Insights into the relationship between hydraulic safety, hydraulic efficiency and tree structural complexity from terrestrial laser scanning and fractal analysis

The potential of trees to adapt to drier and hotter climates will determine the future state of forests in the wake of a changing climate. Attributes connected to trees’ hydraulic network are likely to determine their ability to endure drought. However, how a tree's architectural attributes relate to its drought tolerance remains understudied. We set out to quantify the relationship between tree structural complexity and drought tolerance. We used terrestrial laser scanning (TLS) to scan 71 trees of 18 species and generated 3D attributes of each tree. We constructed quantitative structure models (QSMs) to characterize the branching patterns of all study trees.

Additionally, the box-dimension approach from fractal analysis was used to assess overall structural complexity of the trees. Three measures of xylem safety, i.e., the water potential at 12%, 50%, and 88% loss of hydraulic conductance (P₁₂, P₅₀, P₈₈), were measured to characterize drought tolerance of the trees’ hydraulic systems, completed by data on specific hydraulic conductivity (K_s). Our findings revealed a significant relationship between the structural complexity (D_b) and the three measures of xylem safety as well as K_s. Tree species with low structural complexity developed embolism-resistant xylem at the cost of hydraulic efficiency. Our findings also revealed that the D_b had a more pronounced and significant relationship with branch hydraulic safety and efficiency than all other tested structural attributes. Our findings indicate that the D_b is a robust and easy-to-measure descriptor of tree architecture that relates to important branch hydraulic properties of a tree.