Trends in forest attributes are typically assessed using long-term forest inventories, but trends can only be assessed when inventory methods are compatible over time. This study demonstrated an appropriate method of comparing historical to current inventory data, showing that comparisons not accounting for changing inventory methods can produce misleading results about forest trends in western states.
Forest managers and policymakers often rely on forest inventory data collected and compiled by the Forest Inventory and Analysis (FIA) program to provide information about changes in forest characteristics. Use of FIA data from different time periods assumes that inventory methods were consistent over time, yet this assumption is not always realistic.
Since 2000, FIA’s definitions, sample designs, field protocols, and estimation procedures have remained consistent, but they differed from those of the historical inventories, which were often biased toward timber forest types and specific ownership groups. Therefore, FIA cautions users against making comparisons between historical and recent inventory estimates without accounting for differences in protocol. An example of an appropriate comparison is evaluating tree volume, growth, and mortality per unit area, as measured only on plots that were surveyed during both historical and current inventories.
This study shows that comparisons based on entire inventories, and not accounting for different methods, may show changes in volume, growth, and mortality that differ in magnitude and sometimes in direction (gain or loss) from comparisons based only on plots surveyed at both time periods. These discrepancies illustrate that historical inventories may provide an incomplete picture of reference conditions in some western states.