Record growth of Sphagnum papillosum in Georgia (Transcaucasus): rain frequency, temperature and microhabitat as key drivers in natural bogs

(1) Peatmoss (Sphagnum) growth has been studied widely, in particular at temperate and boreal latitudes > 45 °N, where productivity is mainly controlled by mean annual temperature and precipitation. We studied the growth of Sphagnum papillosum and S. palustre in four peatlands in the year-round warm and humid Kolkheti Lowlands (Georgia, Transcaucasus, eastern end of the Black Sea, latitude 41–42 °N). (2) Productivity, site conditions and climate in Kolkheti are included in a worldwide analysis of studies on the growth of S. papillosum to identify driving factors for its growth. (3) The productivity of S. papillosum and S. palustre under natural conditions is extraordinarily high in Kolkheti, reaching 269–548 g m-2 yr-1 and 387–788 g m-2 yr-1 (mean of various sites), respectively. Rates of increase in length are up to 30.3 cm yr-1, with the largest values for S. palustre. (4) Rate of increase in length and biomass productivity differed between years, with better growth being explained by higher number of rain days and shorter periods without precipitation. Regular rainfall is essential for continuous Sphagnum growth as low water table prevents permanent water supply by capillary rise. (5) The analysis of international studies on Sphagnum papillosum productivity confirms the decisive role of rain frequency, next to microhabitat. Productivity increases further with mean temperature during growth periods, the near-largest values being for Kolkheti. As temperature is correlated with latitude, Sphagnum productivity may increase towards the equator if the water supply is sufficient. (6) Substantial Sphagnum growth was observed during winter, confirming year-round growth in the virtual absence of frost. Growth in lawns, rain frequency and almost optimal nutrient availability (N, P, K) were the only site conditions found to explain the high rates of Sphagnum growth in Kolkheti.