Tree growth is correlated with hydraulic efficiency and safety across 22 tree species in a subtropical karst forest

A karst forest is a habitat where access to soil water can be challenging for plants. Thus, safe and efficient xylem water transport and large internal water storage may benefit tree growth. In this study, we selected 22 tree species from a primary subtropical karst forest in southern China and measured their xylem anatomical traits, saturated water content, hydraulic conductivity, and embolism resistance. Additionally, we monitored diameter growth in 440 individual trees of various sizes over three consecutive years. Our objective was to analyze the relationships among xylem structure, hydraulic efficiency, safety, water storage, and growth of karst tree species. The results showed structural but no significant hydraulic trait differences between deciduous and evergreen species. Large vessel diameter, paratracheal parenchyma, and high saturated water content correlated with high xylem hydraulic conductivity. Embolism resistance was not correlated with studied anatomical traits, and no trade-off with hydraulic conductivity was observed. In small trees (5-15 cm diameter at breast height), diameter growth rate was independent of hydraulic traits. In large trees (> 15 cm diameter at breast height), high hydraulic conductivity and high embolism resistance accounted for high diameter growth rate. From lower to greater embolism resistance, the size-growth relationship shifted from growth deceleration to acceleration with increasing tree size in eight of the 22 species. Our study highlights the vital contributions of xylem hydraulic efficiency and safety to growth rate and dynamics in karst tree species; therefore, we strongly recommend their integration into trait-based forest dynamic models.