Trends in U.S. forest biomass and carbon are assessed using Forest Inventory and Analysis (FIA) data relative to baseline assessments from the 1990s. The integrity of baseline data varies by state and depends largely on the comparability of periodic versus annual forest inventory data. In most states in the Interior West FIA region, the periodic inventory's sample design, plot configuration, estimation procedures, and definitions were different from those for the annual inventory, which are nationally consistent. Direct comparisons of periodic versus annual inventory data are therefore tenuous and may reflect changing protocols rather than actual changes, yet they comprise the best available method of assessing recent trends in some states. This study attempts to clarify trends in aboveground tree biomass in the Interior West region by comparing estimates at matched plots that were sampled during both periodic and annual inventories. To illustrate the ramifications of ignoring changes in inventory protocols, mean trends at paired plots were compared to those demonstrated by unpaired comparisons of entire periodic and annual inventories. In some states, the results produced by the two methods are contradictory. This demonstrates the importance of reassessing the use of estimates based on periodic forest inventories as reference conditions.