Leaves intercept light and acquire carbon, while stems transport water with nutrients and mechanically support the leaves, thus leaf and stem traits are expected to be highly coordinated biomechanically and physiologically. However, it is still unclear how xylem hydraulic and mechanical properties varied with stem/leaf dimensions, and if these changes could shape the stem–leaf size relationships. 

Researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and Wageningen University conducted a study to test the hypothesis that there is a trade-off between hydraulic and mechanical function of stems, and the trade-off has compensated effect on the twig–leaf size allometric relationship. 

The researchers measured leaf and stem architectures – including stem area/mass, petiole area/mass, lamina area/mass, leaf number, specific leaf area, and mass-based leafing intensity – on the current-year branches for 28 Ficus species growing in XTBG. They also measured leaf anatomical traits, stem wood density, and stem anatomical and mechanical properties of these species. 

They analyzed the allometric relationships among those traits, both across species and across phylogenetically independent contrasts. 

They found that leaf–stem size followed an isometric relationship across 28 Ficus species in XTBG, indicating that the increase of total or individual leaf area/mass was generally proportional to the increase of stem area/mass.  

Leaf size scaled negatively and isometrically with mass-based leafing intensity (LI, i.e. number of leaves per unit of stem mass). 

Large leaves tend to have a larger fractional biomass investment in petioles. Larger stems have wider vessel lumens in the xylem and achieve higher stem specific hydraulic conductivity, but the function of mechanical strength is diminished with less dense wood. 

The results suggested that the trade-off between hydraulic efficiency and mechanical strength in xylem may shape the leaf–stem allometric relationships. 

The study entitled "Tradeoff between Stem Hydraulic Efficiency and Mechanical Strength Affects Leaf–Stem Allometry in 28 Ficus Tree Species" has been published in Frontiers in Plant Science.