The influence of water level fluctuations on the potential for convective flow in the emergent macrophytes Typha domingensis and Phragmites australis

This paper examines the influence of the amplitude of water level fluctuations and elevation on the gas space anatomy and potential for convective flow in the emergent macrophytes Typha domingensis Pers. and Phragmites australis (Cav.) Trin. ex Steud. Plants were grown under a range of amplitudes of cyclic water level fluctuations: static, ±15, ±30 and ±45 cm. The water level of each treatment fluctuated around an initial and average depth of 60 cm. Within each amplitude treatment, plants were grown at three elevations: sediment surface 20, 40 and 60 cm above the base of the pond.The gas space anatomy of T. domingensis showed a range of modifications in response to experimental treatments. Lacunal cross-sectional area of the leaves increased with decreasing elevation as a product of changes in both fractional porosity and total cross-sectional area. Lacunal cross-sectional area of leaves was maintained across the gradient in amplitude through changes in fractional porosity despite decreasing total cross-sectional area. As a result of these adaptations in gas space anatomy, specific resistance in T. domingensis decreased with decreasing elevation but was unaffected by amplitude. In contrast, no unequivocal anatomical modifications were detected in P. australis and the specific resistance was constant across treatments.Estimates of the total resistance to convection in the above-ground parts of the plant suggests the adaptations in the gas space anatomy by T. domingensis largely compensated for increases in the pathlength of convection due to decreasing elevation. Total resistance increased significantly with increasing amplitude. P. australis had a low specific resistance which minimised the effect of increasing amplitude on total resistance but, lacking the ability to adapt gas space anatomy, the total resistance was strongly affected by elevation. The differential impact of elevation and amplitude suggests the two species have contrasting aptitudes for aeration across gradients in elevation and amplitude. T. domingensis is better suited to tolerate a range of static water depths whereas P. australis is better suited to fluctuating water levels especially when growing at high elevations.