Height-related changes in xylem hydraulics and anatomy of leader shoots of Picea abies trees

In trees, water is conducted from roots to leaves under tension. Increasing tree height constrains water transport in terms of both safety and efficiency. From one hand, the gravitational pressure drop imposes an extra tensile strength to the xylem water equal to -0.01 MPa every m of height, with potential negative consequences due to higher risks of embolism formation. On the other hand, the total root-to-leaves path length increases with increasing tree height, with potential negative effects on water flow due to the increased frictional forces. However, it is still unknown how these requirements of hydraulic safety and efficiency are managed in taller trees in order to avoid critical water potentials and/or the risk of embolism formation. We assessed the vulnerability curves (VCs) and measured xylem anatomical traits of the leader shoots of Picea abies varying from 2 to 37 m of height, in two different sites in the Dolomites (Italian Eastern Alps). Both xylem hydraulics and anatomy changed with tree height. The xylem water potential triggering 50% of loss of conductivity (P50) significantly increased from small (-5.86 MPa) in to tall trees (-3.40 MPa) (P=0.007); the total tracheid number (N) and the tracheid hydraulic diameter (Dh) significantly increased with tree height (P=0.005 and P=0.007 respectively), with Dh varying from 10.92 in small to 14.88 μm in tall trees. We found a strong trade off between efficiency vs. safety, with Kh (total xylem conductivity) significantly varying with P50 (R2=0.60, P<0.001). Taller trees prioritize xylem efficiency vs. safety, with the xylem becoming more vulnerable to cavitation with increasing tree height. These results could represent a mechanistic explanation of why taller trees are commonly more prone to top dieback and vigour decline compared to smaller ones. Therefore, we suppose that these phenomena will more frequently occur with the forecasted climate change scenarios of increasing occurrence of drought events.