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A stem-map model for predicting tree canopy cover of Forest Inventory and Analysis (FIA) plots

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Toney, Chris; Shaw, John D.; Nelson, Mark D.

Year Published

2009

Publication

In: McWilliams, Will; Moisen, Gretchen; Czaplewski, Ray, comps. Forest Inventory and Analysis (FIA) Symposium 2008; October 21-23, 2008; Park City, UT. Proc. RMRS-P-56CD. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 19 p.

Abstract

Tree canopy cover is an important stand characteristic that affects understory light, fuel moisture, decomposition rates, wind speed, and wildlife habitat. Canopy cover also is a component of most definitions of forest land used by US and international agencies. The USDA Forest Service Forest Inventory and Analysis (FIA) Program currently does not provide a national standard measurement of tree canopy cover, and most regional FIA units do not measure canopy cover in the field.

This paper describes a model for predicting canopy cover of FIA plots by mapping the locations of trees ≥ 5.0 in. diameter within the plot, and statistical modeling of sapling contribution to total cover. The model was developed with an operational focus, including the requirement that it scale efficiently to national applications. Coefficients for species-specific crown width equations have been stored in lookup tables with surrogates assigned to FIA tree species lacking equations. Modeling was supported by field measurements on 12,070 FIA plots distributed across the eight-state Interior West FIA region. Refinements to the model included adjustments to crown width equations for small-diameter trees, stem-mapping of microplot subsamples to support cover estimation of sapling-stage plots, and the use of spatial statistics to derive predictor variables describing the spatial pattern of overstory trees. Model predictions were compared to field measurements of canopy cover by line-intercept method on 1,454 single-condition plots from the Interior West FIA 2006 field season. The mean absolute difference between field-measured and model-predicted values was ± 7.9% canopy cover, with mean bias of -0.7% canopy cover. The relationship between field-measured and predicted values was approximately linear with approximately constant variance and a correlation coefficient r = 0.875.

FIA produces estimates of forest land area based on a definition of forest land that includes a minimum threshold of tree stocking. Proposed changes to the FIA definition from one based on stocking to one based on canopy cover could affect estimates of forest land area, but the amount and variability of this change is not fully understood. We made a preliminary assessment of the effect of a canopy cover-based definition on forest area estimates for a subset of states within the Northern Research Station FIA unit.

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Citation

Toney, Chris; Shaw, John D.; Nelson, Mark D. 2009. A stem-map model for predicting tree canopy cover of Forest Inventory and Analysis (FIA) plots. In: McWilliams, Will; Moisen, Gretchen; Czaplewski, Ray, comps. Forest Inventory and Analysis (FIA) Symposium 2008; October 21-23, 2008; Park City, UT. Proc. RMRS-P-56CD. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 19 p.

Last updated on: January 21, 2010