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Bryophyte diversity and niche relations along a 3000 m gradient in Sequoia National Park
Patterns of diversity in bryophytes were studied along a 3000-m gradient, on the western slope of the Sierra Nevada in Sequoia National Park (Sequoia). In Sequoia, climate varies from the foothills with their dry summers and cool winters, to the alpine zone with its short summers and deep snow cover. For bryophytes, richness and cover have been hypothesized to reach a maximum at temperatures that are cool and humid. Species richness of bryophytes was predicted to decrease with elevation in Sequoia. The lower elevations present highly suitable winters compared to the higher elevations (and the bryophytes are dormant during California's summer dry season). The effect of elevation was hypothesized to apply best to microhabitats that dry out quickly, whereas it ought to apply less well to bryophytes that live in streams. Similar theorizing addresses how other aspects of the microhabitat adjust the suitability of the environment to bryophytes. Elevations from 380-3556 m a.s.l. were surveyed in plots of 25 m2. Within each of 250 plots, every bryophyte was identified along with the wetness tendency of its microsite, shadiness, incline, and the type of substrate. Species richness within plots decreased with increasing elevation. As plots became wetter and shadier, richness increased. Incline did not have an effect on richness independent of shadiness and wetness. The effect of elevation on species richness and cover interacted with several microsite tendencies. For example, species richness decreased with elevation in dry microsites but remained the same in wet microsites across elevation. Sunnier and inclined microsites also decreased in richness with increasing elevation. Epiphytes disappeared above about 2000 m. Bryophytes on rotten wood had highest richness at intermediate elevations. Rock microsites remained relatively unchanged in species richness across elevations, and soil bryophytes had greatest richness at the lowest elevations. The explanation of diversity patterns based on suitability has been phylogenetically blind, but families and orders of bryophytes differed in how they related to the ecological variables. The moss family Pottiaceae was richer in species at lower elevations, whereas the Mniaceae was richer at mid elevations. The liverwort order Marchantiales was richer at lower elevations, whereas the Jungermanniales was richer at higher elevations. Thus, phylogenetic niche conservatism presents an alternative accounting for the richness patterns found along the gradient. The lower elevations are richer in species because the families and orders that do well at lower elevations are regionally species rich. Nevertheless, the phylogenetic structure is complex with patterns of particular species differing from general patterns.