Date Published: February 12, 2018
Publisher: Public Library of Science
Author(s): Kunwar K. Singh, Marguerite Madden, Josh Gray, Ross K. Meentemeyer, Krishna Prasad Vadrevu.
Urban ecosystem assessments increasingly rely on widely available map products, such as the U.S. Geological Service (USGS) National Land Cover Database (NLCD), and datasets that use generic classification schemes to detect and model large-scale impacts of land-cover change. However, utilizing existing map products or schemes without identifying relevant urban class types such as semi-natural, yet managed land areas that account for differences in ecological functions due to their pervious surfaces may severely constrain assessments. To address this gap, we introduce the managed clearings land-cover type–semi-natural, vegetated land surfaces with varying degrees of management practices–for urbanizing landscapes. We explore the extent to which managed clearings are common and spatially distributed in three rapidly urbanizing areas of the Charlanta megaregion, USA. We visually interpreted and mapped fine-scale land cover with special attention to managed clearings using 2012 U.S. Department of Agriculture (USDA) National Agriculture Imagery Program (NAIP) images within 150 randomly selected 1-km2 blocks in the cities of Atlanta, Charlotte, and Raleigh, and compared our maps with National Land Cover Database (NLCD) data. We estimated the abundance of managed clearings relative to other land use and land cover types, and the proportion of land-cover types in the NLCD that are similar to managed clearings. Our study reveals that managed clearings are the most common land cover type in these cities, covering 28% of the total sampled land area– 6.2% higher than the total area of impervious surfaces. Managed clearings, when combined with forest cover, constitutes 69% of pervious surfaces in the sampled region. We observed variability in area estimates of managed clearings between the NAIP-derived and NLCD data. This suggests using high-resolution remote sensing imagery (e.g., NAIP) instead of modifying NLCD data for improved representation of spatial heterogeneity and mapping of managed clearings in urbanizing landscapes. Our findings also demonstrate the need to more carefully consider managed clearings and their critical ecological functions in landscape- to regional-scale studies of urbanizing ecosystems.
Studies of human-modified ecosystems often rely on existing land use and land cover (LULC) products, such as the U.S. Geological Survey (USGS) National Land Cover Database (NLCD) [1, 2], or datasets that were produced using generic LULC classification schemes . Utilizing existing products or schemes are useful for many science questions related to land change, but general-purpose LULC schemes may fail to adequately represent ecological functions that are important for particular studies, and/or may altogether omit important classes. For example, land uses such as road right-of-ways and utility lines that are pervious, yet regularly managed by mechanical clearing and herbicide applications, are often labeled as ‘developed’ in many classification schemes. The notion of developed implies impervious surface and the possibility of redevelopment, infill development, and future change; contrary to the characteristics of these semi-natural land-cover types and their contribution to ecosystem services. Therefore, LULC data without meaningful land cover types that differentiate ecological functions may imperil our efforts to achieve realism in urban ecosystems studies. Unfortunately, our present understanding of managed clearings is insufficient and represents an under-explored opportunity that could benefit realistic assessments of urbanizing ecosystems.
Managed clearings, such as lawns, public parks, and highway medians, are an integral part of urban and suburban landscapes. As the name implies, these areas require management in the form of mowing, tree/shrub removal, and/or application of herbicides and other maintenance in order to remain in a relatively cleared state. LULC data for urban landscapes that does not account for managed clearings may limit our ability to adequately model and address human-modified ecosystem problems, in turn leading to erroneous or misguided policies and management practices. In this study, we investigated the abundance of managed clearings relative to other land-cover types using NAIP imagery, and we compared this to the proportion of land-cover types in NLCD data. Managed clearings in NAIP-derived data is the second most dominant land-cover type after forest and comprises more than one-fourth of the total mapped area in the Charlanta megaregion. We observed a large variation in area estimates between NAIP-derived LULC data and NLCD data suggesting the use of high-resolution remote sensing data for mapping managed clearings versus modifying NLCD data. These findings validate our assumption that a higher proportion of maintained, ecologically functional, pervious managed clearings exist in urbanizing landscapes with inadequate representation in various standard data products and traditional classification schemes. Substantial differences in total land-cover areas between digitized 1-m NAIP images and 30-m NLCD data, while not surprising [25, 26], do suggest including managed clearings in urban landscape mapping to better capture ecologically functional land-cover types.
Managed clearings are a semi-natural, vegetated and ecologically important land cover with varying degrees of management practices used to maintain their cleared and open status in urbanizing landscapes. Our results highlight managed clearings as a dominant land-cover type comprising more than one-fourth of the urban composition in our sample area. Our study also demonstrates the need to derive managed clearings using high-resolution remote sensing data for obtaining true representations of urbanizing landscapes. Because managed clearings have important functional differences (vegetated, pervious, managed) compared to the developed impervious surface urban categories in which they are usually aggregated, the addition of managed clearings to classification schemes is expected to improve urban growth predictions, ecological simulations, and modeling of urban ecosystems.