Precipitation determines the persistence of hollow Sphagnum species on hummock.

On raised bogs, the distribution of Sphagnum species is determined by their distance to the water table, but occasionally species are able to survive outside their niche. Hollow species that persist in hummock vegetation are assumed to profit from the higher water content of the surrounding hummock species, although the mechanism responsible is unclear. In this study we elucidated the role of lateral hummock water transport (LHWT) and precipitation on the water content of hollow species occurring in hummocks. This was tested using a full factorial field transplantation experiment with cores of Sphagnum cuspidatum in a high and a low hummock. Treatments included direct precipitation (present or absent) and LHWT (present or absent). Fresh weights of the cores were measured at regular time intervals. Our results show a relatively large effect of precipitation on the water content in both the high and low hummock, whereas LHWT only seemed to be an important source of water in the high hummock, which was relatively dry. Furthermore, LHWT played an important role only after large precipitation events, suggesting that lateral water transport is indirectly affected by rain. This study showed that precipitation alone could explain the persistence of hollow species in high hummocks, whereas it was less important for hollow species in low hummocks. Our data suggest that survival and potential expansion of hollow species in higher hummocks is strongly depending on the intensity and frequency of rain events. Changes in precipitation patterns may result in a loss of Sphagnum diversity in hummocks.