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 (DeltaPsi(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 ((Ψ) over tilde (L) = Psi(L) - rho(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, (Ψ) over tilde (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 (Ψ) over tilde (L) was maintained above a minimum threshold value of -2.0 MPa independently of tree height, thus limiting the range of compensatory change in DeltaPsi(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 growth-limiting responses (e.g. soil nutrient sequestration, increased respiratory costs) cannot be excluded.