Response of liquid flow resistance to soil drying in seedlings of four deciduous angiosperms

Soil-leaf resistance to liquid water flow (R) in moist and drying soil was compared in three-month-old seedlings of two drought tolerant (white [Quercus alba L.], post oak [Q. stellata Wangenh.]) and two drought sensitive forest species (sugar maple [Acer saccharum Marsh.], black walnut [Juglans nigra L.]). At high soil moisture (Ψ_s≥-0.3 MPa), R was higher in J. nigra than in the other species, and as soil water was depleted R increased most in this species. In contrast, the lowest resistance at all levels of soil moisture was observed in Q. stellata. At Ψ_s of -1.5 MPa, R of drought-sensitive J. nigra and A. saccharum was about twice as high as that of the two drought-tolerant Quercus species. The difference in R between the two Quercus species was much smaller than that between this pair and the other two species. These differences among species in flow resistance may be attributable to: 1) variation in the balance between root surface area and leaf area, 2) variation in the inherent absorption capacity of the root systems and in xylem water conducting systems or 3) differences in root permeability, shrinkage and mortality in severely stressed seedlings. As the soil dried, seedlings of all species exhibited pronounced reductions in transpiration rate, which prevented development of large water potential gradients between leaves and the soil. Reduction in transpiration in J. nigra was especially pronounced, resulting in a decrease in the soil-to-leaf water potential gradient in dry soil despite high flow resistance. The observed differences among species in flow resistance are correlated with natural distribution patterns.