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 highperched 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°06 0 N, 4°16 0 E). Roots unevenly colonized the soil down to 1.6 m: fine roots (diameter < 5 mm) were mostly distributed on the surface horizons (0-0.3 m) and around 1.3 m deep, despite dense clayey horizons appearing at 0.35 m 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.Abbreviations: EW -Extractable water (mm); FC -Field capacity; MT -Moisture tension (kPa); PETPotential evapotranspiration (mm); REW -Relative extractable water; W -Gravimetric soil water content (g g )1 ); WP -Wilting point; Y 0 -Predawn leaf water potential (kPa)

Evapotranspiration of a declining Quercus robur (L.) stand from 1999 to 2001. II. Daily actual evapotranspiration and soil water reserve

“…Soils are dystric Cambisol [10]. Those soils present severe signs of hydromorphy: a temporary ground water table is present from late fall to late spring [39]. The forest stand is dominated by Quercus robur L., with also Fraxinus excelsior L., Quercus rubra L., Betula sp., Acer pseudoplatanus L. The forest floor vegetation is constituted mainly by Circaea lutetiana L., Stachys sylvatica L., Carex pendula Huds., Athyrium filix femina (L.) Roth and Rubus fruticosus L., the latest being the most covering.…”

“…the sum of tree and understorey transpiration, soil evaporation and rain interception. This calculation was made in condition of no drainage (a poorly weathered schist stratum, located at 1.7-2.1 m depth, strongly reduces drainage and contributes to the formation of the water table during autumn and winter; [39]). …”

Mapping soil water holding capacity over large areas to predict potential production of forest stands

“…However, most studies have focused on the creation of PTFs to predict water hydraulic soil properties but only a few have evaluated the performance of such PTFs, particularly when they are used for vegetation modelling (Nemes et al, 2006). The validity of PTFs to predict the hydraulic properties of forested soils is thus poorly known, most samples used for their elaboration originating from cultivated soils, whose characteristics are different (Vincke and Delvaux, 2005). In this context, little research has focused on mapping the SWHC despite a clear interest for ecological studies and forest management (Romano and Santini, 1997;Orfanus and Mikulec, 2005).…”

Evapotranspiration of a declining Quercus robur (L.) stand from 1999 to 2001. I. Trees and forest floor daily transpiration

“…Those soils present severe signs of hydromorphy. Though the soil horizons are dense at depth > 0.45 m, roots were observed down to 1.6 m [47]. The soil water reserve was estimated to be 600 mm at field capacity by the use of soil moisture measurements (Thetaprobes, Delta-T, Cambridge, UK; data not shown).…”