Bryophyte Richness and Habitat Occupancy Along an Elevation Gradient Starting in a Mediterranean Climate

Bryophytes were surveyed from the foothills (summers dry, winters mild) to the alpine zone (summers short, winters with snow cover) on the western slope of California's Sierra Nevada. The lower elevations of the transect are within a mediterranean climate, with mild winters suitable to bryophyte physiology, while during the hot, dry summers, bryophytes are dormant. Bryophyte richness and abundance was predicted to decrease with elevation. The effect of elevation was hypothesized to apply more strongly to microhabitats that dry quickly compared to spots that are moist for months at a time. Plots of 25 m2 were chosen from 380–3578 m above sea level. Bryophyte richness and abundance, along with microsite wetness, shadiness, incline, and substrate type were recorded for each of 253 plots. Richness decreased with increasing elevation. The effect of elevation depended on microhabitat, with bryophytes of dry microsites abundant at low elevations and those of wet microsites evenly abundant across elevations. Epiphytes disappeared above ∼2000 m. Bryophytes on rotten wood were most prominent at intermediate elevations. Soil bryophytes had greatest richness at low elevations. Not all groups of bryophytes have the same relation to climate: Pottiaceae and Marchantiales were richer at lower elevations, whereas Mniaceae and Jungermanniales were richer at high elevations. In general, bryophytes do best at low elevations, where winters in the California Mediterranean are highly suitable. The effect of elevation is most pronounced for taxa that do best in dry microsites.