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Increasing aridity is one of the most important trends of current climate change. Leaf functional traits suggest a substantial basis for assessing the aridity effects on vegetation. However, since plants possess diverse leaf morphology and anatomy due to different evolutionary history of taxa, the effect of aridity can hardly be revealed in a multispecies analysis. We studied leaf functional traits for 317 samples of 193 plant species in steppe and desert communities along a 1600-km latitudinal gradient in Southern Siberia (Transbaikalia, Russia) and Mongolia. We determined morphological leaf traits, quantitative anatomical parameters, physiological parameters, and photosynthetic pigments content. Different relevance of leaf traits for indication of plant response to climate has been demonstrated. The clearest changes in site-mean values along the aridity gradient were shown for leaf thickness, total chloroplast number per leaf area (N-chl/A) and total surface area of chloroplasts (A(chl)/A) and cells (A(mes)/A) per leaf area. Unlike leaf size and leaf mass per area, these quantitative mesophyll parameters related to plant photosynthetic capacity were strongly correlated with climate. We found no evidence for a decrease in sizes of mesophyll cells with aridity, but cell volume as well as chloroplast number per cell were linked with plant functional type (PFT). We revealed an increase in N-chl/A and A(chl)/A in desert-steppe species in comparison to steppe and forest-steppe vegetation types within each PFT of C-3-plants (C-3-dicot herbs, C-3-dicot shrubs, C-3-monocots and C-3-succulents). C-4-plants were generally characterized by low A(chl)/A and A(mes)/A, but had higher rate of CO2-transfer through mesophyll and chloroplast surfaces. C-3- and C-4-plants differed in response to aridity and showed opposite trends in changes of leaf traits along the aridity gradient. We conclude that leaf mesophyll traits contribute to important mechanism of climatic adaptation in different PFTs along a large latitudinal gradient.