Developing a Field-Tested Wetland Indicator Rating for Blue Spruce (Picea Pungens) in the Southern Rocky Mountains

To be identified as a wetland under U.S. Federal regulations, a site must, under normal circumstances, support vegetation dominated by hydrophytes. A list of hydrophytes and their wetland indicator rating is published by the U.S. Army Corps of Engineers as the National Wetland Plant List (NWPL) and is the basis for assessing the vegetation criteria of Federal wetland delineation manuals. Ratings are primarily based on expert opinion and few empirical studies have been done, particularly at landscape scales. In this study, we developed an approach for quantifying plant indicator ratings at broad spatial scales and used it to estimate the frequency that Picea pungens Engelm. (Colorado blue spruce) occurs in wetlands across a 22,921 km(2) study area in the southern Rocky Mountains. Species distribution models were developed and used to inform a multistage field sampling design. Wetland soil and hydrology indicators were assessed around 423 randomly selected trees in 22 HUC12 watersheds. Only 16.5 % of trees occurred in wetlands, suggesting that a rating of facultative upland (FACU) is more appropriate than the currently published rating of facultative (FAC) for our study area. This study demonstrates that it is feasible to quantitatively evaluate ratings for species even at broad landscape scales.