The role of tree size, wood anatomical and leaf stomatal traits in shaping tree hydraulic efficiency and safety in a South Asian tropical moist forest

Efficient water transport through xylem determines tree performance particularly when trees are under water stress. Yet, the hydraulic behaviour of the tropical moist forest of Bangladesh has not been adequately understood. The aim of this study was to evaluate the hydraulic conductivity and hydraulic vulnerability of tree species from a nature reserve of Bangladesh (The Rema-Kalenga Wildlife Sanctuary, RKWS) and to assess their relationship with tree size, wood anatomical and leaf stomatal traits. Increment cores and leaf samples were collected from a total of 325 individual trees representing 56 species distributed in 55 randomly selected sampling plots. Following a standard protocol, xylem anatomical parameters such as vessel radial diameter (RD), vessel tangential diameter (TD), and vessel density (VD) were measured. Stomatal density (SD) and stomatal guard cell length (SGCL) were measured using the light microscope from the abaxial surfaces of leaves. Potential hydraulic conductivity (Kp) and vulnerability index (Vx) were determined from the vessel measurements. Demographic variables were measured directly in the field. Results showed that both Kp and Vx varied among the species. We found a significant differences in Kp (F=11.695, P < 0.001) and Vx (F=3.6045, P < 0.05) among the trees of different phenological groups- deciduous, evergreen, and brevi-deciduous. The independent sample t-test showed a significantly higher Vx in light demanding species than in shade tolerant species (t = 3.85, p < 0.001). However, there was no significant difference in (Kp) between species group with different shade tolerance which is consistent with the results derived from nested ANOVA testing the relative importance of each functional group. Kp was positively related with SD, crown surface area (CSA), tree height, and DBH and negatively related with wood density (WD). However, we interpret the Kp-WD relationship with caution since a few species with low density had more influence on this relationship. Our study demonstrates that plant functional type is an important factor in regulating hydraulic efficiency and safety in moist tropical forests.