Nitrogen addition and ecosystem functioning: Both species abundances and traits alter community structure and function

Johannes M. H. Knops,Anna R. Tatarko

doi:10.1002/ecs2.2087

Abstract

AbstractIncreased 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 leaf functional trait changes in a full factorial nitrogen (N), phosphorus (P), and 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 specific leaf area (SLA) by 19%, leaf chlorophyll content by 34%, height by 26%, and leaf dry matter content (LDMC) decreased by 11% 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 addition. These intraspecific changes in functional traits account for 51–71% of the community‐level changes in SLA, chlorophyll, plant height, and LDMC. The remaining change is due to species abundance changes; the two most abundant species (Bouteloua gracilis and Carex filifolia) decreased in abundance with 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 both functional traits and species abundances magnify ANPP changes in LAI and canopy chlorophyll density. Therefore, ANPP underestimates N addition‐induced ecosystem‐level changes in the canopy vegetation.

Highlights

Human activities are altering the availability of resources in ecosystems worldwide, and environmental change can cause shifts in plant community structure and function (Vitousek et al 1997, Peters and Meybeck 2000, Baer et al 2004)
Aboveground functional traits, which combined, are key in determining photosynthetic capacity, growth, and productivity: specific leaf area (SLA; Reich et al 1997, Garnier et al 2004), leaf chlorophyll content (Chapman and Barreto 1993, Wright et al 2004), plant height (Tilman 1987, Fonseca et al 2000, PerezHarguindeguy et al 2013), leaf thickness, leaf toughness, and leaf dry matter content (LDMC; Choong et al 1992, Cunningham et al 1999, Perez-Harguindeguy et al 2013)
Community SLA increased by 19% with N addition (v2 = 6.3, P = 0.012)

Summary

Introduction

Human activities are altering the availability of resources in ecosystems worldwide, and environmental change can cause shifts in plant community structure and function (Vitousek et al 1997, Peters and Meybeck 2000, Baer et al 2004). Many studies examining the effects of environmental change on plant communities focus on its isolated impact on community composition, functional trait values, or aboveground net primary productivity (ANPP; Stevens et al 2004, 2015, Yavitt et al 2011, Fay et al 2015, Harpole et al 2016). Environmental change can simultaneously effect species functional traits and abundances, shifting overall community structure and function (La Pierre and Smith 2014, Siefert and Ritchie 2016). We examine how shifts in both plant functional traits and species abundances contribute to community-level changes in plant resource acquisition–resource use strategies. Aboveground functional traits, which combined, are key in determining photosynthetic capacity, growth, and productivity: specific leaf area (SLA; Reich et al 1997, Garnier et al 2004), leaf chlorophyll content (Chapman and Barreto 1993, Wright et al 2004), plant height (Tilman 1987, Fonseca et al 2000, PerezHarguindeguy et al 2013), leaf thickness, leaf toughness, and leaf dry matter content (LDMC; Choong et al 1992, Cunningham et al 1999, Perez-Harguindeguy et al 2013)

Methods

Results

Conclusion