Habitat differentiation and environmental adaptability contribute to leaf size variations globally in C₃ and C₄...

The grass family (Poaceae) dominates ~43 % of Earth's land area and contributes 33 % of terrestrial primary productivity that is critical to naturally regulating atmosphere CO₂ concentration and global climate change. Currently grasses comprise ~11,780 species and ~50 % of them (~6000 species) utilize C₄ photosynthetic pathway. Generally, grass species have smaller leaves under colder and drier environments, but it is unclear whether the primary drivers of leaf size differ between C₃ and C₄ grasses on a global scale. Here, we analyzed 34 environmental variables, such as latitude, elevation, mean annual temperature, mean annual precipitation, and solar radiation etc., through a comparatively comprehensive database of ~3.0 million occurrence records from 1380 C₃ and 978 C₄ grass species (2358 species in total). Results from this study confirm that C₄ grasses have occupied habitats with lower latitudes and elevations, characterized by warmer, sunnier, drier and less fertile environmental conditions. Grass leaf size correlates positively with mean annual temperature and precipitation as expected. Our results also demonstrate that the mean temperature of the wettest quarter of the year is the primary control for C₃ leaf size, whereas C₄ leaf size is negatively correlated with the difference between summer and winter temperatures. For C₄ grasses, phylogeny exerts a significant effect on leaf size but is less important than environmental factors. Our findings highlight the importance of evolutionarily contrasting variations in leaf size between C₃ and C₄ grasses for shaping their geographical distribution and habitat suitability at the global scale.