Increased nutrient inputs can cause shifts in plant community composition and plant functional traits, both of which affect ecosystem function. However, community-level changes in functional structure may be due to either species abundance, or intraspecific plant trait changes, or both. Studies focusing only on ANPP or species abundance/diversity changes under novel environmental conditions may underestimate the importance of intraspecific trait changes. Further, species-level responses to environmental change are not uniform and certain species (or groups of species) may differ in their response. We studied community and species-level shifts in specific leaf area (SLA), chlorophyll content, plant height, leaf dry matter content (LDMC), leaf thickness, leaf toughness, in a full factorial NPK (nitrogen, phosphorus, potassium) fertilization experiment. Community weighted traits were compared using linear mixed-effects models to determine whether fertilization predicts the community weighted trait. We partitioned the changes in community weighted traits to intraspecific trait changes and species abundance changes. We used the same mixed-effects model approach to determine whether fertilization predicted changes in ANPP. Finally, we used generalized linear models to determine if species-level changes were best explained by unique groups of species.
Only N influenced the suite of community functional traits. Community weighted SLA, chlorophyll, and height increased with N addition, while LDMC decreased with N addition. Leaf thickness and toughness did not change significantly. At the species level, most species followed the community trend and increased in SLA, chlorophyll, height, and declined in LDMC with N addition. These intraspecific changes in functional trait account for 51% - 71% of the community-level change in SLA, chlorophyll, plant height, and LDMC. The remaining change is due to species compositional changes; the two most abundant species (Bouteloua gracilis and Carex filifolia) decreased in abundance under N addition while subdominant species increased in abundance. Aboveground net primary productivity (ANPP) increased by 13% with N addition, but the increase was not significant. These functional trait changes are associated with faster growth rate and higher resource use which are functional changes that can scale up to effect ecosystem function. These results demonstrate the importance of considering intraspecific trait changes and community structure and function as they are crucial steps in how environmental change scales up to influence ecosystem functions such as nutrient, energy, and water cycling.