Do Small Mammals and Vegetation Metacommunity Dynamics Determine the Extent and Pattern of Treeline in the High Elevation Zone of the Sierra Nevada Mountain Range?

Abstract:

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 analyze potential state changes of meadows and the role seed and seedling predation play in conifer expansion in the high elevation zone of the Sierra Nevada mountain range of the western United States. The observational component consists of 256 plots spanning 3 degrees of latitude and an elevation range from 3000 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 of seeds at five seed densities (1, 2, 3, 5 and 10 seeds per 0.25 m²) and one seedling cohort were placed within and immediately outside the exclosures at each site. Trend surface and distance decay analyses of community composition indicate vegetation communities in the high elevation zone have not assembled predictably along environmental or spatial gradients. Rather, we have found strong support for neutral dynamics, implying that communities assemble more stochastically as a result of dispersal limitation or priority effects. Density of mature and sapling conifers decrease as a function of distance from conifer patches, but seedling density has no relationship with distance from conifer patches. Germination of seeds outside of the exclosures was 19% compared to 65% within, and these 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 of the seedlings planted outside the exclosures were removed within a year. Collectively, these findings indicate a highly patchy rather than uniform pattern of treeline extension in the high elevation zone of the Sierra Nevada. Moreover, smaller mammals appear to be playing a critical role in slowing or decoupling colonization of conifers into high elevation meadows and other vegetation types.