Timothy G. Wade

Fragmentation of Continental United States Forests

AbstractWe report a multiple-scale analysis of forest fragmentation based on 30-m (0.09 ha pixel−1) land-cover maps for the conterminous United States. Each 0.09-ha unit of forest was classified according to fragmentation indexes measured within the surrounding landscape, for five landscape sizes including 2.25, 7.29, 65.61, 590.49, and 5314.41 ha. Most forest is found in fragmented landscapes. With 65.61-ha landscapes, for example, only 9.9% of all forest was contained in a fully forested landscape, and only 46.9% was in a landscape that was more than 90% forested. Overall, 43.5% of forest was located within 90 m of forest edge and 61.8% of forest was located within 150 m of forest edge. Nevertheless, where forest existed, it was usually dominant—at least 72.9% of all forest was in landscapes that were at least 60% forested for all landscape sizes. Small (less than 7.29 ha) perforations in otherwise continuous forest cover accounted for about half of the fragmentation. These results suggest that forests are connected over large regions, but fragmentation is so pervasive that edge effects potentially influence ecological processes on most forested lands.

Mapping Spatial Patterns with Morphological Image Processing

We use morphological image processing for classifying spatial patterns at the pixel level on binary land-cover maps. Land-cover pattern is classified as ‘perforated,’ ‘edge,’ ‘patch,’ and ‘core’ with higher spatial precision and thematic accuracy compared to a previous approach based on image convolution, while retaining the capability to label these features at the pixel level for any scale of observation. The implementation of morphological image processing is explained and then demonstrated, with comparisons to results from image convolution, for a forest map of the Val Grande National Park in North Italy.

Distribution and Causes of Global Forest Fragmentation

Because human land uses tend to expand over time, forests that share a high proportion of their borders with anthropogenic uses are at higher risk of further degradation than forests that share a high proportion of their borders with non-forest, natural land cover (e.g., wetland). Using 1-km advanced very high resolution radiometer (AVHRR) satellite-based land cover, we present a method to separate forest fragmentation into natural and anthropogenic components, and report results for all inhabited continents summarized by World Wildlife Fund biomes. Globally, over half of the temperate broadleaf and mixed forest biome and nearly one quarter of the tropical rainforest biome have been fragmented or removed by humans, as opposed to only 4% of the boreal forest. Overall, Europe had the most human-caused fragmentation and South America the least. This method may allow for improved risk assessments and better targeting for protection and remediation by identifying areas with high amounts of human-caused fragmentation.

The effect of Appalachian mountaintop mining on interior forest

Southern Appalachian forests are predominantly interior because they are spatially extensive with little disturbance imposed by other uses of the land. Appalachian mountaintop mining increased substantially during the 1990s, posing a threat to the interior character of the forest. We used spatial convolution to identify interior forest at multiple scales on circa 1992 and 2001 land-cover maps of the Southern Appalachians. Our analyses show that interior forest loss was 1.75–5.0 times greater than the direct forest loss attributable to mountaintop mining. Mountaintop mining in the southern Appalachians has reduced forest interior area more extensively than the reduction that would be expected based on changes in overall forest area alone. The loss of Southern Appalachian interior forest is of global significance because of the worldwide rarity of large expanses of temperate deciduous forest.

Informing landscape planning and design for sustaining ecosystem services from existing spatial patterns and knowledge

Over the last decade we have seen an increased emphasis in environmental management and policies aimed at maintaining and restoring multiple ecosystem services at landscape scales. This emphasis has resulted from the recognition that management of specific environmental targets and ecosystem services requires an understanding of landscape processes and the spatial scales that maintain those targets and services. Moreover, we have become increasingly aware of the influence of broad-scale drivers such as climate change on landscape processes and the ecosystem services they support. Studies and assessments on the relative success of environmental policies and landscape designs in maintaining landscape processes and ecosystem services is mostly lacking. This likely reflects the relatively high cost of maintaining a commitment to implement and maintain monitoring programs that document responses of landscape processes and ecosystem services to different landscape policies and designs. However, we argue that there is considerable variation in natural and human-caused landscape pattern at local to continental scales and that this variation may facilitate analyses of how environmental targets and ecosystem services have responded to such patterns. Moreover, wall-to-wall spatial data on land cover and land use at national scales may permit characterization and mapping of different landscape pattern gradients. We discuss four broad and interrelated focus areas that should enhance our understanding of how landscape pattern influences ecosystem services: (1) characterizing and mapping landscape pattern gradients; (2) quantifying relationships between landscape patterns and environmental targets and ecosystem services, (3) evaluating landscape patterns with regards to multiple ecosystem services, and (4) applying adaptive management concepts to improve the effectiveness of specific landscape designs in sustaining ecosystem services. We discuss opportunities as well as challenges in each of these four areas. We believe that this agenda could lead to spatially explicit solutions in managing a range of environmental targets and ecosystem services. Spatially explicit options are critical in managing and protecting landscapes, especially given that communities and organizations are often limited in their capacity to make changes at landscape scales. The issues and potential solutions discussed in this paper expand upon the call by Nassauer and Opdam (Landscape Ecol 23:633–644, 2008) to include design as a fundamental element in landscape ecology research by evaluating natural and human-caused (planned or designed) landscape patterns and their influence on ecosystem services. It also expands upon the idea of “learning by doing” to include “learning from what has already been done.”

DESERTIFICATION EVALUATED USING AN INTEGRATED ENVIRONMENTAL ASSESSMENT MODEL

Desertification has been defined as land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors, including climatic variations and human activities (United Nations, 1992). A technique for identifying and assessing areas at risk fordesertification in the arid, semi-arid, and subhumid regionsof the United States was developed by the Desert Research Institute and the U.S. Environmental Protection Agency (EPA), using selected environmental indicators integrated into a Geographic Information System (GIS). Five indicators were selected: potential erosion, grazing pressure, climatic stress (expressed as a function of changesin the Palmer Drought Severity Index [PDSI]), change invegetation greenness (derived from the Normalized DifferenceVegetation Index [NDVI]), and weedy invasives as a percentof total plant cover. The data were integrated over aregional geographic setting using a GIS, which facilitateddata display, development and exploration of data relationships, including manipulation and simulation testing. By combining all five data layers, landscapes having a varying risk for land degradation were identified, providing a tool which could be used to improve landmanagement efficiency.

Transitions in forest fragmentation: implications for restoration opportunities at regional scales

Where the potential natural vegetation is continuous forest (e.g., eastern US), a region can be divided into smaller units (e.g., counties, watersheds), and a graph of the proportion of forest in the largest patch versus the proportion in anthropogenic cover can be used as an index of forest fragmentation. If forests are not fragmented beyond that converted to anthropogenic cover, there would be only one patch in the unit and its proportional size would equal 1 minus the percentage of anthropogenic cover. For a set of 130 watersheds in the mid-Atlantic region, there was a transition in forest fragmentation between 15 and 20% anthropogenic cover. The potential for mitigating fragmentation by connecting two or more disjunct forest patches was low when percent anthropogenic cover was low, highest at moderate proportions of anthropogenic cover, and again low as the proportion of anthropogenic cover increased toward 100%. This fragmentation index could be used to prioritize locations for restoration by targeting watersheds where there would be the greatest increase in the size of the largest forest patch.

Evaluating the relative roles of ecological regions and land-cover composition for guiding establishment of nutrient criteria

The continuing degradation of United States surface waters by excessive nutrient loads has motivated the establishment of nutrient criteria for streams, lakes, and estuaries as a means to protect aquatic resources. Nutrient criteria have been established based on ecoregional differences, recognizing that geographic variation in climate, topography, geology, and land use require use of different criteria values for different regions of the continental United States. Several studies have demonstrated that land-cover composition also strongly influences nutrient concentrations and yields. We examined the relative importance of ecoregions and watershed land-cover composition in explaining variability in nitrogen (N) and phosphorus (P) concentrations by re-analyzing the National Eutrophication Survey (NES) data reported by Omernik (1977). The variance of N concentrations among land-cover composition classes within ecoregions was six times larger than the variance among ecoregions. For P concentrations, land-cover composition within ecoregions accounted for three times more variance than ecoregions themselves. Variance across ecoregions was only weakly significant after accounting for variance in land-cover composition within ecoregions. The results suggest that the relationship between land-cover composition and nutrient concentrations in aquatic systems should also be used to help guide establishment of nutrient criteria.

Assessing Landscape Condition Relative to Water Resources in the Western United States: A Strategic Approach

The Environmental Monitoring and Assessment Program (EMAP) is proposing an ambitious agenda to assess the status of streams and estuaries in a 12-State area of the western United States by the end of 2003. Additionally, EMAP is proposing to access landscape conditions as they relate to stream and estuary conditions across the west. The goal of this landscape project is to develop a landscape model that can be used to identify the relative risks of streams and estuaries to potential declines due to watershed-scale, landscape conditions across the west. To do so, requires an understanding of quantitative relationships between landscape composition and pattern metrics and parameters of stream and estuary conditions. This paper describes a strategic approach for evaluating the degree to which landscape composition and pattern influence stream and estuary condition, and the development and implementation of a spatially-distributed, landscape analysis approach.

Watershed level risk assessment of nitrogen and phosphorus export

Land cover composition across a watershed is a principal factor in controlling the amount of nitrogen and phosphorus exported from a watershed. A well developed literature of nutrient export coefficients by land-cover class was used to model the risk of equaling or exceeding specified levels of nutrient export. The model was applied to about 1000 comparatively small watersheds mapped for the state of Maryland for environmental analysis and planning. Risk estimates generally increased from west to east, but numerous areas of high variability were evident. Risk of exceeding specified levels of nitrogen and phosphorus export were nonlinearly related to the amount of forest in the watershed. Risk increased more dramatically for phosphorus and nitrogen when forest dropped below between 90 and 95%, respectively. Bifurcations in this nonlinear relationship were the result of the relative abundance of agriculture and urban land in the watershed. The nonlinear relationship between percentage forest and risk increased more dramatically for phosphorus and less dramatically for nitrogen when urban was relatively more abundant than agriculture. Regional-scale variation in risk is discussed in terms of its relevance to environmental management.