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| 5-pointRegional FIA Cluster shown in grey |
In an effort to standardize the differing data collection methodologies among regions, and between NFS and other forest lands (private, BLM, NPS, etc), the US Forest Service implemented a new national “Forest Inventory and Analysis (FIA) Core Methodology, with region “add-ons” in early 2001. The idea was to standardize national forest data acquisition needs while allowing for region-specific needs. This paper provides a comparative summary of major differences between the two inventory data collection methodologies.
The transition resulted in significant changes to Region 5’s FIA program. The 5-point “Regional FIA” variable radius cluster plot, for which complete current California data collection had just been completed at that time, was replaced by a 4-point fixed radius “hex” design. The California 3.4 mile FIA “x-y” grid was replaced by the national hex (honeycomb-shaped) grid, which samples at a slightly more intensive rate (1 FIA plot per 6000 acres vs. 1 Region 5 FIA plot per 6500 acres). In an effort to provide continuity between the 5-point and the new 4-point systems, when a 5-point plot fell anywhere within the hex grid overlay, the origin (subplot #1) of new 4-subplot cluster was co-located at the exact location of the origin of the previously installed 5-point cluster plot. Therefore point/subplot 1 of both methods measured at least some of the same trees and understory vegetation. Point/subplot 2, to the north, overlaps to a significant extent. These overlaps are useful for monitoring change occurring during the transition from regional to the national plot design. Though some overlap exists, most trees on points 3 and 4 of the hex plots were not measured on the old Region 5 plot (see figure 1 below).
All trees on the hex subplots are measured on fixed plots, whereas most live trees had formerly been sampled using variable plot methods. Both protocols measured dead trees and snags as fixed-area plots, although of differing sizes. The vegetation plot records regeneration trees, shrubs, forbs and grasses on a 1/24th acre rather than a 1/4 acre plot. The photo series guides fuels plots used by Region 5, which gave a general fuels estimate at the subplot-specific level, were replaced by Brown’s transects, which provide good overall statistics while not always adequately representing the individual plot. As a “regional add-on”, hex plots which fell within the Northwest Forest Plan area, and some within the Sierra Nevada Framework area, were measured to include a “hectare plot” of 185.1’ radius (centered around subplot 1) to obtain measurements on all live trees and snags 32” and larger in diameter.
The hex design also includes azimuth and distance for every tree from one of the known plot origin points. Therefore tree spatial distribution on every point can be accurately mapped. Similarly condition changes, roads, and non-forest inclusions are now accurately mapped on every hex plot.
As part of this process, during initial hex plot installation, the 5 points of the Region 5 FIA plot are visited and all formerly live trees are checked to monitor mortality. For instance, in recently revisiting plots on the San Bernardino National Forest (2004) we determined that 15% of the trees we measured in 1995 are now dead .
Figure 1 below illustrates plot geometry and the overlap area. The major methods for each inventory protocol are summarized in Table 1 below.
Discussion
The implications of this transition are many. The Region 5 plots are not directly comparable to the hex plot so direct monitoring is limited. The Region 5 data set is more “user friendly” and understandable. Software is available for processing the Region 5 FIA data, while the hex plots must be processed using a central database processing system which is, 4 years later, still under development. Nonetheless, with simple programming researchers and forest managers can derive similar analyses, such as forest inventories, basal area, vegetation and fuels summaries, canopy closure calculations, and create summary spreadsheets from both data sets. Although the fixed area hex plot results in measurement of many more trees than the variable plot method, mid-sized trees—those from 14-22” in particular—may be best inventoried using variable plot sampling as they are sparsely sampled using the 1/24th acre hex subplot. The hectare plot identifies the presence of large trees on individual plots more accurately, while, from an overall analysis or summary perspective, large tree inventories using either the hectare plots or the cluster of ¼ acre plots (either method) return essentially identical volumes and numbers. The linear Brown’s fuels transects often will not adequately describe individual plots, although, on average, they will provide an adequate fuels and down log inventory for a forest stratum or a large number of plots. The plot photography is valuable for a variety of non-empirical analyses, for education and illustrative purposes, and Region 5 continues to obtain photography as an add-on. The vegetation plot is more comprehensive when obtained from the ¼ acre subplots, but uncommon grasses, shrubs and forbs are rarely observed, because most plots are not visited during the flowering season.
In both sets of plots the USFS inventory staff has worked diligently to assure quality of measurement. Plots are permanently monumented. Plot checks are routinely checked for quality and accuracy of measurement. Few trees or common plants have been “missed” in either inventory. Both methodologies take about the same amount of time to install the cluster plot, but the hex plot is more time-consuming to install, because 10% of all plots on all forests are remeasured annually, versus 100% remeasurement each decade one forest at a time. The hex method therefore vastly increases travel time, especially to remote plots in wilderness areas.
Summary
For the fisher rest site habitat suitability model we chose to use the 5-point Regional FIA dataset because a full inventory existed for NFS lands throughout the state—over 3000 cluster plots. The national core hex formatted data sets should be adequate for future analyses.
See Table 1: Summary Comparison of National Core Hex Plot and Region 5 FIA Designs
References
Remote Sensing Lab. USDA Forest Service, Region 5. Forest Inventory and Analysis Field Guide. June 1999. Download guide2002.zip at http://www.fs.fed.us/r5/rsl/projects/inventory/tools.shtml and review Chapter 400.
PNW FIA Program. 2004 Field Instructions for the Annual Inventory of Washington, Oregon and California. Available at http://www.fs.fed.us/pnw/fia/local-resources/pdf/field_manuals/2004_annual_manual_final.pdf