Methods for estimating status and change in presence and foliar cover area of invasive plants using national forest...

The influence of invasive plant species (IPS) on forest ecosystems has been of sufficient concern to warrant data acquisition on their presence and abundance. These data are needed at various spatial scales for both land management and policy purposes. One potential source of data are national forest inventories (NFIs) that often record IPS observations on sample plots that span relatively large spatial and temporal extents, thus providing data users with a wide range of analytical opportunities to meet desired information needs. In the NFI of the USA, which is conducted by the USA Forest Service's Forest Inventory and Analysis (FIA) program, IPS presence and foliar cover area are visually observed and recorded nationally on a spatially balanced subset of inventory plots measured each year. As with any sample-based assessment, the implementation of statistically appropriate methods is imperative for credibility of the final estimates. In this paper, statistical methods for calculating population estimates and associated measures of uncertainty for current status and change in IPS attributes are presented. The estimators address both presence and cover area estimation, as well as estimation of area proportions relative to total area domains. Also included are data and various numeric examples that assist in understanding the estimation steps and provide for verification of accuracy in results. The methods should be applicable to most IPS data collected in other forest inventories with little or no modification.