Responses of the functional traits in Cleistogenes squarrosa to nitrogen addition and drought

Aims Plant functional traits have been widely used to study the responses of plant to environmental change. Cleistogenes squarrosa is an important C₄ species in Nei Mongol grassland. How its functional traits would respond to varied nitrogen and water conditions have rarely been studied. Our subject was to examine the responses of the whole-plant traits and leaf morphological and physiological traits to nitrogen addition and drought in this species.
Methods We conducted a pot experiment with a gradient of N addition (0, 10.5, 35.0, and 56.0 g·m^–2·a^–1) and water treatments (natural precipitation vs. 70% of mean monthly precipitation) in 2013. The whole-plant traits (e.g., root depth and stem-leaf biomass ratio), leaf morphological (e.g., leaf area and specific leaf area) and physiological traits (e.g., photosynthetic rate, water use efficiency, and leaf N content) were investigated.
Important findings N addition had a significant effect on the whole-plant traits in C. squarrosa. The effects of N addition, water treatments, and an interaction between N addition and water treatments on leaf morphological and physiological traits were highly significant in most cases. The patterns of functional traits in response to N addition differed between plants under natural precipitation and with reduced mean monthly precipitation. Root depth, stem biomass, and stem-leaf biomass ratio were increased in treatments with low and intermediate N additions under reduced precipitation, but not changed under natural precipitation. Specific leaf area increased along the N addition gradient under drought, but did not change under natural precipitation. High N addition stimulated photosynthetic rate and transpiration rate and increased water use efficiency under natural precipitation, but had no effect on under reduced precipitation. Leaf N content on area basis increased slightly with the increases in N addition under natural precipitation, but decreased significantly under reduced precipitation. N addition influenced mainly the leaf morphological and physiological traits under natural precipitation and the whole-plant traits and leaf morphological traits under reduced precipitation. In conclusion, our results indicate that the functional traits in C. squarrosa respond to N addition and the patterns of responses differ under different water conditions, reflecting the adaptation to changes in N and water availability.