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FIA Procedures

This section describes the inventory procedures used to collect forest resource data in Southern States. Dates of surveys for each State are in the section, 1970s to 1999. Inventory procedures between 1972 and 1995 differed slightly from procedures in 1997 through 1999. Descriptions of both methods follow.

Inventories procedures between 1972 and 1995

Estimates of forest and nonforest areas were based on the ground classification of sample clusters systematically spaced on the most recent aerial photographs. A subsample of 16-point clusters was ground-checked, and a linear regression was fitted to the data to develop the relationship between the photo and ground classification of the subsample. This procedure provided a means for adjusting initial estimates of area for changes in land use since date of photography and for errors in photo interpretation.

Estimates of timber volume and forest classification were based on measurements recorded at ground-sample locations systematically distributed on timberland. The plot design at each location was based on a cluster of 10 points. In most cases, variable plots, established by using a basal-area factor of 37.5 square feet per acre, were systematically spaced in a single forest condition at 5 of the 10 cluster points. Trees less than 5 inches d.b.h. were tallied on a fixed-radius plot at the center of each point.

Equations prepared from detailed measurements collected on standing trees in each State, and similar measurements taken throughout the Southeast, were used to compute the volume of individual tally trees. A mirror caliper and sectional aluminum poles were used to obtain the additional measurements required to construct volume equations. Forest biomass was estimated with equations developed by the Ecology and Genetics of Southern Pine Ecosystems Research Work Unit of the SRS in Athens, GA. In addition, felled trees were measured at several active cutting operations in each State to provide utilization factors for the different timber products and species groups, and to supplement the standing-tree volume study.

In each State, growth, removals, and mortality were estimated from the remeasurement of permanent sample plots established at the time of the previous inventory. Periodic surveys of timber products output conducted in cooperation with State agencies, along with the annual pulpwood production study for the South, provided additional information for breakdowns of removals by product.

Ownership information was collected from correspondence, public records, and local contacts in each Southern State. In counties where the sample missed a particular ownership class, temporary samples were added and measured to describe forest conditions in the ownership class.

All field data were sent to the Southern Research Station (SRS) FIA Unit for editing and were stored for processing. Final estimates were based on statistical summaries of the data.

Inventory methods for Georgia (1997) and Tennessee (1999)

The SRS-FIA unit currently uses a two-phase sample of aerial-photo points and permanent ground plots. The area of forest land in each county is determined by interpreting aerial-photo point clusters. Initial estimates of forest and nonforest land are based on the classification of sample clusters systematically spaced on the most recent aerial photographs. A subsample of the photo clusters is ground-checked so initial area estimates can be adjusted for changes in land use since the date of photography and for errors in photo interpretation.

The plot design at each ground sample location is based on a cluster of four points spaced 120 feet apart. Each point is the center of a 1/24-acre circular subplot used to sample trees 5.0 inches d.b.h. and larger. A 1/300-acre circular microplot, located at the center of the subplot, is used to sample trees 1.0 through 4.9 inches d.b.h. and seedlings (trees less than 1.0 inch d.b.h.). These fixed-radius sample plots are located without regard to land use or forest cover. Forest and nonforest condition classes are delineated and recorded. Condition classes are defined by six attributes: land use, forest type, stand origin, stand size, stand density, and major ownership category. All trees tallied were assigned to their respective condition class.

Estimates of timber volume and forest classification are derived from tree measurements and classifications made at the ground sample locations. Volumes for individual tally trees are computed using equations for each of the major species in the State. The equations were developed from detailed measurements collected on standing trees in each State and throughout the region.

Growth, removals, and mortality are estimated from the remeasurement of permanent sample plots established in the previous inventory. Plot design for the previous inventory has already been described.