Porosity and available water of temporarily waterlogged soils in a Quercus robur (L.) declining stand

Abstract

Pedunculate oak (Quercus robur L.) is particularly sensitive to decline in clayey soils presenting a high-perched temporary water table. These soils induce two successive constraints in one-year cycle: water excess (and hypoxy) in winter and early spring, and water shortage in summer (water stress being more restrictive to oak). We determined the porosity and water properties of temporarily waterlogged clayey soils supporting forest stands of declining Quercus robur trees in a 101 yr-old oak stand in Belgium (50 degrees 06'N 4 degrees 16'E). Roots unevenly colonized the soil down to 1.6 in: fine roots (diameter < 5 mm) were mostly distributed on the surface horizons (0-0.3 in) and around 1.3 in deep, despite dense clayey horizons appearing at 0.35 in depth. Clay content below this depth reached 46-51%. Illite and vermiculite were the dominant clay minerals. The clayey horizons exhibited marked shrink-swell properties: bulk density at 30 kPa increased from 1.41 to 1.88 g cm(-3) from the surface to 2 m depth. There was also evidence of hypoxic conditions caused by water saturation of pore space in the rooting zone from October to mid-April. Extractable water (EW), calculated between moisture tensions of 5 and 1600 kPa was 152.8 mm. The level of perched temporary water table strongly depended on the seasonal rhythm of water uptake by trees and on the shrink-swell behaviour of soil.