Weak co-ordination between vein and stomatal densities in 105 angiosperm tree species along altitudinal gradients in Southwest China

Leaf-level water balance, as revealed by a correlation between stomatal density (SD) and vein density (VD), has been reported in some plants. However, the generality of this correlation and how it may be affected by altitude changes are unclear. Here, we investigated whether this balance is maintained across tree species of diverse families along a large altitudinal gradient. We measured leaf area (LA), SD, stomata length (SL), and VD in 105 angiosperm species across two altitudinal ranges, 800-1400 m above sea level (a.s.l.) in tropical montane forests (TMF) and 2000-2600 m a.s.l. in subtropical montane forests (SMF) in Yunnan, South-west China. The average SD was independent of altitude in both regions. Similarly, the average VD within either SMF or TMF was also not significantly different. However, overall, TMF had significantly larger VD and LA but smaller SL than SMF. Vein density was positively correlated with SD across SMF species, with a weaker correlation for TMF species and all species combined. Stomatal length was negatively correlated with SD and VD across all species. Our results extend the leaf water balance theory to diverse angiosperm tree species, and indicate decoupled adaptation of SD and VD in these species along a large altitudinal gradient.