Overview

This tool is designed to synthesize information about nitrogen deposition, species sensitivity, site and current climate conditions into an easy-to-use, web-based geospatial tool (N-CLAS) to facilitate effective resource management across the northern forest region.

The N-CLAS tool allows land managers to determine critical loads (CL), quantify exceedance, and set targets for the purposes of land management and restoration. N-CLAS can be used to address questions about selected areas such as:

• What proportion of my area is at risk from air pollution?
• How does the sensitivity of my area compare with other locations?
• What nitrogen deposition level would ensure protection of my forests?
• Which species are the most sensitive?

The interactive web interface provides maximum flexibility for the user to assess heterogeneous landscapes for specific areas and species of interest. Outputs include downloadable maps, tables and figures summarizing critical loads and exceedance based on a range of customizable options, including output products that reflect the minimum CL value, range of CL values, or percent of the target area protected.

You can also download base layer maps of N deposition, climate, soil, and topographic parameters.

How it Works

Nitrogen Critical Load Assessment by Site (N-CLAS) is a GIS-based tool designed for resource managers and policy makers to assess the effects of nitrogen deposition and climate change on forest ecosystems in the Northeastern United States. N-CLAS uses geospatial data layers for key topographic, climactic, soil abiotic factors, and nitrogen deposition levels to predict whether growth at a site will be optimal or suboptimal for each of 23 significant tree species. N-CLAS can then be used to assess critical loads and exceedance for individual species or summarized for all species combined for user selected areas.

Customize your output by setting your preferences for area unit (acres or hectares), and tree species name (Latin or common).

Step 1: Select your target Area

N-CLAS includes data aggregation by state, ecoregion, National Forest, National Park, and wilderness area classes. You can select one, or multiple areas for comparison within each class.

Step 2: Select your Species of interest

A total of 23 tree species are included in the N-CLAS assessment. You may select a single species of interest, any combination of key species, or all species to better understand the forest as a whole.

Step 3: Select your preferred Output

Three levels of detail are provided for output maps, figures, and tables:

• Basic: Summary figures and tables are limited to assessments for the most protective critical load of most sensitive species.
  
• Tailored: A set of screening questions about the key critical load and exceedance questions helps to guide the selection of the most informative output tables, figures, and maps.
  
• Comprehensive: All figures and tables. This selection will include outputs to address all range values of critical loads, and output data products.
  
  
• You can preview example output and alter selected figures prior to running the analysis for all three levels.
  

Step 4: View your generated Results

The results window allows users to explore a variety of graphs, tables, and figures generated according to their customized settings.

Outputs

The tool's output includes downloadable summary tables and figures as well as spatially explicit maps of CL and exceedance values across the landscape of interest. Some outputs are designed to specifically address common questions (see overview above), while others are designed to provide detailed comparisons across sites or species.

Various types of map products, data aggregation and summary statistics include:

• Basic descriptive metrics for the area in exceedance and magnitude of exceedance for each species and area of interest
• Cumulative frequency graphs to visualize the distribution of various CL and exceedance values across your selected landscape
• Risk assessment (area protected or exceeded) at various deposition levels
• High resolution maps of critical load and exceedance for individual species, as well as a summary of all species combined for a range of CL thresholds

N-CLAS Overview

N-CLAS is a GIS-based tool designed for resource managers to assess the effects of nitrogen deposition and determine critical loads and exceedance for forest ecosystems in the Northeastern United States.

• What factors does N-CLAS consider when determining CL?

N-CLAS can be used to determine critical loads and exceedance for individual tree species or all tree species present for user selected areas, including national forests and parks, wilderness areas, and ecoregions (areas with similar ecosystem structure and function as defined by the Commission for Environmental Cooperation (CEC)).

N-CLAS uses geospatial data of abiotic modifying factors (topographic, climatic, and soil parameters) to predict whether growth in each 30 m x 30 m pixel is likely to be optimal or sub-optimal for a given species. Sub-optimal growth conditions are expected to increase sensitivity to N deposition, whereas optimal growth conditions are expected to result in increased N demand and higher tolerance of N deposition.

N-CLAS combines the effect of the abiotic modifying factors on growth to determine if the critical load for a species will be in the bottom half or upper half of the species' documented critical load range.

• In N-CLAS, suboptimal growth conditions are expected to increase sensitivity to N deposition and thus shift the critical load to the bottom half of the species' critical loads range.


• Optimal growth conditions are expected to result in increased N demand and thus shift the critical load to the upper half of the critical loads range.

• What tree species does N-CLAS include?

N-CLAS includes 23 species of management concern, of commercial interest and dominant species in the northern forest.


• What areas does N-CLAS include?

N-CLAS can be used to determine critical loads and exceedance for individual tree species or all tree species present at user-selected areas, including national forests and parks, wilderness areas, and ecoregions (areas with similar ecosystem structure and function as defined by the Commission for Environmental Cooperation (CEC)).


• What factors modify the CL in N-CLAS?

N-CLAS uses geospatial data for topographic, climatic, and soil parameters to assess whether growth is likely to be optimal or sub-optimal for each species at a given location. N-CLAS then modifies the critical load for each species based on these factors.

• Topographic factors
• Elevation
• Aspect
• Slope gradient
• Climatic
• January, July and May-September temperature
• Annual and May-September precipitation
• Soil
• pH
• Clay and sand content
• Permeability
• Depth to bedrock
• How do optimal growth conditions affect the CL in N-CLAS?
• Optimal growth conditions are expected to result in increased N demand and thus shift the critical load to the upper half of the species' documented critical loads range.

• Sub-optimal growth conditions are expected to increase sensitivity to N deposition and thus shift the critical load to the bottom half of the species' documented critical loads range.
• The combined effect of all the abiotic modifying factors on growth determines whether the critical load for a species will be in the bottom half or upper half of the species' critical load range.

N-CLAS Approach

N-CLAS uses a 5-step process to calculate critical loads.

PDF containing detailed methodology

N-CLAS critical loads can be reported for individual species or aggregated for all tree species present.

Steps N-CLAS uses to calculate the critical load:

1. Identify the species present

• How does N-CLAS identify the species present?

N-CLAS uses LANDFIRE geospatial data, which provides SAF (Society of American Foresters) forest cover type fore existing vegetation.

• What species are included in each SAF Forest Cover Type?

Species composition of each forest cover type is derived from Eyre (1980), the USDA Fire Effects Information System, and Little's Little's species range maps. We created a list of species included in each SAF forest cover type used by LANDFIRE.




2. Evaluate whether abiotic modifying factors are more likely to lead to optimal or sub-optimal conditions for growth

• What is the optimal range for growth for each modifying factor?

For each tree species, Robin-Abbott and Pardo (2017) identified the optimal range for growth for specified topographic, climatic, and soil abiotic modifying factors.

• How is the optimal range for growth determined?

The optimal range for growth for specified topographic, climatic, and soil abiotic modifying factors is based on importance values and distribution data from the Climate Change Tree Atlas (U.S. Forest Service, n.d.) and information from other sources, including the Forest Service silvics handbook "Silvics of North America" (Burns and Honkala, 1990); the PLANTS database (NRCS 2014); and additional literature sources.


• What is the spatial scale of N-CLAS?

N-CLAS makes calculations for each 30 m x 30 m pixel for each species present at that location.




3. Weigh the strength of evidence for the response of each abiotic factor

• What is the weight of evidence?

The weight of evidence is a way of quantifying the strength of the scientific evidence currently available for a species' response to an abiotic factor.

• How is the weight of evidence determined?

The weight of evidence for abiotic modifying factor thresholds is set using FIA-based importance value data from the Climate Change Atlas.
The overall uncertainty rating was determined through the arithmatic mean of three weighting criteria:

• Number of samples included in the FIA assessment
• Shape of the importance value response curve over the range of each factor
• Accuracy of factor values

The matrix used to assign the weight of evidence for abiotic factors included in the Climate Change Atlas is described in Robin-Abbott and Pardo (2017).
Another matrix was used to assign the weight of evidence for abiotic modifying thresholds set using data from the Forest Service silvics handbook, "Silvics of North America" (Burns and Honkala, 1990), PLANTS database (NRCS 2014), or other literature sources.

• How does N-CLAS determine the net effect of the abiotic modifying factors?

Values for the weight of evidence are summed across all factors in the optimal range and then compared to the sum of the weights of evidence for all factors in the sub-optimal range. If the sum of the weight of evidence of sub-optimal factors is higher, overall growth is likely to be sub-optimal. If the sum of the weight of evidence of optimal factors is higher, overall growth is likely to be optimal.


• How was the weighting system developed?

This weighting system is based on the endorsement theory, which is described in detail in Hall and Wadsworth (2010)




4. Incorporate the relative ecological significance of each abiotic factor




5. Adjust the critical load value based on the combined effect of all abiotic modifying factors

• Why is the CL reported as a range?

The critical load is reported as a range to incorporate the variation in response to N deposition across sites and individuals.

• What does it mean if the adjusted CL is in the bottom half of the CL range for that species?

If the adjusted critical load is in the bottom half of the CL range, it means that in this area, the species is more sensitive to N deposition. Suboptimal growth conditions are expected to increase sensitivity to N deposition and thus shift the critical load to the bottom half of the species' critical loads range.

• What does it mean if the adjusted CL is in the upper half of the CL range for that species?

If the adjusted critical load is in the upper half of the CL range, it means that in this area, the species is less sensitive to N deposition. Optimal growth conditions are expected to result in increased N demand and thus shift the critical load to the upper half of the critical loads range.

Instructions for Use

This interactive web interface provides maximum flexibility for the user to assess heterogeneous landscapes. To get the most from the outputs of the N-CLAS tool and match user needs, there are several customization steps that must be completed before viewing outputs. This includes selecting your target areas of interest, which of the 23 forest species to include in the analyses, and the type of response variable, data aggregation and summary statistics to be reported in the output. The N-CLAS tool walks users through each of these customization steps:

Step 1: Select your target Area

N-CLAS includes data aggregation by state, ecoregions (levels 2 and 3), National Forest, National Park, and wilderness area classes. You can select one, or multiple areas for comparison within each class.

Step 2: Select your Species of interest

A total of 23 tree species are included in the N-CLAS assessment. You may select a single species of interest, any combination of key species, or all species to better understand the forest as a whole.

Step 3: Select your preferred Output format

Three levels of detail are provided for output maps, figures and tables:

• Basic: Summary figures and tables are limited to assessments for the most protective critical load of the most sensitive species.
  This can be used as an efficient assessment of the maximum forest risk from nitrogen deposition.
  
• Tailored: Users select outputs tailored to meet their specific needs by answering a set of screening questions.
  This may be the best option for many stakeholders to ensure the appropriate level of information without unnecessary outputs.
  
• Comprehensive: This selection will include outputs to address all ranges of values of critical loads, and output data products.
  This may be most useful for those who wish to have the complete set of information available to identify questions or issues that may not have been obvious prior to analysis (for "data mining").
  

Output products can be previewed to ensure that all required results are included, with the ability to remove unnecessary outputs or select additional outputs.

Step 4: View your generated Results

The results window allows users to explore a variety of graphs, tables, and figures generated according to their customized settings.

These products are nested in the results window on the right side of the map interface. Select and expand various result tabs, hover for additional information on each output product, or click for a zoomed view.

All products are individually downloadable, and (coming soon) can be used to generate a common report for all selected outputs.

Tips for New Users

• When in doubt, hover. Hovering will generally open a brief description of the item of interest.
  
• Many items in the web app contain an info icon (). Hovering over these icons provides a summary of the function of each item.
  
• The preferences icon () will allow you to change the unit of area measurement to acres or hectares, and the tree species names to Latin or common for produced output.
  
• Clicking on output products (tables or figures) in the results pane will enlarge those in the main viewer.
  
• The main viewer can be expanded by collapsing the left and right side panels by clicking on the side arrows.
  
• To clear settings select the refresh button () in the upper right corner
  

Tutorials (coming soon)

• N-CLAS starter - various intro buttons
  
• Exploring abiotic factor map layers
  
• User scenarios:
  
• Basic
  
• Example 2
  
• Example 3
  
• Video guides and scenarios
  

Frequently Asked Questions

Glossary