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The U.S. Forest Service’s inventory programs have been in continuous operation since 1930. Then known as the “Forest Survey,” this program completed periodic rural forest inventories for all the lower 48 states by the 1960s. The Forest Survey was later renamed in the mid-1970s as the “Renewable Resource Evaluation” and then “Forest Inventory and Analysis” in the mid-1980s. Recent notable events in the FIA program include:

  • 1980s: USFS forest inventories were conducted under the Forest Health Monitoring (FHM) program along with the FIA program under different plot designs over a national grid system, collecting data periodically at plots (uneven plot remeasurement intervals)
  • 1992: CSFS assists the USFS with measuring FHM plots in Colorado
  • 1995: FIA adopts the 4-point plot design of the FHM program
  • 1999: FIA merges with the FHM program and starts the transition to an annual inventory of plots into the early 2000s (each plot remeasured every 10 years in the Rocky Mountain region)
  • 2002: Annual inventory begins in Colorado; with CSFS staff conducting data collection
  • 2011: Annual inventory begins in Wyoming; with CSFS staff conducting data collection

    2017: CSFS begins Urban FIA data collection in Colorado cities

Traditionally, the USFS inventory programs were conducted in a rural setting. However, with the provisions set forth by the 2014 Farm Bill, an additional component accounting for urban forests was added. The decision to incorporate urban forests into the inventory will help provide a more holistic understanding of our nation’s forest resources and the benefits they provide to almost every facet of our lives. FIA also implemented inventory programs for interior Alaska, Hawaii, and island territories of the United States in 2014.

More information on the history of the FIA Program: FIA: In Context and A History of the Forest Survey in the United States 1830-2004.

Information collected from surveys of FIA plots is used in scientific research and this data is available to the public. Data is evaluated by USFS FIA analysis staff to produce reports, mandated by the U.S. Congress for each state. FIA analysts also produce estimation applications and raw data files for public use (FIA Data and Tools). FIA data is used for research into:

  • Potential wildfire hazard
  • Condition of wildlife habitat
  • Tree mortality causes like insects or diseases
  • Biomass estimates and carbon storage
  • Timber volume estimates
  • Many other scientific efforts by the USFS, other Federal agencies, state agencies, universities, and private industry

Each plot represents a potential sample area of 2.47 acres and is comprised of a nested sampling design: hectare plot (not shown), macroplot, subplot, microplot, and transects. The data collected on this portion of land is then extrapolated to represent approximately 6000 acres. FIA adopted this 4-point plot design nationwide in 1995.

The Interior West FIA (IW-FIA) unit, part of the USFS Rocky Mountain Research Station, conducts and continuously updates a comprehensive inventory and analysis of the present and prospective conditions of the renewable resources of the forest and rangelands of the Interior West. The Interior West includes inventories in Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, and Wyoming.

FIA Regions

The Farm Bill of 1998 mandated a change in survey procedures and integrated the Forest Inventory and Analysis (FIA) program with the similar Forest Health Monitoring (FHM) program. To reduce bias and ensure that the data collected is significantly sound, a hexagonal lattice framework was laid across the conterminous United States.

A diagram showing the FIA sample design based on tiled hexagons.

Example of Hexagonal grid plot selection

Each hexagon is spaced 3.27 miles apart from center and is approximately 6000 acres in area. A FIA plot is placed in each hexagon without regards to landscape features or land ownership. FIA also incorporates older generation FHM plots into the hexagonal design to preserve historical data and track change over time. Once these plots are located, there are two phases to conducting the FIA program.

Phase 1 (P1)

This phase involves office work to remotely determine which plots are potentially forested. As the plot locations are essentially random, it is inevitable that plots fall in open areas such as water, agricultural fields, open meadows, and parking lots. These non-forested plots are removed from the survey, while plots that are potentially forested are then sent to the next phase.

Phase 2 (P2)

The plots that make it through P1 are then visited by field inventory crews. Once on site, experienced crews determine if the plot location qualifies as forested land. To be considered forested, FIA protocols require the area to be at least one acre in size, 120 feet wide, and 10% canopy cover of qualifying tree species. If a plot is forested, FIA crews then proceed to establish the plot and collect data on numerous variables.

Phase 2 + Soils (P2 + Soils)

P2+soils plots are a subset of P2 plots on which additional data on soil composition, depth to restrictive layer, and duff/litter nutrient content are collected.

Plot Design

Each plot represents a potential sample area of 2.47 acres and is comprised of a nested sampling design: hectare plot (not shown), macroplot, subplot, microplot, and transects. The data collected on this portion of land is then extrapolated to represent approximately 6000 acres. FIA adopted this 4-point plot design nationwide in 1995.

A diagram showing the CSFS Forest Inventory Analysis team's plot design for forest sample areas.
Diagram of FIA plot design

As part of the National FIA Program, each region collects the same core set of variables. Additionally, each region has the option to collect additional variables to address specific policy questions, research studies, and changes in ecosystem services. Examples of variables include, but are not limited to, the following:

  • Tree Diameters & Lengths (trees 5 inches and greater in diameter)
  • Sapling Diameters & Lengths (tree between 1 inch and 5 inches in diameter)
  • Seedling Counts (trees under 1 inch diameter)
  • Insect & Disease Damage
  • Tree Mortality Causes
  • Form Defect
  • Crown Ratios
  • Area of Crown Coverage
  • Bare Ground Coverage
  • Understory Vegetation (Species & Area Coverage)
  • Vertical Structure of Vegetation
  • Duff & Litter Depths
  • Down Woody Materials (wildfire fuels)
  • Residue Piles (from timber cutting activity)
  • Soil Composition, Structure, and Compaction

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