The extent to which hydraulic limitation of tree height can be applied as a general constraint to growth is incompletely understood. To test the hydraulic limitation theory’s regional applicability, four mature White Pine trees (Pinus strobus) in Northern Minnesota, approaching the regional species-specific height maximum, were analyzed for the presence of three water stress signatures with height:  water potential gradients, carbon isotope (¹³C ) discrimination, and changes in leaf anatomy.   

Results/Conclusions

Midday water potentials decreased with height, reaching pressures approaching -2.0 MPa at the top of each canopy during midday.  The water potentials measured may be sufficiently negative to induce stomatal closure and inhibit photosynthesis in the upper fraction of the canopies. Preliminary carbon isotope analysis results showed decreasing ¹³C discrimination with height, indicative of stress-induced stomatal closure over longer time scales.  Predawn water potentials also decreased with height, suggesting potentially reduced turgor pressure available for leaf expansion and cell division. Decreased turgor pressure may result in measurable leaf anatomical variation with height.  The results are consistent with the hydraulic limitation hypothesis and suggest that the regional asymptotic maximum height of Pinus strobus may be strongly constrained by water relations.