Rapid tufa deposition and bryophyte growth rates in Irish petrifying springs

Rates of deposition of tufa in petrifying springs are complex and highly variable, and the growth rates of associated bryophytes are poorly known. Tufa deposition and bryophyte growth rates were measured using novel methods at six petrifying spring sites in Ireland, north-western Europe. Tufa height was measured at 293 point locations, from 2011 to 2013, using fixed bars mounted above actively growing deposits. Most samples were bryophyte-dominated and the height increments of individual species were measured. Environmental factors were recorded and the chemical composition of spring water was analysed. Tufa deposits increased in height by 20.5 ± 1.1 mm.yr⁻¹ on average. The pleurocarpous moss <i>Palustriella commutata</i>, in particular, was associated with rapid deposition. Mixed effects modelling revealed a significant interaction between <i>P. commutata</i> and the presence of surface water: together, they increased annual deposition of tufa by 5.7 ± 1.9 mm. The mean height increment for bare, unvegetated tufa was only 16.5 ± 3.0 mm.yr⁻¹. <i>Palustriella commutata</i> can outgrow the mean rate of tufa deposition, with a mean annual increment of 27.6 ± 1.9 mm in depth of sward. The smaller acrocarpous mosses <i>Didymodon tophaceus</i> and <i>Eucladium verticillatum</i> grew in height by only 9.1 ± 1.6 mm.yr⁻¹ and 9.5 ± 1.3 mm.yr⁻¹ respectively; these two species cannot, therefore, contend with the most rapid rates of tufa deposition. A colonisation cycle was observed, involving competitive displacement of the smaller species by <i>P. commutata</i>. Non-intrusive methods enabled rapid deposition rates to be detected.