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Title: Soil vital signs: A new Soil Quality Index (SQI) for assessing forest soil health

Author
item AMACHER, MICHAEL
item O Neill, Katherine
item PERRY, C

Submitted to: USDA Forest Service Research Notes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2006
Publication Date: 2/27/2007
Citation: Amacher, M., O Neill, K.P., Perry, C.H. 2007. Soil vital signs: A new Soil Quality Index (SQI) for assessing forest soil health. Res. Pap. RMRS-RP-65WWW. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, 12p.

Interpretive Summary: The Forest Inventory and Analysis (FIA) program of the USDA Forest Service collects soil samples as part of its national forest monitoring program in order to address specific questions about the conservation of soil resources. However, without advanced training in soil science, many users find it difficult to integrate these analyses into assessments of forest health. To assist in these efforts, we developed an index of forest soil quality (SQI) that integrates 19 measured physical and chemical properties of forest soils into a single number that can function as a baseline for assessing changes in key soil properties over time. The purpose of this report is to present an overview of the current stage of development of the soil quality index and to provide guidance to analysts interested in using FIA soil data to address questions related to sustainable forest management. Further research is needed to determine quantitative relationships between SQI values and other indicators of forest health measured on FIA plots (e.g., including mortality, damage, dieback, crown transparency, biomass, productivity, species diversity, etc.). However, following this additional testing and refinement, we anticipate that calculation of SQI values will be included as part of the FIA soil database and made available for state, regional, and national forest health analysts.

Technical Abstract: The Forest Inventory and Analysis (FIA) program measures a number of chemical and physical soil properties to address specific questions about forest soil quality, or health. However, without advanced training in soil science, many potential users find it difficult to integrate variables from the soil indicator database to assess the overall state of forest soil quality. To overcome this limitation, we developed a new index of forest soil health, the soil quality index (SQI), that integrates 19 measured physical and chemical properties of forest soils into a single number that serves as the soil's 'vital sign' of overall soil quality. Each physical and chemical property was assigned an index number depending on whether it was above or below an assigned threshold value. Thresholds were selected either on the basis of known physiological responses of vegetation to soil properties or on the relative distribution of soil property values in the FIA soil database. Individual index values were then summed to obtain a total index, which could have a maximum value of 26. The index total was expressed as a percentage of the total number of measured parameters. Thus, missing values do not contribute to the index. Because the percentage index can exceed 100 % as a result of assigning 'bonus points' for optimal levels of certain properties, the percentage index was divided by the maximum possible percentage (137 %) to re-normalize the scale to 0 to 100 %. Calculated SQI's for the 2001 and 2002 FIA soils database ranged from less than 30% to 100 %. Western soils tended to have higher SQI's than soils from the northeast and southeast regions of the U.S. because of their higher pH, higher exchangeable Ca, and lower exchangeable Al levels. The 0-10 cm soil layer tended to have higher SQI's than the 10-20 cm layer because of higher organic matter and nutrient contents and lower bulk density values. Because the SQI integrates 19 measures of soil physical and chemical properties into a single indicator of overall forest soil health, it may prove to be a promising tool for detecting forest health trends. The index provides a mechanism for exploring relationships between forest soil quality and other measures of forest health such as dieback, productivity, and mortality. A strong positive correlation between SQI levels and measures of declining forest health would indicate the need for more intensive site monitoring to isolate causative agents of decline. Lack of an association between SQI levels and measures of forest health may indicate that inaccurate threshold values were selected, that other soil properties not measured in the FIA program are contributing to forest decline, or that the causative agent of decline is not soils related.