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 geneti cally related and even-aged plantations growing in loam versus sand soil. W ater use was evaluated relative to the maximum transpiration rate (E-crit) allowed by the soil-leaf continuum. We expected that trees on both soils wo uld approach E-crit during drought. Trees in sand, however, should face gre ater drought limitation because of steeply declining hydraulic conductivity in sand at high soil water potential (Psi(S)). Transport considerations su ggest that trees in sand should have higher root to leaf area ratios (A(R): A(L)): less negative leaf xylem pressure (Psi(L)), and be more vulnerable t o xylem cavitation than trees in loam. The A(R):A(L) was greater in sand ve rsus 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 d eeply 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 fail ure. Midday Psi(L) was constant for days of high evaporative demand, but wa s less negative in sand (-1.6 MPa) versus loam (-2.1 MPa). Xylem was more v ulnerable 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-crit during drought, but at much higher Psi(S) in sand (<-0.4 MPa) than in loam (<-1.0 MPa). Results suggest considerable phenotypic plasticity in water use trait s for P. taeda which are adaptive to differences in soil porosity.