The association of leaf size with climate is one of the most striking global ecological patterns. This pattern is well known for eudicotyledonous plants: species with smaller leaves are more common in drier or colder climates. Two dominant hypotheses for the functional advantages of smaller leaves might explain this global pattern. The thinner boundary layer of smaller leaves facilitates coupling of leaf and air temperature, preventing heat and chilling damage and facilitating high gas exchange. Further, smaller leaves are developmentally constrained to have denser and narrower major veins, traits that provide cold and drought tolerance and also permit high gas exchange. Yet, this pattern and the developmental and functional mechanisms underlying it have not been tested in grasses, an exceptionally important lineage of > 11,000 species, distributed across climates globally, that dominate > 40% of the terrestrial surface, and from which the bulk of crop production is derived. We tested for a worldwide association of leaf size with climate for grasses, using a global database for 1752 species from 373 genera. Further, we tested for the developmental constraints and trait-based mechanisms that underlie this pattern in eudicots using a global database and a greenhouse common garden with 27 C3 and C4 species.
Results/Conclusions
Worldwide, grasses in colder and drier climates have shorter, narrower and overall smaller leaves. The scaling of leaf vein traits with leaf size was consistent with a novel synthetic grass leaf developmental model. Modeling showed that the biophysical advantage of stronger coupling with air temperature in small leaves in colder and/or more arid climates would be enabled and critically expanded by the benefits of their vein traits, i.e., the higher density of major veins and narrower major vein diameters in smaller leaves. The worldwide distribution of grass leaf size exemplifies how a major macro-ecological pattern can arise from differential climate adaptation of a conserved developmental process.