Influence of soil porosity on water use in Pinus taeda.

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.