Air-conducting porosity in slash pine roots from saturated soils
1990
Fisher, H.M. | Stone, E.L.
On many sites, slash pine taproot systems extend into zones of perennially saturated, anoxic soil. Tap and sinker roots, submerged for weeks or months by periodic rising of the water table, appear to be undamaged by anoxia. On such sites, green wood of large-diameter sinkers and a taproot had unusually high air contents (48% to 69% of green volume), a consequence of low wood substance and low moisture volumes, Green root sections conducted air longitudinally when small pressures (0.06 to 0.98 kPa) were applied at one end. Mass air flow through root wood increased linearly with the pressure applied and was approximately proportional to root cross-sectional area. Since sinker roots taper, root length influenced mass flow by affecting both pressure gradient and the conducting cross-section. Using Darcy's Law, mass air flow through each root was characterized by K, an air-permeability factor, which incorporates the unique anatomic or morphologic features of each root section. The average K for the population of green sinker and taproot sections was 68.6 cm2 kPa-1 min-1, which is the same magnitude of Darcy K estimates reported for mass air flow through dried pine stem wood. The findings suggest that vertical roots simultaneously conduct water up from, and air down to, submerged distal ends of the root system. The large gas-filled volume in wood likely serves as a pool for diffusive gas exchange between the atmosphere and O2-depleted tissues.
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