Nitrogen Addition and Ecosystem Functioning: Changes in Species Composition Leaf Traits Amplify Increases in Leaf Area Index and Canopy Chlorophyll

Increased nutrient inputs can cause shifts in plant community composition and plant functional traits, both of which affect ecosystem function. We studied community- and species-level changes in specific leaf area (SLA), chlorophyll, leaf thickness, leaf toughness, plant height and leaf dry matter content (LDMC) in a full factorial nitrogen (N), phosphorus (P), potassium (K) fertilization experiment in a semi-arid grassland. Nitrogen was the only nutrient addition to significantly affect leaf functional traits, and N addition increased community weighted SLA by 19%, leaf chlorophyll content by 34%, height by 26%, and resulted in an 11% decrease in LDMC while leaf thickness and toughness did not change significantly. At the species level, most species contributed to the community weighted trait and increased in SLA, chlorophyll, height and LDMC with N. These intraspecific changes in functional traits account for 51% - 71% of the community-level increase in SLA and chlorophyll and plant height and decrease in LDMC. The remaining change is due to species abundance changes; the two most abundant species (Bouteloua gracilis and Carex filifolia) decreased in abundance under N addition while subdominant species increased in abundance. We also found annual variation in SLA, chlorophyll, plant height, and LDMC to be as important in influencing traits as N addition, likely due to differences in precipitation. Aboveground net primary productivity (ANPP) did not change significantly with N addition. However, N addition caused a 34% increase in leaf area index (LAI) and a 67% increase in canopy chlorophyll density. We demonstrate that nitrogen-induced changes in functional traits and species abundances can have profound effects on community structure and function which can magnify changes ANPP as reflected by LAI and canopy chlorophyll density. Therefore, ANPP may underestimate ecosystem level changes in the canopy vegetation