  Beta-diversity has been repeatedly shown to decline with increasing
elevation, but the causes of this pattern remain unclear, partly because they
are confounded by coincident variation in alpha- and gamma-diversity. We used
8,795 forest vegetation-plot records from the Czech National Phytosociological
Database to compare the observed patterns of beta diversity to null-model
expectations (beta-deviation) controlling for the effects of alpha- and
gamma-diversity. We tested whether \b{eta}-diversity patterns along a 1,200 m
elevation gradient exclusively depend on the effect of varying species pool
size, or also on the variation of the magnitude of community assembly
mechanisms determining the distribution of species across communities (e.g.,
environmental filtering, dispersal limitation). The null model we used is a
novel extension of an existing null-model designed for presence/absence data
and was specifically designed to disrupt the effect of community assembly
mechanisms, while retaining some key features of observed communities such as
average species richness and species abundance distribution. Analyses were
replicated in ten subregions with comparable elevation ranges. Beta-diversity
declined along the elevation gradient due to a decrease in gamma-diversity,
which was steeper than the decrease in alpha-diversity. This pattern persisted
after controlling for alpha- and gamma-diversity variation, and the results
were robust when different resampling schemes and diversity metrics were used.
We conclude that in temperate forests the pattern of decreasing beta-diversity
with elevation does not exclusively depend on variation in species pool size,
as has been hypothesized, but also on variation in community assembly
mechanisms. The results were consistent across resampling schemes and diversity
measures, thus supporting the use of vegetation plot databases for
understanding...
