Bryophyte diversity in Californian grasslands in relation to substrate quality, exotic vascular plants and disturbance

Bryophyte communities of semi-arid grassland ecosystems are little studied, and it is poorly known how they vary in relation to soil quality, disturbance and the invasion status of vascular plants. By analogy with short-statured, competitively inferior native vascular plants, we hypothesized that bryophytes would show high beta diversity and increasing alpha diversity along a gradient from productive non-serpentine grasslands dominated by exotics to low productivity serpentine grasslands dominated by native vascular plants. We also predicted that disturbance by gophers would benefit bryophyte alpha and beta diversity, especially at the productive end of the soil fertility gradient. We sampled bryophytes in 48 sites in a 3,000 ha landscape in the Inner Coast Range of California, USA. We used several multivariate (non-metric multidimensional scaling, PERMDISP) and univariate analysis methods (generalized linear models). As predicted, we found high beta diversity of bryophytes along the soil gradient, and higher cover and richness in unproductive rocky serpentine grasslands than productive and exotic-dominated non-serpentine grasslands. Gopher burrowing had a unimodal influence on bryophyte alpha diversity. Our results show that bryophyte species composition and diversity respond strongly to the same soil gradient that controls vascular grassland species. Bryophytes are likely imperiled by the continued spread of exotic vascular plants, but promoted by moderate disturbances such as gopher burrowing.