Abstract: Leaf nitrogen (N) and phosphorus (P) levels provide critical strategies for plant adaptions to changing environments. However, it is unclear whether leaf N and P levels of different plant functional groups (e.g., monocots and dicots) respond to environmental gradients in a generalizable pattern. Here, we used a global database of leaf N and P to determine whether monocots and dicots might have evolved contrasting strategies to balance N and P in response to changes in climate and soil nutrient availability. Specifically, we characterized global patterns of leaf N, P and N/P ratio in monocots and dicots, and explored the sensitivity of stoichiometry to environment factors in these plants. Our results indicate that leaf N and P levels responded to environmental factors differently in monocots than in dicots. In dicots, variations of leaf N, P and N/P ratio were significantly correlated to temperature and precipitation. In monocots, leaf N/P ratio was not significantly affected by temperature or precipitation. This indicates that leaf N, P and N/P ratio are less sensitive to environmental dynamics in monocots. We also found that in both monocots and dicots N/P ratios are associated with the availability of soil total P rather than soil total N, indicating that P limitation on plant growth is pervasive globally. In addition, there were significant phylogenetic signals for leaf N (λ = 0.65), P (λ = 0.57) and N/P ratio (λ = 0.46) in dicots, however, only significant phylogenetic signals for leaf P in monocots. Taken together, our findings indicate that monocots exhibit a “conservative” strategy (high stoichiometric homeostasis and weak phylogenetic signals in stoichiometry) to maintain their growth in stressful conditions with lower water and soil nutrients. In contrast, dicots exhibit lower stoichiometric homeostasis in changing environments because of their wide climate-soil niches and significant phylogenetic signals in stoichiometry.

Key words: Adaptation and evolution, Ecological stoichiometry, Environmental gradients, Global scale, Niche hypervolume, Plant functional groups