Modelling approach to the analysis of pressure-flow in Phragmites stands

Field studies have shown that resistance to convective gas-flow within the culm-rhizome gas-space system of Phragmites tends to be higher in 'die-back ' as opposed to healthy stands. However, the collection of reliable data an d its interpretation is complicated by many factors. To help interpret fiel d measurements and further our understanding of convective flows in general , we have developed mathematical models based on the humidity-induced conve ctive-flow generating potential of culms. Among other things, these make it possible to study the effects on pressure flows of increasing flow resista nces in rhizomes and between culms, of different pressure-generating potent ials of interlinked culms and of different numbers of efflux culms. A multi -culm and rhizome model is described together with some simple examples of the way it can prove helpful in interpreting some of the field observations from healthy and die-back sites. Increased venting resistance reduced flows curvilinearly; increasing the co unterpressure to mimic those induced by other interlinked living culms redu ced flows linearly. In some of the multi-culm examples shown counter-pressu re exerted an effect approximately the same as that of rhizome and venting resistance, but as culm numbers declined it assumed even greater importance . The value of conductivity derived from applying pressures to the stubble of excised culms and measuring the flows induced, proved to be a composite measure of the effective conductivity of the whole rhizome-culm train rathe r than that of the rhizome plus major vent. The expression 1 - (P-d/P-s) - the delivery coefficient - where Pd is the dynamic pressure in the base of the intact culm, and P-s the static pressure developed by the culm with the outflow blocked, was identified as a useful and easily obtained measure of the ability of an individual intact culm to contribute to the convective g as-flow in a stand, and one which should be relatively unaffected by weathe r conditions. For identical interlinked culms it was shown that the flows f rom all culms fitted exactly along the same line of declining flow versus d ynamic pressure irrespective of the numbers of culms or number and position of outflow vents. (C) 2001 Elsevier Science B.V. All rights reserved.