Citation: Glassman, S. I., K. C. Lubetkin, J. A. Chung, and T. D. Bruns. 2017. The theory of island biogeography applies to ectomycorrhizal fungi in subalpine tree "islands" at a fine scale. Ecosphere 8(2):e01677. 10.1002/ecs2.1677Abstract. The theory of island biogeography, which predicts that species richness is a function of island size and distance from the mainland, is well tested with macro-fauna and flora. Yet, in many ways, microbes are more appropriate for testing this and other ecological theories due to their small size and short generation times that translate to an ease of replication. We used a natural experimental system of isolated "host islands" to test the generality of the theory of island biogeography. Specifically, we tested whether ectomycorrhizal fungal (EMF) richness increased with tree size and decreased with distance from forest in a subalpine basin in Yosemite National Park for two congeneric pine species, Pinus albicaulis and Pinus contorta. We determined EMF richness with next-generation sequencing, measured the size and age of each tree island (n = 40), and calculated geographic distances from each tree to the nearest forest edge. We found that EMF richness increased with island size (as measured by tree volume) and tree age for both pine species and decreased with distance from forest edge for P. albicaulis. Thus, we show the applicability of the theory to microbial symbionts in harsh, dry, and likely non-equilibrium systems. In addition, we found that despite the fact that our tree islands had a mean age of 65 yr, a pioneer community of EMF dominated. We interpret this as evidence that water stress interacts with succession to create a sustained period of early-stage fungi even in mature trees.

Mountain meadows have high biodiversity and help regulate stream water release following the snowmelt pulse. However, many meadows are experiencing woody plant encroachment, threatening these ecosystem services. While there have been field surveys of individual meadows and remote sensing-based landscape-scale studies of encroachment, what is missing is a broad-scale, ground-based study to understand common regional drivers, especially at high elevations, where land management has often played a less direct role. With this study, we ask: What are the climate and landscape conditions conducive to woody plant encroachment at the landscape scale, and how has historical climate variation affected tree recruitment in subalpine meadows over time? We measured density of encroaching trees across 340 subalpine meadows in the central Sierra Nevada, California, USA, and used generalized additive models (GAMs) to determine the relationship between landscape-scale patterns of encroachment and meadow environmental properties. We determined ages of trees in 30 survey meadows, used observed climate and GAMs to model the relationship between timing of recruitment and climate since the early 1900s, and extrapolated recruitment patterns into the future using downscaled climate scenarios. Encroachment was high among meadows with lodgepole pine (Pinus contorta Douglas ex Loudon var. murrayana (Balf.) Engelm.) in the immediate vicinity, at lower elevations, with physical conditions favoring strong soil drying, and with maximum temperatures above or below average. Climatic conditions during the year of germination were unimportant, with tree recruitment instead depending on a 3-yr seed production period prior to germination and a 6-yr seedling establishment period following germination. Recruitment was high when the seed production period had high snowpack, and when the seedling establishment period had warm summer maximum temperatures, high summer precipitation, and high snowpack. Applying our temporal model to downscaled output from four global climate models indicated that the average meadow will shift to forest by the end of the 21st century. Sierra Nevada meadow encroachment by conifers is ubiquitous and associated with climate conditions increasingly favorable for tree recruitment, which will lead to substantial changes in subalpine meadows and the ecosystem services they provide.

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