People

Michael Scherer-Lorenzen, Project P.I.
Bernhard Schmid, Project P.I.
Ernst-Detlef Schulze, Project P.I.
Christiane Roscher, PostDoc
Alex Fergus, Ph.D.-student
Lisa Marquard, Ph.D.-student
Peter Mwangi, Ph.D.-student
Jana Petermann, Ph.D.-student
Martin Schmitz, Ph.D.-student

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Rationale

Recent biodiversity research has focused on the relationship between manipulated biodiversity and ecosystem functioning but did not consider processes of community assembly, broadly defined as changes in species abundances over time. Discrepancies between experimental and observational studies indicate that e.g. high diversity-productivity combinations may not be sustainable in naturally assembling communities. The goal of this subproject is to study how and by which mechanisms biodiversity affects plant community assembly directly or via ecosystem functioning (e.g. biomass production). We will: a) analyze plant community compositional changes in terms of biomass and module populations from year 4 to 6 of the Jena Experiment (main and dominance plots) and relate these to variation in biodiversity and ecosystem functioning; b) test the effects of relaxing biodiversity manipulations on community composition and in turn resulting biodiversity-ecosystem functioning equilibria (invasion / succession subplots in main plots); c) test effects of management intensity on ecosystem functioning and community composition (associated experiment in subplots of main plots). Furthermore, we will d) assess plant life-history traits, in particular reproduction and recruitment from seed, at the different diversity levels to explain community shifts via population processes (main plots).

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Phase I (2002-2005):

Density x Evenness Experiment (Martin Schmitz)

By manipulation of density and evenness across all the diversity levels of the Jena-Experiment, the following hypotheses were tested:

• biodiversity-ecosystem functioning relationships are weaker at high community sowing density because of reduced evenness;
• biodiversity-ecosystem functioning relationships are stronger if species abundance distributions are flatter (high evenness).

Method

Density and evenness were manipulated in quadrants of 3.5 x 3.5 m plots (small replicates of the main experiment).

Some results

• Community productivity was positively related to species richness in all treatments.
• Realized plant density increased with increasing species richness.
• Different sowing densities resulted in different plant densities but similar biomass of experimental communities.
• Species abundance distributions rapidly converged between different density and evenness treatments.

Phytometer Experiment (Peter Mwangi)

Marked in-situ and transplanted "phytometers" were used to assess diversity effects at the level of single plant individuals and how individual responses might explain community responses to increasing biodiversity.

We tested the following hypotheses:

• individual plants (in-situ phytometers) grow better with increasing plant diversity, because con-specific neighbours are replaced by different ones, reducing niche overlap;
• "invaders" (transplanted phytometers) grow worse with increasing diversity, because on average there is a greater chance that other species with similar niches are already present in the community.

Method

1. In-situ phytometers

Selected individuals of four target species were marked. They belonged to the Dominance Experiment in which each species occurs in eight mixtures (each with two replicates) in all species richness levels (see also subproject "Community structure and plant competition in the dominance experiment (Weigelt)").

Among these are the dominance experiment plots with 1, 2, 3, 4, 6 and 9
species richness levels where individuals of four out of the pool of nine species
were marked and monitored.

2. Transplanted phytometers

Five individuals of Festuca pratense, Plantago lanceolata, Knautia arvensis and Trifolium pratense were transplanted into a 2 x 2 m subplot in the 20 x 20 m main plots.

Transplanted phytometers.

Some results

• Aboveground biomass of in-situ phytometers did not increase with increasing species richness and decreased for G. pratense.
• Aboveground plant biomass progressively decreased with increasing species richness. Grasses had a strong negative effect while legumes had a positive effect on the introduced plants except for the legume introductions.

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Phase II (2005-2008): Ongoing research

Invasion / Succession experiment (Christiane Roscher, Alexander Fergus, Jana Petermann)

The invasion / succession study is an additional experiment nested within the large plots of the main experiment.
The following hypotheses are tested with the invasion/succession experiment:

• randomly assembled experimental communities of above or below "natural" diversity-productivity values are instable and converge towards a "natural" level if biodiversity manipulations are discontinued (allowing succession via invasion and extinction processes;
• natural community assembly does not lead to maximum diversity-productivity values because a species' persistence ability is poorly related to its ability to contribute to high ecosystem functioning at high biodiversity;
• even after succession there will not be a single dominant species, but rather a species mixture will persist whose composition will reflect influences of initial composition of the designed communities.

Specifically, we will also test, if

• invasion resistance increases with species and functional richness;
• non-weeded communities are more invasion resistant;
• sown invaders are less successful if they belong to a functional group already present in the initial community;
• forced invasion leads to community convergence.

Method

The experiment consists of six treatments in equally-sized subplots: two subplots were never weeded (early succession treatment), two subplots were weeded in phase one, but weeding has ceased (cessation of weeding treatment), and two subplots have been regularly weeded throughout the experiment (continuous weeding treatment). In each of the three pairs of treatment plots, one subplot (forced invasion) received the complete species pool via sowing in April 2005, the other subplot received only spontaneously invading seeds (spontaneous invasion), with the exception of the continuous weeding treatment.

Community convergence (Alexander Fergus)

We will attempt to test Egler`s (1954) theory of Initial Floristic Composition, which stipulates that all species likely to be involved in a succession are present from the beginning. The continuous seed addition and the presence of a control sub-plot will permit us to analyse whether or not species richness, ecosystem functioning, and community composition converge rapidly. Such a result would indicate that the Initial Floristic Composition model does not operate in grassland communities and perhaps not in other higher communities also.

Method

Seeds of all species in the experimental species pool will be sown at medium and high densities, four times a year, into randomised subplots (50x50cm) of the small replicate plots, at all diversity levels. A control subplot of the same size will receive no seed addition, and will be weeded. Spontaneous invaders which do not belong to the experimental species pool will be weeded in all subplots. Species compositions will be followed via sampling of cover and biomass.

Temporal changes in experimental grassland communities (Elisabeth Marquard)

The second phase of the Jena Experiment offers the opportunity to study the response of experimental plant communities to different diversity levels over a time period of several years. The focus is on population-ecological aspects such as species composition, genetic diversity, and demographic structure. In a community-based approach, the following hypotheses will be tested:

• compositional changes (shifting species abundance distributions, dominance or evenness indices) are related to initial species richness and functional diversity;
• selection as well as non-adaptive processes such as drift and inbreeding differ across diversity levels and lead to changes in genetic diversity within plant communities;
• the demographic structure of the experimental communities is influenced by their species or functional richness and contributes causally to the observed chances in composition or aboveground productivity.

Method

Measurements involve vegetation cover estimates, biomass harvests and censuses of life phases as well as quantitative and molecular genetics.

Estimation of cover by a modified point-intersect method.

Management experiment (Michael Scherer-Lorenzen)

In a management experiment, different mowing and fertilization treatments are applied in order to test the following hypotheses:

• the addition of nutrients will increase productivity, and change species composition (dominance of nutrient-demanding species), leading to higher forage quality;
• frequent mowing will alleviate the dominance of few species under higher fertilization due to reduction of competitive strength of dominant species;
• fertilizer application increases and mowing (also a simulated grazing treatment) decreases the slope of biodiversity–ecosystem functioning relationships;
• the increased/decreased ecosystem functioning leads to more/less asymmetric plant competition and less/more even abundance distributions.

Method

Four treatments will be applied on 1.6 x 4 m subplots within the main plots of the Jena-Experiment:

• No fertilizer, 1x mowing
• No fertilizer, 2x mowing
• NPK-fertilization (corresponding to 100 kg N ha^-1 yr^-1), 2x mowing
• NPK-fertilization (corresponding to 100 kg N ha^-1 yr^-1), 4x mowing
• NPK-fertilization (corresponding to 200 kg N ha^-1 yr^-1), 4x mowing

Application of NPK fertilizer to the experimental plots in April 2006

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