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ForestERA Data Layer Detail - Crown Bulk Density (kg / m³)

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Description

Crown Bulk Density is a measure of canopy fuels used in fire behavior modeling applications. Typically it is the weight of fine canopy fuels (leaves, needles, smaller branches, etc.) divided by the total canopy volume. This layer is provided in units of kg / m³ (multiply by 0.0624 to change to lb / ft³), and has a resolution of 90m (0.8 ha or 2 acres).

Purpose

This data layer was created as part of the ForestERA project to support landscape-scale forest restoration planning efforts by a broad group of stakeholders including federal and state agencies, academic institutions, and non-governmental entities. These data are intended for regional analyses over spatial extents on the order of tens to hundreds of thousands of acres, and were not developed for use at finer spatial scales, although they may be useful for some applications at finer scales.

Development

This layer was created using the basal area, tree density, and dominant overstory vegetation layers developed by the ForestERA team. Crown bulk density (hereafter CBD) cannot be directly measured using readily available types of remote-sensing imagery. Because of this, it was necessary to use allometric equations to develop the CBD layer. For the mixed-conifer, and ponderosa pine vegetation types we used equations developed by Cruz et al. (2003) that derive CBD from basal area and tree density. The equations are:

In ponderosa pine: ln(CBD) = (0.435 * ln(basal area)) + (0.579 * ln(tree density)) - 6.649.

In mixed-conifer: ln(CBD) = (0.319 * ln(basal area)) + ((0.859 * ln(tree density)) - 8.445.

The same equations were used for mixtures of ponderosa pine with aspen or oak, and of mixed-conifer with aspen, as the coniferous component of these vegetation types is typically dominant. For pinyon-juniper woodlands and juniper-dominated woodlands we based CBD on percent canopy cover using the equation (CBD = percent canopy cover / 100). This methodology is a modification of the methodology used by Keane et al. (2000) to assign CBD values to pinyon-juniper woodlands in the Gila National Forest of New Mexico. Keane et al. suggested CBD values of 0.03 for areas of low (< 30%) canopy cover and 0.05 for areas of moderate (30% - 60%) canopy cover in pinyon-juniper woodlands. Keane et al. (2000) and Pollard (1971) suggested values of 0.01 for CBD in stands of pure quaking aspen and we followed their suggestion by assigning this value for CBD to aspen stands.

Accuracy Assessment

We used the equations given above for CBD in ponderosa pine to obtain CBD estimates based on ground measurements using basal area and tree density measurements from forty-three 90 m (0.81 ha) plots. We also obtained CBD estimates using the values for basal area and tree density from our predictive maps of these attributes created using Enhanced Thematic Mapper (ETM) satellite imagery. We then used linear regression analysis to determine if there were statistically significant relationships between the values of CBD as estimated from the ground data and the values for CBD as estimated from the layers produced using ETM imagery. The results of this regression analysis indicated that a highly significant relationship exists between the two measures (r² = 0.601, P < 0.0001). The slope of the line from this relationship is 1.01 indicating that the CBD estimates from the ETM derived layers are unbiased with respect to the estimates from the ground layers. Analysis of the differences between the estimated values for CBD indicate that over 50% of the estimated CBD values based on the ETM derived layers lie within 0.03 kg / m3 of the estimated values based on the ground data and over 80% of the estimated CBD values based on the ETM derived layers lie within 0.05 kg / m3 of the estimated values based on the ground data. We did not have enough ground data to do a regression analysis or estimation of the errors in CBD in mixed-conifer areas, but the differences in estimates of CBD for 8 plots of mixed conifer vegetation measured showed a similar range of errors.

Sources of errors: For our purposes it was less important that we develop a completely accurate layer than that the development of this layer was undertaken using as consistent a methodology as possible. By using a consistent methodology, the layer can be used as a relative measure of the canopy fuels across the landscape, even if it is not a completely accurate measure. Cruz et al. found that their allometric equations were highly accurate for the ponderosa pine and mixed-conifer vegetation types so we feel that use of these equations is appropriate. We decided not to adjust CBD in areas with mixtures of aspen or oak because the conifers tend to be dominant in these areas. However, we note that CBD is likely to be overestimated in areas with mixtures of aspen and oak with conifers.

The use of other equations for pinyon-juniper woodlands and aspen stands suggests that fire behavior in these vegetation types may not be directly comparable to that in the ponderosa pine and mixed-conifer vegetation types. However, we note that pinyon-juniper stands almost never have active crown fire, and aspen stands have crown fire only under very rare conditions. The fire behavior that we predict under the fire modeling procedures for pinyon-juniper woodlands and aspen stands is consistent with actual fire behavior under natural conditions.

It was also important that the range of values for CBD were consistent with values measured in the field for the same types of vegetation. The predicted values of CBD for the major vegetation classes in our area lie within the range reported by researchers that have measured these values in the field (e.g., Brown 1978, Cruz et al. 2003, Pollard 1972). For example, predicted CBD in ponderosa pine areas our study area ranged from 0.01 to 0.5, with a mean of 0.14. Brown (1978) measured a range of CBD from 0.01 to 0.40 in ponderosa pine, and Cruz et al. (2003) measured a range of CBD from 0.01 to 0.76 in ponderosa pine with a mean CBD of 0.18.

Recommendations

We recommend that this layer be used at a minimum resolution of 90m (0.8 ha or 2 acres) for purposes of analysis and display. However, ForestERA data layers were not designed for analyses at the level of individual pixels, and uncertainty in the data will generally decline over greater spatial extents. Therefore, we recommend using larger analysis units, with groupings of at least 50 cells (40 ha or 100 acres). Finally, we reiterate that ForestERA data layers were developed for the purpose of regional landscape-level planning, and we suggest that the analyses be applied over spatial extents of tens to hundreds of thousands of acres. We recognize, however, that this layer may be useful for analyses over smaller spatial extents depending on the type and purpose of those analyses.

References

Brown, J. K. 1978. Weight and density of crowns of Rocky Mountain conifers. USDA Forest Service Research Paper INT-RP-197.

Cruz, M. G., M. E. Alexander., and R. H. Wakimoto. 2003. Assessing canopy fuel stratum characteristics in crown fire prone fuel types of western North America. International Journal of Wildland Fire 12: 39-50.

Keane, R. E., S. A. Mincemoyer, K. M. Schmidt, D. G. Long, and J. L. Garner. 2000. Mapping vegetation and fuels for fire management on the Gila National Forest complex, New Mexico. USDA Forest Service General Technical Report RMRS-GTR-46-CD.

Pollard, D.F.W. 1972. Above-ground dry matter production in three stands of trembling aspen. Canadian Journal of Forest Research 27: 27-33.

Last updated February 23, 2005