Pattern of leaf vein density and climate relationship of Quercus variabilis populations remains unchanged with environmental changes

The leaf vein traits of plants result partially from adaptations to environmental factors during the long-term evolution. However, no general conclusion on the variation trend of the vein density along a climate gradient or the response of such vein density–climate relationship pattern to climate changes has been made. We examined the variations in leaf vein density and other leaf traits of oriental oak (Quercus variabilis) in 10 in situ populations (in situ populations) across temperate-subtropical biomes and the response of the leaf vein density to environmental changes in 7 populations grown in a common garden (garden populations). The results show that the minor vein density of the in situ populations (≥3rd order) significantly decreased with increasing latitude (r ²  = 0.44 and P = 0.04). This pattern remained unchanged for the garden populations (r ²  = 0.67 and P = 0.02). The minor vein densities of both the garden and in situ populations were positively correlated to the mean annual temperature (MAT) of the origins (r ²  = 0.66 and P = 0.03 for the garden populations; r ²  = 0.37 and P = 0.06 for the in situ populations), but their correlation to the mean annual precipitation (MAP) of the origins was not significant. Compared with the MAT and MAP, the vein density displayed a significantly lower correlation to climate variables in the current year or the current-year growing seasons. For the garden populations, the minor vein density significantly increased with leaf dry mass per area and decreased with petiole length and leaf length. These results imply that leaf vein density is genotypically fixed and is therefore not responsive to temporal changes in the growing conditions.