Developmental and biophysical determinants of grass leaf size worldwide.

One of the most notable ecological trends-described more than 2,300  years ago by Theophrastus-is the association of small leaves with dry and cold climates, which has recently been recognized for eudicotyledonous plants at a global scale1-3. For eudicotyledons, this pattern has been attributed to the fact that small leaves have a thinner boundary layer that helps to avoid extreme leaf temperatures4 and their leaf development results in vein traits that improve water transport under cold or dry climates5,6. However, the global distribution of leaf size and its adaptive basis have not been tested in the grasses, which represent a diverse lineage that is distinct in leaf morphology and that contributes 33% of 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 leaf size in grasses 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.

C4 photosynthesis evolved in warm climates but promoted migration to cooler ones.

C4 photosynthesis is considered an adaptation to warm climates, where its functional benefits are greatest and C4 plants achieve their highest diversity and dominance. However, whether inherent physiological barriers impede the persistence of C4 species in cool environments remains debated. Here, we use large grass phylogenetic and geographical distribution data sets to test whether (1) temperature influences the rate of C4 origins, (2) photosynthetic types affect the rate of migration among climatic zones, and (3) C4 evolution changes the breadth of the temperature niche. Our analyses show that C4 photosynthesis in grasses originated in tropical climates, and that C3 grasses were more likely to colonise cold climates. However, migration rates among tropical and temperate climates were higher in C4 grasses. Therefore, while the origins of C4 photosynthesis were concentrated in tropical climates, its physiological benefits across a broad temperature range expanded the niche into warmer climates and enabled diversification into cooler environments.