3.5 AI drives the hydraulic function by affecting xylem anatomical traits
The value of P 50 root - branch decreased significantly with AI (r 2 = 0.14, P< 0.001; Figure 4a) and varied significantly (one-way ANOVA,F = 18.5, P < 0.001) across four biomes (tropical seasonal forest, temperate seasonal forest, woodland/shrubland, and desert). On average, the species from desert had the highest P 50 root - branch (3.06 MPa), while, species from tropical seasonal forests had the lowest (0.87 MPa). Notably, the variation in P 50 root - branchincreased from the most humid biome (i.e., tropical seasonal forests) to driest (i.e., desert). The trend lines fitted between AI and theanatomical traits of branches and roots had significantly different slopes and intercepts (Figure 4b, 4c). There was a significantly positive correlation between D h and AI in branches (r 2 = 0.12, P < 0.01; Figure 4c), while the correlation between D h and AI in roots was not significant (r 2 = 0.00,P > 0.05).