An assessment of the selection criteria, roles, and representativeness of indicator trees in the nationwide forest inventory of the United States

In forest inventories, trees are often singled out to represent forest attributes and other environmental conditions. These “indicator” trees may be selected to estimate site level attributes, including site productivity, stand age, climate history, and growth rates. Here, we use data from the nationwide forest inventory (NFI) in the United States (US) to assess how indicator tree attributes compare to tally tree attributes across space and through time. First, we use contemporary field guides and NFI data to describe indicator tree selection criteria across regions of the US. Second, we compare indicator tree diameter, height, and species to two subsets of associated tally trees: a) all live tally trees and b) tally trees conforming to indicator tree selection criteria. Finally, we use annual ring width information from indicator trees to compare growth rates, as well as diameter and species to compare site trees and tally trees from historic inventories in the northeastern US. Contemporary and historic indicator tree attributes are rarely equivalent to either subset of tally tree attributes, but the differences are smaller when comparing to tally trees with consistent selection criteria. Across regions of the US, the differences between indicator tree and tally tree attributes are often close to or centered around zero, suggesting they may represent population-level tree attributes. With the increasing use of NFI data, it is important to understand the original intent for data collection, how the data were collected, and potential limits that might impact the efficacy of the data in new contexts.