Growth of Sphagnum is strongly rhythmic: Contribution of the seasonal, circalunar and third components

: Among the bryophytes, biological growth rhythms have not yet been identified due to a lack of long-term precision observations. Here, we carry out precision field monitoring of the growth of the peat moss Sphagnum riparium using the recent geotropic curvature method. For 4 years, using the observation intervals of 2-5 days, we measured 116 469 shoots and received 4493 estimates of growth rates. We found three rhythmic growth components. The seasonal rhythm of growth has a period of about 180 days, which coincides with the seasonal temperature cycle. The circalunar growth rhythm has a period of about 29.5 days and coincides with the synodic lunar cycle. There is an acceleration near the new moon, and a slowdown in growth near the full moon. The third rhythm has a period of 7-16 days and is always synchronized with the circalunar growth rhythm. From the models of the sums of sinusoids, we found that the total contribution of all three rhythms to the growth rate is R2  = 0.51-0.78, and without taking into account the seasonal rhythm, R2  = 0.36-0.42. Thus, our study provides the first data on the biological rhythms and the contribution of these rhythms to the growth process of bryophytes. We attribute the unexpectedly high contribution of rhythms to the synchronous growth of the Sphagnum mat, which is necessary to reduce the loss of moisture.