Soil Quality

Soil as an indicator of forest health

The Forest Inventory and Analysis Program of the USDA Forest Service surveys Soil Quality across all forested areas of the United States.

The USDA Forest Service's Forest Inventory and Analysis (FIA) program has implemented a national soil monitoring program to address specific questions related to the current status of soil resources and the contribution of forest soils to the global carbon cycle.

As the first and only nationally consistent effort to monitor forest soil quality in the United States, this program provides critical baseline information on the current status of the soil resources and the potential effects of natural and human disturbance on forest health and productivity.

Specific questions we seek to address with this indicator include:

• Can declines in forest productivity be correlated with changes in the availability of soil nutrients and water to plants?
• What impacts are pollution (e.g., acid deposition) having on soil chemical properties?
• How much carbon is currently stored in forest soils and is this quantity changing over time?
• What percent of U.S. forestland is impacted by soil compaction and soil erosion?
• How much topsoil and forest floor material may be lost each year due to accelerated erosion?

Data from the soil quality indicator are not intended to be used alone when making statements about forest health. However, once the presence of a potential soil quality issue has been identified, these indicator data can be considered in association with other FIA measurements to assess the impact of soil quality on forest health.

Background

The USDA Forest Service's Forest Inventory and Analysis program began sampling related to the Soil Quality Indicator in 2001.

The FIA sampling design consists of three phases, of which Soil Quality constitutes part of the third phase. During the first phase of inventory, sample points are randomly placed within a hexagonal grid across the entire United States. Each sample point is assigned to either non forest, forest/non forest, or forest classes based on remotely sensed imagery.

The second phase involves field crews visiting all forest/non forest and forested sample points to establish permanent sample plots. Each sample plot consists of a cluster of four subplots, each subplot having a fixed-radius of 24 feet. On all these sample plots, individual tree information (e.g., diameter and height) as well as site information (e.g., slope and land use) is collected for the forested portions of the sample plot.

During the third phase, a randomly selected sub sample of field established phase 2 plots are visited to collect forest health information. Critical data are collected pertaining to specific forest health issues such as soils, understory vegetation, and tree damage. These specific forest health issues are referred to as "Indicators of Forest Health."

The Soil Quality Indicator collects information through field measurements on Forest Inventory and Analysis (FIA) sample plots and laboratory analyses. Soil condition indicators such as erosion, compaction, and soil chemistry are monitored over time and used to demonstrate trends.

This data can be used to characterize soil status on all forest land across the United States, and can be used with subsequent measurements to identify and evaluate change.