Forschergruppe Biodiversität - Experimental design

The role of biodiversity for element cycling and trophic interactions

an experimental approach in a grassland community

General Considerations

1. Plot Size

The size of experimental plots (20 m x 20 m) in our experiment is larger than in any previous biodiversity experiments. There are several advantages of such a large plot size:

Plots must be large enough to be able to study population dynamics and interactions between species. In small plots the contribution of a few large individuals may be exaggerated.
Large plots decrease the impact of scientific investigations on the establishment and dynamics of the experimental plant communities, thus enabling long-term observations.
Several subexperiments can be nested within the large plots, allowing for additional experimental manipulations to detect mechanisms of diversity effects and interactions between trophic levels.

2. Experimental Factors

Most previous biodiversity experiments found a positive relationship between species number and ecosystem processes provoking the need to better understand which functional characteristics of plants contribute to these positive effects.

Beside manipulating species richness our experiment therefore focuses on manipulation of functional diversity. To this end the total species pool has been classified into four functional groups. Species richness and functional group richness are varied as independently as possible and balanced representation of functional groups is ensured on all species richness levels.

This enables

decoupling of sampling and complementarity effects to the greatest possible extent
detection of general complementarity effects between functional groups
an increase of the probability to detect particular effect of special functional groups.

3. Experimental Mixtures

The following tables summarize the composition of experimental mixtures. For each species mixture(column) it gives the total number of species and how this level of species richness is composed of species from different functional groups.

Species Number

Functional
Group

1

1

1

1

2

2

2

2

2

2

2

2

4

4

4

4

4

4

4

4

4

4

4

4

4

Grasses

1

2

1

1

4

2

2

2

1

1

1

Small herbs

1

2

1

1

4

2

2

1

1

2

1

Tall Herbs

1

2

1

1

4

2

2

1

2

1

1

Legumes

1

2

1

1

4

2

2

1

2

1

1

Number of
Replicates

4

4

4

4

2

2

2

2

2

2

2

2

1

1

1

1

1

1

1

1

1

1

1

1

4

Species Number

Functional
Group

8

8

8

8

8

8

8

8

8

8

8

8

8

16

16

16

16

16

16

16

16

16

16

16

60

Grasses

8

4

4

2

3

3

2

16

8

8

5

5

6

4

16

Small herbs

8

4

4

3

3

2

2

8

8

5

5

6

4

12

Tall Herbs

8

4

4

3

2

3

2

16

8

8

6

5

5

4

18

Legumes

8

4

4

3

2

3

2

8

8

6

5

5

4

12

Number of
Replicates

1

1

1

1

1

1

1

1

1

1

1

1

4

1

1

1

1

1

1

1

1

1

1

4

4

Constrained by this functional group composition species have been randomly allocated to mixtures.

4. Blocking

Preliminary examination of soil characteristics revealed heterogeneity of the experimental field with respect to various relevant parameters (e.g. carbon content, C and N). The experimental plots are therefore arranged in four plot to account for this source of variability. Different species mixtures comprising the same combination of Species Richness x Functional Group Richness were randomly allocated to blocks. Detailed information about experimental species mixtures can be found here.

5. Sub-Plots

To assess scale effects and variability among plots of identical species composition, all species mixtures of the large plots were replicated on small 3.5 m x 3.5 m plots (Replication). In addition, replicates of monocultures of all 60 plant species were established on small plots, to provide a basis for measuring species specific traits and to calculate expected values such as relative yields for diversity mixtures based on the performance of the constituent species in monocultures (Monocultures). Finally, a Dominance Experiment was set up on additional 188 small plots using nine dominant species from the species pool (Dominance). It was specifically designed to disentangle the effects of species richness per se and the presence/absence of particular species or species pairs.