Get them while they are young: How mammals can decouple transitions of alpine meadows to woody dominated communities
There has been a general expectation that warming temperatures will facilitate transformation of high elevation meadows to woody dominated communities. We have been using observational and experimental approaches to test the hypothesis that seed and seedling predation by mammals could limit potential state changes of meadows in the alpine and upper subalpine zones of the Sierra Nevada mountain range in the western United States. The observational component of the study includes mammal (2008 - 2012) and vegetation surveys (2010 - 2012) across three degrees of latitude and an elevation range of 2800 m to 4000 m. The experimental component consists of mammal seed predator exclosures (N = 252) allocated among three arrays at each of two sites separated by > 100 km. Three cohorts (2010 - 2012) of seeds at five seed densities (1, 2, 3, 5 and 10 per 0.25 m2) and one seedling cohort were placed within and immediately outside the exclosures at each site.
The densities of mature and sapling conifers in meadow vegetation plots were low and decreased as a function of distance from conifer patches. There were very few conifer seedlings in the meadow plots and there was no relationship between their density and distance from conifer patches. This suggests a broad pattern of strong seedling limitation and low establishment rates of conifers in the meadows. Germination of seeds outside of the experimental exclosures was 19% compared to 65% within, and those that germinated outside the exclosures were mainly at densities of 1 seed per 0.25 m2. None of the seeds that germinated outside the exclosures survived more than 1.5 years compared to 23% within the exclosures. Virtually all seedlings that were planted outside the exclosures were removed within a year. Soil moisture had a strong effect on germination and seedling survival, but only within the exclosures. Densities of four mammal species (yellow-bellied marmot, American pika, Belding’s ground squirrel, and golden-mantled ground squirrel) were individually or cumulatively high throughout the range, and all four were observed feeding on seeds and seedlings in the experimental sites. Collectively, these findings suggest mammals are playing a critical role in slowing or decoupling colonization of conifers into high elevation meadows. Abiotic factors potentially influence establishment rates of conifers into meadows, but biotic interactions appear to be the primary limiting force in the upper subalpine and alpine zones.