Inventory of Fens in West Central Colorado

As part of on-going resource management and forest planning activities, the Grand Mesa, Uncompahgre, and Gunnison National Forests (GMUG) began an effort in 2008 to better understand the abundance and distribution of fens on lands managed by the GMUG. To complete this effort, the forest assembled a multi-disciplinary team composed of specialists in soil science, geology/hydrogeology, hydrology, botany, and range management. This group was directed by forest leadership to provide information in three areas:

•  Distribution and characterization of fens
•  Evaluation of the condition of fens
•  Land management implications for fens

This report details the results of efforts to better characterize the unique and important fen resource present on GMUG lands. It is intended to inform local resource specialists on the GMUG, as well as others interested in wetland and fen research, of the methods and results of all fen investigation efforts that have been conducted on the GMUG.

This report is the result of three years of investigation. Photointerpretation of the entire Grand Mesa, Uncompahgre, and Gunnison National Forests for potential fens was completed in 2009, identifying 3,270 potential fen sites covering 17,485 acres, about 0.65% of the Forest.

Prior to the selection of a field verification sample set, the Forest was divided into twelve landscape areas based on similarities of geologic and hydrologic settings, climate, and glaciation. About two hundred 1 × 1 km cells across the Forest were selected for inventory using a spatially balanced sampling process. During the field seasons of 2009 and 2010, 204 of those cells and 336 potential fens were visited and sampled. One hundred forty-seven fens were documented and complete data collected. From this sample it can be estimated with 95% confidence that there are approximately 1,738 (±827)[1] fens covering 11,034 (±6,936) acres on GMUG lands.

Some general spatial attributes were common to the fens visited. About half the fens found are less than four acres; 20% of them are less than one acre. Most of the fens found are fen-wetland complexes with several to many different communities in the same wetland. The majority (90%) of the fens found are between 9,000 ft and 11,900 ft elevation. Most of the fen acres are associated with unconsolidated glacial drift or mass wasting geologic map units.

Most fens were observed to have some form of disturbance, with a wide variety of different disturbance factors present. The most common general disturbances documented during the field work, in order of frequency of occurrence, were browsing, grazing, trampling, trails, beaver activity, flooding, and vehicle tracks. However, disturbances such as flooding, de-watering, and the presence of vehicle tracks, though less frequent are of greater consequence because they are much more likely to disturb or threaten the functioning quality of the fen. Six fens had no apparent disturbances.

Disturbances were also investigated in a 100-meter buffer (outward from the edge of the fen-wetland complex). The most common general disturbances in the buffer, in order of frequency of occurrence, are browsing, grazing, trails, roads, erosion, tree cutting, trampling, and vehicle tracks. Again, disturbances such as tracks, roads, and campsites, while being less often encountered, are of much greater consequence than the more frequently observed disturbances because they increase the risk to fen function.

Using similar factors used by other scientists, a rating system for the assessment of fen condition was devised. Based on this system, 81% of the fens we investigated in 2009-2010 would be classed as high condition, 18% in moderate condition, and 1% in low condition. To further test this system, data from other sources describing lower-condition “modified fens” (or “former fens”), were ranked according to the score sheet, and the rating system seems to correctly characterize those sites as well.