Vertical gradient in bryophyte diversity and species composition in tropical and subtropical forests in Yunnan, SW China

Questions: We sampled epiphytic bryophytes along the entire vertical gradient, from tree base to upper canopy, in tropical and subtropical forests in order to answer the following questions: (a) how do diversity, composition and life forms differ between forests; (b) how do communities change along the vertical gradient in each forest; (c) what are the likely environmental drivers of this; and (d) what is the conservation significance of the results?. Location: Canopy crane sites in tropical lowland seasonal rain forest (TRF) and subtropical montane moist forest (STF) in Yunnan, southwest China. Methods: Bryophytes were sampled from 1,600–2,400 cm² of bark surface in 14 vertical segments in three canopy layers on 142 trees, using canopy cranes. Microclimate was measured at five levels on the crane towers. Diversity was compared within and between forests using ANOVA. NMDS was used to compare species composition and assess potential drivers. Results: In total, 184 species were recorded in 106 genera and 39 families. STF had a richer bryoflora than TRF. More species occurred in the canopy than the sub-canopy and understorey in both sites. The fan life form was dominant in the lower two layers of both forests, while mats were more common in the canopy of TRF and tall turfs in STF. The main axis of variation in species composition was associated with the vertical gradient of declining humidity and vapour pressure, and increasing temperature, light and wind speed. Conclusions: This study demonstrates the value of canopy cranes for complete sampling of the epiphytic bryoflora in complex forests. Both forest types had very high species richness, reflecting the diversity of microhabitats along the vertical gradient. Observed and projected climate change may threaten this bryoflora as a result of rising temperatures and, in some areas, declining rainfall and fog frequency.