The subproject aims to better understand the mechanisms behind overall diversity effects by studying individual plant species and plant functional types (PFT). We aim to identify and quantify ecological traits, their plasticity in response to the species richness gradient and their importance for grassland functioning (i.e. net primary productivity, resource use, evapotranspiration, N₂-fixation, and plant species interactions) and for the provision of ecosystem services (i.e., soil fertility/nutrient retention, water use, herbage quality).

Each plant functional type (PFT) is represented by the following number of different plant species in the Jena Experiment: 16 grass species, 12 small herb species, 20 tall herb species, and 12 legume species. Although a principle component analysis showed that these PFT represent different traits in the overall design of the experiment (Roscher et al. 2004), the species combined into each group still contain very different traits: some are forming stolons or rhizomes, others form major storage organs or are rosette plants with a simple root system. Some species perform a short life cycle, while still others use different temporal niches (overwintering green vs. summergreen).

The following objectives are addressed:

to quantify relevant competitive plant traits and their plasticity for each plant functional type, based on assessments of individual plant species,
Earlier results of the Jena Experiment, but also of other studies point to the fact that the performance of different species at the same diversity level as well as their contributions to the overall diversity effect differ. However, which functional traits are the most relevant and how those functional traits of individual plant species change when grown as single individuals, in monoculture or in different mixtures is less clear.
to identify the functional interactions among plant species, and among plant functional types, at the level of plant individuals,
Functional interactions, e.g., facilitation between legumes and non-legumes, but also complementary resource use for light, nitrogen and water, appear to be key mechanisms to explain the observed positive biodiversity - productivity relationships. We therefore aim to quantify facilitation as well as resource complementarity among different species by using the naturally occurring differences in the stable nitrogen isotope ratios between N₂ fixing legumes and non-legumes as well as tracer labeling studies with ¹³C and ¹⁵N enriched compounds. Spatial niche partitioning, i.e., canopy structure, seems also important to explain the observed patterns. Thus, vertical profiles of relevant parameters (e.g., light, N allocation) are addressed.
to test the concept of niche complementarity based on the information gained above, and to quantify the role of major plant functional types on ecosystem functioning.

The subproject is organized in close cooperation between

Marlén Gubsch (ETH Zurich) with focus on grasses
Annett Winkler (University of Zurich, MPI-BGC) with focus on non-leguminous herbs
Christiane Roscher (MPI-BGC) with focus on legumes

Top Top of page

Methods

The assessment of plant functional traits and their plasticity under competitive pressure comprises the determination of many traits for plant growth strategy, above-ground biomass allocation and foliage parameters, often in relation to light or resource use. It is assumed that one of the main effects of competition will be through shading and overtopping, supported by sufficient resources. This competitive advantage of certain species will affect their morphology and physiology.

Plant functional traits and their plasticity are measured for all plant species in monocultures (under intraspecific competition) and in mixtures (under interspecific competition). Additionally, we established a "plant individual garden" where all species are grown as individuals with enough space to avoid competitive interactions between plant individuals.

Plant individual garden

Basic measurements taken for all species

Measurements are taken at the individual plant level with five to ten randomly selected plant modules twice a year (at peak biomass and second growth in late summer). Plant modules are studied under consideration of their vertical arrangement in the vegetation. Measurements are taken according to three sections (see Fig.)

The following traits are measured:

shoot height and stretched length
leaf number, leaf angle and leaf area
length of stem internodes
number of inflorescences (or length of inflorescences for grasses)
biomass of plant compartments (stems, leaves, inflorescences)

Plant material is analysed for carbon and nitrogen concentrations (plant compartments), and isotope ratios of C and N (δ¹⁵N,δ¹³C).

Specific measurements

Plant life history

Survival of plant individuals of selected non-leguminous herbs (Campanula patula, Crepis biennis, Plantago lanceolata, Taraxacum officinale) and legumes (Lotus corniculatus, Onobrychis viciifolia, Trifolium hybridum, Trifolium pratense) which we expect to have stronger population cycles than grasses and other herbs (many with clonal propagation) is followed by marking plant individuals. Ten individuals per species are marked in each mixture of the main experiment where these species occur. We will assess plant growth and reproductive variables of these individuals.

Light foraging

Plant individuals of creeping herbs with stolons (Ajuga reptans, Glechoma hederacea, Prunella vulgaris, Ranunculus repens, Veronica chamaedrys) are marked in all mixtures to follow the formation of stolons in relation to the light environment of the plant individuals.

Leaf turnover and resource allocation

Shoots of legumes and grass species are marked in monocultures to observe the seasonal rhythm of foliage. Additional measurements are taken to characterize resource allocation of grass species during the winter period. Above- and belowground biomass are harvested, separated into living and dead plant material and analysed for non-structural carbohydrates.

Functional traits and plasticity of Plant Functional Types (PFT)in experimental grasslands

People

Rationale

Methods

Functional traits and plasticity of Plant Functional Types (PFT)
in experimental grasslands