P. ASNER Carnegie Institution for Science, Stanford, California CHO-YING HUANG National Taiwan University, Taipei Remote sensing has become a key tool for mapping ecosystems, but it is relatively new in studies of alien invasive plants. With the advent of high spatial, temporal, and spectral resolution sensors since 1990, the role of remote sensing in invasive species research and management has gradually increased. Different types of sensors offer varying information on invasive species or the habits they invade. This entry highlights the major types of sensors currently available, as well as new systems, and how they are being used for alien invasive species work. REMOTE SENSING TECHNOLOGIES AND APPROACHES There are many types of field, airborne, and satellite remote sensing technologies, but a few have proven useful for alien invasive species detection and mapping. The most intuitive and straightforward remote sensing approach for alien plant detection is to use high spatial resolution images (<10 m pixel size; Table 1) to visually inspect the spatial distribution of nonnative species. High spatial resolution imagery is usually acquired from aircraft, but a few satellite sensors also offer this capability. Independently of the image source, the goal with high spatial resolution imagery is to pinpoint species based on their unique spatial patterns or phenological characteristics. For example, in 1995 Everitt and colleagues used aerial photographs taken during the flowering seasons of Eurasian Euphorbia esula and Asian Tamarix chinensis, finding the visible reflectance ( nm) of the invaders to be significantly higher than surrounding vegetation due to their bright inflorescences. Color infrared (CIR) aerial photographs are also often used, with resolutions ranging from a few centimeters to about 2 m. In 2005 Müllerová and colleagues used CIR aerial photographs acquired during the onset of Heracleum mantegazzianum invasion (the largest central European forb native to the western Caucasus) in the Czech Republic. The brightness of H. mantegazzianum was much greater than that of the neighboring vegetation, not only during the flowering, but also in the early fruiting season, due to its distinct structure of fruiting umbels. At times, the spatial configuration or structure of the invader can be delineated from that of its native neighbors. In 2005 Fuller used multispectral IKONOS satellite imagery with a 4 m spatial resolution to map an Australian tree, Melaleuca quinquenervia, in southern Florida. He found that the spatial pattern of Melaleuca was highly aggregated and regularly shaped, compared to the nearby patches of woody plants, which were likely associated with infrastructural development. However, Fuller s analysis pointed out that the 4 m spatial resolution of IKONOS was insufficient to identify the alien tree when canopy cover dropped below 50 percent. A variety of other high spatial resolution satellite sensors have been used to estimate the location and extent of invasive plants, but most (if not all) fall short in detecting single individuals or clusters prior to reaching about 50 percent cover. In contrast, others have worked with low spatial resolution (>250 m) imagery, which allows for frequent, often daily, imaging from space (Table 1). These high temporal resolution or time-series data have been used for studies of invasive plants and their habitats. The very common Normalized Difference Vegetation Index (NDVI) enhances the signal of photosynthetically active vegetation with a combination of visible and near-infrared spectral bands. Several phenological metrics such as vegetation onset time, duration of growth, end time, and rate of vegetation green-up and senescence can be derived from NDVI time-series data. Using the NOAA Advanced Very High Resolution Radiometer (AVHRR), Bradley and Mustard showed in 2005 that the highly invasive grass Bromus tectorum could be readily mapped in the Great Basin of the United States, based on its unique phenological response to precipitation relative to neighborhood 580 REMOTE SENSING From Daniel Simberloff and Marcel Rejmánek, editors, Encyclopedia of Biological Invasions, Berkeley and Los Angeles: University of California Press, 2011.

2 TABLE 1 Sensor Specifications and Applications as Applied to Biological Invasion of Terrestrial Ecosystems Sensor Specifications Spectral Species Traits and Remote Spatial (m) Temporal (days) (bands) Examples Sensing Strategies Limitations Moderate Spatial/Spectral <20 ASTER, SPOT, Large stands; differentiate Coarse spatial and spectral resolution Landsat TM/TEM phenology of coexisting incompatible with invasive species + satellite sensors plants; select images detection among native species acquired in the right season High Spatial <10 <10 5 Aerial photographs, Unique spatial patterns; Inflexibility of data collection; pixel Quickbird and flowing phenology spacing still not fine enough to IKONOS satellite observe plants at the species level; sensors cannot detect plants lacking distinct flowering pattern; impractical for large-area monitoring High Temporal <40 AVHRR, MODIS, Unique phenological Insufficient spectral bands to extract VEGETATION characteristics; combination non-native species from large pixels; satellite sensors of models and time-series difficult to conduct field validation vegetation indices derived due to the large pixel size; nonfrom the images native species signals overwritten by climatic variations such as precipitation Hyperspectral Varied (0.5 30) Variable >75 AVIRIS airborne and Unique spectral signatures; Limited geographical coverage; Hyperion satellite spectral mixture analysis; spectral similarity of species sensors biochemical analysis Active Remote Sensing Variable 1 (3-D imaging) LiDAR, RADAR Monitoring structural No spectral information so species are aircraft and changes of stand during hard to detect without other satellite sensors invasion from repeat information coverage Image Fusion 0.5 Variable 50+ (3-D view) Carnegie Airborne Unique spectral High performance computing Observatory signatures of species; required; limited spatial coverage; detection of non-native pushing the limits of spatial, species of different height spectral, and dimensional levels (overstory and resolutions understory) at the very fine spatial scale vegetation. In 2006, Morisette and colleagues combined NDVI and land cover from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) sensor with thousands of field points to predict the habitat suitability for nonnative Tamarix spp. in the mainland United States. The analysis highlighted the Southwest as the most suitable region for these Asian trees and shrubs. Imaging spectrometers (also called hyperspectral imagers) collect continuous data across a wide region of the visible ( nm), near-infrared (700 1,200 nm), and shortwave infrared (1,200 2,500 nm) spectra, often using hundreds of spectral bands ( 10 nm resolution) (Table 1). Some of the most commonly used sensors for scientific and management purposes include the REMOTE SENSING 581

3 NASA Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) and the Compact Airborne Spectrographic Imager (CASI). There has been one civilian research satellite, Earth Observing-1 (EO-1), which has provided hyperspectral imagery from space. Although the history of hyperspectral remote sensing is relatively short (less than 25 years) compared to other types of remote sensing (e.g., aerial photographs, which have been around for more than 100 years; satellite images, around for about 50 years), hyperspectral images are currently the most heavily used source of remotely sensed data for invasive species research. The main advantages of hyperspectral data are that detailed spectral profiles can be developed for native and nonnative plants, and that specific spectral regions can be analyzed that are most sensitive to the abundance of the species of interest. In 2002 Williams and Hunt used hyperspectral data from the AVIRIS sensor to map a European herbaceous perennial plant Euphorbia esula in the Great Plains of the United States. Similarly, Underwood and colleagues used airborne hyperspectral imagery to map invasive plants in California in Uses of hyperspectral data are not limited to studies of the presence or abundance of alien species, but can be further extended to study the ecosystem effects of invasion. In 2005 Asner and Vitousek used AVIRIS with canopy models to investigate how the invasive nitrogenfixing tree Morella (Myrica) faya and the understory herb Hedychium gardnerianum affect the canopy chemistry of Hawaiian montane forests. They found that M. faya significantly increases canopy nitrogen concentrations and water content in invaded sites. In contrast, H. gardnerianum substantially reduces nitrogen concentrations in the overstory tree canopies (presumably through competition for nitrogen in soils). In 2008 Asner and colleagues further studied the uniqueness of hyperspectral signatures of 43 Hawaiian native and alien trees (Fig. 1). They found that invasive trees were spectrally unique from native trees, resulting from differences in leaf pigments, nutrients, and structural and biophysical properties of invasive and native species. Hyperspectral images from the spaceborne EO-1 Hyperion sensor are less commonly used for alien species mapping, probably due to the coarse spatial resolution (30 m) and lower instrument performance compared to airborne imaging spectrometers such as AVIRIS. Nonetheless, Hyperion provides an opportunity for systematic data collection over a remote region on a stable platform (EO-1 satellite) and at much lower cost than many airborne mapping efforts. The data have been utilized to FIGURE 1 Hyperspectral reflectance signatures grouped as Hawaiian native (H) and invasive (I) tree species are distinct from one another, making it possible to map them using current airborne imaging spectrometer technology. The black and gray lines indicate mean and range of spectral reflectance in each group, respectively. The unique spectral properties of each group arise from the unique biochemical and structural properties of the species, especially those which tend to be successful invaders of Hawaiian ecosystems. (Derived from Asner et al., 2008.) map coastal alien plants with demonstrably satisfactory accuracies (overall accuracy 81%). Light detection and ranging (LiDAR) is another optical remote sensing technology that measures the distance between the sensor and a target surface, obtained by determining the elapsed time between the emission of a short duration laser pulse and the arrival of the reflection of that pulse at the LiDAR receiver. It has been widely used to estimate the three-dimensional structure of individual plants and vegetation. LiDAR has not yet proven to be an efficient tool for detecting alien plants; however, recent efforts have integrated hyperspectral and LiDAR instrument systems to improve the detection of invasive species based on their chemical 582 REMOTE SENSING

4 utility of images for alien plant monitoring especially in early stages of invasion. Data patterns in hyperspectral images can be complicated and hard to unravel. In many cases, data reduction techniques are required for generalization, and they usually involve sophisticated algorithms that mine for invasive species signals in the high-dimensional spectral space. An imaging spectrometer is also usually flown onboard aircraft, so the swath width of a hyperspectral image is usually narrow, which limits its ability for largescale invasive plant mapping. Also, scheduling of aircraft and related regulations for different countries could make data acquisition inflexible. FIGURE 2 New airborne three-dimensional (3-D) remote sensing of SEE ALSO THE FOLLOWING ARTICLES invasive species and their canopy structural properties. This image was collected by the Carnegie Airborne Observatory (CAO) over a lowland moist tropical forest region on Hawaii Island. Spectroscopic analysis in the lower panel indicates canopy species composition of the forest Disturbance / Early Detection and Rapid Response / Fire Regimes / Habitat Compatibility / Hawaiian Islands: Invasions / Landscape Patterns of Plant Invasions / Range Modeling (shown as different colors), while LiDAR analysis in the upper panel provides quantitative measurements of the 3-D structure of the canopies. Infestations of the highly invasive tree Psidium cattleianum is highlighted. (From G. Asner, unpublished.) and structural properties. One of the only systems in the world with this capability is the Carnegie Airborne Observatory (CAO; In 2008, using a laser-guided spectroscopy approach, Asner and colleagues mapped individual crowns of five highly invasive tree species throughout Hawaii Island (Fig. 2). This study showed that different invasive tree species alter the fundamental structure of the forests they invade, but in different ways, either from the ground up or from the top down. CURRENT LIMITATIONS The information provided by moderate spatial and spectral resolution satellite images (e.g., Landsat Thematic Mapper) is usually insufficient to delineate the distribution of alien plants or their functional traits in ecosystems. Aerial photographs and some high spatial resolution satellite imagery techniques (e.g., IKONOS, GeoEye-1) have been used to assess the extent of some invasive species with distinct phenological characteristics, but not in a systematic way that might allow large-scale application. Sensors with large area coverage such as MODIS have large pixels (low spatial resolution), and thus the images collected by these sensors capture not only the species of interest but other components as well, such as untargeted plants, surface soils, and senescent vegetation; this limits the FURTHER READING Asner, G. P., R. F. Hughes, P. M. Vitousek, D. E. Knapp, T. KennedyBowdoin, J. Boardman, R. E. Martin, M. Eastwood, and R. O. Green Invasive plants transform the three-dimensional structure of rain forests. Proceedings of the National Academy of Sciences, USA 105: Asner, G. P., M. O. Jones, R. E. Martin, D. E. Knapp, and R. F. Hughes Remote sensing of native and invasive species in Hawaiian forests. Remote Sensing of Environment 112: Asner, G. P., and P. M. Vitousek Remote analysis of biological invasion and biogeochemical change. Proceedings of the National Academy of Sciences, USA 102: Bradley, B. A., and J. F. Mustard Identifying land cover variability distinct from land cover change: Cheatgrass in the Great Basin. Remote Sensing of Environment 94: Cuneo, P., C. R. Jacobson, and M. R. Leishman Landscape-scale detection and mapping of invasive African Olive (Olea europaea L. ssp. cuspidata Wall ex G. Don Ciferri) in SW Sydney, Australia using satellite remote sensing. Applied Vegetation Science 12: Everitt, J. H., G. L. Anderson, D. E. Escobar, M. R. Davis, N. R. Spencer, and R. J. Andrascik Use of remote sensing for detecting and mapping leafy spurge (Euphorbia esula). Weed Technology 9: Fuller, D. O Remote detection of invasive Melaleuca trees (Melaleuca quinquenervia) in South Florida with multispectral IKONOS imagery. International Journal of Remote Sensing 26: Huang, C., and G. P. Asner Applications of remote sensing to alien invasive plant studies. Sensors 9: Morisette, J. T., C. S. Jarnevich, A. Ullah, W. Cai, J. Pedelty, J. E. Gentle, T. J. Stohlgren, and J. L. Schnase A tamarisk habitat suitability map for the continental United States. Frontiers in Ecology and the Environment 4: Müllerová, J., P. Pyšek, V. Jarošík, and J. Pergl Aerial photographs as a tool for assessing the regional dynamics of the invasive plant species Heracleum mantegazzianum. Journal of Applied Ecology 42: Pengra, B. W., C. A. Johnston, and T. R. Loveland Mapping an invasive plant, Phragmites australis, in coastal wetlands using the EO-1 Hyperion hyperspectral sensor. Remote Sensing of Environment 108: Robinson, T. P., R. D. van Klinken, and G. Metternicht Spatial and temporal rates and patterns of mesquite (Prosopis species) invasion in Western Australia. Journal of Arid Environments 72: REMOTE SENSING 16_Simberloff10_R_p indd /14/10 3:20:43 PM

5 Underwood, E., S. Ustin, and D. DiPietro Mapping nonnative plants using hyperspectral imagery. Remote Sensing of Environment 86: Williams, A. P., and E. R. Hunt Jr Estimation of leafy spurge cover from hyperspectral imagery using mixture tuned matched filtering. Remote Sensing of Environment 82: REPRODUCTIVE SYSTEMS, PLANT SPENCER C. H. BARRETT University of Toronto, Ontario, Canada The reproductive systems of plants concern the diverse strategies by which populations reproduce, including the balance between sexual and asexual reproduction, and the structures and processes governing the union of gametes and the transmission of genes between generations. The reproductive system is a particularly important life history trait for invasive species, because the quantity and genetic quality of propagules produced by reproduction are important for successful colonization and spread in novel environments. Flowering plants (angiosperms) display a wide range of reproductive systems, including a diversity of pollination and mating strategies, as well as striking variation in the relative importance of sexual versus asexual reproduction. Reproductive systems differ in their ecological and biogeographical consequences, particularly when colonizing individuals occur at low density without mates or pollinators following dispersal. Under these demographic circumstances, uniparental reproductive systems are often favored over biparental reproduction, especially for species with short life histories. Reproductive systems also determine opportunities for adaptive evolution, because they influence key populationgenetic parameters such as genetic recombination, effective population size, gene flow, and the partitioning of genetic diversity within and among populations. REPRODUCTIVE DIVERSITY IN PLANTS Three distinctive features of plant biology complicate reproduction. First, being immobile, plants require vectors to transfer pollen (pollination) between individuals to facilitate mating. This has promoted the evolution of diverse floral adaptations associated with the agents of pollen dispersal (animals, wind, water) and has given rise to the field of pollination biology. Second, most plants exhibit hermaphroditic sex expression; thus many are capable of self-fertilization (selfing) as well as cross-fertilization (outcrossing). For sexual species, selfing requires that plants are self-compatible, which means that they have the ability to set seed following self-pollination. In contrast, outcrossing is most commonly enforced by self-incompatibility, a genetically determined physiological mechanism that prevents self-fertilization. Lastly, due to the modular structure of plants and to the fact that they often exhibit a high degree of phenotypic plasticity, male and female gametes can be combined in diverse ways at the flower, inflorescence, and plant levels. This variation in sex allocation results in diverse gender strategies despite the basic hermaphroditic condition of most plants. These distinctive features of plant reproduction result in a high level of sexual promiscuity within plant populations, and individuals often mate with many sexual partners (multiple paternity), including themselves. Recent research in reproductive biology is concerned with using genetic markers to determine who mates with whom, and, through ecological experiments, with evaluating the fitness consequences of different levels of outcrossing and selfing in populations. In addition to reproducing sexually, many plants also reproduce asexually. In such cases, the parents produce genetically identical copies of themselves through either vegetative reproduction or apomixis (agamospermy). Vegetative reproduction, also termed clonal growth or clonal propagation, is very common among perennial plants, and the morphological structures involved include axillary shoots, stolons, runners, bulbs, corms, and tubers. A functionally important distinction between the different forms of clonal growth is whether clonal offspring become physiologically autonomous and capable of dispersal away from the parental plant. Clonal propagation often dominates at geographical range limits in species with broad distributions and is also a characteristic feature of aquatic plants, for whom floating clonal propagules are an important means of dispersal. Rates of vegetative reproduction vary widely among species, depending on local environmental conditions, but there are very few examples, if any, in which clonal propagation is the exclusive means of reproduction throughout a species geographical range. Apomixis, the production of clonal seed in the absence of fertilization, is less common than vegetative reproduction. Nevertheless, it has been reported from at least 300 species from over 40 families and is especially common in the Asteraceae, Poaceae, and Rosaceae, all families in which invasive species are well represented. Apomixis can be either obligate or facultative and can be 584 REPRODUCTIVE SYSTEMS, PLANT