Reduction in leaf size at higher altitudes across 39 broad-leaved herbaceous species on the northeastern Qinghai-Tibetan Plateau

Leaf size varies conspicuously within and among species under different environments. However, it is unclear how leaf size would change with elevation, whether there is a general elevational pattern, and what determines the altitudinal variation of leaf size. We thus aimed to address these questions by focusing on the broad-leaved herbaceous species at high altitudes on the northeastern Qinghai-Tibetan Plateau. We measured the leaf size, leaf length, leaf width and leaf mass per area for 39 broad-leaved herbaceous species inhabited in the open areas along two mountain slopes from 3200 to 4400 m at the Lenglongling and the Daban Mountain, the northeastern Qinghai-Tibetan Plateau. We analyzed the altitudinal patterns in leaf size in relation to leaf inclination and leaf surface features, and applied a leaf energy balance model to discuss the underlying mechanisms. Leaf size decreased significantly at higher altitudes. The altitudinal reduction of leaf size was mainly attributed to the reduction of leaf length, and differed in different species, and in leaves with different inclination and leaf surface features. A leaf energy balance model with local environmental measurements demonstrates that leaf temperature tracks air temperature more closely in small than in large leaves, and that the leaf-size impact is stronger at higher latitudes. Based on the observational findings, we propose that the distribution upper-limit for broad-leaved herbaceous species would be at an elevation of about 5400 m on the northeastern Qinghai-Tibetan Plateau.