One of the most striking ecological trends is the association of small leaves with dry and cold climates, described 2400 years ago by Theophrastus, and recently recognized for eudicotyledonous plants at the global scale 1-3 . For eudicotyledons, this pattern is attributed 24 to small leaves having a thinner boundary layer to avoid extreme leaf temperatures 4 , and 25 their developing vein traits that improve water transport under cold or dry climates 5,6 . Yet, 26 the global distribution of leaf size and its mechanisms have not been tested in grasses, an 27 extraordinarily diverse lineage, distinct in leaf morphology, which contributes 33% of 28 terrestrial primary productivity, including the bulk of crop production 7 . Here we demonstrate that grasses have shorter and narrower leaves under colder and drier climates worldwide. We show that small grass leaves have thermal advantages and vein development that contrast with those of eudicotyledons, but that also explain the abundance of small leaves in cold and dry climates. The worldwide distribution of grass leaf size exemplifies how biophysical and developmental processes result in convergence across major lineages in adaptation to climate globally, and highlights the importance of leaf size and venation architecture for grass performance in past, present and future ecosystems. Data Fig. 1, SupplementaryTable 3). We tested whether developmental scaling would confer 64 small leaves with potential climatic advantages. 65 4 66 Box 1. Synthetic model of grass leaf vein development based on published data for 20 species (Supplementary Tables 5-6), conferring small leaves with traits advantageous under cold and dry climates Grass leaf development includes five phases based on developmental zones: Phase P (formation and expansion of the primordium, P): "Founder cells" in the periphery of the shoot apical meristem generate the leaf primordium. Cell divisions drive growth of a hood-like structure, in which the central 1° vein (midvein) and the large 2° veins are initiated early and extend acropetally, enabling their prolonged diameter growth (Box 1 Fig. 1a, c, e). Henceforth, discrete spatial growth zones develop at the leaf base and drive leaf expansion laterally and longitudinally. Phase D (formation of the cell division zone, DZ):The basal cell division zone (DZ) expands slightly, driving minimal growth (Box 1 Fig. 1a, b). The 1° and 2° vein orders (major veins) complete their patterning basipetally along the leaf blade and increase in diameter (Box 1 Fig. 1c, e). Meanwhile, beginning at the lamina tip, C 3 species form a single order of small longitudinal minor veins, i.e., 3° veins, as do most C 4 species, i.e., C 4-3L species. Some C 4 species of the subfamily Panicoideae additionally form smaller 4° veins, i.e., C 4-4L species 15 (Box 1 Fig. 1c). Phase D-E (DZ, and formation of the expansion zone, EZ):Cells from the DZ transition to a distinct, distal expansion zone (EZ).In the EZ, cell expansion in width and length spaces apart the 1° and 2° veins, resulting in th...

Multi-dimensional trait mechanisms underlying community assembly at regional scales are largely unclear. In this study, we measured leaf economic, hydraulic and anatomical traits of 394 tree species from tropical to cold temperate forests, from which we calculated the leaf trait moments (mean, variance, skewness, and kurtosis) using community-weighted methods. Economic and hydraulic traits were decoupled at the species level, but coupled at the community level, and relationships between leaf traits in observed communities were stronger than that in null communities, suggesting that the adaptive mechanisms of plant species may be different. Furthermore, leaf economic traits were distributed more evenly across species occupying communities with lower temperature and precipitation, whereas hydraulic traits were distributed more evenly under lower water availability. This suggests that limiting similarity of specific leaf traits within communities would be enhanced when related-resources are limited, and highlights the independent assembly of leaf economics and hydraulic traits in terms of functional evenness. Importantly, the moments of leaf economic and hydraulic traits of observed communities explained more variation in ecosystem productivity than that of null communities, indicating ecosystem productivity depended on traitbased community assembly. Our results highlight the principles of community assembly regarding multi-dimensionsional traits in natural forests at a regional scale.

Developmental phenotypic plasticity can allow plants to buffer the effects of abiotic and biotic environmental stressors. Therefore, it is vital to improve our understanding of how phenotypic plasticity in ecological functional traits is coordinated with variation in physiological performance in plants. To identify coordinated leaf responses to low-water (LW) versus low-light (LL) availability, we measured leaf mass per area (LMA), leaf anatomical characteristics and leaf gas exchange of juvenile Populus tremuloides Michx. trees. Spongy mesophyll tissue surface area (Asmes/A) was correlated with intrinsic water-use efficiency (WUEi: photosynthesis, (Aarea)/stomatal conductance (gs)). Under LW availability, these changes occurred at the cost of greater leaf tissue density and reduced expansive growth, as leaves were denser but were only 20% the final area of control leaves, resulting in elevated LMA and elevated WUEi. Low light resulted in reduced palisade mesophyll surface area (Apmes/A) while spongy mesophyll surface area was maintained (Asmes/A), with no changes to WUEi. These leaf morphological changes may be a plastic strategy to increase laminar light capture while maintaining WUEi. With reduced density and thickness, however, leaves were 50% the area of control leaves, ultimately resulting in reduced LMA. Our results illustrate that P. tremuloides saplings partially maintain physiological function in response to water and light limitation by inducing developmental plasticity in LMA with underlying anatomical changes. We discuss additional implications of these results in the context of developmental plasticity, growth trade-offs and the ecological impacts of climate change.