A known issue in FIA's EVALIDator causes a mismatch between rFIA and EVALIDator estimates of tree growth. Estimates output by rFIA have been compared against updated estimates produced with EVALIDator (not yet publicly available), and tend to differ by approximately +/- 1%. Current development seeks to identify the cause of this remaining discrepancy.
Estimation of attributes follows the procedures documented in Bechtold and Patterson (2005). Average annual diameter, basal area, height, and net volume growth of a stem is computed using a sample-based ratio of means estimator of total diameter (basal area, height, net volume) growth / total trees, and average annual basal area and net volume growth per acre is computed as total basal area (net volume) growth / total area. All estimates are returned as average annual rates. Only conditions which were forest in time 1 and in time 2 are included in estimates (excluding converted stands). Only stems 5 inches DBH or greater are included in estimates.
As estimates are of net growth rates, they may attain a negative value. Negative growth estimates most likely indicate a substantial change in an attribute of the tree or area between time 1 and time 2, which caused the attribute to decrease. Implementation of the growth accounting method allows us to more accurately representshifts in forest attributes (biomass) between classified groups (size classes) over time.
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).