Functional significance of variation in bryophyte canopy structure

Abstract

In most bryophytes, the thickness of boundary layers (i.e., unstirred layers) that surrounds plant surfaces governs rates of water loss. Architectural features of canopies that influence boundary layer thickness affect the water balance of bryophytes. Using field samples (9.3 cm diameter cushions) from 12 populations (11 species) of mosses and liverworts, we evaluated the relationship between canopy structure and boundary layer properties. Canopy structure was characterized using a contact surface probe to measure canopy depth along perpendicular transects at spatial scales ranging from 0.8 to 30 mm on 186 points per sample. Semivariance in depth measurements at different spatial scales was used to estimate three architectural properties: surface roughness (L(r)), the scale of roughness elements (S(r)), and fine-scale surface texture, the latter characterized by the fractal dimension (D) of the canopy profile. Boundary layer properties were assessed by evaporation of ethanol from samples in a wind-tunnel at wind speeds from 0.6 to 4.2 m/s and applied to characterize mass transfer using principles of dynamic similarity (i.e., using dimensionless representations of conductance and flow). In addition, particle image velocimetry (PIV) was used to visualize and quantify flow over two species. All cushions exhibited the characteristics of turbulent as opposed to laminar boundary layers, and conductance increased with surface roughness. Bryophyte canopies with higher L(r) had greater conductances at all wind speeds. Particle image velocimetry analysis verified that roughness elements interacted with flow and caused turbulent eddies to enter canopies, enhancing evaporation. All three morphological features were significantly associated with evaporation. When L(r), S(r), and D were incorporated with a flow parameter into a conductance model using multiple linear regression, the model accounted for 91% of the variation in mass transfer.

To concentrate or ventilate? Carbon acquisition, isotope discrimination and physiological ecology of early land plant life forms

“…(a) Co-limitation of CO 2 uptake and water loss for bryophyte life forms The lower overall conductances ( g liq , liquid phase and, even more so g int , mesophyll) for the non-ventilated thalli were consistent with gas exchange characteristics showing diffusion limitation (Slavík 1965;Proctor 1980;Green & Lange 1994;Williams & Flanagan 1996;Rice et al 2001;Griffiths et al 2004;Fletcher et al 2005). By contrast, resistance to CO 2 uptake was significantly lower for the ventilated thalli, suggesting that surface morphological features, as well as internal airspaces, and possibly biochemical differences, all help to maximize CO 2 assimilation and support the notion of an evolutionary progression from simple to complex thalli.…”

Role of morphological structure and layering ofSphagnumandTomenthypnummosses on moss productivity and evaporation rates

“…As the surface of the Tomenthypnum canopy is more porous than Sphagnum, the depth of turbulence penetration is likely greater (cf. Rice et al 2001). Consequently, while capillary flow is required to sustain evaporation, it does not need to reach the surface of the tomenta, as vapour exchanges can occur slightly deeper in the moss shoots, and the aerial portions of the plant can desiccate.…”

“…Because Tomenthypnum can lose considerable amounts of water before desiccation (Busby and Whitfield 1978) it is likely inherent in its physiological design to tolerate desiccation and photosynthesize at low water contents (Rice et al 2001). However, since u and GEP relationships were similar to Busby and Whitfield (1978) and productivity ceased with low water tables (Fig.…”

“…Connectivity within moss and peat structures is a function of pore-size distribution, geometry, and tortuosity (Rezanezhad et al 2010), which affect water content and ultimately unsaturated hydraulic conductivity (Price et al 2008). The upward capillary water movement from underlying peat substrates is driven by soil-water pressure gradients within the moss structure caused by atmospheric demand at the canopy surface Rice et al 2001). However, when soil-water pressures become too low for free capillary rise, capillary water is evaporated and the water pressure of the moss cells drops rapidly and equilibrates with the surrounding air, thereby desiccating and ceasing all photosynthetic activity , and reducing evaporative losses to maintain saturated conditions in the peat profile below (Kettridge and Waddington 2014).…”

Measurement and modelling of bryophyte evaporation in a boreal forest chronosequence

“…As a poikilohydric plant, a bryophyte-more specifically the water vapour partial pressure of the plant body-is always in equilibrium with ambient humidity (Green and Lange, 1994); this lack of active control over plant water status implies fundamentally different ecosystemlevel dynamics as well (Longton, 1992;Turetsky, 2003). The morphology of the bryophyte canopy influences the development of a boundary layer adjacent to the plant surface (Campbell and Norman, 1998), which in turn greatly influences the plant water and carbon budgets (Zotz et al, 2000;Rice et al, 2001). Bryophyte water status is a function of factors spanning a wide range of scales: cell turgidity, osmotic potential, local hydrology, distance from the water table and thallus water content (Dilks and Proctor, 1979;Hayward and Clymo, 1982;Proctor, 1982Proctor, , 2000aProctor and Tuba, 2002).…”