Overview

This project will test stoichiometric controls over the influence of plant functional diversity on ecosystem functioning across three trophic levels (plants, herbivores, and soil microbes) in the Inner Mongolian Grassland, part of the largest grassland in the world. It synthesizes core principles derived from biodiversity and ecosystem functioning (BEF) research and ecological stoichiometry (ES) to clarify some of the ambiguities over functional diversity and niche complementarity, and employs a removal design to eliminate several experimental artifacts associated with traditional BEF studies.

Direct and indirect modifications of biotic influences over ecosystem functioning through anthropogenic drivers such as habitat change, species invasion, climate change, pollution, and overexploitation have significantly altered ecosystem functioning and the services ecosystems provide. Understanding these impacts has proven to be an enormous challenge, and considerable research over the last decade has provided a variety of evidence for the influence of plant biodiversity, especially plant functional diversity, over ecosystem functioning . But our current understanding is neither precise nor complete.

Experimental Design

Our project provides four important advances in current BEF research.

First, while BEF studies have been conducted using a wide variety of organisms and approaches, experimental manipulations of species or functional diversity have been almost exclusively on European or North American grassland plots. It remains unclear if these studies from old fields, meadows, wetlands, and pasturelands can be applied to natural grassland ecosystems.

Second, while current evidence suggests that the chief mechanism for effects of biodiversity on ecosystem function is niche complementarity, neither an explicit quantification of complementarity among plant species nor a clear definition of its mechanisms has been accomplished. We address this limitation by using the theory of ecological stoichiometry as a basis for defining plant functional types (PFTs), for quantitatively assessing niche differentiation, and for deriving explicit hypotheses for predicting the response of the IMG to loss of plant functional diversity.

Third, past experimental approaches have had several limitations to applying results to natural systems. Traditional designs typically use topsoil removal or methyl bromylation to eliminate seedbanks, disking to remove vegetation, and seeding to regenerate vegetation, all of which represent major disturbances to the study system. Removal experiments provide the logical means to get around this problem, but they have been used only in a limited way and have other shortcomings. We address these shortcomings by employing a novel design in which PFTs will be manipulated in a removal, random loss design.

Fourth, much of grassland BEF research has focused on plants and aboveground production, but how increases in production through niche complementarity interact with other factors, such as N supply rate and herbivory remain unclear. We address this limitation by simultaneously manipulating N at 3 levels (ambient and two enrichment levels) while also manipulating livestock grazing. Furthermore, we examine the consequences of these manipulations for a variety of components, including insect herbivores (grasshoppers) and livestock (sheep).

Research Goals

Our primary goals are to experimentally determine how plant functional diversity (as delineated by stoichiometric parameters) affects ecosystem processes, and to assess how these effects are influenced by nutrient availability and the presence of higher trophic levels in the Inner Mongolian Grassland. To achieve these goals, our research program is structured by 4 hypotheses for which several specific predictions will be tested. In this study, we will measure functional diversity in terms of both the number of PFTs and Shannon diversity index of PFTs. Ecosystem functioning will be measured as aboveground net primary production (ANPP), belowground net primary production (BNPP), microbial biomass, C storage, N retention and N use efficiency, N mineralization, litter decomposition, and suitability of primary production for growth of higher trophic levels. Although our hypotheses below use EF as a general term, we will test these hypotheses with specific EF variables.