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Title: A multi-scale evaluation of pack stack effects on subalpine meadow plant communities in the Sierra Nevada

Author
item LEE, STEVEN - Us Geological Survey (USGS)
item BERLOW, ERIC - Us Geological Survey (USGS)
item Ostoja, Steven
item BROOKS, MATTHEW - Us Geological Survey (USGS)
item GENIN, ALEXANDRE - University Of Montpellier
item MATCHETT, JOHN - Us Geological Survey (USGS)
item HART, STEPHAN - University Of California

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/15/2017
Publication Date: 6/13/2017
Citation: Lee, S.R., Berlow, E.L., Ostoja, S.M., Brooks, M.L., Genin, A., Matchett, J.R., Hart, S.C. 2017. A multi-scale evaluation of pack stack effects on subalpine meadow plant communities in the Sierra Nevada. PLoS One. 12(6):e0178536. doi: 10.1371/journal.pone.0178536.

Interpretive Summary: Meadows in the Sierra Nevada of California have inspired art, literature and modern day environmental ethics and conservation. While meadows only make up a small percentage of the land area (4-7%) in the Sierra Nevada they support the majority of biodiversity and attract recreational uses including packstock groups for their scenic beauty and supply of forage and water for the horses and mules. At the same time, packstock have long been blamed for site degradation and have been suggested to impact sensitive plant and animal resources, although very little data is available to support such claims. We compared the condition of vegetation in sites used by packstock animals compared to those that received no use and found very little difference except at the very highest use area. Although there was some increase in the amount of bare ground at the driest sites but it’s unclear to the degree by which this finding is ecologically meaningful or not.

Technical Abstract: We evaluated the influence of pack stock (i.e., horse and mule) use on meadow plant communities in Sequoia and Yosemite National Parks in the Sierra Nevada mountains of California. Meadows were sampled to account for inherent variability across multiple scales by: 1) controlling for among-meadow variability by using remotely sensed hydro-climatic and geospatial data to pair stock use meadows with similar non-stock (reference) sites, 2) accounting for within-meadow variation in the local hydrology using in-situ soil moisture readings, and 3) incorporating variation in stock use intensity by sampling across the entire available gradient of pack stock use. Increased cover of bare ground was detected only within “dry” meadow areas at the two most heavily used pack stock meadows. There was no difference in plant community composition for any level of soil moisture or pack stock use. Increased local-scale spatial variability in plant community composition (species dispersion) was detected in “wet” meadow areas at the two most heavily used meadows. These results suggest that at the meadow scale, plant communities are generally resistant to the contemporary levels of recreational pack stock use. However, within-meadow responses such as increased bare ground can be a function of local-scale hydrological conditions. By adopting multiple plant community indices, and considering local moisture regimes wilderness managers can improve monitoring of disturbance in Sierra Nevada meadows.