Influence of soil porosity on water use in Pinus taeda

Abstract

We analyzed the hydraulic constraints imposed on water uptake from soils of different porosities in loblolly pine (Pinus taeda L.) by comparing genetically related and even-aged plantations growing in loam versus sand soil. Water use was evaluated relative to the maximum transpiration rate (E ) allowed by the soil-leaf continuum. We expected that trees on both soils would approach E during drought. Trees in sand, however, should face greater drought limitation because of steeply declining hydraulic conductivity in sand at high soil water potential (Ψ ). Transport considerations suggest that trees in sand should have higher root to leaf area ratios (A:A ), less negative leaf xylem pressure (Ψ), and be more vulnerable to xylem cavitation than trees in loam. The A :A was greater in sand versus loam (9.8 vs 1.7, respectively). This adjustment maintained about 86% of the water extraction potential for both soils. Trees in sand were more deeply rooted (>1.9 m) than in loam (95% of roots <0.2 m), allowing them to shift water uptake to deeper layers during drought and avoid hydraulic failure. Midday Ψ was constant for days of high evaporative demand, but was less negative in sand (-1.6 MPa) versus loam (-2.1 MPa). Xylem was more vulnerable to cavitation in sand versus loam trees. Roots in both soils were more vulnerable than stems, and experienced the greatest predicted loss of conductivity during drought. Trees on both soils approached E during drought, but at much higher Ψ in sand (<-0.4 MPa) than in loam (<-1.0 MPa). Results suggest considerable phenotypic plasticity in water use traits for P. taeda which are adaptive to differences in soil porosity.

Hydraulic architecture of trees: main concepts and results

“…It is worth noting, however, that in this model, the radial resistance of roots is not taken into account. Hacke et al [52] argued that because this model gives "a reasonable fit to the seasonal pattern of transpiration and water potential data, ..., by implication [it] suggests that changes in hydraulic conductance caused by processes not modeled, such as changes in radial conductance of roots, ..., were less important to the overall continuum conductance than changes modeled in soil and xylem". However, compensation effects of processes not taken into account in this model could lead to the same conclusion.…”

Testing an optimality-based model of rooting zone water storage capacity in temperate forests

“…7) are pine stands growing on sandy soils. Pinus roots often show a high degree of adaptation to soil conditions (Hacke et al, 2000;Kutschera and Lichtenegger, 2002 to be closer to the calibrated value. However, both !…”

Does overshoot in leaf development of ponderosa pine in wet years leads to bark beetle outbreaks on fine-textured soils in drier years?

“…Therefore, it may be that the amount of rain or snowmelt that actually reached the available root-zone water supply during drier periods was lower at site I than site II, because of differences in percolation rates between coarse and fine-textured soils. Water in coarse soils is acted upon only by gravitational forces, and therefore it travels easily through the soil profile and is readily extracted by tree roots (Hacke et al 2000). Conversely, water in clayey soils is held by capillary forces of cohesion and adhesion, which make drainage to deeper soil layers slower and uptake by tree roots more difficult.…”