Hydraulic responses to height growth in maritime pine trees

As trees grow taller, decreased xylem path conductance imposes a major constraint on plant water and carbon balance, and is thus a key factor underlying forest productivity decline with age. The responses of stomatal conductance, leaf area: sapwood area ratio ( A L : A S ) and soil–leaf water potential gradient ( DY S–L ) to height growth were investigated in maritime pine trees. Extensive measurements of in situ sap flow, stomatal conductance and (non-gravitational) needle water potential ( L = Y L r w gh ) were made during 2 years in a chronosequence of four even-aged stands, under both wet and dry soil conditions. Under wet soil conditions, L was systematically lower in taller trees on account of differences in gravitational potential. In contrast, under dry soil conditions, our measurements clearly showed that L was maintained above a minimum threshold value of 2.0 MPa independently of tree height, thus limiting the range of compensatory change in DY S–L . Although a decrease in the A L : A S ratio occurred with tree height, this compensation was not sufficient to prevent a decline in leaf-specific hydraulic conductance, K L (50% lower in 30 m trees than in 10 m trees). An associated decline in stomatal conductance with tree height thus occurred to maintain a balance between water supply and demand. Both the increased investment in non-productive versus productive tissues ( A S : A L ) and stomatal closure may have contributed to the observed decrease in tree growth efficiency with increasing tree height (by a factor of three from smallest to tallest trees), although other growthlimiting responses (e.g. soil nutrient sequestration, increased respiratory costs) cannot be excluded. Key-words : Pinus pinaster ; hydraulic conductance; stomatal regulation; tree height; water potential.