Carbon storage along headwater stream segments with differing valley geometry in Rocky Mountain National Park, Colorado

Background/Question/Methods

Headwater streams are known to be important in the global carbon cycle, yet few studies have investigated carbon (C) storage along mountainous stream-riparian corridors.  To better understand the magnitude and spatial distribution of C stocks along headwater fluvial networks, we estimated above- and below-ground C pools in 100-m-long reaches in six different valley types in Rocky Mountain National Park, Colorado.  Valley types were distinguished based on downstream gradient and valley-bottom width relative to active channel width (valley geometry) and the presence of biotic drivers, notably beaver dams and channel-spanning logjams associated with old-growth forest that contribute to development of multi-thread channel patterns.  Three valley types were associated with old-growth riparian forest: laterally confined; laterally unconfined with multi-thread channels; and laterally unconfined with single-thread channels. A laterally confined valley type occurring in younger forest was also sampled. Two valley types were distinguished as low-gradient, laterally unconfined stream segments: beaver-meadow complexes with multi-thread channels and mixed riparian vegetation (conifer, willow, and herbaceous); and abandoned beaver-meadow complexes with single-thread channels, dominated by herbaceous vegetation.  From field measurements, we estimated carbon stored in the riparian vegetation (live and dead), forest floor, instream and floodplain large wood, and floodplain sediment for each 100-m-long valley segment.  

Results/Conclusions

In the forested reaches, total C pools that summed vegetation biomass (live and dead), forest floor, and downed large wood  ranged from 190 Mg/ha to 450 Mg/ha.  Highest biomass C stocks were found in the laterally unconfined valley segment with multi-thread channels and dominated by riparian old-growth forest.  C pools for forest floor components and floodplain large wood were consistently higher than upland forests with similar species composition.  Belowground C pools were highest in the beaver-meadow complexes (approximately 1000 to 3080 Mg/ha), reflecting the substantial storage potential of wide, low-gradient valleys and the high C content of floodplain sediments.  Results indicate that large quantities of terrestrial carbon are concentrated in predictable segments of the stream-riparian corridor.  Multi-thread channel segments influenced by beaver or logjams cover less than 25 percent of the total length of stream networks in the study area, yet they may account for more than three-quarters of the stored C.  Historical removal of beavers and old-growth forests has likely resulted in a reduction of total ecosystem C pools and potential for C storage along headwater river networks in the Colorado Front Range.