Estimation of attributes follows the procedures documented in Bechtold and Patterson (2005). Specifically, the percent land area occupied by forest in each stand structural stage are computed using a sample-based ratio-of-means estimator of total area in structural stage / total land area within the domain of interest. Stand structural stage is classified based on the relative basal area of canopy stems in various size classes (defined below). Only stems which are identified on-site dominant, subdominant, or intermdediate crown-classes are used to classify stand structural stage.
Diameter Classes
Pole: 11 - 25.9 cm
Mature: 26 - 45.9 cm
Large: 46+ cm
Structural Stage Classification
Pole Stage: > 67% BA in pole and mature classes, with more BA in pole than mature.
Mature Stage: > 67% BA in pole and mature classes, with more BA in mature than pole OR > 67% BA in mature and large classes, with more BA in mature.
Late-Successional Stage: > 67% BA in mature and large classes, with more in large
Mosiac: Any plot not meeting above criteria.
Stratified random sampling techniques are most often employed to compute estimates in recent inventories, although double sampling and simple random sampling may be employed for early inventories. Estimates are adjusted for non-response bias by assuming attributes of non-response plot locations to be equal to the mean of other plots included within thier respective stratum or population.
Forest land must be at least 10-percent stocked by trees of any size, including land that formerly had such tree cover and that will be naturally or artificially regenerated. Forest land includes transition zones, such as areas between heavily forested and nonforested lands that are at least 10-percent stocked with trees and forest areas adjacent to urban and builtup lands. The minimum area for classification of forest land is 1 acre and 120 feet wide measured stem-to-stem from the outer-most edge. Unimproved roads and trails, streams, and clearings in forest areas are classified as forest if less than 120 feet wide. Timber land is a subset of forest land that is producing or is capable of producing crops of industrial wood and not withdrawn from timber utilization by statute or administrative regulation. (Note: Areas qualifying as timberland are capable of producing at least 20 cubic feet per acre per year of industrial wood in natural stands. Currently inaccessible and inoperable areas are NOT included).
Easy, efficient parallelization is implemented with the parallel package. Users must only specify the nCores argument with a value greater than 1 in order to implement parallel processing on their machines. Parallel implementation is achieved using a snow type cluster on any Windows OS, and with multicore forking on any Unix OS (Linux, Mac). Implementing parallel processing may substantially decrease free memory during processing, particularly on Windows OS. Thus, users should be cautious when running in parallel, and consider implementing serial processing for this task if computational resources are limited (nCores = 1).