Implementing the measurement interval midpoint method for change estimation.

The adoption of nationally consistent estimation procedures for the Forest Inventory and Analysis (FIA) program mandates changes in the methods used to develop resource trend information. Particularly, it is prescribed that changes in tree status occur at the midpoint of the measurement interval to minimize potential bias. The individual-tree characteristics requiring midpoint values depend on the predictor variables needed to compute tree volume. Tree diameter change models are used to predict midpoint values for both future and past conditions. These updated diameters are used in conjunction with other information in a height model to obtain midpoint merchantable heights. These estimated diameter and height values are used to predict tree cubic-foot volume at the measurement interval midpoint. Limitations encountered in implementing this system included lack of information for some trees and inconsistencies between observed and updated values. A comparison is made between the previous method and the newly adopted technique, and effects on components of change are examined. Net change is unaffected by the new methodology.