Measurement and modelling of bryophyte evaporation in a boreal forest chronosequence

The effects of changing climate and disturbance on forest water cycling are not well understood. In particular, bryophytes contribute significantly to forest evapotranspiration in poorly drained boreal forests, but few studies have directly measured this flux and how it changes with stand age and soil drainage. We measured bryophyte evaporation (E) in the field (in CanadianPicea marianaforests of varying ages and soil drainages) and under controlled laboratory conditions, and modelled dailyEusing site‐specific meteorological data to drive a Penman–Monteith‐based model. Field measurements ofEaveraged 0·37 mm day−1and ranged from 0·03 (Pleurozium schreberiiin a 77‐year‐old dry stand) to 1·43 mm day−1(Sphagnum ripariumin a 43‐year‐old bog). In the laboratory, moss canopy resistance (which ranged from ∼0 to 1500 s m−1) was constant until a moss water content of ∼6 g g−1and then climbed sharply with further drying; unexpectedly, no difference was observed between the three moss groups (feather mosses, hollow mosses and hummock mosses) tested. Modelled annualEranged from 0·4 mm day−1, in the well‐drained stands, to ∼1 mm day−1in the 43‐year‐old bog. The Penman–Monteith modelling approach used was relatively insensitive to most parameters but only explained 35% of the variability in field measurements. BryophyteEwas greater in bogs than in upland stands, was driven by low‐lying mosses and varied with stand age only in the poorly drained stands; this suggests that bryophytes may provide a buffering effect to fire‐driven changes in tree transpiration.