Elucidating the effects of atmospheric nitrogen (N) deposition on the photosynthetic capacity of plants is critical to understand forest growth and conservation under global change. However, studies on this topic generally consider only understory N addition, which ignores the effect of canopy interception. In this study, we conducted a field experiment in a subtropical forest to compare the effects of canopy vs. understory N addition on the photosynthetic rate of canopy and understory species. We found that canopy N addition enhanced the photosynthetic rate of canopy species by increasing leaf hydraulic conductivity and shortening the distance of CO2 transportation. In contrast, understory N addition had non-significant effects on the photosynthetic rate of canopy species. Moreover, the photosynthetic rate of understory species was not affected by canopy or understory N addition. Interestingly, changes in hydraulic conductivity contributed more to accelerating the photosynthetic rate than changes in CO2 transport distance. Our results provide important insights into the dissimilar effects of canopy and understory N addition on the photosynthetic rates of species in subtropical forests. Based on our findings, we highlighted the urgent need to consider canopy processes in future studies on N deposition.

Leaf water storage capacity and osmotic strength are important traits enabling species to adapt to environments that are often moisture limited. However, whether these drought tolerance traits are correlated with the species diversification rate (DR) of plant lineages is yet to be determined. In this study, we selected a species‐rich genus (Primulina) of plants widely distributed in karst regions in which species frequently experience variable periods of drought. We measured water storage capacity‐related traits (including leaf thickness and water content per mass) and saturated osmotic strength in the leaves of 58 Primulina species growing in a common garden. Subsequently, using phylogenetic methods, we examined the relationships between the rate of species diversification and the drought tolerance traits and between the species DR and evolutionary rates of these traits. We found that neither water storage capacity nor saturated osmotic strength showed significant relationships with the rate of species diversification. However, the evolutionary rate of saturated osmotic strength showed a significant correlation with the species DR, although no comparable significant relationship was detected regarding the evolutionary rate of water storage capacity. Our study indicates that the diversification among Primulina species has typically been accompanied by an extensive divergence of leaf osmotic strength but not a divergence in leaf water storage capacity. These findings will enhance our current understanding of how drought tolerance influences the diversification of plant species in karst regions.

Diversification rates are critically important for understanding patterns of species richness among clades. However, the effects of climatic niche width on plant diversification rates remain to be elucidated. Based on the phylogenetic, climatic, and distributional information of angiosperms in China, a total of 26 906 species from 182 families were included in this study. We aimed to test relationships between diversification rate and climatic niche width and climatic niche width related variables (including climatic niche divergence, climatic niche position, geographic extent, and climatic niche evolutionary rate) using phylogenetic methods. We found that climatic niche divergence had the largest unique contribution to the diversification rate, while the unique effects of climatic niche width, climatic niche position, geographic extent, and climatic niche evolutionary rate on the diversification rate were negligible. We also observed that the relationship between diversification rate and climatic niche divergence was significantly stronger than the null assumption (artefactual relationship between diversification and clade-level climatic niche width by sampling more species). Our study supports the hypothesis that wider family climatic niche widths explain faster diversification rates through a higher climatic niche divergence rather than through higher geographic extent, higher climatic niche evolutionary rate, or separated climatic niche position. Hence, the results provide a potential explanation for large-scale diversity patterns within families of plants.