Vertical stratification of soil water storage and release dynamics in Pacific Northwest coniferous forests

We characterized vertical variation in the seasonal release of stored soil moisture in old-growth ponderosa pine (OG-PP, xeric), and young and old-growth Douglas-fir (Y-DF, OG-DF, mesic) forests to evaluate changes in water availability for root uptake. Soil water potential (ψ) and volumetric water content (θ) were measured concurrently at 10 cm intervals to 1 m depth to create in situ soil water retention curves (SWRC) under drying conditions. Non-linear regression was used to fit SWRC specific to each depth and site. We also quantified root biomass, soil texture, and hydraulic redistribution (HR) of soil water by roots to identify factors affecting the seasonal dynamics of root water uptake and depletion from the soil profile. Soil θ measured at a particular ψ increased with soil depth, and was strongly dependent upon soil texture. For example, when ψ was -0.1 MPa, θ ranged from 13% at 20 cm to 35% at 100 cm for the OG-DF forest. Soil texture and bulk density accounted for 60-90% of the variation in the SWRC. As the summer drought progressed, water extraction shifted to the deeper layers, and recharge from HR approached 0.15 mm day-1 in the upper 60 cm for all sites. Total water use from the upper 2 m at all sites peaked between 1.5-2.5 mm day-1 in mid-July and then declined to 0.5-1.0 mm day-1 by the end of the dry season. Total fine root biomass in the upper 1 m was 0.77 kg m-2 (OG-PP), 1.08 kg m-2 (OG-DF) and 1.15 kg m-2 (Y-DF), with 40% (PP) to 60% (DF) of fine roots located in the upper 20 cm. However, the upper 20 cm only accounted for 20% of total water depletion from the upper 2 m at peak water uptake, declining to 4-6% later in the season, illustrating the contribution of deeper roots to water uptake. Nevertheless, daily water uptake from the entire 2 m profile was strongly dependent on water potential at 20 cm, indicating that fine roots in the upper soil may play an important role in regulating water uptake through hydraulic effects on stomatal conductance.

Duke Scholars

Author Jean Christophe Domec Environmental Sciences and Policy