Evaporation from Sphagnum monoliths under controlled water table and humidity conditions

English Abstract:

Despite a long interest in characterizing evaporation from bryophytes there remains a poor understanding of the mechanisms of water delivery, and of the rates of loss according to species and setting. We sampled three shallow (25 cm) monoliths (15 cm diameter) each of Sphagnum fuscum and S. rubellum. Along with an open water “column”, the evaporation rate (E) from each Sphagnum monolith was monitored in an environmental chamber with relative humidity (RH) at 80, 50 and 30%, with water table (wt) set at 0, 5, 10 and 20 cm below the Sphagnum surface for each humidity level. Total E over the 65-day period encompassing all RH and wt levels was the most variable for S. rubellum, which ranged from 63 to 128 mm (avg. 100 mm), whereas E was between 59 to 98 mm (avg. 76 mm) for S. fuscum. Open water evaporation was 89 mm. E was far more sensitive to RH than wt. For example, the variation of E_rubellum at different wt when RH was 30%, was ±0.3 mm/d. In contrast, E_­rubellum dropped from 2.9 to 0.7 mm/d as RH increased from 30 to 80%. The hydraulic properties of S. rubellum within a profile and between monoliths was much more variable than in S. fuscum. In fact, the variability in E in all monoliths was easier to explain on the basis of the hydraulic properties of individual monoliths, rather than by species. S. rubellum had higher bulk density than S. fuscum, so at a soil matric potential of -30 cm their respective water retentions averaged 0.45 and 0.35. Therefore, with more water in the pores the geometric mean unsaturated hydraulic conductivity across all depths and samples (ψ = -5 to -30 cm) was higher for S. rubellum (1.2e-03 cm/s) compared to S. fuscum (and 0.6e-03 cm/s), facilitating more effective water transport in S. rubellum, hence higher E. While the E behavior for given RH and wt settings was best explained by the monolith’s hydraulic properties, a much larger sample size is required to generalize about differences between these two species.