Effects of nutrient addition on plant community composition: a functional trait analysis in a long-term experiment

Abstract

The effects of nutrient availability on plant community composition and diversity have been well-documented, but the mechanisms behind the community response remain unclear. Plant species interact with variation in the environment though a suite of morphological, biochemical, and physiological traits known as functional traits. Analysis of functional traits can provide insights into the resource use strategies that allow plants to be successful in different environments. At two ends of a spectrum, species may exhibit conservative or exploitative strategies that differ in the rates at which they acquire and invest resources in structures and functions. Some functional traits have been shown to be related to resource use strategy. Additionally, functional traits can exhibit phenotypic response to changes in the environmental factors. The degree of phenotypic response may be ecologically important and relate to resource strategy, with exploitative species expected to have higher amounts of phenotypic response. This study, which takes place at a long-term experiment in a protected wetland site, examined eight functional traits of plant species, building upon the previously collected community data from the past 14 years. The long-term experiment was set up to study the effects of nutrient addition (fertilization) and disturbance (mowing) on plant community composition. The design, a 2x2 factorial, replicates fertilization and mowing treatments on eight blocks. A drainage ditch is also present and runs along one edge of the experimental array. Functional trait data were collected on 46 of the most common species at the site from plants in mowed/fertilized and mowed/unfertilized plots. Functional traits from three categories were sampled: leaf traits, leaf nutrient traits, and plant size traits. Data on species abundance and functional traits were integrated to calculate community-weighted trait means to provide insight into the mechanism behind changes in community composition due nutrient enrichment. Consistent with previous studies, our results showed that, in addition to the documented species composition differences between treatments, trait composition of the plots was different between fertilized and unfertilized plots. We found that mean community trait values in the fertilized plots were shifted in the direction expected for an exploitative resource use and acquisition strategy. We also found that more conservative trait values were present in the wetter plots found farther away from, and presumably less well drained by, the ditch. Traits and species varied in their amount of intraspecific variation, and overall trait composition was heavily influenced by phenotypic response. On average, phenotypic response to fertilization was in the direction expected of exploitative species. Our results suggest that community assembly in the long-term experiment is influenced by an environmental filter for species that exhibit exploitative traits or express such traits in response to fertilization. In contrast, we found no significant relationship across species between effect size of response in abundance to fertilization and mean trait values. We found no support for the hypothesis that species with high amounts of phenotypic response were more dominant in the fertilized plots or that species with an exploitative strategy exhibit higher amounts of phenotypic response. These results have implications for predicating how species and trait composition will change in response to anthropogenic influences on nutrient cycling and deposition to the environment.