Forests are substantial land cover sources for new urbanization both in the U.S. and globally [1
]. Cumulatively, the increase in urban land and related types of development, such as roads and other exurban infrastructure, can cause a reduction of forest extent, fragmentation of wildlife habitat, and changes to hydrology and other regulating ecosystem services, such as carbon storage [4
]. Due to geographic differences in human population and demographics, biophysical settings, and other factors, the impact of forest land cover conversion to new developed built-up land can be highly variable. Replacement of forest by urban development is also one of the most permanent changes to the environment [5
] and may become even more important in regards to climate change effects on a growing number of people [6
The growth of urban areas has been inescapable for decades, has tended to be sprawling, and is expected to continue to have substantial impact on land cover in the future [5
]. However, mitigation is increasingly recognized as important, and there are new approaches to planning and managing urban ecological systems that could impact future trends, including consideration of urban forests and the sustainability of surrounding landscapes [10
]. The role of forested land cover within, and surrounding, urban areas, and how best to mitigate the ongoing negative externalities of forest to urban developed land cover change is just one of the management pieces needed in understanding changed forest conditions in the near future [14
Urbanization is a major driver of forest land cover change that needs renewed focus to analyze its widespread implications and potential impacts to human well-being [15
]. A number of studies have conducted assessments of forest to urban developed land cover conversion either as their main emphasis or as part of the overall aspect of increased urbanization but these works tend to be scale limited by metropolitan area [16
] or by region [20
] or by temporal interval if done at a near national scale [24
]. This research is the first to access near-national scale (CONUS) forest land cover to urban land change across a much longer time span (1973–2011) using similar remote sensing-derived datasets for six time-step intervals. Although near-national in overall scale, results are presented using a meso-scale ecoregional geographic framework that links similar land forms, vegetation, soils, and land use [25
]. Using several proportional and rate conversion metrics, this work shows what ecoregions have been heavily impacted by forest to urban developed land cover conversion during the study period and where this type of land-use change has been much less of an issue.
A plurality of the ecoregions (35 out of 84) had conditions where at least 40% of their new developed land cover came from upland forest land cover at least one time during the study period. Geographically, these ecoregions tended to be clustered in the eastern U.S. outside of Florida (Figure 3
). Other large regional clusters include the Pacific Northwest, the South-central U.S., and the Great Lakes North Woods as well as the “Texas Hill country” (Edwards Plateau, ecoregion #30 in Figure 1
) and scattered ecoregions across the Inter-Mountain West, although most of the ones there were infrequent in occurrence. A number of these 35 ecoregions also had small area amounts of land being converted to developed land cover from forest (Table 1
), making them appear more impressive on a map based on percentage of overall newly urban developed land than area affected.
The number of ecoregions where forest was the source of at least 40% of the new urban development in every time period was more limited (11 out of 84). The number of clusters shrunk as well with only the Puget Lowland (ecoregion #2 in Figure 1
) found in the Western U.S., a two-ecoregion cluster in the South-central U.S., three ecoregions in the Northeast, and five ecoregions scattered across the Appalachian Mountains and foothills (Figure 3
). The Piedmont, Northeastern Coastal Zone, Puget Lowlands, and South Central Plains (ecoregions #45, #59, #2, and #35, respectively, in Figure 1
) consistently had the most forest to urban developed land cover by area across time.
A majority of the ecoregions (58 out of 84) had at least one time interval where the proportion of forest to urban development in overall new urban land exceeded the proportion of forest land cover within the ecoregion. This is a useful metric because it can indicate where forested land is targeted more for conversion than other land covers within an ecoregion, and without replacement from another land cover forested land may face noticeable losses. The geographic pattern was more widespread and diffuse (Figure 4
) than that seen in the forest as a substantial source of new urban developed land cover (Figure 3
). However, in many ecoregions where this metric occurred, the threshold was met only occasionally (Table 2
The number of ecoregions where the proportion of forest to urban developed land cover change exceeded the proportion of forest within the ecoregion every interval was far fewer (6 out of 84,) than those exceeding it occasionally and only about half the ecoregions where forest was a substantial source of new urbanization every time interval. There was less geographic clustering of the six ecoregions that exceeded their proportion of forest every interval except the three along the eastern seaboard from southern Maine through Northern Florida (Figure 4
), the Mississippi Alluvial Plain and Western Gulf Coastal Plain (ecoregions #73 and #34, respectively, in Figure 1
), and the Puget Lowland (ecoregion #2 in Figure 1
) in the Pacific Northwest. The six ecoregions that exceeded their forests’ proportions when converting to urban development generally did so substantially.
Even though the annual rate of forest to developed land cover change was set at a fairly conservative number of 0.2%, only a minority of the ecoregions (12 out of 84) met or exceeded this rate at any time during the study period. Geographically, four clusters and one additional ecoregion are visible (Figure 5
) although several of the clusters merge to create even larger contiguous regions. All of the ecoregions that front the Atlantic Ocean or Gulf of Mexico shoreline had a rate of 0.2% or greater annual change of upland forest converting to urban developed land cover at least once during the study period. Inland, the Northern Piedmont (ecoregion #64 in Figure 1
) links highly urbanized areas of the Northeast coastal ecoregions and the Piedmont (ecoregion #45 in Figure 1
) cities along the Fall Line and the foothills of the Appalachian Mountains, Gottmann’s older “Megalopolis” of interspersed mosaics of urban, forest, and agricultural land covers [55
] meeting up with Hart’s and Morgan’s emerging southern “Spersopolis” of low-density, but nearly continuous, residential housing along highways linking urban centers [56
]. Another cluster is centered on the Erie Drift Plains and the Eastern Corn Belt Plains (ecoregions #61 and #55 respectively in Figure 1
), whereas the Puget Lowland (ecoregion #2 in Figure 1
) is the only ecoregion in the Western U.S.
The rate of “high” annual forest to urban developed land cover change ranged from three ecoregions reaching 0.2% at least during one time interval to the Southern Florida Coastal Plain (ecoregion #76 in Figure 1
) reaching 0.61% annually during the 1986 to 1992 interval (Table 3
). This ecoregion exceeded or nearly exceeded 0.5% annual change during the first three intervals of the LC Trends era, although with forest to urban developed land cover change declining to near zero during the NLCD intervals may bring into question the issue of how forest cover is classified as either “upland” or “wetland” between the two datasets. The Atlantic Coastal Pine Barrens (ecoregion #84 in Figure 1
), which includes the center of the New York metropolitan area, was the only ecoregion to reach or exceed the 0.2% annual rate during all the time intervals.
The results of the composite metric shows that there are three ecoregions (Puget Lowland, Northeastern Coastal Zone, and the Atlantic Coastal Pine Barrens—ecoregions #2, #59, and #84, respectively, in Figure 1
) that had 15 or above out of 18 “points” (Figure 6
). Each of these are small ecoregions in size, heavily urbanized, and where continued urbanization has either been the leading or co-leading stories of land cover change during the study period.
Forest land cover across the U.S. is dynamic because of the geographic and temporal variability of many human and natural drivers including harvesting-replanting cycles (timber management), agricultural clearance or abandonment, natural disturbances, including wind throw, fire, and insects and disease, climate change and drought, as well as urbanization [1
]. Monitoring and understanding these changes requires a long-term view. This analysis of the urban growth effects on regional forest land cover shows some of these long-term spatial dynamics.
Upland forest land cover at the ecoregion scale within a national context has not been heavily impacted by forest cover loss to urban development during the study period, and certainly not as cartographically displayed by Clement et al. [24
] for the 2001–2006 interval. Small, already heavily urbanized ecoregions such as the Northeastern Coastal Zone and the Atlantic Coastal Pine Barrens of the northeast and the Puget Lowlands of the northwest U.S. may be the exceptions and may have been impacted the most. This does not mean that the loss in specific ecosystem services of former forested land, especially those services not found or found in greater amounts than in urban tree cover, in moderately affected ecoregions should be overlooked or discounted in importance. Land-cover modeling efforts for future dates, such as 2050, or even 2100, show sustained losses of forest land cover to urban development at both regional [57
] and national scales [58
]. Research into the quantification of ecosystem services provided by undeveloped forest land cover should continue to be encouraged. The growth or maintenance of urban forests may mitigate and moderate some of the loss of undeveloped forested lands in various ecosystems services, but do they truly replace their undeveloped counterparts in all aspects? Multi-scale land-use policies protecting more forest or slowing the rates of conversion may need to be augmented or even created, depending on location, to balance forest land cover ecosystem services with the opportunities and amenities found in urban regions. These multi-scale forest retention land-use policies may have special relevance because most people in the U.S. and, increasingly, around the world, live in cities for specific reasons. Increased forest land-cover preservation may clash with efforts to protect farmland and other natural or non-built-up land covers and land uses because urban areas continue to expand in size even with the efforts to increase density within existing developed land cover [24
]. Americans have long pushed the boundaries of their cities and it is something not easily culturally undone [47
]. The dilemma on how best to keep the most undeveloped land covers from being converted to highly urbanized conditions while cities expand in size will not be easily solved and will remain an issue into the future.
A way to improve the multi-scale regionalization of mapping forest to urban developed land cover conversion may be the use of Level IV ecoregions using available multi-date wall-to-wall land cover datasets. Drummond et al. [61
] used this scale for the 2001–2006 era within two Level III ecoregions in the Southeast U.S. and showed urban growth at a finer scale without losing the next scale up in geographic size. Forested land preservation planning may be better articulated and discussed using the results from land change mapping using multi-scale ecoregions that commonly cross local and even state political jurisdictions. The impacts of land cover change from individual or multiple urban areas may be seen more clearly using Level IV ecoregions and wall-to-wall land cover data.
The inclusion of forested wetland land cover change to urban developed land may be a way to provide a more comprehensive overview of forested land conversion to urban areas especially in the Southeast coastal region of the U.S. where Xian et al. [54
] reported that “woody wetlands” was a leading source of newly urbanized land cover. This has not been the case in other ecoregions, such as the Northeastern Coastal Zone, where wetlands conversion to urban developed land cover was a minor source of increased urbanization [62
]. The inclusion of Anderson II “woody wetlands” with current and future wall-to-wall land cover mapping would negate the issue of whether forest is correctly classified as “upland” or “wetland” and provide a better indication of the total contribution of “forest” land cover as a source of new urban land.