Basal area is a measure of the cross-sectional area of trees calculated by multiplying the foresters’ constant (0.005454) by the squared diameter of each tree to determine a measure of tree area (ft2 or m2) per unit area (acre or ha). Similar to an assessment of forest canopy cover, basal area provides a measure of horizontal structure, and is closely associated with measures of vertical structure. When assessing condition of upland hardwood woodland and forest systems, the GCPO LCC Integrated Science Agenda targets a large proportion of the basal area in those systems to be composed of oak and hickory species. This endpoint targets >90% of basal area as oak and hickory for upland hardwood woodland systems and >70% for upland hardwood forest systems, and results from the desire for these systems to contain ample hardwood mast-producing trees. However, in consultations with the GCPO Adaptation Science Management Team during conservation blueprint development, the team determined that an additional endpoint that examined total live tree basal area for woodland and forest systems would provide a more comprehensive assessment of basal area when used in combination of proportional oak-hickory composition. We therefore added a component to the basal area endpoint that targets 30 - 80 ft2/ac total live tree basal area for upland hardwood woodlands and 80-100 ft2/ac for upland hardwood forests. Assessing total basal area allow for better assessment of forest structure in addition to forest composition, and reduces the risk that forest systems with undesirable high basal area but that reflect the proper composition of oaks and hickories being included in the assessment. We used plot-level imputed data from the FIA program at 250 m resolution to assess the proportion of total live tree basal area composed of oak and hickory species and total live tree basal area in the GCPO (USDA Forest Service Remote Sensing Applications Center, personal communication). The USFS per-species (Wilson et al. 2013) and total live tree basal area data product provides raster maps for the conterminous U.S. generated using a weighted k-nearest neighbor and canonical correspondence analysis from a combination of vegetation phenology data produced from 250 m resolution MODIS satellite imagery, ancillary environmental data, NLCD tree canopy cover data, and 2000-2009 plot-level field data from the FIA program (Wilson et al. 2012). Note live tree basal area estimates were calculated on a per-acre-of-land basis, though forested lands were the primary sampling frame. Proportion of basal area comprised of oak and hickory species was developed using the per-species basal area to select out oak and hickory tree species, summing oak and hickory basal area, then calculating the proportion of the total live tree basal area comprised of oak and hickory species on a per-250 m pixel basis. We used an extract by mask function in ArcGIS to delineate pixels where total basal area and proportion of basal area oak and hickory were within the target ranges in upland hardwood woodland and forest systems, using the USFS imputed basal area layers as input data and the woodland and forest data as masks. We then reclassified each product to pull out pixels with total basal area 30 - 80 ft2/ac and proportion of basal area oak-hickory >90% for upland hardwood woodlands. We did the same for upland hardwood forests, pulling out pixels with total basal area 80-100 ft2/ac and proportion of basal area oak-hickory >70%. We assessed acreage by summing the count of pixels within each geographic construct and multiplying by pixel resolution (250 x 250 m = 62,500 m2) and converting to acres. For display we calculated the proportional area (acres upland hardwood (basal area target/acres HUC 12) within each HUC 12 watershed using zonal statistics in ArcGIS. Wilson, B. T., A. J. Lister, R. I. Riemann, and D. M. Griffith. 2013. Live tree species basal area of the contiguous United States (2000-2009). Newtown Square, PA: USDA Forest Service, Northern Research Station.
